1. CHAPTER 23 EVOLUTION OF POPULATIONS Population Genetics/ Modern Synthesis Hardy-Weinberg Theory (Non-evolving Population) Microevolution (Evolving Populations) –Genetic Drift –Gene Flow –Mutations –Natural Selection

2. One obstacle to understanding evolution is the common misconception that individual organisms evolve, in the Darwinian sense, during their lifetimes. In fact, natural selection does act on individuals; their characteristics affect their chances of survival and their reproductive success. But the evolutionary impact of this natural selection is only apparent in tracking how a population of organisms changes over time. Consider, for example, representatives from a population of marine snails (Liguus fascitus). Their different patterns of coloration represent genetic variations within that population. If predators feed preferentially on snails having a particular coloration, then the proportion of individuals with that coloration probably will decline from one generation to the next because such snails will produce fewer offspring. Thus, it is the population, not its individuals, that evolves; some characteristics become more common within the overall population, while other characteristics decline. Population Genetics

3. Evolution on the smallest scale, or microevolution, can be defined as a change in the allele frequencies of a population We begin our study of microevolution by tracing how biologists finally began to understand Darwin’s theory of natural selection during the first half of the 20th century. Individuals are selected, but populations evolve. The bent grass (Agrostis tenuis) in the foreground is growing on the tailings of an abandoned mine in Wales. These plants tolerate concentrations of heavy metals that are toxic to other plants of the same species growing just meters away, in the pasture on the other side of the fence. Each year, many seeds land on the mine tailings, but most are unable to grow successfully there. The only plants that germinate, grow, and reproduce are those that inherited genes enabling them to tolerate metallic soil. Thus, this adaptation does not evolve by individual plants becoming more metal-tolerant during their lifetimes. We can only see the evolution of this population by observing the proportions of metal-tolerant plants in successive generations. Example of Microevolution:

4. The Origin of Species convinced most biologists that species are products of evolution, but Darwin was not nearly so successful in gaining acceptance for his idea that natural selection is the main mechanism of evolution. Natural selection requires hereditary processes that Darwin could not explain. His theory was based on what seems like a paradox of inheritance: Like begets like--but not exactly. Although Gregor Mendel and Charles Darwin were contemporaries, Mendel’s discoveries were unappreciated at the time, and apparently no one noticed that he had elucidated the very principles of inheritance that could have resolved Darwin’s paradox and given credibility to natural selection. What was missing in Darwin’s explanations was an understanding of inheritance that could explain how chance variations arise in a population while also accounting for the precise transmission of these variations from parents to offspring.

5. Ironically, when Mendel’s research article was rediscovered and reassessed at the beginning of the 20th century, many geneticists believed that the laws of inheritance were at odds with Darwin’s theory of natural selection. Darwin considered the raw material for natural selection to be quantitative characters, those characteristics in a population that vary along a continuum, such as fur length in mammals or the speed with which an animal can flee from a predator.

6. We know today that quantitative characters are influenced by multiple genetic loci. But Mendel (and later the geneticists of the early 20th century) recognized only discrete "either-or" traits, such as purple or white flowers in pea plants, as heritable. Thus, there seemed to be no genetic basis for natural selection to work on the more subtle variations within a population that were central to Darwin’s theory. C C CC aaBbCC

7. An important turning point for evolutionary theory was the birth of population genetics, which emphasizes the extensive genetic variation within populations and recognizes the importance of quantitative characters. With progress in population genetics in the 1930s, Mendelism and Darwinism were reconciled, and the genetic basis of variation and natural selection was worked out. Mendelism genetic variation Darwinism natural selection

8. A comprehensive theory of evolution that became known as the modern synthesis began to take form in the early 1940s. It is called a synthesis because it integrates discoveries and ideas from many different fields, including paleontology, taxonomy, biogeography, and, of course, population genetics. The architects of this modern synthesis included geneticists Theodosius Dobzhansky (1900-1975) and Sewall Wright (1889-1988), biogeographer and taxonomist Ernst Mayr (1904-), paleontologist George Gaylord Simpson (1902-1984), and botanist G. Ledyard Stebbins (1906-2000). Dobzhansky Wright Mayr Simpson Stebbins

9. The modern synthesis emphasizes the importance of populations as the units of evolution, the central role of natural selection as the most important mechanism of evolution, and the idea of gradualism to explain how large changes can evolve as an accumulation of small changes occurring over long periods of time. PopulationsNatural Selection Gradualism

10. A population’s gene pool is defined by its allele frequencies A population is a localized group of individuals belonging to the same species. For now, we will define a species as a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature K = Animalia P = Chordata C = Mammalia O = Carnivora F = Felidae G = Panthera S = leo K = Animalia P = Chordata C = Mammalia O = Carnivora F = Felidae G = Panthera S = tigris

11. Each species is distributed over a certain geographic range, but within this range individuals are usually concentrated in several localized populations. A population may be isolated from other populations of the same species, exchanging genetic material only rarely. Such isolation is particularly common for populations confined to widely separated islands, unconnected lakes, or mountain ranges separated by lowlands.

12. However, populations are not always isolated, nor do they necessarily have sharp boundaries. One dense population center may blur into another in an intermediate region where members of the species occur but are less numerous. Although these populations are not isolated, individuals are still concentrated in centers and are more likely to breed with members of the same population than with members of other populations. Therefore, individuals near a population center are, on average, more closely related to one another than to members of other populations

13. The total aggregate of genes in a population at any one time is called the population’s gene pool. It consists of all alleles at all gene loci in all individuals of the population. For a diploid species, each locus is represented twice in the genome of an individual, who may be either homozygous or heterozygous for those homologous loci. If all members of a population are homozygous for the same allele, that allele is said to be fixed in the gene pool. Often, however, there are two or more alleles for a gene, each having a relative frequency (proportion) in the gene pool. Recall that homozygous individuals have two identical alleles for a given character, whereas heterozygous individuals have two different alleles for that character.

14. Imagine a wildflower population with two varieties contrasting in flower color. An allele for red flowers, which we will symbolize by R, is completely dominant to an allele for white flowers, symbolized by r. For our simplified situation, these are the only two alleles for this locus in the population. RR Rr rr

15. Suppose an imaginary population has 500 plants, and 20 of these plants have white flowers because they are homozygous for the recessive allele; their genotype is rr. The other 480 plants have red flowers; some of them will be homozygous (RR ) and others will be heterozygous (Rr ). Suppose that 320 plants are RR homozygotes and 160 are Rr heterozygotes.

16. Because these are diploid organisms, there are a total of 1,000 copies of genes for flower color in the population of 500 individuals. The dominant allele (R ) accounts for 800 of these genes: 320 X 2 = 640 for RR plants, plus 160 X 1 = 160 for Rr individuals. Thus, the frequency of the R allele in the gene pool of this population is 800/1,000 = 0.8 = 80%. And because there are only two allelic forms of the gene, the r allele must have a frequency of 0.2, or 20%.

17. The Hardy-Weinberg theorem describes a nonevolving population Before we consider the mechanisms that cause a population to evolve, it will be helpful to examine, for comparison, the gene pool of a nonevolving population. Such a gene pool is described by the Hardy-Weinberg theorem, named for the two scientists who derived the principle independently in 1908. G. H. Hardy Wilhelm Weinberg

18. The theorem states that the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than Mendelian segregation and recombination of alleles. Put another way, the shuffling of alleles due to meiosis and random fertilization has no effect on the overall gene pool of a population. Crossing Over Independent Assortment Random Fertilization

19. To apply the Hardy-Weinberg theorem, let’s return to our imaginary wildflower population of 500 plants Recall that 80% (0.8) of the flower-color loci in the gene pool have the R allele and 20% (0.2) have the r allele. How will meiosis during sexual reproduction affect the frequencies of the two alleles in the next generation of our wildflower population?

20. We will assume that the union of sperm and ova in the population is completely random; that is, all male-female mating combinations are equally likely. The situation is analogous to mixing all gametes in a sack and then drawing them randomly, two at a time, to determine the genotype for each zygote (fertilized egg). Each gamete has one allele for flower color, and the allele frequencies of the gametes will be the same as the allele frequencies in the parent population. Every time a gamete is drawn from the pool at random, the chance that the gamete will bear an R allele is 0.8, and the chance that the gamete will have an r allele is 0.2. Random Mating

21. The probability of picking two R alleles from the pool of gametes is 0.8 X 0.8 = 0.64. Thus, about 64% of the plants in the next generation will have the genotype RR. The frequency of rr individuals will be about 0.04 (0.2 X 0.2 = 0.04), or 4%. And 32%, or 0.32, of the plants will be heterozygous--that is, Rr or rR, depending on whether it is the sperm or ovum that supplies the dominant allele (frequency of Rr = 0.8 X 0.2 = 0.16; frequency of rR = 0.2 X 0.8 = 0.16; frequency of Rr + rR = 0.32). Using the rule of multiplication (see Chapter 14), we can calculate the frequencies of the three possible genotypes in the next generation of the population

22. Hardy-Weinberg Equilibrium Notice in Figures below that the sexual processes of meiosis and random fertilization have maintained the same allele and genotype frequencies that existed in the previous generation of the wildflower population. For the flower-color locus, the population’s gene pool is in a state of equilibrium--referred to as Hardy-Weinberg equilibrium. Theoretically, the allele frequencies could remain constant at 0.8 for R and 0.2 for r forever (though in reality, some other factor always intervenes). ____

23. The Hardy-Weinberg theorem describes how the Mendelian system has no tendency to alter allele frequencies. For instance, the dominant allele (R ) has no tendency to increase in frequency from one generation to the next relative to the recessive allele (r ). The system operates somewhat like shuffling a deck of cards: No matter how many times the deck is reshuffled to deal out new hands, the deck itself remains the same. Aces do not grow more numerous than jacks. And the repeated shuffling of a population’s gene pool over the generations cannot, in itself, increase the frequency of one allele relative to another.

24. The Hardy-Weinberg Equation We can use our imaginary wildflower population to describe the Hardy-Weinberg theorem in more general terms. We will restrict our analysis to the simplest case of only two alleles, one dominant over the other. However, the Hardy-Weinberg theorem also applies to situations in which there are three or more alleles for a particular locus and no clear-cut dominance. R completely dominant over r

25. For a gene locus where only two alleles occur in a population, population geneticists use the letter p to represent the frequency of one allele and the letter q to represent the frequency of the other allele. In our imaginary wildflower population, p = 0.8 and q = 0.2

26. Note that p + q = 1; the combined frequencies of all possible alleles must add to 100% for that locus in the population. If there are only two alleles and we know the frequency of one, the frequency of the other can be calculated: If p + q = 1 then: 0.8 + 0.2 = 1 p = 1 – 0.2 = 0.8 p = 1 - q q = 1 - p and q = 1 – 0.8 = 0.2

27. When gametes combine their alleles to form zygotes, the probability of generating an RR genotype is p 2 (an application of the rule of multiplication). In our wildflower population, p = 0.8, and p 2 = 0.64, the probability of an R sperm fertilizing an R ovum to produce an RR zygote. The frequency of individuals homozygous for the other allele (rr ) is q 2, or 0.2 X 0.2 = 0.04 for the wildflower population. There are two ways in which an Rr genotype can arise, depending on which parent contributes the dominant allele. Therefore, the frequency of heterozygous individuals in the population is 2pq: (2 X 0.8 X 0.2 = 0.32 in our example). If we have included all possible genotypes, the genotype frequencies add up to 1: p 2 +2pq2pq+q 2 = 1 Frequency of RR genotype Frequency of Rr plus rR genotype Frequency of rr genotype For our wildflowers, this is: 0.64 + 0.32 + 0.04 = 1

28. Population geneticists refer to this general formula as the Hardy-Weinberg equation. The equation enables us to calculate frequencies of alleles in a gene pool if we know frequencies of genotypes, and vice versa.

29. Population Genetics and Health Science We can use the Hardy-Weinberg equation to estimate the percentage of the human population that carries the allele for a particular inherited disease. For instance, one out of approximately 10,000 babies in the United States is born with phenylketonuria (PKU), a metabolic disorder that, left untreated, results in mental retardation and other problems. (Newborn babies are now routinely tested for PKU, and symptoms can be prevented by following a strict diet.)

30. The disease is caused by a recessive allele; thus, the frequency of individuals in the U.S. population born with PKU corresponds to q 2 in the Hardy-Weinberg equation (q 2 = frequency of the homozygous recessive genotype). Given one PKU occurrence per 10,000 births, q 2 = 0.0001. Therefore, assuming Hardy-Weinberg proportions, the frequency of the recessive allele for PKU in the population is and the frequency of the dominant allele is The frequency of carriers, heterozygous people who do not have PKU but may pass the PKU allele on to offspring, is Thus, about 2% of the U.S. population carries the PKU allele.

31. The Hardy-Weinberg Theorem and Genetic Variation The Hardy-Weinberg theorem is important conceptually and historically because it shows how Mendel’s theory of inheritance plugs a hole in Darwin’s theory of natural selection. Natural selection requires genetic variation; it cannot act in a genetically uniform population. The Hardy-Weinberg theorem explains how Mendelian inheritance preserves genetic variation from one generation to the next.

32. Pre-Mendelian theories of inheritance were mainly "blending" theories, in which the hereditary factors in the offspring were thought to be a blend of the hereditary factors inherited from the two parents. If a red flower mates with a white one, blending theory predicts that the offspring will be a paler red and will now have hereditary factors for this paler red color. Genetic variation has been eliminated, since the two kinds of factors in the parents have been reduced to only one kind in the offspring. x = ? Such a hereditary mechanism would soon produce a uniform population.

33. In Mendelian inheritance, however, the hereditary mechanism has no tendency by itself to reduce genetic variation. The set of alleles inherited by each generation from its parents are in turn passed on when that generation breeds. This nonblending mechanism of inheritance preserves the genetic variation upon which natural selection acts.

34. The Assumptions of the Hardy-Weinberg Theorem For a population to be in Hardy-Weinberg equilibrium, it must satisfy five main conditions: 1. Very large population size. In a population of finite size, especially if that size is small, genetic drift, which is chance fluctuation in the gene pool, can cause genotype frequencies to change over time. 2.No migration. Gene flow, the transfer of alleles between populations due to the movement of individuals or gametes, can increase the frequency of any genotype that is in high frequency among the immigrants. 3.No net mutations. By changing one allele into another, mutations alter the gene pool. 4.Random mating. If individuals pick mates with certain genotypes, then the random mixing of gametes required for Hardy-Weinberg equilibrium does not occur. 5.No natural selection. Differential survival and reproductive success of genotypes will alter their frequencies and may cause a detectable deviation from frequencies predicted by the Hardy- Weinberg equation. Thus, we do not really expect a natural population to be in Hardy-Weinberg equilibrium. And a deviation from the stability of a gene pool--and from Hardy-Weinberg equilibrium--usually results in evolution.