All Hail the Apple Maggot!

Next Monday — Nov. 24 — is the 149th anniversary of the day that Charles Darwin's masterpiece, "On the Origin of Species," was first published. In honor of that, I thought I'd look at a remark that a friend recently made: "We spend so much time wailing about extinction, but we never celebrate new species."

Hand-colored lithograph of passenger pigeons from J. J. Audubon's "The Birds of America." (The New York Public Library)

Good point. But there are several reasons for the asymmetry. The most obvious is that extinction is easier to see. In the 18th century, the passenger pigeon was one of the most numerous birds on Earth. Flocks of birds several miles long would fly over, blocking out the sun like an immense cloud. Some observers said the effect was like an eclipse. But by 1880, the numbers had plummeted; by 1915, the passenger pigeon had gone.

The appearance of a new species is not so dramatic. The first members of a new species will typically be indistinguishable — to us — from the species they have evolved from. And while extinction has a clear final moment — the last member of a species dies — the formation of a new species does not usually happen in a single recognizable instant. Which is why we haven't yet raised our glasses to celebrate, say, Rhagoletis pomonella, the apple maggot fly.

Rhagoletis pomonella, the apple maggot fly. (Wikimedia Commons)

This species is in the process of splitting into two. Until the mid-1800s, R. pomonella was a hawthorn fly: adults met at hawthorn fruits to mate and lay eggs. But then apples were introduced to North America. Some haw flies found these fruits attractive places to gather, and began to mate and lay their eggs on apples instead.

Today, flies that like apples have become genetically distinct from those that like haw. There are a couple of reasons why. First, flies meet each other at fruits. Since most flies have a preference for one fruit over the other, haw-preferring flies tend to meet other haw-preferring flies, and ditto for apple flies.

http://judson.blogs.nytimes.com/2008/11/18/all-hail-the-apple-maggot/?th&emc=th