Parallel speciation

In biology, parallel speciation is a type of speciation where there is repeated evolution of reproductively isolating traits via the same mechanisms occurring between separate yet closely related species inhabiting different environments. This leads to a circumstance where independently evolved lineages have developed reproductive isolation from their ancestral lineage, but not from other independent lineages that inhabit similar environments. In order for parallel speciation to be confirmed, there is a set of three requirements that has been established that must be met: there must be phylogenetic independence between the separate populations inhabiting similar environments to ensure that the traits responsible for reproductive isolation evolved separately, there must be reproductive isolation not only between the ancestral population and the descendent population, but also between descendent populations that inhabit dissimilar environments, and descendent populations that inhabit similar environments must not be reproductively isolated from one another. To determine if natural selection specifically is the cause of parallel speciation, a fourth requirement has been established that includes identifying and testing an adaptive mechanism, which eliminates the possibility of a genetic factor such as polyploidy being the responsible agent.

Parallel Speciation Vs. Parallel Evolution

Parallel evolution is a common phenomenon that occurs when separate yet closely related lineages evolve the same, non-ancestral trait as a result of inhabiting the same environment, and thus, facing the same selection pressures. An example given of parallel evolution is the independent development of small body sizes in two or more descendent populations in a new, similar environment that diverged from the same ancestral population. Parallel speciation differs from this slightly, as it is a form of parallel evolution, but the traits that are independently evolving in these differing lineages are those that are responsible for reproductive isolation. Using the previous example of independently evolved small body sizes, it changes from parallel evolution to parallel speciation when the descendent populations that have both evolved small body sizes due to their similar environments have become reproductively isolated from their ancestral population, but they are not reproductively isolated from one another.

Problems in Detecting Parallel Speciation

The required analysis of several variables, including genetic markers, morphology, and ecology of these independent populations, makes it hard to attribute speciation events specifically to parallel speciation. Failing to address all three of these contributing factors could incorrectly attribute the event to some other form of speciation, when in fact, parallel speciation was the occurring process. It can also be difficult to assess due to populations of the same species that may have a small amount of gene flow occurring between them despite living in different areas, but do not have physical barriers to overcome. Without these physical barriers, gene flow cannot be considered insignificant, which can further conceal the evidence of parallel speciation taking place.