Evolution of Populations

1. Evolution of Populations Chapter 16

2. Genes and Variation16-1 • Darwin did not have the leisure of understanding how inheritance works when he was studying and developing his theory of evolution • Evolutionary biologists later connected Mendel’s work to Darwin’s ideas • We now define fitness, adaptation, species, and evolutionary change in genetic terms

3. How common is genetic variation? • Many genes have at least two forms, or alleles. • All organisms have genetic variation. Some is “invisible” because it involves biochemical processes, some is visible (color, fur length, etc).

4. Variation and Gene Pools • Genetic variation is studied in populations. • Population = a group of individuals of the same species that interbreed, living in the same place at the same time. • Gene pool = all the genes, including all the different alleles, that are present in a population.

5. Sample Population Frequency of Alleles allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown • Relative frequency = the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. • expressed as a percentage

6. Relative frequency has nothing to do with whether the allele is dominant or recessive • In this population, the recessive allele is more common • In genetic terms, evolution is any change in the relative frequency of alleles in a population • If the relative frequency of the B allele in this mouse population changed over time, the population is evolving.

7. Sources of Genetic Variation • The two main sources of genetic variation are mutations and genetic shuffling from sexual reproduction. • Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.

8. Single-Gene and Polygenic Traits • The number of phenotypes produced for a given trait depends on how many genes control the trait. • A single-gene trait = controlled by one gene that has two alleles. • Variation in this gene leads to only two possible phenotypes.

9. Polygenic trait = controlled by two or more genes • One polygenic trait can have many possible genotypes and phenotypes. • Height in humans is a polygenic trait. • A bell-shaped curve is typical of polygenic traits.

10. Evolution as Genetic Change16-2 • Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. • EX: A population of normally brown lizards. Mutations produce new color choices. • If red lizards are more visible to predators, they might be less likely to survive. • Black lizards absorb more heat to warm up faster on cold days so they can move faster to get food and avoid predators. The allele for black may increase in frequency.

12. Natural Selection on Polygenic Traits • Natural Selection of polygenic traits can affect the distribution of phenotypes in any of three ways • Directional Selection • Stabilizing Selection • Disruptive Selection

13. Directional Selection • Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

14. EXAMPLE OF DIRECTIONAL SELECTION Beak size varies in a population Birds with bigger beaks can feed more easily on harder, thicker shelled seeds. Suppose a food shortage causessmall and medium size seeds to run low. Birds with bigger beaks would be selected for and increase in numbers in population.

15. Stabilizing Selection • Individuals near center of curve have higher fitness than individuals at either end of the curve.

16. EXAMPLE OF STABILIZING SELECTION Stabilizing Selection Male birds with showier, brightly- colored plumage also attract predators, and are less likely to live long enough to find a mate. The most fit, then, is the male bird in the middle-- showy, but not too showy. Male birds use their plumage to attract mates. Male birds in the population with less brilliant and showy plumage are less likely to attract a mate, while male birds with showy plumage are more likely to attract a mate. Key Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Brightness of Feather Color

17. Disruptive Selection • Individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle

18. EXAMPLE OF DISRUPTIVE SELECTION Suppose bird population lives in area where climate change causes medium size seeds become scarce while large and small seeds are still plentiful. Birds with bigger or smaller beaks would have greater fitness and the population may split into TWO GROUPS. One that eats small seeds and one that eats large seeds.

19. Genetic Drift • Natural selection is not the only source of evolutionary change – sometimes things happen by chance • Genetic Drift =random change in allele frequency due to changes in small populations • Some individuals may randomly leave more offspring than is typical • A small group carrying a different relative frequency of alleles may migrate  founder effect

20. Genetic Drift

21. Genetic Drift

22. Genetic Drift Descendants Population A Population B

23. Evolution vs. Genetic Equilibrium • The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. • Genetic equilibrium = when allele frequencies remain constant

24. Five conditions are required to maintain genetic equilibrium from generation to generation: • there must be random mating  ensures individuals an equal chance of passing on alleles to offspring • the population must be very large  genetic drift has less effect than it would on a small population • there can be no movement into or out of the population  want to resist adding or removing alleles • there can be no mutations  don’t want to add new alleles • there can be no natural selection  there needs to be an equal opportunity for different genotypes and phenotypes to be passed on

25. Hardy-Weinberg Model predicts a relationship between allele frequency in a gene pool and the genotype frequency in the next generation • The equation: p2 + 2pq + q2 = 1 p = dominant allele q = recessive allele p + q = 1 represents all the alleles in the population for a trait p2 = # homo. dominant organisms in a gene pool 2pq = # heterozygous organisms in a gene pool q2= # homo. recessive organisms in a gene pool

26. Hardy-Weinberg allows you to: • Calculate all the genotype frequencies • Show that the allele frequencies tend to stay stable over time so the genetic variation in a population tends to be unchanged over time • Many of the assumptions are unrealistic, but the model does work in the real world. Genotype frequencies are usually close to those predicted by the model.

27. The Process of Speciation16-3 • Speciation is the formation of new species. • Species= group of organisms that breed with one another and produce fertile offspring IN NATURE. • The gene pools of two populations must become separated for them to become new species. • When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

28. Isolating Mechanisms • Reproductive isolation can develop in a variety of ways, including: • behavioral isolation • geographic isolation • temporal isolation

29. Behavioral isolation = when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.

30. Geographic isolation = when two populations are separated by geographic barriers such as rivers or mountains. • Geographic barriers do not guarantee the formation of new species.

31. Temporal isolation or seasonal isolation = when two or more species reproduce at different times of the year.

32. Speciation in the Galápagos finches occurred by: • founding of a new population • geographic isolation • changes in new population's gene pool • reproductive isolation • ecological competition