Aphid Speciation on Pinyon Pines

The process of one species evolving to become two is called speciation. There are a variety of possible causes of speciation, but the best studied involves two populations of a single species that become separated from each other so that they are free to evolve in different directions independently of each other. This separation can happen if individuals migrate across a significant barrier--the sea between islands, over a mountain range, across a desert--and found a new population. However, as long as they do not interbreed, they do not have to be geographically isolated. Speciation can also occur, for instance, if some part of a population begins to feed on a different kind of host, thereby separating them in time and space from the rest of the population on the old host.

Discerning if two insects are different species is generally straightforward, but the more closely related they are, the more they tend to look alike and the harder it becomes to discriminate between them. At some point it becomes impossible to tell definitively if two insects are indeed different species, or just different individuals of the same species; if two populations are in the process of speciating, where (or when) can you draw the line?

As hosts, pinyon pines can isolate aphid populations from each other, and they can isolate them twofold: 1) there are two principal pinyons in the U.S., the singleleaf pinyon and the two-needle pinyon; 2) pinyons grow only in the mountains of the Southwest, and so are separated by vast boundaries of desert between ranges. Fossil evidence suggests that these pinyons moved into the U.S. from Mexico 5,000-9,000 years ago (during the warming and drying following the last ice age), which is just enough time for the aphids that feed on them to begin to speciate.

Pinyon-feeding aphids of the genus Cinara were collected from all over the U.S. pinyon range, from southwest Texas to east-central California, and studied to discriminate between the various species. One group of aphid collections has yielded an interesting trichotomy: what had been a single species at one time, thousands of years ago, today is three. Although the three species are superficially identical, they can be differentiated using three pieces of complementary evidence: 1) they feed on three different pinyons (the third are Mexican pinyons that extend into southern Texas and Arizona); 2) one of their genes has three different DNA sequences; 3) statistical analysis of measurements of 15 different aphid parts, 2 of which are the lengths of the third antenna segment and hair on the tibia, clusters the aphids into three distinct anatomical groups. The species on two-needle pinyon has already been officially described, but the other two are new to science and will have to be described and given names. Also, interestingly, the closest living relatives of these three pinyon aphids are species that feed on ponderosa pine, a nonpinyon. Researchers have typically grouped conifer-feeding aphids based on their host, but the fact that the next closest relatives of these pinyon aphids are not themselves pinyon-feeding is cause to reevaluate this practice.

Another group of superficially identical aphids is not as clearly separated. Two distinct DNA sequences correspond to aphids on the two principal pinyon species, but there is some overlap in their anatomy. Some insect species that cannot be differentiated on the basis of anatomy have been named and described solely on genetic evidence, and this may be required here as well. There are several other pinyon-feeding Cinara groups at various earlier stages of speciation. Assessing the geographic, genetic, and anatomical divergence among populations of these species will help elucidate the rate at which speciation is occurring.

Of course the easiest thing to do would be to wait several thousand more years and see what has happened to them. Provided the incipient species do not go extinct and remain isolated from each other, they will continue to evolve apart and eventually become distinct species. In the meantime, studying the process will help illuminate a poorly understood area of evolutionary biology.

Colin Favret, Center for Biodiversity

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