Jeff's Lunchbreak

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A little while ago, I came across a question on Quora, When gorillas and humans split from their common ancestor was there a 'new' individual of each new species born at that time?. Since other people had already discussed human evolution specifically, I decided to go with a more concrete example of an ongoing speciation event happening before our very eyes. Here's that answer, slightly edited.

I'm going to give a concrete example - apple maggot flies. A few hundred years ago, there were no apple maggot flies, because there were no apples in the Americas. Apples weren't brought to the Americas until the 17th century. Now, there were obviously other types of trees and fruits in the Americas before that, and different types of insects that fed on them. One type of fruit bearing tree was the hawthorne, which was fed upon by hawthorne flies. You can probably see where this is heading.

Once Europeans brought apple trees to the Americas, there was a whole new potential food source for these flies. Some time in the early 1800s, some hawthorne flies began eating apples. They were still the same species as their relatives that ate hawthornes, without any drastic differences. They just had a few mutations that made them preferentially seek out apples as opposed to hawthornes. If anyone had put them in close contact with flies that still ate hawthornes, they wouldn't have had any trouble breeding.

Well, as the years went by, the flies that preferred apples began to get more and more distant from their hawthorne eating relatives. For one, since they were physically on apple trees more often than hawthornes, they mated more often with other apple eating flies. And since apples ripen earlier than hawthornes, the apple eating flies timed their emergence from their pupae to occur earlier in the year, introducing a separation in time, as well (obviously, this timing was based on genetic mutations, not conscious choice by the flies). Since each group of flies was more likely to mate with flies that ate the same type of fruit, any mutations that were appearing in the two different groups were more likely to remain in that particular group, and not be shared with the other group. So, the populations of apple maggot flies and hawthorne flies have been growing more genetically distinct.

Today, they are still classified as the same species, because flies from one group can still mate with flies from the other group to produce fertile offspring. But those hybrid offspring are less likely to survive because they'll have a mix of genetics from their parents, making them less specialized for eating either hawthornes or apples. The hybrids aren't completely doomed to dying without mating themselves, but because their chances are lower than non-hybrids, it further reduces the the gene flow between the two populations. If things continue as they are, these two populations will probably end up becoming two distinct species.

That's how speciation occurs. It's not something that occurs over a single generation. It's a process that takes many generations to complete. And the two incipient species are initially very similar. It's only many, many generations later, once each lineage has had a chance to evolve independently, that the two species start to show bigger differences.

To answer the original Quora question regarding human evolution, millions of years ago when a certain population of apes gave rise to two different lineages - one that would eventually give rise to gorillas, and the other that would eventually give rise to chimps, bonobos, and us, it would have occurred in much the same way. At first there were two populations that were still the same species but just separated somehow. Once separated, they acquired unique mutations in each lineage. Eventually, many generations later, they were two distinct populations that could no longer interbreed - two separate, but closely related and very similar species. Only that speciation event would have taken longer than the current ongoing hawthorne/apple maggot fly event, because generations in apes are much longer than those of fruit flies.

More Info:

• All Hail the Apple Maggot!
• Rapid evolutionary change seen in fruit fly
• The apple maggot fly--how an altered sense of smell could drive the formation of new species

Image Source: Wikipedia

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