Migration itself does not guarantee gene flow. If individuals are locally adapted to their natal environment, the reproductive fitness of those individuals in a different population residing in a new environment may be quite low, leading to an effective isolation of the populations. Selection may act differentially on parts of the genome that are most directly affected by environmental differences. For example, if an individual migrates into a population and successfully reproduces with a native individual, sexual recombination of gametes will generate offspring with combinations of 'native' and immigrant gene copies. Considering two physically unlinked genes A and B, where different copies at the A gene are selectively neutral but alleles at the B gene are not, many offspring may survive in the population that carry the immigrant allele at the A gene, but few or none may survive if they carry the immigrant allele at the B gene. Here, isolation is again quantitative in that the isolation may only be for particular elements of an organism's genome, rather than isolating two populations or species entirely.
The net result of these varying types of isolation is that to a certain extent, indirect techniques such as measuring allelic diversity at a variety of genetic loci in multiple populations can be used to characterize the degree to which populations fit the equilibrium neutral model governed by mutation, drift, and migration. There are a number of ways in which this model may be violated, particularly in cases where one or more populations being compared is or has recently expanded from a founder population (e.g., range expansions or species introductions); nevertheless, the comparison of isolation measures across many markers may be indicative of both demographic and selective forces that promote the evolutionary divergence of populations in isolation.
Isolation is not a static characteristic of populations. The Isthmus of Panama is an excellent case of complete contemporary isolation of marine populations on either side; there is no gene flow, no migration, and no interaction between individuals from the tropical eastern Pacific and the Caribbean. However, as the Isthmus formed over the course of a million years or more, populations that were initially freely mixing were slowly but increasingly isolated by the uplift of land masses and reduction of currents by freshwater inflows and mangrove swamps, and the final reduction in the number of pathways by which individuals could travel from one side to the other. This more realistic scenario of isolation allows for migration to persist beyond the initiation of the isolating event. Known as the isolation-migration model, this statistical approach allows more complete description of the cause and timing of isolation between populations. Without considering both factors, it is difficult to distinguish whether two populations share many alleles due to high migration, or recent (complete) isolation, or a mixture of intermediate migration and intermediate levels of isolation.
Complex scenarios may arise due to the interaction between historical and contemporary causes of isolation.
While the variance in allele frequencies from one population to another is a standard method of measuring isolation it is not always clear whether that isolation is truly due to an equilibrium of mutation, drift, and migration - the isolation by distance model. It is also possible that ancestral events could separate a single population into two or more disjunct populations; subsequent environmental change, again permitting migration between the two areas, would cause a pattern of secondary gene flow. Geographic areas where two genetically or morphologically distinct groups interact, with incomplete isolation, are called clines. Many well-studied clines are caused by the secondary interaction between historically isolated populations.
One case study that illustrates both isolation by distance and the interaction of historically isolated lineages involves a 'ring species' of warblers in Asia. The geographic range of Phylloscopus trochiloides wraps around the Tibetan Plateau, and limited dispersal from the nest site results in populations from western Siberia through Tibet that exhibit a strong correlation of the geographic distance between sampled sites and the genetic distance measured at a mitochondrial gene. With greater geographic distance around the Tibetan Plateau, genetic distance gradually increases. However, there is a zone of overlap in central Siberia in which populations at one end of the 'ring' encounter populations from the other end -but in this region, the birds differ significantly in terms of plumage and song. This case is made more interesting by the isolation of eastern Siberian populations from those further south on the Tibetan Plateau due to deforestation -eventually, without any means of dispersal between the two sites, the east Siberia populations of P. trochiloides could become completely distinct (ecologically and demographically isolated).
Isolation and its subsequent effect on the evolutionary trajectory of a species has been the central theme of speciation literature over the past century. Only recently has this topic begun to be regularly incorporated into the study and theory of ecology and demography. Much of the diversity in form and function observed in our present-day environment can be thought to be the result of past isolation events and should therefore be considered a topic of much importance in fields of study other than evolutionary biology. Isolation events and their underlying mechanisms can have wide-ranging effects on a species. The effects of isolation may be detectable in genetic, ecological, and demographic patterns. Therefore, these topics should be of increasing relevance to current research focused on population ecology, local adaptive processes, and conservation biology.
See also: Adaptation; Clines; Pedosphere.
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