26 May 2006

How many species?

I've got a little question to keep people busy over the long weekend.

There are three populations of an organism. The populations are physically separated from each other as a result of geographical factors. Geographically, they are arranged in a more or less linear fashion. The geographic details are as follows:

Population A is the northwestern population. Population B lies to the southeast, and is eparated from population A by a minimum of ~14km. Population C is southeast of population B, and separated by about 50km.

Populations A & B are identical to each other in appearance and in a key reproductive characteristic. Population C differs very slightly in appearance, but is substantially different in the reproductive characteristic.

The organisms (flying insects) were captured and bred in the laboratory. Experimental crosses were made for the different combinations of these three populations, with the following results:

Male from A x Female from A:
91% of male progeny fertile

Male from B x Female from B:
99% of male progeny fertile

Male from C x Female from C:
92% of male progeny fertile

Male from A x Female from B:
98% of male progeny fertile

Male from B x Female from A:
93% of male progeny fertile

Male from B x Female from C:
0% of male progeny fertile

Male from C x Female from B:
0% of male progeny fertile

Male from A x Female from C:
0% of male progeny fertile

Male from C x Female from A:
69% of male progeny fertile

Female offspring had somewhat better fertility than the males, which is not unexpected in this group, for reasons I'll discuss in another post. Female offspring were fertile in the cases where the males were sterile, but the number of offspring surviving to maturity was greatly reduced in those crosses overall (around 10% of what was seen in the control crosses) and the fertility of the female hybrids was reduced compared to the control crosses.

My question for you is this: How many different species should these three populations be grouped in? Provide an explanation for your answer. Oh, and if you hadn't guessed, this isn't a hypothetical case. I've removed the names so that you can't see what the "right" answer is.

I'll talk about the currently accepted scientific grouping sometime on Tuesday.
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