Ne is not equal to N total population 1 Biased primary sex ratio not necessarily a 1 1 male to female ratio 2 Biased operational sex ratio one gender may mature before the other due to a Sex specific growth rates b Differential mortality survivorship 3 Sexual selection social factors b Female selection a Male male competition males who lose will not be part of Ne number 1 2 3 increased variance in reproductive success increase potential for evolution D Gene Flow migration The movement of genetic factors from one population s gene pool to another due to It results in an averaging or homogenizing effect within a species Example genetic and phenotypic variation become more uniform among populations within a species so differentiation is inhibited When a small population receives migrants this process opposes drift and inbreeding Gene flow is not equal to dispersal Gene flow is genetically effective dispersal migrant must mate and having surviving offspring for this to happen What causes genetically ineffective dispersal 1 If individual migrants disperse at random then they do not always find conspecifics to mate with conspecific individual of the same species 2 Social factors may keep immigrants from interbreeding and may lower their survivorship a Example a male lion joins a pride of lions but the alpha male lion prevents him from dispersing 3 A dispersal event may not introduce an novel genes so little consequence for evolution Methods for estimating rates of gene flow Direct methods 1 Release large numbers of individuals with unique markers then sample multiple populations to see how far the genetic marker has moved see if a heterozygote for that gene surfaces which will mean unambiguous evidence for effective dispersal a Potential pitfalls must release a very large number of individuals at one place to have hope of detecting subsequent gene flow 2 Mark and recapture all individuals in ponds A B C sample over time to see if a Potential pitfalls judgment call on whether or not effective dispersal has taken exchange has taken place place Indirect Method of Gene Flow Determination Most commonly used method 3 Evaluate the distribution of genetic markers example allozymes or microsatellites within and among population of a species a Fst is one measure of differences of allele frequencies among populations b Fst can be directly estimated from frequencies of genotypes detected by molecular techniques c Fst ranges from 0 to 1 If we assume m is relatively small and some other assumptions then Fst 1 4Nm 1 Where m proportion of migrants per generation N Ne effective population size Nm average number of immigrants into each population per generation Rearrange the equation Nm 1 Fst 1 4 Therefore estimating the divergence among populations Fst by using genetic techniques can be used to indirectly estimate gene flow Nm If populations are genetically homogeneous low Fst high Nm high gene flow If populations are genetically heterogeneous high Fst low Nm low gene flow Nm values 1 are thought sufficient to prevent population differentiation due to genetic This idea is based on theory and computer simulations not direct observation of natural drift populations Why is the amount of divergence Fst among populations dependent on the absolute number of migrants Nm and not the proportion of exchange among demes m In the absence of selection the rate of divergence the rate of increase in Fst results from the opposing forces of gene flow and genetic drift Therefore the larger populations are the more slowly they diverge due to drift and thus proportionally fewer migrants are required to counteract the effects of genetic drift Small populations diverge rapidly due to genetic drift thus proportionally more migrants are required to counteract drift Advantages 1 Much less consuming since sampling populations over time is not necessary 2 Able to detect historical gene flow gene flow averaged over time while numbers 1 and 2 only detect current gene flow at best Disadvatages 1 Comes with many assumptions to use Nm 1 Fst 1 4 to estimate gene flow a Assumes an island model of population structure Island model of population structure Individuals may move from any population to any other with equal probability as example shows it can go right next door or completely across Stepping stone model More likely to go next to each other than farther away 2 Alleles that we calculate frequencies for must be selectively neutral 3 Allele frequencies must have reached equilibrium between gene flow and genetic drift a Implies a steady state but is that usually the case with natural populations most likely not b Able to detect historical gene flow only for example i Say Fst 01 then Nm 24 75 ii Suggests relatively high levels of gene flow iii But at this point in time gene flow actually could be zero due to a newly arisen barrier to dispersal for instance a dam in a stream river for example Gene flow levels are relatively hard to quantify accurately since 1 Can be episodic vs steady state 2 It has both a current and historical component Calculating Fst form genetic data in a one locus 2 allele system Fst Ht Hs Ht Ht expected heterozygosity from HWECP within a species Ht 2poqo where o observed frequency Where po the frequency of A1 allele in the species qo the frequency of A2 allele in the species Hs the average observed heterozygosity among populations Example See material Fst Gene Flow HWCE Xsq Fit selection figs pdf