GENETIC DRIFT 2

GENETIC DRIFT 2

1. effective population size can be smaller than census size for several reasons

   • 1. variation in number of progeny produced by females, males, or both reduces Ne
     • elephant seal represents extreme example
   • 2. sex ratio different to 1:1 lowers EPS
   • 3. natural selection can lower Ne by increasing variation in progeny number
     • eg if larger individuals have more offspring than smaller ones, rate of genetic drift may increase at all neutral loci because small individuals contribute fewer gene copies to subsequent generations
   • 4. if generations overlap offspring may mate with parents
     • since these pairs carry identical copies of same genes, effective number of genes propagated reduced
   • 5. perhaps most importantly, fluctuations in population size reduce Ne, which is more strongly affected by smaller than larger sizes
     • eg if number of breeding adults in 5 successive generations is 100, 150, 25, 150 & 125, Ne is approx. 70 (harmonic mean) rather than 110 (arithmetic mean)

2. average Ne over long term estimated from heterozgosity given estimate of neutral mutation rate

   • equilibrial level of genetic variation set by balance between gain of variation attributable to mutation & loss attributable to genetic drift

3. bottleneck - can occur when new population established by small number of colonists eg single mating pair

   • random genetic drift that ensues is founder effect
     • if new population grows rapidly to large size, allele frequencies & heterozygosity will not greatly alter
     • if colony remains small, genetic drift will alter allele frequencies & erode genetic variation
       • if colony persists & grows, new mutations eventually restore heterozygosity to higher levels
   • generally, in genetic drift, bottlenecks increase levels of linkage disequilibrium

4. genetic features of natural populations based on inferred causes of evolution, such as genetic drift or natural selection, described by interpreting patterns

   • such inferences only possible on basis of theories that tell of patterns to expect if one or another cause most important

5. genetic drift - most important driver of non-adaptive evolution in the wild

   • entirely random as driven by chance fluctuations in allele frequencies within given population from one generation to next
     • often results in permanent evolutionary change as it can eventually cause one allele to attain frequency of 1.0 within population, while all other alleles go extinct

6. inbreeding depression most important reason why low genetic diversity can be detrimental to populations in short-term

   • reduced genetic variation can also threaten survival of populations in longer term by reducing ability to adapt & evolve in response to changing environmental conditions