Evolution in Small Population Flashcards
Large populations
High fitness and strong stabilising selection, mildly deleterious mutations. Unlikely to become fixed by drift.
Declining populations
Possibly lower fitness. Weaker stabilising selection, mildly deleterious mutations that are only weakly selected against, can become fixed by drift.
Small populations
Low Fitness, very weak stabilising selection. Mildly deleterious mutations that are very likely to become fixed due to larger effects of drift.
Definition of small population
ICUN criteria: vulnerable =
Chance (Drift)
Inheritance: Offspring receive one randomly selected allele from each parent.
Allele Freq: Some alleles are common and others are rare.
Chance: In small populations, rarer alleles may not be passed on from the parents, simply by chance, drift.
New Freq: Relative freq will differ greatly from those of parental generation.
Random genetic drift = big effect in small populations.
3 major implications of chance
A) Large random change in allele freq between generations.
B) Loss of genetic diversity and fixation
C) Diversification among replicate populations from the same source (Fragmented populations)
Modelling Chance. Problem with randomness
Chance is a random event. Outcome of a single event is almost impossible to predict. but we can calculate all possible outcomes.
Fixation and loss of fixation
Loss of alleles happens per generation. Allelic loss depends on population size.
Effect of population bottlenecks
Bottlenecks reduce genetic diversity. Populations can emerge from a bottleneck, or can be founded from them. Example- Isle Royale Grey Wolf, a single pair reached an island over an ice bridge and got stranded. Thickness of bottleneck refers to number of generations that the population is at its smallest size.
Inbreeding in small populations
With time every individual becomes related, simply due to the small number of founders or small population size. Larger populations can become inbred, it would just take longer.
Selection in small populations
Least effective in small populations. Shows greater variability in outcome across replicates. Since small populations loose genetic diversity, selection- response (R) will be reduced, no diversity means nothing for selection to act upon.
Selection response is critical for populations to survive. Selection is less efficient in smaller populations. Deleterious alleles are less likely to be removed by selection.
Mutation
Mutation events are rare (even in large populations). Most mutations are lost from the population soon after they arrive! In small populations the probability of fixation is much bigger.
Mutation -selection balance in large populations
Elimination of deleterious mutations takes a long time and accumulation of new mutations. Equilibrium between loss of alleles by selection and gain of alleles by mutation = Mutation load. Large outbred populations have low mutation load.
Mutation- selection balance in small populations
Average frequency of deleterious alleles change in small populations. BUT, the variance is increased, Meaning some del. alleles drift to high freq by chance and others drift to low, again by chance. Example of Californian condors.
