Population Evolution: Microevolution

1. Population Evolution:Microevolution Individuals do not evolve, populations do.

2. Microevolution • A change in a population’s gene pool over a secession of generations. • Evolutionary changes in single species over relatively brief periods of geological time.

3. Five Mechanisms of Microevolution 1. Genetic drift: Change in the gene pools allele frequencies of a small population due to chance. • Two examples that lead to small populations: • Bottleneck effect • Founder effect

4. Bottleneck Effect • Genetic drift (reduction of alleles in a population) resulting from an event that drastically reduces population size. • Bottleneck causes: 1. Earthquake 2. Volcano 3. Asteroid impact 4. Human contact 5. Flood 6. Tsunami

5. Examples of bottlenecks • By 1900 hunting of the northern elephant seal off the Pacific coast had reduced its population to only 20 survivors. Since hunting ended, the population has rebounded from this population bottleneck to some 100,000 animals today. However, these animals are homozygous at every one of the gene loci that have been examined. • Cheetahs have passed through a similar period of small population size with its accompanying genetic drift. Examination of 52 different loci has failed to reveal any polymorphisms; that is, these animals are homozygous at all 52 loci. The lack of genetic variability is so profound that cheetahs will accept skin grafts from each other just as identical twins (and inbred mouse strains) do.

6. Founder Effect • Genetic drift resulting from the colonization of a new location by a small number of individuals. • Results in random change of the gene pool. • Examples: 1. Islands (first Darwin finch) 2. Settling of newly exposed areas (volcanic, ice sheet, fire damaged) 3. Settlers of open areas.

7. Examples of Founder Effect • In the 1680s Ariaantje and Gerrit Jansz emigrated from Holland to South Africa, one of them bringing along an allele for the mild metabolic disease porphyria. Today more than 30,000 South Africans carry this allele and, in every case examined, can trace it back to this couple, a remarkable example of the founder effect.

8. Example

9. Five Mechanisms of Microevolution 2.Gene Flow: The gain or loss ofalleles from a population by the movement of individuals. • Immigration (in) or emigration (out).

10. Five Mechanisms of Microevolution 3. Mutation: Change in an organism’s DNA that creates a new allele. 4. Non-random mating: The selection of mates other than by chance. 5. Natural selection: Differential reproduction.

11. Number of Individuals Small Large Size of individuals Types of selection • Natural selection has three modes of selection: 1. Stabilizing selection 2. Directional selection 3. Disruptive selection

12. Number of Individuals Small Large Size of individuals 1. Stabilizing Selection • Pushes both ends in (extremes), removes upper and lower variation in species.

13. Examples • Moths that are easily seen on trunk of tree if too light or too dark. • Siberian husky: too big will sink into snow, too light won’t be able to pull sled. • Human birth weights.

14. Number of Individuals Small Large Size of individuals 2. Directional Selection • Pushes towards one end, removes upper or lower variations.

15. Examples • Several mammals, like horses, have become larger over time. • Frogs ability to jump farther. • Humans getting taller in industrial nations.

16. Number of Individuals Small Large Size of individuals 3. Disruptive Selection • Removes middle (median) variation, pushes towards both end • Can cause development of two new species.

17. Examples • Tadpoles developing either with a dark tail or a light tail. • Grasses on mine tailings. • Finches on the Galapagos.