Flying Towards a New Species

Changes in a Single Gene Divide Two Groups of Birds

by Denise Chen

July 16, 2009

A new study shows that small changes in a single gene affect the colors of a group of flycatcher birds. This color change makes these birds of a different feather not flock together. Scientists predict that this may be the first step towards these two groups of birds becoming different species.

The flycatchers from the Solomon Islands include two varieties—one with black and chestnut feathers and one with all black feathers. The black flycatcher only competes and mates with other black birds. Same too with the black and chestnut birds who only interact with their own kind.

Individual differences come about from
small changes in specific genes.

Another way to say this is that even though these two groups of birds live near one another, they are isolated from each other. And isolation is a key way for new species to form.

Usually people think about isolation as physical. Maybe a storm sweeps a group of birds to a distant island. Or a weather change keeps animals in adjacent valleys from mixing with one another.

But sometimes the separation is behavioral rather than physical. In this case, the birds live near one another but don’t interact. It’s already as if they are two different bird species.

The interesting part of the story is how simple it was for this isolation to happen. Basically, differences in a single gene caused these two groups of birds to ignore one another. And changes in single genes are very common.

What this shows is that there may be simple ways to isolate groups of animals from each other. It also suggests that getting new species is not as hard as many people might think.

Flycatchers Care About Looks, not Song

Researchers in this new study wanted to see how birds use certain traits to identify others of their kind. They looked at two bird subspecies from the Solomon Islands that live on neighboring islands. Though they have similar anatomies, one kind has chestnut and black feathers and the other has all black feathers. They also sing differently structured songs.

The researchers made stuffed birds that looked like either subspecies and recorded the songs of both. Then they observed how local birds on either island reacted to each of these decoy birds:

Chestnut bird singing chestnut bird song
Chestnut bird singing black bird song
Black bird singing chestnut bird song
Black bird singing black bird song

Species form from the build
up of many small DNA
differences.

The researchers used these combinations to figure out whether color, song, both, or neither are used by birds to identify their same kind. This also allows them to see if either color or song is a stronger identification mark.

The researchers scored the reactions of local birds toward the decoy birds. As expected, the black birds responded most to the one that looked most like themselves, #4, and the black and chestnut birds did the same with #1. They would approach these decoys to attack, sing back, or investigate.

The locals cared the least about decoys with a different color and song. The black birds couldn’t care less about #1 and the black and chestnut birds ignored #4. Each bird responded somewhere in between towards #2 and #3.

Researchers found, though, that color seemed more important than song. For example, the black birds responded more to black birds singing chestnut songs (#3) than they did to chestnut birds singing black songs (#2). And vice versa.

All of these results point towards these birds preferring to flock together with like birds. And to ignore birds that are a bit different from themselves.

The researchers decided to dive deeper into what caused the color difference between these two subspecies. If the change was a small one, then it would be relatively simple to see how these two birds would isolate themselves from each other. And eventually become two new species.

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When One Gene Makes Two Subspecies

As an educated guess, the researchers picked MC1R as a gene likely involved in giving these two birds different colored feathers. MC1R is associated with skin and hair color in birds and mammals. Mutations in MC1R are usually linked to darker skin and hair color (an exception is humans who end up with red hair and lighter skin).

The researchers compared the MC1R genes in both groups of birds and found three small DNA differences (or polymorphisms). All the black birds had three MC1R changes that none of the chestnut colored birds had.

Having different versions of the same genes is very common. Most plants and animals of the same species share the exact same set of genes. What makes each individual of a species unique are these small differences in individual genes.

These differences come about as a natural consequence of being alive. Sometimes DNA damage doesn’t get fixed and is passed down to the next generation. This is where most of the wonderful variety of nature came from! And these differences are important for species survival and formation.

Natural Selection and Survival

Small differences in the same genes
account for differences within a species.

A wide variety of traits is important for species survival. What might be useful now could be harmful and need to be replaced later. This is why genes within a successful species can vary so much.

Think about it like this. Imagine a bunch of people all dressed in different pants. Some have sturdy rugged jeans, others have designer jeans while still others have khakis.

The people in the sturdy jeans will do better than the other two working as a ranch hand. And the ones in designer jeans will be more suited to go out clubbing.

Genes in a species work similarly. Lots of variety means that species can survive in lots of different situations. No species ends up working on the farm in khakis!

The members of a species that are well adapted to their situation survive and prosper. Their genes become more common. This is called natural selection.

In a place with many trees and no sunlight, a dark feathered bird blends into the shadows better than a light feathered bird. But in another place with short bushes and lots of sunlight, a dark feathered bird sticks out like a sore thumb and probably gets very hot.

So in one environment, the light colored birds do well. They have many kids while the darker birds get eaten or die of heat stroke. After many generations, there are now mostly lightly colored birds. In the other environment, the reverse happens.

As each bird sticks with only birds of the same color, the two species will start to drift apart. Other sorts of changes will become important in the different surroundings.

For example, maybe the sunny place has big hard seeds and the shady place has little soft seeds. The birds in the shade will end up with whichever beak works best for little seeds. And the chestnut birds will end with a beak that works better for big seeds.

Add enough changes like this and a few random ones (called genetic drift) and the two groups become different species. Even if a researcher forced them to mate they would not create viable offspring.

Of course, this hasn’t happened yet for these birds. But they may make for a great laboratory to study this kind of speciation. Perhaps scientists in a few hundred years will be able to use these birds to trace back how at least one case of speciation happened.

Denise Chen

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Content provided by the Department of Genetics, Stanford University.