Topic 5: Genetic Drift Flashcards
What is genetic drift?
- random fluctuations in allele frequencies over time
- changes are entirely due to chance sampling of alleles ( it is random!!)
What is the bottleneck effect?
- drastic reduction in population size
- surviving population has a different allele frequency and little genetic diversity
- reduction in population results in loss of genetic diversity
- ex. Seal hunting in 1800’s lead to bottleneck
What is the founder effect?
- where a few individuals from a population start a new population with a different allele frequency than the original population
- a SUBSET of the original population forms a new population
- could be random or non-random
-ex. Big horn sheep, humans took a selection of them and placed them on an island
What are the two stable allele states for genetic drift? What do they mean?
- Fixation or Extinction
- Fixation is where an allele reaches a frequency of p=1.0, extinction is where it reaches p=0
- Alleles will fluctuate between these two states
- large populations alleles will tend to stay in between these two states, while small populations will drift to either state
“Probability theory shows that, given enough time, genetic drift will always lead to ______ or __________”
“Probability theory shows that, given enough time, genetic drift will always lead to extinction or fixation”
the probability of fixation of an allele is equal to …
… the initial frequency of the allele
What is stochastic process?
a process where individual outcomes are dictated by chance (randomness) but the average of a large number of outcomes can be described as a probability distribution based on initial conditions.
What is the Markov Chain?
- a discrete stochastic process where the probabilities of occurrence of various future states depend only on the present state of the system or on the immediately preceding state and not on the path by which the present state was achieved.
- random, unique periods of time
- periods of time depend only on present state of the system, any past events do NOT influence
What is Neutral theory?
- suggests that most of the genetic variation in populations is the result of mutation and genetic drift, rather than selection.
- most mutations are neutral
- suggests allele frequencies are consequences of equilibrium between mutation and drift
Mutation _____ diversity while drift ____ diversity
mutation ADDS diversity, while drift LOSES diversity
How can we use neutral theory as a null hypothesis?
- if alleles are neutral, no changes should occur
- if there are changes happening, then selection is perhaps occuring
What is the Wright-Fisher model?
- model to examine genetic drift
- same assumptions as Hardy-Weinberg, except for population size.
- this model incorporates a finite population to look at how allele frequencies change, randomly (without selection)
- in small populations, there will be large changes in allele frequency
- ## in large populations, there will be little to no changes
What is census population size? Breeding population size?
Census: Everyone in the population
Breeding: All reproductive adults, with potential to reproduce (generally 1/3 of the population for humans)
What is effective population size? (Ne)
- it is the breeding population size in an idealized population where a number of conditions, such as equal sex ratio and constancy in population size apply.
- it is a theoretical idealized population, that meets specific requirements, and dictate drift.
- EPS is smaller than the census or breeding population
Effective population size is the ideal number of breeding adults, assuming:
- all individuals reproduce equally
- equal numbers of males and females
- generations are discrete (one generation at a time) and there is no inbreeding
- population size is constant and stable
