The Evolution of Populations Chapter 23

1. The Evolution of Populations Chapter 23

2. Populations, NOT Individuals, Evolve! • Natural selection acts on individuals • differential survival • “survival of the fittest” • differential reproductive success • i.e., bearing more offspring • Populations evolve • populations of organisms change over time • traits which offer greater fitness become more frequent in the population

3. Population = Smallest Unit of Evolution • Individuals are selected, Populations evolve • For example, the bent grass (Argrostis tenuis)is growing on the tailings of an abandoned mine, rich in toxic heavy metals. • While many seeds land on the mine tailings each year, the only plants that germinate, grow, and reproduce are those that had already inherited genes enabling them to tolerate metallic soils. • Individual plants do not evolve to become more metal-tolerant during their lifetimes.

4. Modern Synthesis • Evolution since Darwin • comprehensive theory of evolution took form in early 1940s • integration of natural selection & Mendelian inheritance (genetics), aka Neo-Darwinism • R.A. Fisher • J.B.S. Haldane • Theodosius Dobzhansky • Ernst Mayr • Sewall Wright • George Gaylord Simpson • Ledyard Stebbins

5. Populations & Gene Pools • Apopulation is a localized group of potentially interbreeding individuals • A gene pool is the total collection of alleles in the population • remember difference between alleles & genes! • Allele frequency is the frequency of an allele in a population • how many A vs. a in whole population

6. Members of a population are far more likely to breed with members of the same population than with members of other populations. • Individuals near the populations center are, on average, more closely related to one another than to members of other populations.

7. Evolution of Populations • Evolution implies a change in allelic frequencies in a population over time • Hypothetical Question: What would a population look like if allele frequencies didn’t change? • A non-evolving population would have . . . • very large population size (no genetic drift) • no migration (in or out) • no mutation • random mating (no competition) • no natural selection

8. Hardy-Weinberg Equilibrium W. Weinberg physician G.H. Hardy mathematician • Hypothetical, non-evolving population • preserves allelic frequencies despite shuffling of alleles from meiosis and random fertilization • Serves as a model • natural populations rarely in H-W equilibrium • useful model to measure if forces are acting on a population

9. Hardy-Weinberg Theorem • Alleles • frequency of dominant allele = p • frequency of recessive allele = q • frequencies must add to 100%, so: p + q = 1 • Individuals • frequency of homozygous dominant = p2 • frequency of homozygous recessive = q2 • frequency of heterozygotes = 2pq • frequencies must add to 100%, so: p2+ 2pq + q2 = 1

10. Implications of HW Theorem In H-W population, all alleles remain at the same frequencies If allele frequencies change, then population is not in equilibrium & evolution is occurring Population biologists sample individualsand measure changing allelic frequencies from year to year

11. Microevolution Generation to generation change in a population’s allelic frequencies

12. The Two Main Causes of Microevolution are Genetic Drift and Natural Selection Four factors, all departures from HW, can alter the allelic frequencies: 1) genetic drift, 2) natural selection, 3) gene flow, 4) mutation Natural selection is the only factor that generally adapts a population to its environment. The other three may effect populations in positive, negative, or neutral ways.

13. 1) Genetic Drift • Genetic drift = changes in allelic frequencies resulting because of chance events that occur when populations are small. • For example, one would not be too surprised if a coin produced seven heads and three tails in ten tosses, but you would be surprised if you saw 700 heads and 300 tails in 1000 tosses - you expect 500 of each. • Genetic drift at small population sizes often occurs as a result of two situations: the founder effect or the bottleneck effect.

14. Example of Genetic Drift . . . MYBPC3 gene encodes a muscle protein. A 25 bp deletion in that gene is associated with a large increase in the risk of cardiomyopathy ("heart muscle disease").

15. The Founder Effect • When a new population is started by only a few individuals • some rare alleles may be at high frequency; others may be missing • Skews the gene pool of the new population relative to the ancestral population!

16. Example of Founder Effect . . . South & Central American Indians were nearly 100% type O for the ABO blood system. Nothing in nature seems to strongly select for or against this trait, It is likely that most of these people are descendants of a small band of closely related "founders" who also shared this blood type

17. The Bottleneck Effect • When larger population is drastically reduced by a disaster • loss of variation, narrows the gene pool • by chance, some alleles may be overrepresented & others under-represented • among survivors some alleles may be eliminated altogether

18. Example of Bottleneck Effect . . . • All cheetahs share a small number of alleles • less than 1% diversity, diversity comparable to highly inbred lab mice! • as if all cheetahs are identical twins, can perform skin grafts from any 2 cheetahs with no immune response • 2 bottlenecks • 10,000 years ago during last Ice Age • last 100 years from poaching & loss of habitat

19. Applying Population Genetics to Conservation Biology Bottlenecking is an important concept in the conservation of endangered species Populations that have suffered bottleneck incidents have lost alleles from the gene pool reducing individual variation & adaptability

20. 2) Natural selection is clearly a violation of the conditions necessary for the Hardy-Weinberg equilibrium.

21. 3) Gene Flow via Gene Migration(Immigration & Emigration) • Consider a population spread over a large geographic area • Individuals can move from one area to another; however, sub-populations may have different allele frequencies • Migrations cause gene mixing across regions = gene flow • new alleles are moving into gene pool • reduce differences between populations

22. Consider World-Wide Travel in Homo sapiens Gene flow in human populations is increasing today transferring alleles between populations

23. Gene Flow &Human Evolution As cultural bias against interracial marriage erodes, we are moving toward a more blended world . . .

24. 4) Mutation • Mutation creates variation (Raw Materials for Evolution) • New genes & new alleles originate only by mutation • Only mutations to sex cells (germ lines) can be passed on (NOT mutations in Somatic Cells) • Mutation changes DNA sequence • Changes amino acid sequence, possibly changing protein structure, often function • Changes in protein may change phenotype & therefore change fitness! • Most mutations are deleterious (negative) • Mutation in Eukaryotes LOW – 1/100,000 genes per generation

25. Types of Mutations • Point Mutations • Change in 1 base in a gene • Example = Sickle Cell Disease • Most harmless, as genetic code is redundant, and most of eukaryotic genome doesn’t code for proteins • Gene Duplication • Duplication of many loci – entire segments of chromosomes • Example = Mammalian olfactory receptors, remote ancestors carried single gene for detecting odors, mice now have ~1300

26. Salient Point:Mutation & Sexual Recombinationproduces the variation that makesevolution possible

27. Sexual Recombination • Sex spreads variation • Sex causes recombination of alleles • Segregation & independent assortment of gametes • Offspring have new combinations of traits = new phenotypes • Sexual reproduction recombines alleles into new arrangements in every offspring

28. Selection and Variation • Natural selection requires a source of variation within the population • there have to be differences • some individuals are more fit than others • Genetic variation is the substrate for natural selection

29. Types of Selection The effect of selection depending on what is “fit”

30. Directional Selection Environmental conditions favor one extreme • Directional selection for beak size in Galápagos population of medium ground finch • Drier years = thicker shelled seeds = selects for stronger billed birds

31. Diversifying Selection Large Billed = Hard Seeds Small Billed = Soft Seeds Environment favors two extremes (bi-modal distribution)

32. Variation • Discrete vs. Quantitativecharacters • Red vs. White flower color = Discrete (distinct traits) • Usually different alleles at 1 loci • Human height = Quantitative (traits that vary along a continuum) • Effects of polygenic alleles are additive • Polymorphic (meaning “many forms”) • Morphs = distinct types in a population • Geographic variation • Clines

33. Quantitative Character = At least 10 genes contribute to human height

34. Polymorphic

35. Clines, an Example of Geographic Variation Plant height varies with altitude, but still same population

36. Salient Point: Diploidy & Balanced Polymorphism Preserves Variation • Diploidy • Genetic variation— even lethal alleles—are hidden in heterozygotes • Balancing Selection • Balanced polymorphism – maintaining 2 or more phenotypes through selection • Heterozygote advantage • Frequency-dependent selection

37. Heterozygous Advantage • Heterozygoteshave a greater fitness • Maintains both alleles in population • Classic Example: Sickle Cell Anemia

38. Frequency Dependent Selection Decrease in fitness of any morph if it becomes too common Selection against more abundant phenotype Consider action of both predators & parasites

39. Sexual Selection Blue Footed Booby courtship display • Natural selection for mating success • Competition amongst males for females • Ritual displays & battles between males • Courtship displays to attract females • Female choice!

40. Female Choice Rules Animal Kingdom! Sexual Dimorphism

41. Males Go To Extremes!

42. My Personal Fav – Bower Birds

43. How Can Such A Male Evolve?

45. Limitations of Natural Selection • Natural selection cannot fashion perfect organisms • Evolution is limited by genetic constraints • Legacy of ancestral genes • Existing variations may not be ideal • Adaptations are often compromises • Adaptation for one situation may be limitation for another • Chance & natural selection interact • The founders may not be the fittest