What Is the Coalescent Theory?

One part of the modern synthesis of evolutionary theory involves population biology and, on an even smaller level, population genetics. Since evolution is measured in units within populations and only populations can evolve and not individuals, then population biology and population genetics are intricate parts of the Theory of Evolution through Natural Selection.

How the Coalescent Theory Affects the Theory of Evolution

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Modern Evolutionary Synthesis

By Heather Scoville

When Charles Darwin first published his ideas of evolution and natural selection, the field of Genetics had yet to be discovered. Since tracing alleles and genetics is a very important part of population biology and population genetics, Darwin did not fully cover those ideas in his books. Now, with more technology and knowledge under our belts, we can incorporate more population biology and population genetics into the Theory of Evolution.

One way this is done is through the coalescence of alleles. Population biologists look at the gene pool and all available alleles within the population. They then try to trace the origin of these alleles back through time to see where they started. The alleles can be traced back through various lineages on a phylogenetic tree to see where they coalesce or come back together (an alternate way of looking at it is when the alleles branched off from one another). Traits always coalesce at a point called the most recent common ancestor. After the most recent common ancestor, the alleles separated and evolved into new traits and most likely the populations gave rise to new species.

The Coalescent Theory, much like Hardy-Weinberg Equilibrium, has a few assumptions that eliminate changes in alleles through chance events. The Coalescent Theory assumes there is no random genetic flow or genetic drift of alleles into or out of the populations, natural selection is not working on the selected population over the given time period, and there is no recombination of alleles to form new or more complex alleles. If this holds true, then the most recent common ancestor can be found for two different lineages of similar species. If any of the above are in play, then there are several obstacles that have to be overcome before the most recent common ancestor can be pinpointed for those species.

As technology and understanding of the Coalescent Theory become more readily available, the mathematical model that accompanies it has been tweaked. These changes to the mathematical model allow some of the previously inhibitive and complex issues with population biology and population genetics have been taken care of and all types of populations may then be used and examined using the theory.