Update: the BBC reports that North American bird species are getting smaller, likely in response to increased temperatures.
In a Mother Jones article, Julia Whitty says, “Birds are rapidly evolving different shapes to cope with clear-cut forests.” Now, she may be accurately quoting the source paper by André Desrochers, and she’s certainly not the only one to describe evolution this way, but that doesn’t make it any less misleading and stupid.
How Evolution Works (with only a trace of self-importance)
It’s not that hard to get it right. Evolution is the “…change in the genetic material of a population of organisms through successive generations.” [wikipedia] This happens because of two opposing processes:
- Over time, the genetic diversity of a population tends to grow. Taking humans as an example, we have tens of thousands of genes. Each individual is a random mix of the genes of her parents, and is potentially a unique/novel combination. Each individual may also have mutations, making their collection of genes not only not a perfect selection of genes from her parents, but again potentially unique/novel. Finally gene transfer can also increase genetic diversity, although I don’t know of any proven instance of this happening in humans.
- Opposing the increase in genetic diversity is natural selection. If a particular combination of genes is unsuccessful — if the person with those genes has bad eyesight and can’t hunt, or has a bad complexion and can’t get a mate — then those genes don’t get passed on, and assuming some other genes do, the species has evolved. (if other genes don’t get passed on either, the species goes extinct)
You can think of evolution as something like a bush in a topiary garden: left to its own devices it will simply grow larger — that’s genetic diversity at work; but if the gardener (in the form of natural selection) comes along and trims here and there, you end up with an elephant.
This is where the stupid comes in
If you have a four-foot-high bush, no amount of trimming is going to turn it into a ten-foot-tall image of an elephant. The article in Mother Jones starts with “A new study shows how North American birds have changed the shape of their wings in the past century as the landscapes around them have been fragmented by clear-cutting.” Bzzt, wrong. It’s not like at the annual convention the bird-leader said, “Guys, the trees are getting farther apart. We have to fly so far, and it’s tiring! I say let’s switch to those new streamlined wings. All in favor?” Again, think of the bush. The change observed in the birds’ wings took place over the last century; it’s not the bush growing, it’s the gardener trimming.
As another example of this aspect of evolution, consider Thoroughbred racing. The Kentucky Derby has been run at one and a quarter miles for over a hundred years. There is tremendous prestige, not to mention money, involved in producing the fastest horses, and breeders have worked very hard over the last century to improve their mounts’ times. Yet the average time of the last ten winners of the Derby is less than a 6% improvement on the time of the first ten winners. Even that overstates the situation, since training methods and riding techniques have presumably improved significantly over that time span as well. The reason for this modest improvement is simple: you can’t trim the bush taller. If you could, horses would be running the Derby at sixty miles per hour by now.
So what’s almost certainly happening with the birds is that previously there was some amount of genetic diversity in the shape of the birds’ wings. This may or may not have manifested in actual variations in the birds’ wing shapes; it’s possible for significant genetic variation to hide. As a gross simplification, one bird might have gene variations A1, B1, and C1. A1 contributes to more pointed wings, but only when accompanied by B3 and C2. Another bird might have A2, B3, and C3. If the two birds mate, none of their offspring will have more pointed wings because they all have either C1 or C3. Even if one of their offspring has A1, B3, and C3, it might then go on to mate with another bird that has A2, B1, and C1, and produce offspring that have A1, B1, and C1, putting us back at the start.
But some subset of the birds have A1, B3, and C2, and thus have more pointed wings. It doesn’t matter much how many of them there are, or even if any of them exist in a particular generation of birds; we have a century to work with here, and about a hundred generations of birds. As long as somewhere along the way A1, B3, and C2 show up and prove beneficial, we’re set. As we trimmed the forests, the bird population declined, and those with A1, B3, and C2 didn’t decline as much. They were better suited to the new environment, where before they were just average among the population. As birds without A1, B3, and C2 failed to compete as well and died off, the percentage of birds with more pointed wings went up: evolution happened.
The more harsh the environment is to the bird status quo, the faster this die-off and re-jiggering of the genetic population happens. The birds are “rapidly evolving,” but only in the sense of the herd being rapidly thinned. As another example, consider if we lined up every human being on the planet and killed off all of them who couldn’t run a mile in six minutes. We’d be left with a much smaller, much fitter population. But if we killed off everyone who couldn’t run a mile in three minutes, we’d be extinct. Doing it in stages wouldn’t help much. If we started with six minutes, and every ten years lowered the limit by ten seconds, we might find in 150 years that there were a number of people who could run sub-3:30 miles, but the human race would be only a few years away from extinction as the required time continued to drop. Again, you can’t trim the bush taller.
“Evolving” is a transitive verb
While it’s not technically wrong to say the birds are rapidly evolving, it’s more accurate and less prone to misunderstanding to say that we are rapidly evolving the birds. We have a history of doing this, and not just recently: we evolved dogs from wolves, we evolved cattle from aurochs (which we then eradicated), and the list goes on and on.
So don’t say that the birds are rapidly evolving. They aren’t hurrying ahead of us, morphing their genes to stay viable as we remake their world. We are changing their environment, and those least suited to the new situation are dying off. To paraphrase Darwin, the fittest birds are surviving, and they are surviving us.
A note about the use of the term “gardener”
In most instances where it appears in this entry, the “gardener” is us — mankind. In those instances where it isn’t, it is a metaphor for natural selection. Do not make the mistake of thinking this article supports intelligent design, or that I am in any way swayed by the temper tantrums put forth as arguments by its supporters. If you misquote me to support your falsehoods I will hunt you down and lecture you mercilessly.