Evolution and Populations: Variation and Gene Pools

1. Chapter 16 Evolution and Populations

2. Variation and Gene Pools • Variations are differences between members of a population. • A population is a group of individuals of the same species that interbreed. • Because members of a population interbreed they share a common group of genes called a gene pool. • Gene pool: all of the genes including all of the different alleles in a population. • Relative frequency: the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

3. How does variation and gene pools apply to evolution? • Evolution involves changes in populations over time. • Evolution is any change in the relative frequency of alleles in a popultion.

4. What causes genetic variations • Mutations: a change in the sequence of DNA which would alter the protein or trait that is produced. • Mutations are random. • Sometimes they are harmful but sometimes they are helpful. • The helpful ones are passed on to offspring. • Recombination (Crossing-Over): occurs during meiosis where gene pairs cross over during Prophase I.

5. Single-Gene and Polygenic Traits • Single-Gene traits: controlled by a single gene that has 2 alleles. • Ex. Widow’s peak in humans. • Polygenic traits: controlled by 2 or more genes. • Ex. Height in humans • Usually a bell curve or normal distribution of individuals in a population occurs in polygenic traits (Most people are average height; neither very tall or very short). • Most human traits are polygenic.

6. Widow’s Peak

7. Bell-Curve

8. Natural Selection on Single-Gene Traits • Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. • Ex. Black versus red lizards • If red lizards are more visible to predators then they will most likely not live long enough to reproduce. • Black lizards might absorb more sun and warm up faster on cold days which would allow them to move faster to feed and avoid predators. • The allele for black color might increase in frequency.

9. Natural Selection on Polygenic Traits • Natural selection can effect Polygenic traits in 3 ways: • Directional Selection • Stabilizing Selection • Disruptive Selection

10. Directional Selection • When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. • The range of phenotypes will shift as some individuals fail to survive and reproduce while others succeed.

11. Example of Directional Selection • Beak size in Darwin’s finches. • Birds with bigger beaks can feed more easily on larger, harder, thicker-shelled seeds. • Suppose a food shortage causes the supply of small and medium sized seeds to run low, leaving only larger seeds. • Birds with larger beaks would have better access to food therefore the average beak size of the population would increase.

12. Stabilizing Selection • Occurs when individuals near the center of the curve have higher fitness than individuals at either end of the curve. • The situation keeps the center of the curve in its current position.

13. Example of Stabilizing Selection • Ex: Infant birth weight. • Human babies born smaller than average are likely to less healthy and less likely to survive. • Babies that are much larger than average have more difficulty being born. • The fitness of these larger or smaller individuals is lower than that of more average-sized babies.

14. Disruptive Selection • Occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. • Selection acts against those individuals of intermediate type. • Can cause a single curve to split in two or create 2 distinct phenotypes.

15. Genetic Drift • Random change in allele frequency in a population. • In small populations, individuals that carry a particular allele may leave more descendents than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

16. Founder effect • Genetic drift may occur when a small group of individuals colonize a new habitat. • These individuals may carry alleles in different relative frequencies than did the larger, original population. • If so, the population the “found” will be genetically different from the parent (original) population. • Darwin’s Finches

17. Hardy-Weinberg Principle • States that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. • The situation where allele frequencies remain constant is called genetic equilibrium. • If the allele frequencies do not change the population will not evolve.

18. 5 Conditions are Required to Maintain Genetic Equilibrium • Random mating • Large population • No movement into or out of the population • No mutations • No natural selection

19. Speciation • The process by which new species are produced. • New species are produced by reproductive isolation, where members of 2 populations cannot interbreed and produce fertile offspring. • 3 causes of reproductive isolation: • Behavioral isolation • Geographic Isolation • Temporal Isolation

20. Behavioral Isolation • Occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. • Ex. Eastern and Western Meadowlarks: use different songs to attract mates.

21. Eastern and Western Meadowlark

22. Geographic Isolation • 2 populations are separated by geographic barriers such as rivers, mountains, or bodies of water. • 2 separate gene pools form where genetic changes that appeared in one group are not passed to the other. • Natural selection worked separately on each group forming distinct subspecies. • Ex. Brown Bear and Polar bear

23. Brown Bear versus Polar Bear

24. Temporal Isolation • 2 or more species reproduce at different times therefore they cannot breed with one another. • Ex. Common in plant species where pollination occurs only at specific times for each species…preventing cross-pollination or interbreeding.