Evolution of Populations

1. Evolution of Populations Ch 16

2. Where does Variation come from? • Mutations • errors in mitosis & meiosis • environmental damage • Meiosis • mixing of genes through crossing over and independent assortment.

3. Natural Selection and Populations -Single gene trait: controlled by single gene with two alleles • Examples: widow’s peak, hitchhiker’s thumb, tongue rolling -Polygenic trait: controlled by 2 or more genes with 2 or more alleles • Examples: height, hair color, skin color, eye color.

4. Low mortality, high fitness Key High mortality, low fitness Food becomes scarce. Natural Selection Affecting Polygenic Traits A. Directional Selection: Individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait • Example: Galapagos finches – beak size

5. Natural Selection Affecting Polygenic Traits B. Stabilizing Selection: individualsat the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve • Example: human birth weight

6. Natural Selection Affecting Polygenic Traits C. Disruptive Selection: individuals at both ends of the curve have highest fitness. • Example: birds where seeds are either large or small

7. Evolution and Speciation • How can natural selection lead to the formation of new species???

8. Speciation • Natural selection and chance events can change the relative frequencies of alleles (# times an allele occurs in the gene pool) in a population and lead to speciation. • Speciation is the formation of new species. • A species is a group of organisms that breed with one another and produce fertile offspring.

9. What Factors are involved in the formation of New species?? • The gene pool of two populations must become separated for them to become new species.

10. Reproductive Isolation • As new species evolve, populations become reproductively isolated from each other. • When members of the two populations can’t breed with each other and produce fertile offspring, reproductive isolation has occured

11. Isolating Mechanisms • How can reproductive isolation occur? • Behavioral Isolation: when two populations are capable of interbreeding, but have differences in courtship rituals or other reproductive behaviors. • Ex: Eastern and western meadow larks have overlapping ranges but don’t breed because they have a different matting call.

12. Isolating Mechanisms How can reproductive isolation occur? 2. Geographic Isolation: two populations are separated by geographic barriers, like rivers, mountains, or bodies of water. Ex: grand canyon squirrels: Colorado river separated 2 populations 10,000 years ago.

13. Isolating Mechanisms How can reproductive isolation occur? • Temporal isolation: when two or more species reproduce at different times (different months or seasons) Ex: Northern Leopard Frog, North American Bullfrog Mates in April Mates in July

14. Genetic Drift • Genetic Drift: random change in allele frequency that occurs in small populations • Genetic drift can be caused by: 1. Founder Effect 2. Bottleneck Effect

15. 1. Founder Effect • When a new population is started by only a small group of individuals. example: colonization of New World • just by chance some rare alleles may be at high frequency; others may be missing • skew the gene pool of new population

16. 2. Bottleneck Effect • When a large population is drastically reduced by a disaster. • Famine, natural disaster, loss of habitat. • Loss of variation by chance event. • Alleles lost NOT due to fitness!

17. Testing Natural Selection & Speciation in Nature • Can natural selection actually be observed in nature?? YES! • Important studies of natural selection in action have been done in the Galapagos Islands, with the descendants of the finches that Darwin first studied • Darwin’s Hypothesis: The finches he saw were different from each other, but he hypothesized that they all had a common ancestor

19. Testing Darwin’s Hypothesis • Two assumptions of Darwin’s finch hypothesis: • For beak size and shape to evolve, there must be enough heritable variation in those traits for natural selection to act on them • Difference in beak size and shape must produce differences in fitness, allowing natural selection to occur

20. Testing Darwin’s Hypothesis The Galapagos Islands • Researchers from Princeton University--Peter and Rosemary Grant--tested Darwin’s finch hypothesis • The Grants focused on the Medium Ground Finch on Daphne Major, one of the Galapagos islands

21. The Grants’ Observations • During the rainy season plenty of food • During droughts food becomes scarce, & birds with bigger beaks have better chance of survival • This shows that beak size can be changed by natural selection

22. A few finches, founders(species A), travel from S. America to the Galapagos • Survive and reproduce • A group of birds from Species A travels to another island in the Galapagos- Geographic Isolation. • The two groups no longer share a gene pool

23. Speciation in Galapagos Finches 3. Seed sizes on the second island favor birds with bigger beaks. Species B evolves over time. 4. If birds from population B migrate back to the original island, they will not mate with birds from population A • Populations A and B are separate species

24. Speciation in Galapagos Finches 5. Ecological Competition: If species A and B compete for seeds on the first island, they continue to evolve in ways that increase the difference between them. • A new species--C-- might evolve

25. Continued Evolution • The process of isolation, genetic change, and reproductive isolation probably repeated itself often across the entire Galapagos Island chain.

26. The End. Answer your review questions. • Name and describe 3 types of selection that affect polygenic traits. • 2) What is speciation? • 3) Name and describe 3 types of reproductive isolation.