Ring Species: A Catalyst for Speciation
After spending time on the Galapagos Islands studying the origin of various species, former divinest Charles R. Darwin proposed the evolutionary theory of natural selection, a mechanism by which advantageous variations in a population are preserved while unfavorable variations are lost (Berkeley, 2008). As a result of these variations, some individuals are better acclimated to their environment than others. This results in the better-suited individuals outcompeting the less fortunate individuals for resources, and eventually causes genetic types to replace others. Natural selection is ultimately a process that can lead to the evolution of species (Zimmer, 2013). One key aspect of natural selection is that it can create variation across the geographical range of a population by the transfer of alleles. This is known as gene flow, and according to scientist James Mallet, “gene flow is caused by…the movement or dispersal of whole organisms or genomes from one population to another.” The rate at which alleles move between populations and the amount of gene flow that occurs depends on how far individual organisms travel (Mallet). Although gene flow is pivotal in creating genetic variation between populations, it is also essential that sexually reproducing species remain somewhat distinct from one another. In order to ensure this, reproducing species demand isolating barriers, in which both extrinsic (geographic) and intrinsic (reproductive) factors reduce gene flow from the individuals of other species (Zimmer, 2013). Both geographic and reproductive isolation can eventually lead to “isolation by distance” introduced by Sewall Wright in 1943 to describe “the accumulation of local genetic differences under geographically restricted dispersal.” This can occur when two populations of a species evolve away from each other at a faster rate than gene flow can connect them. This ultimately results in the continuous divergence of a population. As a population continues to diverge, it may reach the point where the two species that are geographically farthest apart from each other can no longer interbreed because they are incompatible, separate species. Species in this situation are known as “ring species,” and according to David B. Wake, result “when one species grades into two at the overlap of a circular population distribution.” A “ring species” emerges when the two most divergent populations come into contact with each other as a result of wrapping around the geographic barrier. Due to this geographic and therefore reproductive isolation, the two most divergent populations of species can no longer interbreed. However, they are still able to co-exist in the same geographic area (Wake, 2001). Therefore, ring species are “the perfect demonstration of speciation,” (Mayr, 1963) or the “evolutionary process by which new biological species arise” (Science Daily). In his article on ring species and their contributions to speciation, evolutionary biologist, David B. Wake outlines a few reasons why ring species are difficult to identify. He states that in order to identify a ring species, “divergence from a single ancestral source population around some kind of barrier” needs to have occurred. In other words, geographic and/or reproductive isolation resulting in the geographic divergence of populations must occur in order for ring species to develop. Another necessity in identifying a ring species is that in order to maintain “genetic continuity,” gene flow through interbreeding must continue between neighboring populations. The ring must be continuous, and this is difficult because “ranges of species are subject to constant change.” These changes can often lead to neighboring populations becoming separated due to adaptations and then later reconnecting, a mechanism known as secondary contact. Lastly, identifying ring species can be difficult because most of the time, the two divergent populations...
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