Chapter 8 Flashcards
Gene pool
- frequency of each allele within a population
- evolution is the change in the gene pool of a population over time
Define Hardy-Weinberg principle + 5 conditions
- in large populations in which only random chance is at work, allele frequencies are expected to remain constant from generation to generation
1. no natural selection
2. population must be very large
3. no mutations
4. population must be isolated from other populations (no immigration or emigration)
5. random mating and no horizontal gene transfer
Selective pressures
- may result from any number of biotic or abiotic factors
- can result in different patterns of natural selection
Directional selection
- a selection that favors individuals with a more extreme variation of a trait
- results in a shift away from the average
- common in artificial selection
Stabilizing selection
- occurs when the average phenotype within a population is favored
- selection against extreme traits
Disruptive selection
- favours individuals with variations at opposite extremes of a trait over the average phenotype; favours more than one phenotype
Sexual selection
- favours any traits that specifically enhance the mating success of an individual
- leads to males and females of a species evolving appearances and behaviours that differ from each other (sexual dimorphism)
Genetic Drift
changes to allele frequency as a result of chance; effects small population more than large population
Bottleneck Effect
- occurs when a population undergoes a drastic reduction in size
- caused by chance events (volcanic eruption, hurricane)
- bad luck, not bad genes
- can cause big losses of genetic variation in small populations
Example of Bottleneck Effect
- Northern elephant seals have reduced genetic variation
- hunting reduce their population size to 20 individuals at the end of the 19th century
- have since rebounded but have less genetic variation than southern elephant seals
Founder effect
- results when a small number of individuals separate from their original population and establish a new population
- this small population size means that the new population may have…
- reduced genetic variation from original population
- non-random sample of the genes in original population
Example of Founder Effect
- Afrikaner population of dutch settlers in south africa descended from a few colonists
- Today, Afrikaner population have a high frequency of Huntington’s disease because the original settlers carried the gene for Huntington’s disease
Gene flow + 2 types
occurs when individuals migrate to/from one population to/from another and interbreed
immigration - individuals enter a population
- possible addition of new genetic material to the gene pool of a population
emigration - individuals leave a popluation
- possible removal of genetic material from the gene pool of a population
Mutations
random genetic mutations can introduce new alleles into a gene pool
Microevolution
changes in gene (allele) frequencies and phenotypic traits within a population over a relatively short period of time