Lecture 2. How populations evolve

Updated: 1/15/08

Thursday, January 17, 2008. Reading: 13

Evolution occurs at small scales by changes in gene frequencies
- Microevolution is a change in the gene pool of the population.
- Therefore, a population is the minimum unit that can evolve
- The modern synthesis is this view of evolution that includes population genetics.
Hardy Weinberg Equilibrium (HWE) is a “null model” of no evolution within a population, i.e. allele frequencies remain constant through generations.

Certain conditions must be met:
- the population is very large
- no immigration or emigration
- no mutations
- mating is random
- no differential reproductive success
Microevolution occurs in any case that breaks the rules of HWE (*=most typical causes)
- small population=genetic drift* (including bottleneck and founder effects)
- immigration or emigration = gene flow
- mutation
- non-random mating = sexual selection
- differential reproductive success = natural selection*
Selection is an editing process, and therefore requires variation in the gene pool. Lack of variation limits ability to adapt to changing environment
- Variation is generated by mutation and sexual recombination; not all variation is subject to selection
- Variation is maintained by diploidy and balancing selection
- Natural selection typically takes three forms: stabilizing, directional, disruptive
- Selection cannot fashion perfect organisms due to: limits on variation, chance, and interactions among selective regimes
- Perpetuation of genes constitutes evolutionary fitness

A printable syllabus, with course dates, required materials, grading and other policies can be found here.

A one page printable version of the schedule can be found here.