Topic 7B: Populations and Evolution Flashcards
What is a gene pool?
- All the alleles of all the individuals in a population at any one time
What are allele frequencies?
- The number of times an allele occurs within a population
What is the Hardy-Weinberg principle?
- Allele frequencies will remain the same from one generation to the next
What conditions are there in Hardy-Weinberg?
- No selection
- No mutation
- No migration
- Large population size
- Mating is random
What is the first equation?
p + q = 1
What is the first equation used for?
- Predicting allele frequencies
What do p and q mean?
- p = frequency of the dominant allele
- q = frequency of the recessive allele
What is the second equation?
p2 + 2pq+ q2 = 1
What is the second equation used for?
- Predicting genotype and phenotype frequencies
What do p2, 2pq and q2 mean?
- p2 = frequency of homozygous dominant genotype
- 2pq = frequency of heterozygous genotype
- q2 = frequency of homozygous recessive genotype
How does variation occur?
- Mutation
- Crossing over of chromatids
- Independent segregation
- Random fertilisation
Describe the general process of evolution
- Variation arises due to mutation
- Something acts as a selection pressure
- Individuals with favourable alleles more likely to survive and reproduce
- Pass on the favourable allele to offspring
- Over time the frequency of the favourable allele in the population increases
When does stabilising selection occur?
- In stable environmental conditions
What happens in stabilising selection?
- Average phenotype makes individuals more likely to survive and reproduce
- Is selected for
- Extremes selected against
What happens to mean and SD in stabilising selection?
- Same mean
- Smaller SD
When does directional selection happen?
- When an environmental change has occurred
What happens in directional selection?
- One extreme phenotype makes individuals more likely to survive and reproduce
- Is selected for
- Average and other extreme selected against
What happens to mean and SD in directional selection?
- New mean
- Same SD
What happens in disruptive selection?
- Both extreme phenotypes selected for
- Environment favours more than one phenotype
Define species
- Organisms that can breed together to produce fertile offspring
What is speciation?
- Development of a new species from an existing species
How does speciation occur?
- Initially a single population of a species
- Splits to 2 populations that cannot interbreed
- Different environments have different selection pressures
- Each population has variation, different mutations and so different alleles arise
- Some individuals are better adapted to new conditions
- Allele frequencies change in the 2 populations
- Eventually allele differences are so great, the 2 populations cannot interbreed
- 2 different species made
What is allopatric speciation?
- Geographically isolated
- e.g. by river, mountain range etc
What is sympatric speciation?
- No geographical isolation
- Reproductively isolated
- Seasonal changes / temporal shift –> breed at different times (nocturnal) or breed at different times in the season
- Behavioural changes –> changed courtship behaviours
- Mechanical changes –> differences in genitalia / stamens
What is genetic drift?
- Change in allele frequencies due to chance
What size population does genetic drift have a greater effect on and why?
- Smaller
- Less individuals to start with - loss of some has a greater impact
