Chapter 23.

1. Chapter 23. Evolution of Populations

2. Populations evolve • Natural selection acts on individuals • differential survival • “survival of the fittest” • differential reproductive success • who bears more offspring • Populations evolve • genetic makeup of population changes over time • favorable traits (greater fitness) become more common Bent Grass on toxic mine site

3. Individuals DON’T evolve!!!

4. Mutation & Variation • Mutation creates variation • new mutations are constantly appearing • Mutation changes DNA sequence • changes amino acid sequence? • changes protein? • change structure? • change function? • changes in protein may change phenotype & therefore change fitness

5. Sex & Variation • Sex spreads variation • one ancestor can have many descendants • sex causes recombination • offspring have new combinations of traits = new phenotypes • Sexual reproduction recombines alleles into new arrangements in every offspring

6. Variation impacts natural selection • Natural selection requires a source of variation within the population • there have to be differences • some individuals must be more fit than others

7. Changes in populations • Evolution of populations is really measuring changes in allele frequency • all the genes & alleles in a population = gene pool • Factors that alter allele frequencies in a population • natural selection • genetic drift • founder effect • bottleneck effect • gene flow

8. Natural selection • Natural selection adapts a population to its environment • a changing environment • climate change • food source availability • new predators or diseases • combinations of allelesthat provide “fitness” increase in the population

9. Genetic drift • Effect of chance events • founder effect • small group splinters off & starts a new colony • bottleneck • some factor (disaster) reduces population to small number & then population recovers & expands again

10. 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 • skew the gene pool of new population • human populations that started from small group of colonists • example: white people colonizing New World

11. Distribution of blood types • Distribution of the O type blood allele in native populations of the world reflects original settlement

12. Distribution of blood types • Distribution of the B type blood allele in native populations of the world reflects original migration

13. Out of Africa Likely migration paths of humans out of Africa Many patterns of human traits reflect this migration

14. Bottleneck effect • When large population is drastically reduced by a disaster • famine, natural disaster, loss of habitat… • loss of variation by chance • alleles lost from gene pool • narrows the gene pool

15. Cheetahs • All cheetahs share a small number of alleles • less than 1% diversity • as if all cheetahs are identical twins • 2 bottlenecks • 10,000 years ago • Ice Age • last 100 years • poaching & loss of habitat

16. Conservation issues • Bottlenecking is an important concept in conservation biology of endangered species • loss of alleles from gene pool • reduces variation • reduces ability to adapt • at risk populations

17. Gene flow • Population spread over large area • migrations = individuals move from one area to another • sub-populations may have different allele frequencies • Migrations cause genetic mixing across regions = gene flow • new alleles are moving into gene pool • reduce differences between populations

18. Human evolution today • Gene flow in human populations is increasing today • transferring alleles between populations Are we moving towards a blended world?

19. Any Questions??

20. Chapter 23. MeasuringEvolution of Populations

21. Populations & gene pools • Concepts • a population is a localized group of interbreeding individuals • gene pool is collection of alleles in the population • remember difference between alleles & genes! • allele frequencyis how common is that allele in the population • how many A vs. a in whole population

22. Evolution of populations • Evolution = change in allele frequencies in a population • hypothetical: what would it be like if allele frequencies didn’t change? • non-evolving population • very large population size (no genetic drift) • no migration (movement in or out) • no mutation (no genetic change) • random mating (no sexual selection) • no natural selection (no selection)

23. Hardy-Weinberg equilibrium • Hypothetical, non-evolving population • preserves allele frequencies • Serves as a model • natural populations rarely in H-W equilibrium • useful model to measure if forces are acting on a population • measuring evolutionary change G.H. Hardy mathematician W. Weinberg physician

24. Hardy-Weinberg theorem • Alleles • assume 2 alleles = B, b • frequency of dominant allele (B) =p • frequency of recessive allele (b) = q • frequencies must add to 100%, so: p + q = 1 BB Bb bb

25. Hardy-Weinberg theorem • Individuals • frequency of homozygous dominant: p x p = p2 • frequency of homozygous recessive:q x q = q2 • frequency of heterozygotes: (p x q) + (q x p) = 2pq • frequencies of all individuals must add to 100%, so: p2 + 2pq + q2 = 1 BB Bb bb

26. Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 p2=.36 2pq=.48 q2=.16 BB Bb bb Must assume population is in H-W equilibrium!

27. BB Bb bb Using Hardy-Weinberg equation p2=.36 2pq=.48 q2=.16 Assuming H-W equilibrium BB Bb bb Null hypothesis p2=.20 p2=.74 2pq=.64 2pq=.10 q2=.16 q2=.16 Sampled data How do you explain the data? How do you explain the data?

28. How do allele frequencies change? Think of all the factors that would keep a population out of H-W equilibrium!

29. Real world application of H-W • Frequency of allele in human population • Example: • What % of human population carries allele for PKU (phenylketonuria) • Should you screen prospective parents? • ~ 1 in 10,000 babies born in the US is born with PKU • results in mental retardation, if untreated • disease is caused by a recessive allele • PKU = homozygous recessive (aa)

30. H-W & PKU disease • frequency of homozygous recessive individuals q2 (aa) = 1 in 10,000 =0.0001 • frequency of recessive allele (q): q= √0.0001 =0.01 • frequency of dominant allele (p): p (A) = 1 – 0.01 =0.99 • frequency of carriers, heterozygotes: 2pq = 2 x (0.99 x 0.01) =0.0198=~2% • ~2% of the US population carries the PKU allele 300,000,000 x .02 = 6,000,000 people

31. Any Questions??