The Conservation Context in Population Genetics, Part 1

This is the first entry in Crowdsourcing ConGen. This entry is meant to be half of an Introduction which lays out the framework for what conservation genetics is, its philosophical basis in population genetics, and why it’s a meaningful method of inquiry for conservation. This first section is meant to outline foundational concepts in population genetics. It is not meant to be a detailed summery of population genetics, but needs to be accurate and clear.

There is often a functional gap present in our understanding of the interaction between organism, ecosystem, and evolution. Organismal biology uncovers the life history of the organism, its physiology, and behavior. Ecology explains how organisms interact with each other, with predators, prey, and parasites, and maps the web which connects individuals to an ecosystem. Evolution reveals the deep history of life on earth, where species came from, how they are related, and how they’ve adapted to their environment. Between the organism and the ecosystem, where ecology merges with evolution, we find the population. Understanding the distribution of populations, where and how boundaries form between them, and historical patterns of colonization, migration, and isolation is crucial for determining the underlying ecologic and evolutionary processes which guide wise conservation initiatives.

Allele - An allele is one possible variant of a gene or region of the genome being examined (locus, plural loci). In population genetics, alleles are often not variants of a functional gene, but variants of a non-coding locus. Most animals being studied in population genetics are diploid - they have two alleles at each loci. Organism that have the same two alleles are homozygous for that locus. Organism that have two different alleles are heterozygous for that locus.

Population genetics is the study of variation in the distribution of allele frequencies that occurs as groups of organisms from the same species separate from each other in space or time. The alleles chosen for population genetics studies are selectively neutral – they occur in non-coding regions of a genome and are not affected by natural selection. Under ideal circumstances, these alleles associate randomly within a population and are not linked to each other, or to coding regions of the genome. Alleles are not distributed evenly across all populations. Because each population has a different set of alleles that occur in different ratios, the distribution of alleles within a group of individual organisms can be used to define discrete populations and determine the extent of interbreeding that occurs among them.

Basics of Population Genetics. A new population is founded from a source population. By random chance no blue alleles are present in the new population. Later, a reduction in population size kills off all members that possess the green allele. Even after the population returns to it's pre-bottleneck size, there are no green alleles.

Several processes determine how allele frequencies within separate populations differentiate. Conceptually, the simplest way for allele frequencies to diverge is through mutation. New alleles that arise in a population after it has become isolated will only occur in that population. Founder and bottleneck events can also create variation in allele frequencies. A population that has been founded by a small group of individuals isolated from a larger (or more diverse) population may not possess all the alleles that occur in the source population. Likewise, if an isolated population experiences a large decline in numbers, less common alleles may be completely removed from that population, resulting in a reduction in total genetic diversity and a shift in allele ratios.

Genetic drift is a more subtle way for large changes in allele distributions to occur. Two populations which become isolated from each other may begin with the same allele distributions, but because alleles associate randomly and mating is random with respect to those alleles, random changes in allele frequencies will accumulate with each new generation. This accumulation is independent of other populations. Both populations may have the exact same alleles but in completely different ratios. Eventually, drift will cause some alleles to become fixed (they occur in all members of the population), while others disappear.

By examining these patterns of allele distribution, population geneticist can unlock the deep demographic history of a species. Historical patterns of expansion and contraction, migration and colonization, isolation and ultimately speciation, can be uncovered through examining the genetic changes that accumulate in populations.

Any comments regard substance or style are appreciated.

~Southern Fried Scientist

February 12, 2010 • 7:55 pm