Chapter 16: Evolution of Populations

1. Chapter 16: Evolution of Populations

2. HOMEWORK • Chapter 16 Section Assessments: Due Fri. 5/2 • Chapter 16.1 SA: p. 396 (1-5) • Chapter 16.2 SA: p. 402 (1-5) • Chapter 16.3 SA: p. 410 (1-2) • Chapter 16 Assessment: p. 413 (1-10, 12, 17, 18, 19)

3. VOCABULARY (DUE FRI. 5/2)Provide the glossary definition and etymology for the following terms: • Gene pool • Relative Frequency • Single-gene trait • Polygenic trait • Directional selection • Stabilizing selection • Disruptive selection • Genetic drift • Founder effect • Hardy-Weinberg principle • Genetic equilibrium • Speciation • Reproductive isolation • Behavioral isolation • Geographic isolation • Temporal isolation

4. 16.1: Genes and Variation • A weakness in Darwin’s theory stemmed from his lack of understanding in genetic variation. • In the 1930s, evolutionary biologists combined Mendel’s work with that of Darwin’s to support evolutionary change, specifically the process of natural selection. • How do you think Mendel would have felt about that?

5. 16.1 Variation and Gene Pools • Population: a group of individuals of the same species that interbreed. • Gene pool: consists of all genes, including all the different alleles, that are present 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. (Fig. 16-2) • Key Concept: In genetic terms, evolution is any change in the relative frequency of alleles in a population.

6. 16.1 Sources of Genetic Variation • Key Concept: The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. • Remember mutations are changes in the genetic code. Some are expressed, some remain silent. Some mutations can affect an organism’s fitness, while others have not effect. • Crossing over and independent assortment during gamete formation leads to genetic variation. (23 pairs of chromosomes can have 8.4 million different combinations of genes!)

7. 16.2 Evolution as Genetic Change

8. 16.2 Natural Selection on Single-Gene Traits • Key Concept: Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. (Fig. 16-5) • Remember: Evolution is change over time in the relative frequencies of alleles in a population, therefore it is populations that can evolve, not individual organisms.

9. 16.2 Natural Selection on Polygenic Traits • Key Concept: Natural selection can affect the distributions of phenotypes in any of three ways: • Directional Selection: when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. (Fig. 16-6) • Stabilizing Selection: center curve has higher fitness (Fig. 16-7) • Disruptive Selection: upper and lower ends have high fitness (Fig. 16-8)

10. Natural Selection and Normal Distribution One extreme favored Average favored Both extremes favored

11. 16.2 Genetic Drift • Key Concept: In small populations, individuals that carry a particular allele may leave more descendants that 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. (Fig. 16-9) • Genetic Drift: Random change in allele frequencies that occurs in small populations • Founder Effect: A situation in which allele frequencies change as a result of the migration of a small subgroup of a population

12. 16.2 Hardy-Weinberg and Genetic Equilibrium • Genetic Equilibrium: A situation in which allele frequencies remain constant. • Key Concept: Five conditions are required to maintain genetic equilibrium from generation to generation: • Random mating • Large population size • No movement into or out of the population • No mutations • No natural selection

13. 16.2 Hardy-Weinberg Equation (p. 401-402) • p + q = 1 • p = dominant allele frequency (A) • q = recessive allele frequency (a) • p2 + 2pq + q2 = 1 • p2: frequency of AA homozygous • 2pq: frequency of Aa heterozygous • q2: frequency of aa homozygous • 1: sum of frequencies for all genotypes (100%) • Extra Credit Assignment!! Due Fri. 5/2

14. 16.3 The Process of Speciation

15. 16.3 Isolating Mechanisms • Speciation: formation of new species • Key Concept: A new species evolve, populations become reproductively isolated from each other. • Reproductive Isolation: When members of two populations cannot interbreed and produce fertile offspring. They now have separate gene pools.

16. 16.3 Isolating Mechanisms • Causes of reproductive isolation: • Behavioral Isolation: capable of interbreeding, but have different courtship rituals or other reproductive behaviors. • Geographic Isolation: separated by physical barriers (i.e. rivers, mountains, etc…) • Temporal Isolation: reproduce at different times. Is the definition of species constant? “Can vs. Will”

17. 16.3 Testing Natural Selection in Nature • Peter and Rosemary Grant have spent more than 20 years observing collecting data on Galapagos finches. • They found variation in physical characteristics. • They observed natural selection occurring during drought seasons.

18. 16.3 Speciation in Darwin’s Finches • Key Concept: Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition.

19. 16.3 Studying Evolution Since Darwin • “New data from genetics, physics, and biochemistry could have proved him wrong on many counts. They didn’t.” • “…while the Grants observed changes in the size of the finches’ beaks, they did not observe the formation of a new species.” • Read: Unanswered Questions (p. 410) • Why is understanding evolution important? After Break...Origins of the Earth!