The Evolution of Populations

1. The Evolution of Populations Chapter 21

2. Microevolution • Evolutionary changes within a population • Changes in allele frequencies in a population over generations • Population – all members of a species living in the same area, interbreed, produce fertile offspring • Example – industrial melanism and the peppered moth

3. Genetic Variation • Differences among individuals in the composition of the genes • Single gene influence (Mendel) or polygenic • Phenotype – physical traits, can be inherited or influence by environment.

4. Sources of Genetic Variation • Formation of new alleles • Mutation • Altering Gene number or position • Chromosomal changes – deletion, translocation, inversion and duplication • Rapid reproduction – prokaryotes • Sexual Reproduction • Crossing over, independent assortment and random fertilization

5. Hardy-Weinberg principle • p2 +2pq + q2 • Used to calculate the genotype and gene frequencies of a population • States: equilibrium of allele frequencies in a gene pool will remain in effect in each generation of sexually reproducing populations as long as: • No mutations • No gene flow • Random mating • No genetic drift • No selection

6. Hardy-Weinberg principle • Tells us what factors cause evolution • The 5 conditions are hardly ever met • Allele frequencies do change from one generation to another • Evolution can be detected by seeing any deviation from a Hardy-Weinberg equilibrium • Practice problems p.406

7. Causes of microevolution • Opposite of HWP • Genetic mutations – cause for multiple alleles, can be adaptive and include favorable phenotypes • Nonrandom mating – inbreeding or breeding between relatives, decreases the heterozygote

8. Causes of microevolution that alter allele frequency directly Genetic Drift – change in allele frequencies due to chance • Bottleneck effect – natural disaster, reduce in population prevents the majority of genotypes from participating in the production of the next generation • Founder effect – rare alleles occur at a higher frequency in a population isolated from a general population ex. amish

9. Microevolution • Gene Flow – transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

10. Natural selection • Not random – adaptive evolution • Most traits are polygenic, see bell curve in allele frequency • 3 major types of selection - • Directional – extreme phenotype favored • Resistance to antibiotics and pesticides, malaria • Stabilizing – intermediate phenotype is favored • Birth weight survival, sickle cell trait • Disruptive – 2 or more extreme phenotypes are favored

11. Sexual selection • Natural selection in which individuals with certain inherited characteristics are more likely than others to obtain mates. • Sexual dimorphism – differences in males and females (i.e. size, color, …)

12. Balancing selection • Natural selection maintains two or more forms in a population. • Heterozygote advantage – Malaria and sickle cell anemia • Frequency-dependent selection – scale eating fish. Right and left mouthed

13. Why doesn’t Natural Selection create perfect organisms? • Selection can act only on existing variations • Evolution is limited by historical constraints • Adaptations are often compromises • Chance, natural selection and the environment interact