What is genetic drift?
Evolution by random changes, organisms can get lucky or unlucky (unrelated to their phenotype). Ex. Salmon jumping from river and getting caught by bear.
What does genetic drift cause?
Random changes in allele frequencies.
Why does genetic drift occur?
Because of random sampling error, meaning that genetic drift will occur more frequently in smaller populations. (Think of jelly bean example).
What are the three key points to remember about genetic drift?
- Drift tends to reduce genetic variation within populations.
- Drift tends to increase genetic variation between populations.
- Drift is stronger in smaller populations.
Why does genetic drift reduce genetic variation in smaller populations?
Because less individuals = higher chance of random selection eliminating an entire allele.
What is a bottleneck?
Temporary reduction in population size, leading to genetic drift. Ex. Northern elephant seals (Due to hunting, dropped to very low numbers and now there is lower genetic diversity within their population).
What is the founder effect?
- A special case of bottlenecks and genetic drift, where new populations are started by a small number of individuals.
- Often occur on islands.
- “Founding” of new population means that there may be little diversity based on the individuals there.
Explain the founder effect in human history.
- Theory that all non-Africans descended from a small population of humans who lived in Ethiopia.
- Decline in genetic diversity as humans moved farther from Ethiopia.
What is phylogeny in humans?
Human populations closely related to neighbouring populations.
What are the three events that genetic drift causes?
- Allele frequencies to randomly change in populations (evolution).
- Causes reductions in genetic variation within a population (alleles become fixed).
- Causes population divergence (seperation causing differences).
What is natural selection?
Non-random evolutionary change.
Is natural selection needed to create evolution?
No, because genetic drift can also cause evolution. But, it is needed for adaptation.
How is evolution by natural selection defined?
A predictable change in the frequency distribution of a trait between the parental and offspring generations.
What are the three conditions that determine evolution by natural selection?
- Individuals in a population vary in their phenotypes (ex. body size).
- Survival and reproduction are not random, they are associated with individual phenotypes (selection).
- Variation is passed down to offspring (inheritance).
What are the five different patterns of natural selection?
- No selection
- Directional selection
- Stabilizing selection
- Disruptive selection
- Frequency-dependent selection
What is no selection?
- When fitness (survival and reproduction) is independent of phenotype, there’s no selection.
- Graph is a straight line down the middle, allele distribution stays the same.
What is directional selection?
- Evolution of the trait mean, occurs when fitness increases (or decreases), as the trait increases.
- Graph is linear going up, allele distribution moved closer to one of either extremes (ex. large or small body size, not intermediate).
What is stabilizing selection?
- Average individuals have higher fitness than extreme individuals.
- Variance decreases between generations.
- Graph peaks at intermediate, as fitness is lower at extremes.
- Ex. Gall size intermediate is selected for as it maximizes number of offspring while minimizing bird predation.
What is disruptive selection?
- Both extremes are favoured, intermediate selected against.
- Average individuals have lower fitness than extremes.
- Graph peaks at extremes.
What is frequency-dependent selection?
- Fitness of each phenotype depends on how rare or common the phenotype is in the population.
- Graph will either peak or dip in the middle.
- Ex. If a phenotype is rare, it may be selected for.
- Ex. The different coloured lizards.
Why is selection in nature hard to detect with typical study designs?
Evolution by natural selection takes a very long time, but it can have huge changes.
What is the difference between genetic drift and natural selection when looking at population size?
- Genetic drift impacts smaller populations, while it is weaker in large populations - it will extinct an allele faster.
- Natural selection is more powerful in larger populations, as small advantages in fitness can lead to large changes over time where a beneficial allele is selected.
What were the original beliefs about sexual selection? Who discovered the idea?
- Darwin believed that sexual selection arises from variation in ability to acquire and fertilize mates.
- Might favour different traits than natural selection does.
What is the modern view of sexual selection?
- It is a subcategory of natural selection.
- Selection arising from non-random success in competition for gametes or fertilization.
- Precise definition is debated.
