Intro. to Evolution I: Chapter 16 Population Genetics

1. Intro. to Evolution I: Chapter 16 Population Genetics

2. How do we get new species? • Breeders have been affecting plant and animal populations for centuries through artificial selection. • Genetic variation in populations allows breeders to change gene pools. • Also allows populations to adapt to environmental changes

4. Microevolution in Large Populations • Natural selection is one of the most important factors that change gene pools. • English peppered moth: • Nearly all had light-colored wings in 1850. • By 1900, almost all had dark wings. How could this have happened? • Natural selection changed the frequencies in the gene pool and helped the population adapt to its changed environment.

5. Sickle-cell Allele Frequency • Hemoglobin is the protein in red blood cells that carries oxygen throughout the body • A difference in one amino-acid results in the sickle-cell allele • When homozygous, causes problems with delivery of oxygen to tissues • Tend to be anemic

6. In regions of Africa, sickle-cell allele frequency is as high as 20%. • Geneticist J. B. S. Haldane noticed most frequent in areas with high malaria rates • Haldane hypothesized that the sickle-cell allele was advantageous in certain environments because it protected against malaria • He was right! • Natural selection favors heterozygote  malaria resistant and only slightly anemic

8. Mechanisms that Affect Gene Pools • Natural selection: Increased reproductive success leads to increased allele frequency; poor reproductive success leads to decreased allele frequency • Main factor that changes the gene pools of large populations

10. Genetic drift: Random changes in small populations • Gene flow: Change in the gene pool resulting from migration of individuals between populations • Can introduce new alleles or change allele frequencies • Can make gene pools similar between geographically separated populations • Mutation: Direct conversion of one allele to another • Often bad or no effect • Sometimes beneficial and favored by natural selection

11. Microevolution in Small Populations • Same factors affecting large populations also act on small populations • Genetic drift has a substantial impact on small populations, but little effect on large populations • In small populations, the allele frequencies jump around, changing randomly from generation to generation

12. Sometimes a small number of migrants starts a new population • Ex. Darwin’s finches came from S. American mainland • Only a few migrants on islands without any other finches  start new population • Gene pools sometimes very different from mainland populations just by chance

13. Genetic drift that influences new populations is called the founder effect • A new population can have allele frequencies that are very different from its source population if the number of organisms that establish the new gene pool is small

14. Inbreeding • Genetic drift reduces the frequency of heterozygotes over time • Inbreeding is the gradual increase in homozygosity caused by genetic drift in small populations • This can occur in large populations, too, if there is a population bottleneck • Number of organisms drastically reduced for a few generations

16. Inbreeding cont. • Inbreeding can have negative effects on populations • Most gene pools have recessive alleles that are harmless as single copies • Inbreeding increases the frequency of harmful and lethal alleles • Increased homozygosity of these alleles cause inbred populations to suffer from inbreeding depression • Fertility and survival are reduced compared to populations that are not inbred

17. Think About It! What are some of the ways that a loss of genetic variation could prove problematic for a population?