South Sister St. Marys, Tasmania
Peter B. McQuillan
Centre for Environmental Studies
University of Tasmania
A parapatric boundary marks the location where two related species meet but rarely if ever interbreed.
There are three main models of speciation, mainly based on geography -- that is, what are the geographic locations, with respect to one another, of the groups that evolve to be different species. These are:
Allopatric speciation:
speciation that occurs when the groups that evolve to be separate species
are in well separated geographic locations and are isolated geographically
from each other so that individuals cannot move between the different
locations.
Parapatric speciation:
speciation that occurs when the groups that evolve to be separate species
are geographic neighbors; they occupy different areas, but the areas are
next to each other and individuals can potentially move between the
areas.
Sympatric speciation:
speciation that occurs when the groups that evolve to be separate species
occur together in the same geographic area.
Allopatric speciation is by far the most common model seen in nature while there are only a few examples documented in the world of parapatric speciation.
The Giant and the Blind Velvetworms are thus rare examples of this phenomenon. (One of the few others involves parasitic ticks on lizards in South Australia).
We assume that this line marks some subtle environmental difference important to the velvet worms. Clearly, one species is favoured by conditions on one side of the line while the other shows greater fitness on the other side. Importantly, any hybrid between the two species is favoured by neither (and hence has low fitness, and is removed by natural selection).
This line of parapatry is defined from velvet worm collection data over the last two decades. The fact that biologists cannot readily identify what it is about the environment that coincides with this line underlines how little we know about the conditions essential for velvetworm survival in the long term.
Parapatric boundary separating Blind Velvet Worm from Giant Velvet
Worm (Map courtesy of A. Richardson, Head, School of Zoology, University of Tasmania) [red dots for Giant Velvet Worm and large white dots for Blind Velvet Worm] |
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