Dear Dr. Universe,
How is it that every animal is different?
Marisa
Valley Village, California
Entomologist Rich Zack with some of the insects in WSU's collection. Photo Robert Hubner
The fill-in-the-blank answer is “evolution.”
But if you were the type to be satisfied with fill-in-the-blank answers, you wouldn’t have asked this kind of question. So I went to talk again with Rich Zack, who runs the entomology museum here at Washington State University.
In this museum are approximately 1,250,000 dead insects representing about 50,000 species. (A species is a group of organisms with many things in common, including the ability to interbreed.) Most of these species live in the Pacific Northwest and are a fraction of the species that exist worldwide.
Biologist E.O. Wilson of Harvard has estimated that scientists have named 6,300 species of reptiles, 9,040 species of birds and 4,000 species of mammals. The total number of vertebrates (animals with backbones) they have described is around 42,580. In contrast, about one million species of insects have been identified, and some scientists think that there may be anywhere from 8 million to 30 million species of insects out there waiting to be named!
How can this be? How can there possibly be so many kinds of insects? And WHY?
Niches are one reason, says Professor Zack. A niche is an insect’s role in its community. The way an insect finds its niche is through adaptation and natural selection. As their surroundings change, insects best suited to this change survive. They are “naturally selected” to pass on that species’ genes.
A related reason there are so many insect species is they adapt by specializing. Let’s take beetles, for example.
Professor Zack says that over a third of all insect species are beetles, around 400,000 of them. And remember, that’s just the ones that have been identified. Groups, or “populations,” of species can become different for various reasons. Maybe they get separated from the others. Maybe their environment or food supply changes.
Let’s imagine, way back, that one species of beetle liked brontosaurus dung. They were perfectly happy to eat brontosaurus dung for the rest of time. Brontosaurus dung was their world!
But then, brontosauri started disappearing. Maybe a few of these beetles got hungry enough to start sampling a little dung from another plant-eating dinosaur. And maybe some of these beetles were actually able to digest this new dung without getting fatal heartburn.
One thing that makes natural selection work is that EVERY INDIVIDUAL is different. With that in mind, maybe some of the individuals able to digest the new dung were able to pass this ability to their children. And maybe some of their buddies across the swamp developed a taste for yet another variety of dinosaur dung. Not only were they able to get by on it, they even liked it!
As time passed, whatever made these different beetles able to digest different kinds of dungs got passed along to their offspring, so that they all became a new species—once they got so different they could no longer interbreed.
Now let your imagination run for a while. Think of variations on this situation over and over through time, as different populations adapted to different conditions as the environment changed.
Admittedly, insects are small, and we are big. Insects and larger mammals live and evolve at much different scales, of both time and space. Insects can pass through their whole life cycles in weeks, rather than decades, as we do. That means, says Professor Zack, that insects tend to take advantage of dividing up their habitat and becoming specialized, both because they’re smaller and because they can evolve more quickly.
But what about us? Why are humans different from monkeys and elephants and cats? Even though it takes longer for us to change, it’s basically the same idea. Over time we have adapted to the environment we live in. In the process, a BUNCH of different versions of us have come to share this world.
