Evolution of Populations

1. Evolution of Populations Chapter 11

2. Genetic Variation Within Populations • Variation and gene pools • Gene pool – all genes present in a population • Allele Frequency – the number of times an allele occurs in the gene pool, compared to other alleles

3. Genetic Variation Within Populations • In genetic terms, evolution is any change in the relative frequency of alleles in a population.

4. Genetic Variation Within Populations • Scientists now recognize two main sources of genetic variation • Mutations • Recombination

5. Genetic Variation Within Populations • Mutations • Any change in a sequence of DNA • Occurs Because: • Problems with replication • Exposure to radiation or chemicals

6. Genetic Variation Within Populations • Recombination • Parents alleles rearrange when forming gametes • The 23 pairs of chromosomes can produce 8.4 million different combinations!

7. Genetic Variation Within Populations • Crossing Over • Occurs during meiosis • Further increases genetic variation

8. Natural Selection In Populations • Normal distribution • Highest frequency in middle and lowest at the extremes • Also representative for allele freqencies

9. Natural Selection In Populations • Microevolution • Observable changes in allelic frequencies • Occurs in a single population • 3 ways • Directional • Stabilizing • Disruptive

10. Natural Selection In Populations • Directional Selection • Shift in phenotype to one extreme • Mean also shifts

11. Natural Selection In Populations • Stabilizing Selection • Norm is selected for and becomes more common • Ex. Gall Fly and Goldenrod

12. Natural Selection In Populations • Disruptive Selection • Selection for the extremes • Ex. Peppered moths

13. Natural Selection In Populations • Single-gene and polygenic traits • Widows peak is a single gene trait – a single gene with two alleles • Polygenic Traits are controlled by two or more genes • This means that a polygenic trait can have many possible genotypes ant thus phenotypes.

14. 16-2 Evolution as Genetic Change • Natural Selection on Single-Gene Traits

15. Other Mechanisms of Evolution • Gene Flow • Some animals move once they are able • When the move into new population, their alleles become part of gene pool • Occurs with movement between populations • Increases variation • If less movement, the more different two species become

16. Other Mechanisms of Evolution • Genetic Drift – Random change in allele frequency • Bottleneck Effect • Founder effect – when allele frequencies change because of migration

17. Other Mechanisms of Evolution • Sexual Selection • Females the important factor in reproduction • This makes females “picky” • Intrasexual selection • Intersexual selection

18. Hardy-Weinberg Equilibrium • Evolution vs. Genetic Equilibrium • Hardy-Weinberg principle – genotype frequencies will stay constant unless some factor changes the frequency • Genetic Equilibrium – when allele frequencies remain constant

19. Hardy-Weinberg Equilibrium • Five Conditions to Maintain Equilibrium • Random Mating • Large Population • No Movement into or out of the population • No Mutations • No Natural Selection

20. Hardy-Weinberg Equilibrium • Biologists can use an equation for comparison purposes p2+ 2pq + q2 = 1 p = dominant alleles q = recessive alleles

21. Hardy-Weinberg Equilibrium • Five factors leading to evolution • Genetic drift • Gene flow • Sexual selection • Natural selection • Mutations

22. Speciation Through Isolation • Reproductive Isolation – members of populations can no longer mate • Mules or Hinnies • Speciation - when two species come from one existing species

23. Speciation Through Isolation • Behavioral Isolation • Two populations will not breed because of differences in courtship • Fireflies

24. Speciation Through Isolation • Geographical Isolation • Rivers, mountains, or bodies of water separate two populations • Snapping Shrimp and placental vs. marsupial mammals

25. Speciation Through Isolation • Temporal Isolation • Different species mate at different times • Times of day • Times of year • Trees and plants

26. Patterns in Evolution • Convergent Evolution – evolution towards similar characteristics in unrelated species • Analogous structures • Bird and bat wings

27. Patterns in Evolution • Divergent Evolution – closely related species in different directions • Results from adapting to different environments • Kit and Red Fox

28. Patterns in Evolution • Coevolution – two or more species evolve in response to changes in each other • Benefit to both species • Plants and bees

29. Patterns in Evolution • Coevolution can also drive predator-prey relationships • Cheetahs and gazelles

30. Patterns in Evolution • Extinctions – species are eliminated • Background extinctions – occur continously, but at very low rates • Mass extinctions – rare, more intense, involve many species

31. Patterns in Evolution • Punctuated equilibrium – bursts of evolutionary activity followed by periods of inactivity • Adaptive radiation – diversification of multiple species from one ancestral species • Mammals