User:Jsfalconero/Founder Flush

Founder Flush is a phenomenon in which genetic diversity increases during a genetic bottleneck despite low population size. In traditional population genetics a population with a low Effective population size will lose genetic variability over time due to inbreeding. During founder flush the genetic makeup of the founding population (see: propagule) is such that heterozygosity at neutral loci and additave variance at epistatic quantitative loci increases for the first few generations.

Theoretical Basis
Founder flush was first described by H.L Carson in 1968. Carson expanded on a basic model of founder effect proposed by Ernst Mayr in 1952. When a new population is founded, the resultant allele frequency is the product of multinomial sampling of the alleles in the source population. While its genetic makeup will likely be similar to that of its source, it is unlikely that allele frequencies in the daughter population will exactally mirror those of its founder. Carson's founder flush model assumes that after a population is founded, it may rapidly increase in size, mitigating the effects of drift and 'locking in' changes in allele frequency. These changes in conjunction with a disruption of selection due to a combination of bottleneck effects and relaxation of competition based natural selection can break up epistatic gene complexes. This yields a net increase in additive genetic variance. In 1980, A.R. Templeton expounded on the concept of the founder flush by adding his theory of Genetic Transilience. Transilience includes all the aspects of founder flush, but also takes into account recombination and mutation, which also serve to increase diversity and cause divergence among populations.

Neutral Loci
Founder Flush can also affect neutral loci such as microsatellites and mtDNA. Changes in allele frequency associated with random sampling can increase overall diversity. Consider a population with four alleles (p, q, r, and s) at a microsatellite locus. The frequencies are given as follows: p=0.4, q=0.4, r=0.1, s=0.1. In this population, expected heterozygosity is 0.66. If a new population is established and the allele frequencies change to: p=0.25, q=0.25, r=0.25, s=0.25 due to random sampling, then the expected heterozygosity increases to 0.75. Although unusual, increases in genetic diversity after a bottleneck have been observed in wild populations. If the population size remains low, drift will reduce the heterozygosity again, but if a 'flush' occurs, and the population increases rapidly enough to prevent drift then the increase in diversity will be fixed.

Implications for Speciation
After the initial 'flush,' once a population reaches its carrying capacity, density dependent selection is re-established. Although likely facing novel evolutionary pressures, the increase in additave variance allows colonizing populations to rapidly adapt. If the selective pressures faced are extreme enough, the founded population may evolve to the point that speciation occurs relative to their parental population. Speciation may be facilitated by other factors as well. In some cases selection may act to reduce the fitness of heterozygotes, creating two distinct 'adaptive peaks.' When these adaptive peaks are disrupted, a new selective balance can be created, instantly causing divergence between two populations.

In Templeton's transilience model, recombination and mutations that might otherwise be removed by selection are allowed to persist during a bottleneck. Transilience and founder flush are so similar that Templeton combines the two into one term: Genetic Transilience/Founder Flush, GTFF. Critics of GTFF point to a lack of emperical evidence that founder speciation has ever occurred. Templeton responded by citing several examples of populations that exhibited changes in allele frequency and quantitative traits that were consistent with the predictions of a GTFF model; though many were experimental studies with Drosophila and thus did not represent natural populations. Proponents of GTFF suggest that it is extremely rare, but is still an important process in speciation, and point to the fact that in many species, conditions exist that would facilitate GTFF in the event of a bottleneck or founder event.

As a mechanism of speciation, GTFF and founder flush need not operate in opposition to or even independant of adaptive mechanisms as has been suggested by some. Central to the idea of founder flush speciation is that it opens new substrates of diversity upon which adaptive evolution can act.