a question of isolation

I spent a lot of last weekend marking exam papers from my first-year bio students. Most of them chose one of my essay questions (it’s a team-taught paper & they had a choice of 3 essays in my section), & today one of the class told me that she’d really liked that question because it got her thinking. So I thought I would share it with you 🙂

Rhagoletis pomonella is a small fly native to North America. Normally its larvae burrow in and eat hawthorn fruit.

Apple trees were introduced to North America from Europe in the 1800s. There are now two sibling species of Rhagoletis. In one species the larvae feed on hawthorn fruit, and in the other the larvae feed on apples. Neither species feeds on the other’s food, even when hawthorns and apples are found growing in the same area. Female Rhagoletis use scent to identify their host plant’s fruit.
Identify the pattern of speciation shown by Rhagoletis and discuss how this could have occurred. In your answer, you should consider the various isolating mechanisms that could be operating in the Rhagoletis sibling species.

You could argue either way on the mode of speciation, although most of my students answered that it was sympatric. By the way, it’s not going to be enough to say that & no more – you need to show that you know what the term means & why you chose it. ‘Sympatric speciation’ is where a new species forms within the geographical range of the parent species ie there’s no geographical isolation. If those introduced apple trees grew in the same area as the native hawthorn, then you would be looking at sympatric speciation of the flies whose larvae feed on the trees’ fruit. (Some students argued that the hawthorn & apple trees originally grew in separate regions, & that a founder Rhagoletis population was blown to the apple trees during a storm.)

However the speciation happened, the two host plants now grow in the same area. And yet the sibling species of Rhagoletis don’t appear to interbreed. So to answer the second part of the question, you need to discuss the possible reproductive isolating mechanisms (RIMs) operating in these species. NB this does not mean that you should list every single type of RIM that you’ve learned about – not all of them are relevant & if this was a Scholarship question, giving all of them is a fairly clear signal to the examiner (me! in this particular case) that you haven’t thought carefully about what pieces of knowledge are relevant. Such an answer looks a bit like a brain dump – & of course you’re also wasting time by writing it all down.

There are probably two RIMs operating here. One is habitat isolation – each species is found only on its host tree. This would mean that ‘apple’ Rhagoletis wouldn’t normally encounter the ‘hawthorn’ species & vice versa, so the likelihood of them interbreeding is low. Underlying this seems to be a form of behavioural isolation: Rhagoletis females use scent to find their host tree. This could, as some of my students suggested, be due to a mutation in some of the original parent species that affected their olfactory receptors. Or it could be a case of mistaken imprinting – if a female fly was blown (say) to the wrong host & laid her eggs there anyway, then her larvae could imprint on the scent of that host plant & would subsequently return to it when they were ready to breed. Either way, larvae on this new source of food (the introduced apple trees) would face less competition & their population could grow quickly. It’s also possible, if the two host species have slightly different flowering & fruiting times, that temporal isolation is also effective in keeping the flies apart.

And for a really good answer, you’d go on to explain why some of those other RIMs probably aren’t working in this case. For example, apples were introduced to North America a little more than 200 years ago. This may not be sufficient time for the evolution of structural features that physically prevent mating.

(A poor answer, by the way, would ignore the conext information provided in the question & – in addition to the all-too-general list of RIMs – talk vaguely about reproductive isolation in very general terms. Don’t go there!)

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