The geographic isolation is a term used in evolutionary biology and ecology to refer to the spatial separation of a group of organisms. It can occur due to a natural event, such as changes in the geology of the region or due to artificial structures.
In most cases, species are isolated by the presence of natural barriers of different types, be it oceans, lakes, mountains, among others, which can drastically reduce contact between individuals in the population..
Once the two groups of individuals separate, the two environments to which they were exposed exert different selective pressures on the individuals, forcing them to follow different evolutionary paths.
The evolutionary forces of natural selection and gene drift will cause changes in the allele frequencies of the new groups, differentiating them from the parent population..
Depending on the magnitude of the separation and the length of time it is maintained, speciation events may occur: formation of new species, thus increasing the diversity of the group..
In the same way, isolation can also lead to the extinction of a group of individuals either due to lack of genetic diversity or due to inbreeding processes..
The geographic isolation of organisms can be translated into two processes: speciation, where new species emerge or the extinction of the group that experienced the isolation.
Below we will describe each of the processes in depth, understanding speciation as an “advantage”, since it increases diversity and extinction as a “disadvantage”:
The process by which new species are formed is of interest to evolutionary biologists. The ornithologist Ernst Mayr contributed greatly to the description of this phenomenon. According to Mayr, speciation is influenced by two factors: isolation and genetic divergence of the individuals involved..
First, for two populations to be sufficiently differentiated to be considered species, the flow of genes between them must be interrupted. In other words, they must not reproduce.
Second, genetic divergence must appear during the isolation period in such a way that if the individuals meet again - due to a collapse of the barrier that initially separated them - the reproduction process will not be efficient and their descendants will have a fitness relatively lower than its parents.
The effectiveness of the geographic isolation process to produce speciation depends on several factors intrinsic to the group that is separating, such as the ability to move.
The event of geographic isolations that gives rise to speciation processes through the separation of an insurmountable barrier is called allopatric speciation, a term derived from Greek roots that literally means "in another country".
Once species are physically isolated, they face different environmental conditions and selective pressures that guide them along different evolutionary paths..
Let's take as a hypothetical example a population of lizards that is isolated by a river, the climatic conditions on the left side may be colder than those on the right side. Thus, the mechanisms of natural selection and genetic drift will act independently, leading to the progressive differentiation of lizards..
In this way, individuals acquire different characteristics, ecological, ethological, physiological, among others, compared to the parent species. In such a case that the isolation barrier has been sufficient to promote the speciation event, there should be no gene flow if the two resulting species meet again..
There is a consensus among biologists that support the importance of allopatric speciation in the generation of new species, since it effectively restricts the flow of genes between organisms..
When the separation of individuals occurs thanks to barriers that cannot be crossed, some of the groups may become extinct.
When separated from the parent species, the diversity of the group may be low and it will not adapt to the new pressures imposed by the new environment they face..
Similarly, if the population that was separated is represented by a small number of individuals, inbreeding (crossing between close relatives) can have negative consequences..
Charles Darwin himself was already aware of the negative effects of inbreeding on natural populations. When crossing close relatives there is a greater probability that certain deleterious alleles are expressed.
For example, if in a family there is a gene for a certain pathology that is only expressed when the individual has both alleles (homozygous recessive) and two siblings cross, there is a greater probability that the offspring carry both alleles for the disease, unlike a crossing with an individual who does not carry said deleterious allele.
Likewise, when human constructions deprive animals of moving to desired places, their population can decrease due to lack of food..
In the Grand Canyon it is a formation of extraordinary dimensions that has been sculpted for 2,000 years by the Colorado River. It is located in northern Arizona of the United States.
Two species of squirrel inhabit this region, which, according to research, are the product of an allopatric speciation event. One of the species lives in the left region and another in the right, separated by a minimum distance. However, the two species are not able to interbreed..
In contrast, species that have the ability to move freely on both sides of the canyon have shown no evidence of speciation..
The concepts described for aquatic species may thus far be difficult to apply. However it is possible.
Cichlids are a family of fish characterized by immense diversity in the Congo River. This particularity caught the attention of ichthyologists who sought to understand why the river was inhabited by so many species and what factors favored massive speciation events..
After studying the conformation of the river, the scientists came to the conclusion that the hydrology of the river, caused by its turbulent waters, functioned as barriers that prevented the contact - and therefore the gene flow - of fish species that were very close.