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Charles Darwin’s Theory of Evolution

 

As a youth, Charles Darwin was passionately interested in natural history — the study of how different organisms carry out their lives. He briefly studied medicine at Edinburgh but he was nauseated by observing surgery conducted without anesthesia. He gave up medicine to study for a career as a clergyman of the Church of England at Cambridge University. However, he was more interested in natural history than theology, and he became a companion of scientists on the faculty, especially the botanist John Henslow. Darwin was given an unprecedented opportunity when in 1831 Henslow recommended him for a position as ship’s naturalist on the H.M.S. Beagle which was preparing for a survey voyage around the world. Whenever possible during the 5-year voyage, Darwin (who was often seasick) went ashore to observe and collect specimens of plants and animals. He noticed that the species he saw in South America differed strikingly from those of Europe. He observed that the species of the temperate regions of South America (Argentina, Chile) were more similar to those of tropical South America (Brazil) than they were to European species. When he explored the Galápagos Islands, west of Ecuador, he noted that most of its animal species were found nowhere else, but were similar to those of mainland South America, 1,000 kilometers to the east. Darwin also recognized that the animals of the archipelago differed from island to island. He postulated that some animals had dispersed from mainland South America and then evolved differently on each of the islands. When he returned to England in 1836, Darwin continued to ponder his observations. Within a decade he had developed the major features of his theory which had two major components:

- Species are not immutable; they change over time. (In other words, Darwin asserted that evolution is a historical fact that can be demonstrated to have taken place.)

- The agent that produces these changes is natural selection. Darwin wrote a long essay on natural selection and the origin of species in 1844, but, despite urging from his wife and colleagues, he was reluctant to publish it, preferring to assemble more evidence first.

Darwin’s hand was forced in 1858 when he received a letter and manuscript from another traveling naturalist, Alfred Russel Wallace, who was studying plants and animals in the East Indies. Wallace asked Darwin to evaluate the manuscript, in which Wallace proposed a theory of natural selection almost identical to Darwin’s. At first Darwin was dismayed, believing that Wallace had preempted his idea. But parts of Darwin’s 1844 essay, together with Wallace’s manuscript, were presented to the Linnaean Society of London on July 1, 1858, thereby giving credit for the idea to both men. Darwin then worked quickly to finish his own book, The Origin of Species, which was published the next year. Although both men conceived of natural selection independently, Darwin developed his ideas first, and The Origin of Species provided an enormous amount of evidence from many fields to support both the concept of natural selection and evolution itself, which is why these concepts are more closely associated with the name Darwin than Wallace. The facts that Darwin used to conceive and develop his theory of evolution by natural selection were familiar to most contemporary biologists. His unique insight was to perceive the significance of relationships among them. On September 28, 1838, Darwin happened to read An Essay on the Principle of Population by Thomas Malthus, an economist. Malthus argued that because the rate of human population growth is greater than the rate of increase in food production, unchecked growth inevitably leads to famine. Darwin recognized that populations of all species have the potential for exponential increases in numbers. To illustrate this point, he used the following example: Suppose…there are eight pairs of birds, and that only four pairs of them annually…rear only four young and that these go on rearing their young at the same rate, then at the end of seven years…there will be 2048 birds instead of the original sixteen.



Yet such rates of increase are rarely seen in nature. Therefore, Darwin reasoned that death rates in nature must also be high. Without high death rates, even the most slowly reproducing species would quickly reach enormous population sizes. Darwin also observed that, although offspring tend to resemble their parents, the offspring of most organisms are not identical to one another or to their parents. He suggested that slight variations among individuals significantly affect the chance that a given individual will survive and reproduce. Darwin called this differential survival and he called reproduction of individuals natural selection. Darwin may have used the words “natural selection” because he was familiar with the artificial selection of individuals with certain desirable traits by animal and plant breeders. Many of Darwin’s observations on the nature of variation came from domesticated plants and animals. Darwin was a pigeon breeder and he knew firsthand the astonishing diversity in color, size, form and behavior that pigeon breeders could achieve (Figure). He recognized close parallels between selection by breeders and selection in nature. As he argued in The Origin of Species, How can it be doubted, from the struggle each individual has to obtain subsistence, that any minute variation in structure, habits or instincts, adapting that individual better to the new conditions, would tell upon its vigour and health? In the struggle it would have a better chance of surviving; and those of its offspring which inherited the variation, be it ever so slight, would have a better chance. That statement, written almost 150 years ago, still stands as a good expression of the theory of evolution by natural selection. It is important to remember, as Darwin clearly understood, that individuals do not evolve; populations do. A population is a group of individuals of a single species that live in a particular geographic area at the same time. A major consequence of the evolution of populations is that their members become adapted to the environments in which they live. The term adaptation has two meanings in evolutionary biology. The first meaning refers to the processes by which adaptive traits are acquired—that is, the evolutionary mechanisms that produce them. We will discuss those processes in great detail in this chapter. The second meaning refers to traits that enhance the survival and reproductive success of their bearers. For example, wings are adaptations for flight, and a spider’s web is an adaptation for capturing flying insects. Biologists regard an organism as being adapted to a particular environment when they can imagine—or better still, measure the performance of—a slightly different organism that reproduces and survives less well in that environment. To understand adaptation, biologists compare the performance of individuals within or among species that differ in their traits. For example, to investigate the adaptive nature of spiders’ webs, we might try to determine the effectiveness of slightly different web structures in capturing insects. We might also measure changes in the webs of the same species in different environments. With these data, we could understand how variations in web structure influenced the survival and reproductive success of their builders. When Darwin proposed his theory of evolution by natural selection, he had no examples of evolutionary agents operating in nature. Since then many studies of the action of evolutionary agents have been conducted. Similarly, many investigations have documented changes over time in the genetic composition of a population. Darwin understood the importance of heredity for his theory but he knew nothing of the mechanisms of heredity. He devoted considerable time developing a theory of heredity but he failed in this effort. Fortunately, the rediscovery of Gregor Mendel’s publications in the early 1900s paved the way for the development of population genetics, which provides a major underpinning for Darwin’s theory. Population geneticists apply Mendel’s laws to entire populations of organisms. They also study variation within and among species to understand the processes that result in evolutionary changes in species through time. The perspective of population genetics given in this chapter which emphasizes the role of variation in characteristics of adult organisms.

 

Genetic Variation within Populations

For a population to evolve, its members must possess heritable genetic variation which is the raw material on which agents of evolution act. In everyday life, we do not directly observe the genetic compositions of organisms. What we do see in nature are phenotypes, the physical expressions of organisms’ genes. The features of a phenotype are its characters — eye color, for example. The specific form of a character, such as brown eyes, is a trait.Aheritable trait is a characteristic of an organism that is at least partly determined by its genes. The agents of evolution generally act on phenotypes but for the moment we will concentrate on genetic variation within populations. We will do so because genetic variation is what is passed on to offspring via gametes — eggs and sperm. The genetic constitution that governs a character is called its genotype. A population evolves when individuals with different genotypes survive or reproduce at different rates. Those different forms of a gene, called alleles, may exist at a particular locus. A single individual has only some of the alleles found in the population to which it belongs. The sum of all copies of alleles found in the population constitutes its gene pool. The gene pool contains the variation that produces the phenotypic characters on which agents of evolution act. To understand evolution, we need to know how much genetic variation populations have, the sources of that genetic variation, and how genetic variation is maintained and expressed in populations over space and time.

 

Most populations are genetically variable

Nearly all populations contain some level of genetic variation for many characters. Artificial selection on different characters in a European species of wild mustard produced many important crop plants. Plant and animal breeders could achieve such results because the original population had genetic variation for the characters of interest.

Laboratory experiments also demonstrate the existence of considerable genetic variation in populations. In one such experiment, investigators chose fruit flies (Drosophila melanogaster) with either high or low numbers of bristles on their abdomens as parents for subsequent generations of flies. After 35 generations, all flies in both the high-bristle and lowbristle lineages had bristle numbers that fell well outside the range found in the original population. Thus, there must have been considerable variation in the original fruit fly population for selection to act on. The study of the genetic basis of evolution is difficult because genotypes do not uniquely determine phenotypes. With dominance, for example, a particular phenotype can be produced by more than one genotype (e.g., AA and Aa individuals may be phenotypically identical). Similarly, different phenotypes can be produced by a given genotype, depending on the environment encountered during development. For example, the cells of all the leaves on a tree or shrub are normally genetically identical, yet leaves on the same tree often differ in shape and size. Leaves closer to the top of an oak tree, where they receive more wind and sunlight, may be more deeply lobed than the shaded leaves growing lower down on the same tree. The same differences can be seen between the leaves of individuals growing in sunny and in shady sites. Thus, the phenotype of an organism is the outcome of a complex series of developmental processes that are influenced by both the environment and its genes.

How do we measure genetic variation?

A locally interbreeding group within a geographic population is called a Mendelian population. Mendelian populations are often the subjects of evolutionary studies. To measure genetic variation in a Mendelian population precisely, we would need to count every allele at every locus in every individual in it. By doing so, we could determine the relative proportions, or frequencies, of all alleles in the population. Fortunately, we do not need to make such complete measurements, because we can reliably estimate allele frequencies for a given locus by counting alleles in a sample of individuals from the population. The sum of all allele frequencies at a locus is equal to 1, so measures of allele frequency range from 0 to 1. An allele’s frequency is calculated using the following formula:

 

p = number of copies of the allele in the population

sum of alleles in the population

 

If only two alleles (for example, A and a) for a given locus are found among the members of a diploid population, they may combine to form three different genotypes: AA, Aa, and aa. Using the formula above, we can calculate the relative frequencies of alleles A and a in a population of N individuals as follows:

- Let NAA be the number of individuals that are homozygous for the A allele (AA).

- Let NAa be the number that are heterozygous (Aa).

- Let Naa be the number that are homozygous for the a allele (aa).

Note that NAA + NAa + Naa = N, the total number of individuals in the population, and that the total number of copies of both alleles present in the population is 2N because each individual is diploid. Each AA individual has two copies of the A allele, and each Aa individual has one copy of the A allele. Therefore, the total number of A alleles in the population is

2NAA + NAa. Similarly, the total number of a alleles in the population is 2Naa + NAa. If p represents the frequency of A and q represents the frequency of a, then

2NAA+NAa

P=---------

2N

 

fnd

2Naa +NAa

q =---------

2N

To show how this formula works, calculates allele frequencies in two populations, each containing 200 diploid individuals. Population 1 has mostly homozygotes (90 AA, 40 Aa, and 70 aa); population 2 has mostly heterozygotes (45 AA, 130 Aa, and 25 aa). The calculations demonstrate two important points. First, notice that for each population, p + q = 1. If there is only one allele in a population, its frequency is 1. If an allele is missing from a population, its frequency is 0, and the locus in that population is represented by one or more other alleles. Since p + q = 1, then q = 1 – p. So when there are only two alleles at a given locus in a population, we can calculate the frequency of one allele and then easily obtain the second allele’s frequency by subtraction. The second thing to notice is that both population 1 (consisting mostly of homozygotes) and population 2 (consisting mostly of heterozygotes) have the same allele frequencies for A and a. Therefore, they have the same gene pool for this locus.

However, because the alleles in the gene pool are distributed differently, the genotype frequencies of the two populations differ. Genotype frequencies are calculated as the number of individuals that have the genotype divided by the total number of individuals in the population. In population 1 in Figure, the genotype frequencies are 0.45 AA, 0.20 Aa, and 0.35 aa. The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian population describe its genetic structure. Allele frequencies measure the amount of genetic variation in a population; genotype frequencies show how a population’s genetic variation is distributed among its members. With these measurements, it becomes possible to consider how the genetic structure of a population changes or does not change over generations.

 

 

The Hardy – Weinberg Equilibrium

If certain conditions are met, the genetic structure of a population may not change over time. The necessary conditions for such an equilibrium were deduced independently in 1908 by the British mathematician Godfrey Hardy and the German physician Wilhelm Weinberg. Hardy wrote his equations in response to a question posed to him by the geneticist Reginald C. Punnett (the inventor of the Punnett square) at the Cambridge University faculty club. Punnett wondered at the fact that even though the allele for short, stubby fingers (a condition called brachydactyly) was dominant and the allele for normal-length fingers was recessive, most people in Britain have normal-length fingers. Hardy’s equations explain why dominant alleles do not necessarily replace recessive alleles in populations, as well as other features of the genetic structure of populations. The Hardy–Weinberg equilibrium applies to sexually reproducing organisms. The particular example we will illustrate here assumes that the organism in question is diploid, its generations do not overlap, the gene under consideration has two alleles, and allele frequencies are identical in males and females. The Hardy–Weinberg equilibrium also applies if the gene has more than two alleles and generations overlap but in those cases the mathematics is more complicated. Several conditions must be met for a population to be at Hardy–Weinberg equilibrium:

- Mating is random

- Population size is very large

- There is no migration between populations

- There is no mutation

- Natural selection does not affect the alleles under consideration

If these conditions hold, two major consequences follow. First, the frequencies of alleles at a locus will remain constant from generation to generation. And second, after one generation of random mating, the genotype frequencies will remain in the following proportions: Genotype AA Aa aa Frequency p2 2pq q2 Stated another way, the equation for Hardy–Weinberg equilibrium is

p2 + 2pq + q2 = 1

To see why, consider population 1 in Figure 23.6, in which the frequency of A alleles (p) is 0.55. Because we assume that individuals select mates at random, without regard to their genotype, gametes carrying A or a combine at random—that is, as predicted by the frequencies p and q. The probability that a particular sperm or egg in this example will bear an A allele rather than an a allele is 0.55. In other words, 55 out of 100 randomly sampled sperm or eggs will bear an A allele. Because q = 1 – p, the probability that a sperm or egg will bear an a allele is 1 – 0.55 = 0.45. To obtain the probability of two A-bearing gametes coming together at fertilization, we multiply the two independent probabilities of their occurring separately: p p = p2 = (0.55)2 = 0.3025 Therefore, 0.3025, or 30.25 percent, of the offspring in the next generation will have the AA genotype. Similarly, the probability of bringing together two a-bearing gametes is

q q = q2 = (0.45)2 = 0.2025

Thus, 20.25 percent of the next generation will have the aa genotype. Figure also shows that there are two ways of producing a heterozygote: An A sperm may combine with an a egg, the probability of which is p q; or an a sperm may combine with an A egg, the probability of which is q p. Consequently, the overall probability of obtaining a heterozygote is 2pq. It is now easy to show that the allele frequencies p and q remain constant for each generation. If the frequency of A alleles in a randomly mating population is p2 + pq, this frequency becomes p2 + p(1 – p) = p2 + p p2 = p, the original allele frequencies are unchanged, and the population is at Hardy–Weinberg equilibrium.

If some agent, such as emigration, were to alter the allele frequencies, the genotype frequencies would automatically settle into a predictable new set in the next generation. For instance, if only AA and Aa individuals left the population, p and q would change, but there would still be aa individuals in the population.

 

Why is the Hardy–Weinberg equilibrium important?

You may already have realized that populations in nature rarely meet the stringent conditions necessary to maintain them at Hardy–Weinberg equilibrium. Why, then, is the Hardy-Weinberg equilibrium considered so important for the study of evolution? The answer is that without it, we cannot tell whether or not evolutionary agents are operating. The most important message of the Hardy–Weinberg equilibrium is that allele frequencies remain the same from generation to generation unless some agent acts to change them. In order to ascertain that evolutionary agents are in play, we must estimate the actual allele or genotype frequencies present in a population and then compare them with the frequencies that would be expected at Hardy–Weinberg equilibrium. The pattern of deviation from the Hardy–Weinberg expectations tells us which assumptions are violated. Thus, we can identify the agents of evolutionary change on which we should concentrate our attention.

 

Evolutionary Agents and Their Effects

Evolutionary agents are forces that change the genetic structure of a population. In other words, they cause deviations from the Hardy–Weinberg equilibrium. The known evolutionary agents are mutation, gene flow, genetic drift, non-random mating, and natural selection. Although only natural selection results in adaptation, to understand evolutionary processes we need to discuss all of these evolutionary agents before considering natural selection in detail.

 

Mutations are changes in the genetic material

The origin of genetic variation is mutation. A mutation, is any change in an organism’s DNA. Mutations appear to be random with respect to the adaptive needs of organisms. Most mutations are harmful to their bearers or are neutral, but if environmental conditions change, previously harmful alleles may become advantageous. In addition, mutations can restore to populations alleles that other evolutionary agents remove. Thus mutations both create and help maintain genetic variation within populations. Mutation rates are very low for most loci that have been studied. Rates as high as one mutation per locus in a thousand zygotes per generation are rare; one in a million is more typical. Nonetheless, these rates are sufficient to create considerable genetic variation because each of a large number of genes may mutate, frame-shift mutations may change many genes simultaneously, and populations often contain large numbers of individuals. For example, if the probability of a point mutation were 10–9 per base pair per generation, then in each human gamete, the DNA of which contains 3. Therefore, each zygote would carry, on average, six new mutations, and the current human population of about 8 billion people would be expected to carry about 48 billion new mutations that were not present one generation earlier. One condition for Hardy–Weinberg equilibrium is that there be no mutation. Although this condition is never strictly met, the rate at which mutations arise at single loci is usually so low that mutations by themselves result in only very small deviations from Hardy–Weinberg expectations. If large deviations are found, it is appropriate to dismiss mutation as the cause and to look for evidence of other evolutionary agents acting on the population.

 

Movement of individuals or gametes, followed by reproduction, produces gene flow

Few populations are completely isolated from other populations of the same species. Migrations of individuals and movements of gametes between populations are common. If the arriving individuals or gametes reproduce in their new location, they may add new alleles to the gene pool of the population, or they may change the frequencies of alleles already present if they come from a population with different allele frequencies. For a population to be at Hardy–Weinberg equilibrium, there must be no gene flow from populations with different allele frequencies.

 

Genetic drift may cause large changes in small populations

In very small populations, genetic drift — the random loss of individuals and the alleles they possess — may produce large changes in allele frequencies from one generation to the next. Harmful alleles, for example, may increase in frequency because of genetic drift, and rare advantageous alleles may be lost. As we will see later, even in large populations, genetic drift can influence the frequencies of alleles that do not influence the survival and reproductive rates of their bearers. Populations that are normally large may pass through occasional periods when only a small number of individuals survive. During these population bottlenecks, genetic variation can be reduced by genetic drift. Most of the “surviving” beans in the small sample taken from the bean population are, just by chance, red, so the new population has a much higher frequency of red beans than the previous generation had. In a natural population, the allele frequencies would be said to have “drifted.” Suppose we perform a cross of Aa xAa individuals of a species of Drosophila to produce an F1 population in which p = q = 0.5 and in which the genotype frequencies are 0.25 AA, 0.50 Aa, and 0.25 aa. If we randomly select 4 individuals (= 8 copies of the gene) from among the offspring to produce the F2 generation, the allele frequencies in this small sample may differ markedly from p = q = 0.5. If, for example, we happen by chance to draw 2 AA homozygotes and 2 heterozygotes (Aa), the allele frequencies in this “surviving population” will be p = 0.75 (6 out of 8) and q = 0.25 (2 out of 8). If we replicate this sampling experiment 1,000 times, one of the two alleles will be missing entirely from about 8 of the 1,000 “surviving populations.” These numbers show that, as it passes through a bottleneck, a population may lose much of its genetic variation. This is what happened to greater prairie chickens, millions of which lived in the prairies of North America when Europeans first arrived there. As a result of both hunting and habitat destruction, the Illinois population of prairie chickens plummeted from about 100 million birds in 1900 to fewer than 50 individuals in the 1990s. Acomparison of DNA from birds collected in Illinois during the middle of the twentieth century with DNA from the surviving population in the 1990s showed that Illinois prairie chickens had lost most of their genetic diversity. As a result, both hatching success and chick survival were low. To increase the genetic diversity of Illinois prairie chickens, birds from Minnesota, Kansas, and Nebraska were introduced to Illinois. They interbred with the Illinois birds, restoring much of the genetic diversity of that population, which is now increasing in size. When a few pioneering individuals colonize a new region, the resulting population is unlikely to have all the alleles found among members of its source population. The resulting change in genetic variation, called a founder effect, is equivalent to that in a large population reduced by a bottleneck. Scientists were given an opportunity to study the genetic composition of a founding population when Drosophila subobscura, a well-studied European species of fruit fly, was discovered near Puerto Montt, Chile, in 1978 and at Port Townsend, Washington, in 1982. In both South and North America, populations of the flies grew rapidly and expanded their ranges. Today in North America, D. subobscura ranges from British Columbia, Canada, to central California. In Chile it has spread across 23° of latitude, nearly as wide a range as the species has in Europe. The D. subobscura founders probably reached Chile and the United States from Europe aboard the same ship because the two populations are genetically very similar. For example, the North and South American populations have only 20 chromosomal inversions, 19 of which are the same on the two continents, whereas 80 inversions are known from European populations. North and South American populations also have lower allelic diversity at enzyme-producing genes than European populations do. Only alleles that have a frequency higher than 10 percent in European populations are present in the Americas. Thus, as expected for a small founding population, only a small part of the total genetic variation found in Europe reached the Americas. Geneticists estimate that at least ten, but no more than a hundred, flies founded the North and South American populations.

 

Nonrandom mating changes the frequency of homozygotes

Mating patterns may alter genotype frequencies if individuals in a population choose other individuals of certain genotypes as mates. For example, if they mate preferentially with individuals of the same genotype, then homozygous genotypes will be overrepresented, and heterozygous genotypes underrepresented, in the next generation in comparison with Hardy–Weinberg expectations. Alternatively, individuals may mate primarily or exclusively with individuals of different genotypes. An example of such nonrandom mating is provided by plant species, such as primroses (Primula), that bear flowers of two different types. One type, known as pin, has a long style (female reproductive organ) and short stamens (male reproductive organs). The other type, known as thrum, has a short style and long stamens. Pollen grains from pin and thrum flowers are deposited on different parts of the bodies of insects that visit the flowers. When the insects visit other flowers, pollen grains from pin flowers are most likely to come into contact with stigmas of thrum flowers, and vice versa. In most species with this reciprocal arrangement, pollen from one flower type can fertilize only flowers of the other type. Self-fertilization (selfing), another form of nonrandom mating, is common in many groups of organisms, especially plants. Selfing reduces the frequencies of heterozygous individuals below Hardy–Weinberg expectations and increases the frequencies of homozygotes, without changing allele frequencies.

 

Natural selection results in adaptation

The evolutionary agents we have just discussed influence the frequencies of alleles and genotypes in populations. As we saw in the previous chapter, major perturbations, such as colliding continents, volcanic eruptions, and meteorite impacts, also have periodically altered the survival and reproductive rates of organisms. All of these agents dramatically affect the course of life’s evolution on Earth, but none of them result in adaptations. For adaptation to occur, individuals that differ in heritable traits must survive and reproduce with different degrees of success. When some individuals contribute more offspring to the next generation than others, allele frequencies in the population change in a way that adapts individuals to the environments that influenced their success. This process is known as natural selection. The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phenotypes is called its fitness. The word “relative” is critical: The absolute number of offspring produced by an individual does not influence the genetic structure of a population. Changes in absolute numbers of offspring are responsible for increases and decreases in the size of a population, but only the relative success of different phenotypes within a population leads to changes in allele frequencies — that is, to evolution. To contribute genes to subsequent generations, individuals must survive to reproductive age and produce offspring. The relative contribution of individuals of a particular phenotype is determined by the probability that those individuals survive multiplied by the average number of offspring they produce over their lifetimes. In other words, the fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype.

 

The Results of Natural Selection

Natural selection can act on characters with quantitative variation in any one of several different ways, producing quite different results:

- Stabilizing selection preserves the average characteristics of a population by favoring average individuals.

- Directional selection changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population.

- Disruptive selection changes the characteristics of a population by favoring individuals that vary in both directions from the mean of the population.

 

Stabilizing selection

If both the smallest and the largest individuals in a population contribute relatively fewer offspring to the next generation than those closer to the average size do, then stabilizing selection is operating. Stabilizing selection reduces variation, but does not change the mean. Natural selection frequently acts in this way, countering increases in variation brought about by genetic recombination, mutation, or migration. Rates of evolution are typically very slow because natural selection is usually stabilizing. Stabilizing selection operates, for example, on human birth weight. Babies born lighter or heavier than the population mean die at higher rates than babies whose weights are close to the mean. This was especially true before modern medical advances.

 

Directional selection

If individuals at one extreme of a character distribution—the larger ones, for example—contribute more offspring to the next generation than other individuals do, then the average value of that character in the population will shift toward that extreme. In this case, directional selection is operating. If directional selection operates over many generations, an evolutionary trend within the population results. Such directional evolutionary trends often continue for many generations, but they may be reversed when the environment changes and different phenotypes are favored, or they may be halted when an optimum is reached, or when trade-offs oppose further change. The character then falls under stabilizing selection. Directional selection produced the resistance to tetrodotoxin (TTX) by some garter snakes that we discussed at the beginning of this chapter. The common garter snake, Thamnophis sirtalis, is the only predator of the rough-skinned newt, Taricha granulosa, known to be resistant to TTX. Resistance to TTX has evolved independently at least twice within T. sirtalis populations in western North America, once in California and once in Oregon. This resistance is due to genetically based differences in the ability of sodium channels in the snake’s nerves and muscles to continue functioning when exposed to variable concentrations of TTX.

 

Disruptive selection

When disruptive selection operates, individuals at both extremes of a character distribution contribute more offspring to the next generation than do those close to the mean, producing two peaks in the distribution. This type of selection is apparently rare. The strikingly bimodal (two-peaked) distribution of bill sizes in the black-bellied seedcracker (Pyrenestes ostrinus), a West African finch, illustrates how disruptive selection can influence populations in nature. The seeds of two types of sedges (marsh plants) are the most abundant food source for these finches during part of the year. Birds with large bills can readily crack the hard seeds of the sedge Scleria verrucosa. Birds with small bills can crack S. verrucosa seeds only with difficulty, but they feed more efficiently on the soft seeds of S. goossensii than do birds with larger bills. Young finches whose bills deviate markedly from the two predominant bill sizes do not survive as well as finches whose bills are close to one of the two sizes represented by the distribution peaks. Because there are few abundant food sources in the environment, and because the seeds of the two sedges do not overlap in hardness, birds with intermediate- sized bills are inefficient in using either one of the principal food sources. Disruptive selection therefore maintains a bimodal bill size distribution.

 

Sexual selection results in conspicuous traits

In The Origin of Species, Darwin devoted a few pages to sexual selection, a topic that he developed at length in another book, The Descent of Man, and Selection in Relation to Sex, in 1871. Sexual selection was Darwin’s explanation for the evolution of apparently useless but conspicuous traits in males of many species, such as bright colors, long tails, horns, antlers, and elaborate courtship displays. He hypothesized that these traits either improved the ability of their bearers to compete for access to members of the other sex (intrasexual selection) or made their bearers more attractive to members of the other sex (intersexual selection). Darwin argued that female preferences for such features are also the result of sexual selection because “unornamented, or unattractive males would succeed equally in the battle for life and in leaving a numerous progeny, but for the presence of better endowed males.” Sexual selection may result in species that are sexually dimorphic — that is, species in which males and females differ in size, shape, or color. The concept of sexual selection was not well received by Darwin’s contemporaries. However, many examples of sexual selection have been investigated in the century and a half since he first proposed the idea, and Darwin turned out to be right. For example, sexual selection is responsible for different morphological attributes of male birds that compete with each other for available females. One case in point is the remarkable tails of male African long-tailed widowbirds which are longer than their heads and bodies combined. To examine the role of sexual selection in the evolution of widowbird tails, a behavioral ecologist captured some male widowbirds. He shortened the tails of some males by cutting them and lengthened the tails of others by gluing on additional feathers. Male widowbirds normally select and defend from other males, a site where they perform courtship displays to attract females. Both short-tailed and long-tailed males successfully defended their display sites, indicating that a long tail does not confer an advantage in male–male competition. However, males with artificially elongated tails attracted about four times more females than did males with shortened tails. Why do female widowbirds prefer males with long tails? The ability to grow and maintain a costly feature such as a long tail may indicate that the male bearing it is vigorous and healthy. The hypothesis that having well-developed ornamental traits signals vigor and health has been tested experimentally with captive zebra finches. The bright red bills of male zebra finches are the result of red and yellow carotenoid pigments. Zebra finches (and most other animals) cannot synthesize carotenoids and must obtain them from their food. In addition to influencing bill color, carotenoids are antioxidants and components of the immune system. Males in good health may need to allocate fewer carotenoids to immune function than males in poorer health. If so, then females can use the brightness of his bill to assess the health of a male. Investigators manipulated blood levels of carotenoids in male zebra finches by means of carotenoid supplements. Experimental males were given drinking water with carotenoids added; control males were given only distilled water. All the males had access to the same food. After one month, the experimental males had higher levels of carotenoids in their blood, had much brighter bills than the control males and were preferred by female zebra finches. Next, the investigators challenged both groups of males immunologically by injecting phytohemagglutinin (PHA) into the webs of their wings. PHA induces a response by T lymphocytes, resulting in an accumulation of white blood cells and thus a thickening of the skin. Experimental males with enhanced carotenoid levels developed thicker skins because they responded more strongly to PHA than control males did, indicating a heightened immune system. This experiment showed that when a female chooses the male with a bright red bill, she probably gets a mate with a healthy immune system. Such males are less likely to become infected with parasites and diseases, so they are less likely to pass on infections to their mates. Healthier males are also better able to assist with parental care than are males with duller bills.

 

Assessing the Costs of Adaptations

Adaptations typically impose costs as well as benefits, and the evolution of adaptations depends on the trade-off between those costs and benefits. Garter snakes in some populations, for example, can eat rough-skinned newts without being poisoned, but they pay for this ability by sacrificing crawling speed. Determining the costs and benefits of a particular adaptation is difficult because individuals differ not only in the degree to which they possess the adaptation, but also in many other ways. How can investigators study individuals that differ only in the genetically based adaptation of interest? Such individuals can be created by recombinant DNA techniques using cloned or highly inbred populations. In plants, for example, plasmids can be used to transfer specific alleles to experimental individuals. Control individuals also receive plasmids, but those plasmids lack the allele of interest. Plasmid transfer techniques made it possible to measure the cost associated with resistance to the herbicide chlorosulfuron conferred by a single allele in the shale cress, Arabidopsis thaliana. The allele, Csr1-1, results in the production of an enzyme that is insensitive to chlorosulfuron. However, plants with the Csr1-1 allele produce 34 percent fewer seeds than nonresistant plants grown under identical conditions in the absence of the herbicide. The reason for the high cost of resistance is not fully understood, but evidence suggests that the resistance allele results in an accumulation of branched-chain amino acids that interfere with metabolism. Agriculturalists wish to alter the genotypes of plants to give them resistance to herbicides so that the herbicides applied to agricultural fields will kill the weeds, but not the crops. This experiment shows that such benefits may impose a trade-off in terms of crop yield. We saw in the previous section that the possession of certain conspicuous features by males confers reproductive benefits. What kinds of trade-offs do these benefits impose? The cost of long tails was not measured in the experiments with widowbirds but related studies have been done on males of other species. In some mammalian species, including deer, lions and baboons, one male controls reproductive access to many females. These polygynous species tend to be sexually dimorphic— the males appear quite different from the females. Males of these species are significantly larger than females and often bear large weapons (such as horns, antlers, and large canine teeth); size and weaponry are needed to defend a male’s multiple mates against other males of the species. The costs of sexual dimorphism for males of polygynous species were assessed using the comparative method. Such males have higher parasite loads and higher mortality rates than females of their own species because maintaining a large size and bearing large weapons makes them more susceptible to parasites. In addition, when compared to parasite loads in males of closely related monogamous species (in which males and females are essentially monomorphic, appearing quite similar), the dimorphic males carried higher parasite loads in almost every case.

 

Maintaining Genetic Variation

Genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation within populations. Nevertheless, as we have seen, most populations have considerable genetic variation. What maintains so much genetic variation within populations? To answer this question, we will show how sexual recombination, neutral mutations, and frequency-dependent selection can maintain variation within populations, and how variation may be maintained over geographic space.

 

Sexual recombination amplifies the number of possible genotypes

In asexually reproducing organisms, the cells resulting from a mitotic division normally contain identical genotypes. Each new individual is genetically identical to its parent, unless there has been a mutation. When organisms exchange genetic material during sexual reproduction, however, offspring differ from their parents because chromosomes assort randomly during meiosis, crossing-over occurs and fertilization brings together material from two different cells.

Sexual recombination generates an endless variety of genotypic combinations that increases the evolutionary potential of populations. Because it increases the variation among the offspring produced by an individual, sexual recombination may improve the chance that at least some of those offspring will be successful in the varying and often unpredictable environments they will encounter. Sexual recombination does not influence the frequencies of alleles; rather, sexual recombination generates new combinations of alleles on which natural selection can act. It expands variation in a character influenced by alleles at many loci by creating new genotypes. That is why selection for bristle number in Drosophila resulted in flies with more bristles than any flies in the initial population had.

 

Neutral mutations accumulate within species

Some mutations do not affect the functioning of the proteins encoded by the mutated genes. An allele that does not affect the fitness of an organism is called a neutral allele. Such alleles, untouched by natural selection, may be lost, or their frequencies may increase with time, purely by genetic drift. Therefore, neutral alleles often accumulate in a population over time, providing it with considerable genetic variation. Much of the variation in those characters we can observe with our unaided senses is not neutral, but much molecular variation apparently is. Modern molecular techniques enable us to measure variation in neutral alleles and provide the means by which to distinguish adaptive from neutral variation.

 

Frequency-dependent selection maintains genetic variation within populations

Natural selection often preserves variation as a polymorphism: the coexistence within a population, at frequencies greater than mutations can produce, of two or more alleles at a locus. A polymorphism may be maintained when the fitness of a genotype (or phenotype) varies with its frequency relative to that of other genotypes (or phenotypes) in a population. This phenomenon is known as frequency-dependent selection. Asmall fish that lives in Lake Tanganyika, in East Africa, provides an example of frequency-dependent selection. The mouth of this scale-eating fish, Perissodus microlepis, opens either to the right or to the left as a result of an asymmetrical jaw joint; the direction of opening is genetically determined. P. microlepis approaches its prey (another fish) from behind and dashes in to bite off several scales from its flank. “Right-mouthed” individuals always attack from the victim’s left; “left-mouthed” individuals always attack from the victim’s right. The distorted mouth enlarges the area of teeth in contact with the prey’s flank, but only if the scaleeater attacks from the appropriate side. Prey fish are alert to approaching scale-eaters, so attacks are more likely to be successful if the prey must watch both flanks. Vigilance by the prey favors equal numbers of rightmouthed and left-mouthed scale-eaters, because if one form were more common than the other, prey fish would pay more attention to potential attacks from the corresponding flank. Over an 11-year period in which the scale-eaters in Lake Tanganyika were studied, the polymorphism was found to be stable: The two forms of P. microlepis remained at about equal frequencies.

 

Genetic variation is maintained in geographically distinct subpopulations

Much of the genetic variation in large populations is preserved as differences among members in different places (subpopulations). Subpopulations often vary genetically because they are subjected to different selective pressures in different environments. Plant species, for example, may vary geographically in the chemicals they synthesize to defend themselves against herbivores. Some individuals of the clover Trifolium repens produce the poisonous chemical cyanide. Poisonous individuals are less appealing to herbivores — particularly mice and slugs — than are non-poisonous individuals. However, clover plants that produce cyanide are more likely to be killed by frost because freezing damages cell membranes and releases the toxic cyanide into the plant’s own tissues. In populations of Trifolium repens, the frequency of cyanide-producing individuals increases gradually from north to south and from east to west across Europe. Poisonous plants make up a large proportion of clover populations only in areas where winters are mild. Cyanideproducing individuals are rare where winters are cold, even though herbivores graze clovers heavily in those areas.

 

 

2.3.2. Light and time effects as aspects of evolution’ phenomena and “natural” classification

Long-day Plants and Short-day plants

The botanists went on testing to confirm this discovery with many species of plants. Following this single lead, they were able to answer a host of questions that had long troubled both professional botanists and gardeners.

The investigators found that plants are of three general types, which they called day-neutral, short-day, and long day. Day–neutral plants flower without regard to day length. Short-day plants flower in early spring or fall; they must have a light period shorter than a critical length (Table 2.2). Long-day plants, which flower chiefly in the summer, will flower only if the light periods are longer than a critical length (Table 2.3).

Day neutral plants are plants in which flowering is not dependant upon photoperiod. Examples include celery, geranium and tomato.

In 1938, two investigators, Karl C. Humner and James Bonner, began to study Photoperiodism, using the cocklebur as their experimental tool. The cocklebur is a short-day plant requiring 15.5 hours or less of light per 24-hour cycle to flower. It is particularly useful for experimental purposes because a single exposure under laboratory conditions to a short-day cycle will introduce flowering two weeks later, even if the plant is immediately returned to long-day- conditions. The cocklebur can withstand a great deal of rough treatment, surviving even if its leaves are removed. Hamner and Bonner showed that it is the leaf blade of the cocklebur that responds to the photoperiod. A plant stripped of all its leaves cannot be induced to flower. But, if as little as one-eighth of a fully expanded leaf is left on the stem, the single short-day exposure induces flowering.

 

Table 2.2.

 

Species of short day (long night) plant Critical duration of darkness (hours)
Bryophyllum
Chrysanthemum
Cocklebur
Strawberry
Winter rye

 


Table 2.3.

 

Species of long day (short night) plant Critical duration of light (hours)
Dill
Italian ryegrass
Red clover
Spinach
Winter wheat

 

Latitude and season

Photoperiodism affects the geographic distribution of plant species since daylight varies with latitude and season. At the equator, the 12-hour day length alternate with 12-hour periods of darkness throughout the whole year. However, in other parts of the world, seasonal variations occur in the length of the daylight period. The further away from the equator a geographical location is situated, the longer its day length in summer and the shorter its day length in winter.

Short day plants (which need a critical duration of darkness) tend to live near the equator where they can flower all year or live in temperature regions where they flower from late autumn to early spring. They cannot reproduce in arctic environments because the temperature is too low for growth when the nights are long in winter.

Long-day plants (which need a critical duration of light) tend to inhabit extreme northern latitudes of temperature regions where they can flower from late spring to early autumn when the day length is long.

By responding to a photoperiod of particular length, all members of a species produce their flowers at the same time of year. This allows cross-pollination to occur, often on a large scale.

Effect of light on timing of breeding in animals

Behavior in animals is described as rhythmical when it is repeated at definite intervals. Although such behavior is endogenous (under internal control), the time, at which it occurs, is influenced by an external factor.

Many birds and mammals are seasonal breeders. Their gonads (tastes and ovaries) become active only at the certain time of the year. These changes are triggered by the arrival of daily photoperiods of a certain critical length.

In birds, the reproductive activity of long day breeders is stimulated by the increasing day length that occurs in spring. Hormones are secreted which promote the enlargement and activity of gonadal tissues and the production of sex cells.

In larger mammals (e.g. sheep and deer), which require a longer period of gestation, a short day breeding cycle occurs. Gonadal activity and reproductive behavior are triggered by the decreasing photoperiods, which occur in autumn.

In each case, seasonal breeding period is timed so that the offspring will be born in spring during favorable environmental conditions. Several months of plentiful food and mild temperatures during summer will allow the young animals to grow and become strong before winter and unfavorable conditions return.

The onset of shorter day lengths also triggers a series of events that lead to the migration of certain birds (e.g. swallow) and the hibernation of certain mammals (e.g. hedgehog) in autumn. These forms of behavior are further examples of photoperiodism. They are of biological significance since they enable the animal to survive the winter when conditions would be severe and the food would be scarce.

 

Circadian Rhythms

Circadian rhythms – regular rhythms of growth and activity that occur approximately on 24-hour basis.

Some plants have flowers that open in the morning and close at dusk or they spread their leaves in the sunlight and fold them toward the stem at night. More recent studies have shown that less evident activities, such as photosynthesis, auxin production, and the rate of cell division, also have regular daily rhythms, which continue even when all environmental conditions are kept constant. These regular day-night cycles have come to be called circadian rhythms, from the Latin words circa, meaning “about”, and dies, “day”. Circadian rhythms now have been found throughout the plant and animal kingdoms.

Biological Clocks

For a number of years, biologists debated whether it might not be some environmental force, such as cosmic rays, the magnetic field of the Earth, or the Earth’s rotation, that was setting the rhythms. Attempts to settle this recurrent controversy have led to countless experiments under an extraordinary variety of conditions. Organisms were taken down to salt mines, shipped to the South Pole, flown halfway around the world in airplanes, and, most recently, orbited in satellites. Although there is still a vocal minority that believes that circadian rhythms are under the influence of a subtle geophysical factor, most workers now agree that the rhythms are endogenous – that is, they originate within the organism. The strongest evidence in support of this belief is that the rhythms are not exact. Different species and different individuals of the same species often have slightly different, but consistent, rhythms, often as much as an hour or two longer or shorter than 24 hours. Nothing, however, is known about the physical or chemical nature of this internal timing device, which is often referred to as a biological clock.

Biological clocks are believed to play an essential role in many aspects of plant and animal physiology. For instance, insects are more active in the early evening hours. Bats that feed on insects begin to fly each evening just when the insects are most available.

Changing the Rhythms

Although circadian rhythms probably originate within the organisms themselves, they can be modified by external conditions – the fact that is, of course, important to the survival of both individuals and species. For instance, a plant whose natural daily rhythm shows a peak every 26 hours, when grown under continuous dim light can adjust its rhythm to 14 hours of light and 10 of darkness. It can also adjust to 11 hours of light and 11 hours of dark (or 22 hours). Such adjustment to an externally imposed rhythm is known as entrainment. If the new rhythm is too far removed from the original one, however, the organism will ‘escape” the entrained rhythm and revert to its natural one. A plant that kept on an artificial or forced rhythm, even for a long period of time, will revert to its normal internal period when returned to continuous dim light.

 

Adaptations to Climate Change

The angiosperms evolved during a relatively mild period in the Earth’s history. As the climate became colder and, as a consequence, water became locked in snow and ice for part of the year, the angiosperms, which already possessed some adaptations to drought (perhaps because of highland origins), were placed under new environmental stress. Some did not survive, and some were pushed southward. Those that did survive in colder, drier areas did so because of selection for characteristics that offered advantages in these relatively unfavorable environments. Chief among such characteristics is the capacity to remain dormant during periods when water is in short supply and when climatic conditions are unfavorable for delicate growing buds, shoots, new leaves, and root tips. Modern plants are classified as annuals, biennials, and perennials, depending on their characteristic patterns of active growth and dormancy.

Annuals, Biennials, and Perennials

Among annual plants, which include many of our weeds, wild flowers, garden flowers, vegetables, and grasses and most other monocots, the entire cycle from seed to vegetative plant to flower to seed again takes place within a single growing season. All vegetative organs (roots, stems, and leaves) die, and only the dormant seed bridges the gap between one generation and the new one. Plants with no woody stems, such as most annuals, are known as herbs.

 

Constraints on Evolution

The many examples of adaptations that we have just discussed are testimony to the power of natural selection, but evolution is limited by a serious constraint: Evolutionary changes must be based on modifications of previously existing traits, which may come to serve new functions. Engineers are able to design a completely new type of engine (jet) to power an airplane that can replace a previous type (propeller), but evolutionary changes cannot happen that way. A striking example of such constraints on evolution is provided by the evolution of fish that spend most of their time resting on the sea bottom. One lineage, the bottom-dwelling skates and rays, is beautifully symmetrical. These fishes are descended from sharks whose bodies were already somewhat flattened; therefore, skates and rays are able to lie on their bellies. Plaice, sole and flounders, on the other hand, are bottom dwelling descendants of deep-bodied, laterally flattened ancestors. Unlike sharks, these fishes cannot lie on their bellies; they must flop over on their sides. During development, the eyes of plaice and sole are grotesquely twisted around to bring both eyes to one side of the body. Small shifts in the position of one eye probably helped ancestral flatfishes see better resulting in the form found today.

 

Cultural Evolution

Traits can evolve by natural selection only if they are at least partly heritable. However, individuals may acquire new traits via cultural evolution — that is, by learning them from other individuals. Cultural evolution is most highly developed in humans, whose language and remarkable learning abilities enable new innovations to spread and be adopted at rapid rates. But the only requirement for traits to evolve via cultural evolution is that individuals have the ability to learn them. Birds, for example, copy the songs of other individuals, resulting in the evolution of song “dialects.” Many behaviors of the apes (chimpanzees, gorillas, gibbons, and orangutans) are transmitted via learning. In one study, investigators compared the behavior of four orang-utan populations on the island of Borneo and two on Sumatra. The investigators identified 24 behaviors that are restricted to a single population. These behaviors are not correlated with any differences in the environments in which the populations live. Ten of the behaviors are specialized feeding techniques, including tool use. Six are alternative forms of social signals, such as kiss-squeaks. Thus, orangutan populations develop cultural distinctions as individuals copy the behavior of other individuals.

 

Short-Term versus Long-Term Evolution

The short-term changes in allele frequencies wit


Date: 2014-12-22; view: 1233


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