South Sister St. Marys, Tasmania

South Sister - velvet worms

parapatry

Peter B. McQuillan
Centre for Environmental Studies
University of Tasmania

What is special about parapatric boundaries?

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]