Chapter 16: Population Genetics and Evolution

1. Chapter 16: Population Genetics and Evolution Robert E. Ricklefs The Economy of Nature, Fifth Edition

3. Background: Molecular Basis for Genetic Variation • Genetic information is encoded by DNA. • Genetic variation is caused by changes in the nucleotide sequence of DNA. • DNA serves as a template for the manufacturing of proteins and other nucleic acids: • each amino acid in a protein is encoded by a sequence of 3 nucleotides, called a codon • the genetic code contains redundancy because only 20 amino acids need be encoded from 64 possible codons

4. The source of genetic variation is mutation and recombination. • Mutations are errors in the nucleotide sequence of DNA: • substitutions (most common) • deletions, additions, and rearrangements also may occur • Causes of mutations: • random copying errors • highly reactive chemical agents • ionizing radiation

5. Can mutations be beneficial? • Most mutations are harmful: • the altered properties of proteins resulting from mutations are not likely to be beneficial • natural selection weeds out most deleterious genes, leaving only those that suit organisms to their environments • an example is the sickle-cell mutation, which alters the structure of the hemoglobin molecule with deleterious effects for its carriers • Insert figure 16.2

6. More on Mutation • Mutations are likely to be beneficial when the relationship of the organism to its environment changes: • selection for beneficial mutations is the basis for evolutionary change, enabling organisms to exploit new environmental conditions • Processes that cause mutations are blind to selective pressures -- mutation is a random force in evolution, producing genetic variation independently of its fitness consequences.

7. Mutation Rates • The rate of mutation for any nucleotide is low, 1 in 100 million per generation. • (Contextualize it and it changes ….) • Because a complex individual has a trillion or so nucleotides, each individual is likely to sustain one or more mutations. • Rates of expressed gene mutations average about 1 per 100,000 to 1 per million: • rates of expression of phenotypic effects are often higher because they are controlled by many genes.

8. Recombination • Variation is introduced during meiosis when parts of the genetic material inherited by an individual from its mother and father recombine with each other: • recombination is the exchange of homologous sections of maternal and paternal chromosomes • recombination produces new genetic variation rapidly  to know: ‘evolution of body size in Galapagos marine iguanas. Natural and sexual selection have opposing influences on the size of males. Read ‘more on the web’

9. Sources of Genetic Variation • While mutation is the ultimate source of genetic variation: • recombination multiplies this variation • sexual reproduction produces further novel combinations of genetic material • the result is abundant variation upon which natural selection can operate

10. The genotypes of all individuals make up the gene pool. • The gene pool represents the total genetic variation within the population. (all the genes in all the individuals in a population) • Not all combinations of alleles for a given gene will be represented in the gene pool, especially those with low probability. • If a rare combination of alleles confers high fitness, individuals with this combination will produce more offspring, and these alleles will increase in frequency.

11. The Hardy-Weinberg Law • In 1908, Hardy and Weinberg independently described this fundamental law: the frequencies of both alleles and genotypes will remain constant from generation to generation in a population with: • a large number of individuals • random mating • no selection • no mutation • no migration between populations

12. Consequences of Hardy- Weinberg Law (what does it mean?) • No evolutionary change occurs through the process of sexual reproduction itself. • Changes in allele and genotype frequencies can result only from additional forces on the gene pool of a species. • Understanding the nature of these forces is one of the goals of evolutionary biology.

13. Deviations from Hardy-Weinberg Equilibrium 1 • For a gene with two alleles, A1 and A2, that occur in proportions p and q, the proportions of the 3 possible genotypes in the gene pool will be: • A1A1: p2 • A1A2: 2pq • A2A2: q2 • Deviations from these proportions are evidence for the presence of selection, nonrandom mating, or other factors that influence the genetic makeup of a population.

14. Deviations from Hardy-Weinberg Equilibrium 2 • Most natural populations deviate from Hardy-Weinberg equilibrium. • We thus consider some of the forces responsible for such deviations (setting aside mutation and selection): • effects of small population size • nonrandom mating • migration

15. Genetic Drift • Genetic drift is a change in allele frequencies due to random variations in fecundity and mortality in a a population: • genetic drift has its greatest effects in small populations • when all but one allele for a particular gene disappears from a population because of genetic drift, the remaining allele is said to be fixed

16. Founder Events • When a small number of individuals found a new population, they carry only a partial sample of the gene pool of the parent population: • this phenomenon is called a founder event • founding of a population by ten or fewer individuals results in a substantially reduced sample of the total genetic variation

17. Population Bottlenecks • Continued existence at low population size of a recently founded population results in further loss of genetic variation by genetic drift, referred to as a population bottleneck: • such a situation may have occurred in the recent past for the population of cheetahs in East Africa • fragmentation of populations into small subpopulations may eventually reduce their genetic responsiveness to selective pressures of changing environments

18. Assortative Mating • Assortative mating occurs when individuals select mates nonrandomly with respect to their own genotypes: • positive assortative mating pairs like with like • negative assortative mating pairs mates that differ genetically • assortative mating does not change allele frequencies but does affect frequencies of genotypes

19. Positive assortative mating leads to inbreeding. • Positive assortative mating can lead to an overabundance of homozygotes: • one result is the unmasking of deleterious recessive alleles not expressed in heterozygous condition (inbreeding depression) • most species have mechanisms that assist them in avoiding matings with close relatives: • dispersal, recognition of close relatives, negative assortative mating, genetic self-incompatibility

20. Is inbreeding always undesirable? • Inbreeding creates genetic problems, particularly loss of heterozygosity. • In some cases inbreeding may be beneficial: • plants that can self-pollinate are capable of sexual reproduction even when suitable pollinators are absent • when organisms are adapted to local conditions, matings with distant individuals may reduce fitness of progeny

21. Optimal Outcrossing Distance • Mating with individuals located at intermediate distances (optimal outcrossing distance) may be desirable: • nearby individuals are likely to be close relatives, resulting in inbreeding • distant individuals may be adapted to different conditions: • in controlled matings in larkspur plants, crosses between individuals 10 m apart enhanced seed set and seedling survival, compared to selfing and mating with distant individuals

22. Migration and Deviations from Hardy-Weinberg Equilibrium • Mixing individuals from populations with different allele frequencies can result in deviations from genotypic frequencies under the Hardy-Weinberg equilibrium: • such movement of genes is called gene flow • mixing results in under-representation of heterozygotes • this phenomenon is called the Wahlund effect

23. Genotypes vary geographically. • Differences in allelic frequencies between populations can result from: • random changes (genetic drift, founder events) • differences in selective factors • Such differences are particularly evident when there are substantial geographic barriers to gene flow.

24. Ecotypes • The Swedish botanist Göte Turreson used a common garden experiment to show that differences among plants from different localities had a genetic basis: • under identical conditions (in the common garden) plants retained different forms seen in their original habitats • Turreson called these different forms ecotypes

25. Ecotypes may be close to one another or distant. • Although ecotypes may be geographically isolated and found some distance apart, this is not always the case: • if selective pressures between nearby localities are strong relative to the rate of gene flow, ecotypic differences may arise: • plants on mine tailings and uncontaminated soils nearby may differ greatly in their tolerance to toxic metals (copper, lead, zinc, arsenic)

26. Clines and Other Geographic Patterns • Some traits may exhibit patterns of gradual change over distance: • such patterns are referred to as clines • clinal variation usually represents adaptation to gradually changing conditions of the environment • Other genetic patterns may be found: • geographic variation related to random founder effects • differentiation related to abrupt geographic barriers and spatial/temporal variation in these

27. Natural Selection • Natural selection occurs when genetic factors influence survival and fecundity: • individuals with the highest reproductive rate are said to be selected, and the proportion of their genotypes increases over time • Natural selection can take various forms depending on the heterogeneity of, and rate of change in, the environment.

28. Stabilizing Selection • Stabilizing selection occurs when individuals with intermediate, or average, phenotypes have higher reproductive success than those with extreme phenotypes: • favors an optimum or intermediate phenotype, counteracting tendency of phenotypic variation to increase from mutation and gene flow • this is the prevailing mode of selection in unchanging environments

29. Directional Selection • Under directional selection, the fittest individuals have more extreme phenotypes than the average for the population: • individuals producing the most progeny are to one extreme of the population’s distribution of phenotypes • the distribution of phenotypes in succeeding generations shifts toward a new optimum • runaway sexual selection is an excellent example of this phenomenon

30. Disruptive Selection • When individuals at either extreme of the range of phenotypic variation have greater fitness than those near the mean, disruptive selection can take place: • tends to increase phenotypic variation in the population • may lead to bimodal distribution of phenotypes • uncommon, but could result from availability of diverse resources, benefits associated with alternative life histories, or strong competition for a preferred resource

31. Directional selection changes allele frequencies. • Selection changes the makeup of the gene pool. • Selection has several important aspects: • directional selection against a deleterious allele results in a decrease in frequency of that allele, coupled with an increase in frequencies of favorable alleles • the rate of change in the frequencies of alleles is proportional to the selective pressure • evolution stops only when there is no longer any genetic variation to act upon; directional selection thus removes genetic variation from populations

32. Maintenance of Genetic Variation 1 • A paradox: • natural selection cannot produce evolutionary change without genetic variation • however, both stabilizing and directional selection tend to reduce genetic variation: • how does evolution continue under such circumstances? • does availability of genetic variation ever limit the rate of evolutionary change?

33. Maintenance of Genetic Variation 2 • Mutation and migration supply populations with new genetic variation. • Spatial and temporal variation tend to maintain variation by favoring different alleles at different times and places. • When heterozygotes have a higher fitness than homozygotes, the relative fitness of each allele depends on its frequency in the population (frequency-dependent selection): • alleles are selected for when at low frequency and against when at high frequency • heterozygote superiority is also called heterosis

34. How much genetic variation? • About 1/3 of genes that encode enzymes involved in cellular metabolism show variation in most species: • about 10% of these are heterozygous in any given individual • however, most genetic variation is apparently neutral or has negative effects when expressed • thus most variation has no fitness consequences or is subject to stabilizing selection

35. Genetic Variation is Important • Under changing environmental conditions, the reserve of genetic variation may take on positive survival value. • There seems to be enough genetic variation in most populations so that evolutionary change is a constant presence.

36. Evolutionary Changes in Natural Populations • Evolutionary changes have been widely documented, particularly in species that have evolved rapidly in the face of environmental changes caused by humans: • cyanide resistance in scale insects (Chapter 9) • pesticide and herbicide resistance among agricultural pests and disease vectors • increasing resistance of bacteria to antibiotics • In each case, genetic variation in the gene pool allowed these populations to respond to changed conditions.

37. Useful Conclusions from Population Genetics Studies • Every population harbors some genetic variation that influences fitness. • Changes in selective factors in the environment are usually met by evolutionary responses. • Rapid environmental changes caused by humans will often exceed the capacity of a population to respond by evolution; the consequence is extinction.

38. Summary 1 • Mutations are the ultimate source of genetic variability. • Recombination and sexual reproduction result in novel genetic combinations. • The Hardy-Weinberg law predicts stable allelic and genotypic frequencies in certain conditions. • Deviations from Hardy-Weinberg equilibrium are caused by mutation, migration, nonrandom mating, small population size, and selection.

39. Summary 2 • Selection pressures may vary geographically, giving rise to variation in gene frequencies within the geographic range of a species. • Selection may be stabilizing, directional, or disruptive. • Selection tends to remove genetic variation, but mutation, gene flow, and varying selective pressures maintain it.