Measuring Evolution of Populations

1. MeasuringEvolution of Populations

2. Populations & Gene Pools • Concepts • a population is a localized group of the same species that can interbreed • gene pool is collection of alleles in the population • Gene pool is all of the different genes in that population • remember difference between alleles & genes!

3. Variation & Gene Pool • allele frequencyis the number of times that the allele occurs in the population • how many A vs a in whole population • Expressed in percentage (%) • i.e. 40% Black & 60% Brown

4. Evolution of populations • Evolution = change in allele frequencies in a population • hypothetical: what conditions would cause allele frequencies to not change? • non-evolving population REMOVE all agents of evolutionary change • very large population size (no genetic drift) • no migration (no gene flow in or out) • no mutation (no genetic change) • random mating (no sexual selection) • no natural selection (everyone is equally fit)

5. 5 Agents of evolutionary change Mutation Gene Flow Non-random mating Genetic Drift Selection

6. Sources of Variation • Mutations • Caused by error in replication, radiation, chemicals in the environment • Only some mutations change the phenotype & affect fitness • Gene Shuffling: results from sexual reproduction • 23 pairs of chromosomes can make 8.4 million gene combinations • Crossing over causes differences in genes • Gene shuffling doesn’t change the allele frequency • Still have same # of alleles in population, but recombined

7. Single Gene vs Polygenic Traits • The number of phenotypes produced for a given trait depends on how many genes control the trait • Single Gene Trait: controlled by a single gene (2 alleles) • Polygenic Trait: traits controlled by two or more genes • Offers more variation

8. Evolution as Genetic Change • evolutionary fitness is an organism’s success in passing genes to the next generation • an evolutionary adaptation as any genetically controlled physiological, anatomical, or behavioral trait that increases an individuals ability to pass along its genes • Remember that evolution is any change over time in the relative frequency of alleles in a population. • This reminds us that it is populations, not individual organisms that can evolve overtime

9. Natural Selection on Single Gene Trait • Natural selection on single gene traits can lead to changes in allele frequencies and thus to evolution • i.e. Lizards

10. Natural Selection on Polygenic Traits • Natural selection can affect the distributions of phenotypes in any of three ways • Stabilizing Selection • Disruptive Selection • Directional Selection

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

12. Disruptive Selection • When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle • Can create 2 distinct phenotypes

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

14. Genetic Drift • Genetic Drift: the random change in allele frequency • Occurs in small populations that break away from larger groups • In small populations, an allele can become more or less common by chance • Caused by individuals entering & leaving (migrating) • Ex. Founder’s Effect • When the allele frequency changes as a result of migration of a small group

15. Hardy-Weinberg equilibrium • Hypothetical, non-evolving population • preserves allele frequencies • Serves as a model (null hypothesis) • 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

16. Non-Evolving Population • Needs 5 conditions for equilibrium: • Random Mating • Large Population (No Genetic Drift) • No Migration In or Out • No Mutation • No Natural Selection

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

18. Hardy-Weinberg theorem • Counting 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 1 (100%), so: p2 + 2pq + q2 = 1 BB Bb bb

19. B b BB Bb bb H-W formulas • Alleles: p + q = 1 • Individuals: p2 + 2pq + q2 = 1 BB Bb bb

20. 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! What are the genotype frequencies?

21. 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?

22. Speciation • Speciation: formation of a new species • Reproductive Isolation: • As new species evolve, populations become more reproductively isolated from each other. • Isolation Mechanisms: • Temporal Isolated • Two species reproduce at different times • Behaviorally Isolated • Can breed, but have different courtship behaviors • Geographically Isolated • Barriers such as rivers, mountains, bodies of water

23. Temporal Isolation Rana aurora - breeds January - March Rana boylii - breeds late March - May

24. Behavioral Isolation Eastern & Western Meadowlark

25. Geographic Isolation Albert & Kaibab Squirrels