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Drift
 Random change in allele frequencies from generation to
 generation due to sampling error
 Two main consequences:
 1. Allele frequencies will change
 2. Genetic variation will be lost
 Stronger in smaller populations

Reproduction as Poisson process
 • Probability of a given # of events happening in a period
 of time if each event happens independently and with
 a fixed probability of occurrence
 • Probability of k copies in generation 1:

Probability of survival
 • One allele will eventually drift to fixation...
 • Assume neutrality
 • Probability of it being the red one
 = q0 = 1/2N

Founder Effect
 • Change in allele frequencies when a new population
 arises from only a few individuals (the “founders”)
 • E.g. only a few fish make it to an island
 • E.g. only a few individuals are introduced to a new
 habitat

N
= census population (total)

Ne
 = breeding population
 • Factors that affect Ne
 – Unequal sex ratios
 – High variance in reproduc+ve success
 – Changes in population size
 – Overlapping generations
 • Long term average number of successfully
 breeding individuals
 • Less than the census size

Assumptions
 • Equal sex ratio
 • All individuals have an equal chance at
 reproducing
 • Constant population size
 • Nonoverlapping (discrete) generations

Effect of variation
 • High levels of variation =
 low ratio of Ne / N


Effect of population fluctuations
 • Years with small populations =
 low ratio of Ne / N
 • Current population may be large, but
 effective size could be small


Know the three types of survivorship

