On Thursday, I presented a species problem taken from a post over at my old blog. I presented data from experimental matings that were carried out among three insect populations, added a little bit of information about the appearance, behavior, and location of the populations. I asked you to tell me how many species these three populations represented, and promised that I'd give you the "official" answer today. I've decided, though, that it wouldn't be totally fair to answer the question just yet. You see, I withheld relevant data when I presented the original version of the question.
When I gave you the information on Thursday, I just gave you data for some of the populations that were involved. There are two more populations that I didn't tell you about at the time. We'll call them Population D and Population E. Individuals from these populations were included in the original crossing experiments, and we also have data about their reproductive behavior and appearance. I'm going to throw these populations into the mix.
What I'd like to know now is this: how many species exist here in total, and which populations fall into which species? Here's the more complete set of data:
Geographically, the populations all fall more or less along a single line, with individual populations separated by a minimum of 5 km. They're arranged in the following order:
E -> D -> A -> B -> C
In appearance, the populations can be split into two groups. The members of each group are identical in appearance to each other, and differ slightly from the other group:
Appearance 1: Populations A, B, D, and E
Appearance 2: Population C
In behavior, the populations can also be split into two groups. One group will only deposit eggs in a very specific type of plant debris. The other group will deposit eggs in many different types of plant debris.
Specific egg depositing: Populations C, D, and E
Generalist depositing: Populations A and B
Reproductive isolation. Here's where it gets really interesting, and not just because the most commonly recited definition of "species" depends on reproductive isolation:
If you cross an individual from Population C with one from either Population A or Population B, the fertility of the male hybrid offspring is greatly reduced, as is the number of viable female offspring. When Population C is crossed with either Population D or E, the hybrid males are still almost all sterile, but there's much less of a reduction in the viability of hybrid females. However, when the hybrids of those crosses reproduce, many of their offspring are infertile or non-viable.
When Population A is crossed with Population B, there's no significant difference in the hybrid fertility or survival than there is when they're bred within their own population. The same is true when Population D is crossed with Population E.
When Population A or B is crossed with Population D or E, there is a slight drop in male fertility, and no obvious drop in female viability. However, when these hybrids were crossed with each other or with an individual from either parental population, there was a sharp decline in the fertility of the male offspring.
How many species do you think there are now? If you're not sure, what do you think the most reasonable choices would be? On Friday, I'll post my own views of the situation, and provide some references to the peer-reviewed literature that will show how experts have viewed this.