Chap 23

1. Chap 23 The Evolution of Populations Chap 23 The Evolution of Populations

2. Evolution Reminders • Individuals DO NOT evolve, a population evolves through its interaction with the environment. • Evolution can only be measured as changes in relative proportions of heritable variations in a population over a succession of generations. • Natural selection can amplify or diminish heritable traits only. • Environmental factors vary from place to place and time to time.

3. Natural selection produces evolutionary change when some individuals in a population possess certain inherited characteristics and then produce more surviving offspring than individuals lacking these characteristics. • As a result, the population gradually comes to produce more and more individuals with advantageous characteristics. • Therefore the population evolves and becomes better adapted to the local circumstances.

4. A population is defined as a localized group of organisms that are capable of interbreeding and producing fertile offspring. • The study of how populations change over time  Population Genetics. • The collection of genes in a population at any one time  Gene Pool.

5. Hardy-Weinberg Theorem • 1908 – Godfrey Hardy and Wilhelm Weinberg were confused as to why, after many generations, populations didn’t become solely composed of individuals with the dominant phenotype.

6. Their theorem is used as a way to provide a benchmark against which to measure frequency changes. • This theorem states that the frequencies of alleles and genotypes in a populations gene pool remain constant from generation to generation, provided certain assumptions are met.

7. This theorem describes a hypothetical population that is NOT evolving. • This theorem can be used to obtain approximate estimates of allele and genotype frequencies.

8. Conditions for Hardy-Weinberg Equilibrium • Extremely large population size. • No gene flow. • No mutations. • Random mating. • No natural selection. ** this is normally unrealistic which means that most natural populations result in evolution.**

9. Theorem: p2 + 2pq + q2 = 1 p = the frequency of an allele at a loci (dominant) q = the frequency of an allele at a loci (recessive) ** (p + q = 1)

10. The equation for the Hardy-Weinberg equilibrium states that at a locus with 2 alleles, the three genotypes will appear in proportions to the sum of 1. • If a population were in H-W equilibrium, and its members continued to mate randomly generation after generation, the allele and genotype frequencies would remain constant.

11. 3 Factors that Alter Allelic Frequencies • Natural Selection • Genetic Drift : The Bottleneck effect and The Founder effect. • Gene Flow These factors not only affect allelic frequencies but also cause the most evolutionary change.

14. Natural Selection as a Mechanism • Natural selection can alter the frequency of heritable traits depending on which phenotypes in a population are favored. • Directional selection : shifts the frequency curve for some phenotypic character in one direction or the other by favoring individuals that deviate form the average.

15. Disruptive selection: favors those individuals on both extremes of a phenotype over those with an intermediate. • Stabilizing Selection: Acts against extreme phenotypes and favors those that are intermediate.

18. Male African Long-tailed Widow bird

19. Evolution is limited by historical restraints. • Adaptations are often compromises. • Chance and natural selection interact. • Selection can edit only existing variations. As a result evolution cannot fashion the “perfect” organism. As a result we see numerous examples of organisms that are less than “ideally engineered” for their lifestyle.