Why Evolution Is True (29 page)

Mayr lived exactly one hundred years, producing a stream of books and papers up to the day of his death. Among these was his 1963 classic,
Animal Species and Evolution,
the very book that made me want to study evolution. In it Mayr recounted a striking fact. When he totaled up the names that the natives of New Guinea’s Arfak Mountains applied to local birds, he found that they recognized 136 different types. Western zoologists, using traditional methods of taxonomy, recognized 137 species. In other words, both locals and scientists had distinguished the very same species of birds living in the wild. This concordance between two cultural groups with very different backgrounds convinced Mayr, as it should convince us, that the discontinuities of nature are not arbitrary, but an objective fact.
³⁷

Indeed, perhaps the most striking fact about nature is that it is discontinuous. When you look at animals and plants, each individual almost always falls into one of many discrete groups. When we look at a single wild cat, for example, we are immediately able to identify it as either a lion, a cougar, a snow leopard, and so on. All cats do not blur insensibly into one another through a series of feline intermediates. And although there is variation among individuals within a cluster (as all lion researchers know, each lion looks different from every other), the clusters nevertheless remain discrete in “organism space.” We see clusters in all organisms that reproduce sexually.

These discrete clusters are known as
species.
And at first sight, their existence looks like a problem for evolutionary theory. Evolution is, after all, a continuous process, so how can it produce groups of animals and plants that are discrete and discontinuous, separated from others by gaps in appearance and behavior? How these groups arise is the problem of
speciation—
or the origin of species.

That, of course, is the title of Darwin’s most famous book, a title implying that he had a lot to say about speciation. Even in the opening paragraph he claimed that the biogeography of South America would “throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers.” (The “philosopher” was actually the British scientist John Herschel.) Yet Darwin’s magnum opus was largely silent on the “mystery of mysteries,” and what little it did say on this topic is seen by most modern evolutionists as muddled. Darwin apparently didn’t see the discontinuities of nature as a problem to be solved, or thought that these discontinuities would somehow be favored by natural selection. Either way, he failed to explain nature’s clusters in a coherent way.

A better title for
The Origin of Species
, then, would have been
The Origin of Adaptations:
while Darwin did figure out how and why a
single
species changes over time (largely by natural selection), he never explained how one species splits in two. Yet in many ways this problem of splitting is just as important as understanding how a single species evolves. After all, the diversity of nature encompasses millions of species, each with its own unique set of traits. And all of this diversity came from a single ancient ancestor. If we want to explain biodiversity, then, we have to do more than explain how new
traits
arise—we must also explain how new
species
arise. For if speciation didn’t occur, there would be no biodiversity at all—only a single, long-evolved descendant of that very first species.

For years after publication of
The Origin,
biologists struggled, and failed, to explain how a continuous process of evolution produces the discrete groups known as species. The problem of speciation was in fact not seriously addressed until the mid-1930s. Today, well over a century after Darwin’s death, we finally have a reasonably complete picture of what species are and how they arise. And we also have evidence for that process.

But before we can understand the origin of species, we need to figure out exactly what they represent. One obvious answer is based on how we recognize species: as a group of individuals that resemble one another more than they resemble members of other groups. According to this definition, known as the
morphological species concept,
the category “tiger” would be defined something like “that group including all Asian cats whose adults are more than five feet long and have vertical black stripes on an orange body, with white patches around the eyes and mouth.” This is the way that you’ll find species of animals and plants described in field guides, and it is the way that Linnaeus first classified species in 1735.

But this definition has some problems. In sexually dimorphic species, as we saw in the last chapter, males and females can look very different. In fact, early museum researchers working on birds and insects often misclassified males and females of a single species as members of two different species. It’s easy to understand, if you are looking only at museum skins, how male and female peacocks could be classified this way. There is also the problem of variation
within
an interbreeding group. Humans, for example, could be classified into a few discrete groups based on eye color: those with blue eyes, brown eyes, and green eyes. These are almost unambiguously different, so why don’t we consider them different species? The same goes for populations that look different in different places. Humans are again a prime example. The Inuit of Canada look different from the !Kung tribespeople of South Africa, and both look different from Finns. Do we classify all of these populations as different species? Somehow that strikes us as wrong—after all, members of all human populations can successfully interbreed. And what is true for humans is true for many plants and animals. The North American song sparrow, for example, has been classified into thirty-one geographic “races” (sometimes called “subspecies”) based on small differences in plumage and song. Yet members of all these races can mate and produce fertile offspring. At what point are differences between populations large enough to make us call them different species? This concept makes the designation of species an arbitrary exercise, yet we know that species have an objective reality and are not simply arbitrary human constructs.

Conversely, some groups that biologists recognize as different species look either exactly alike or nearly alike. These “cryptic” species are found in most groups of organisms, including birds, mammals, plants, and insects. I study speciation in a group of fruit flies,
Drosophila,
that includes nine species. The females of all these species can’t be told apart, even under the microscope, and males can be classified only by tiny differences in the shape of their genitals. Similarly, the malaria-carrying mosquito
Anopheles gambiae
is one of a group of seven species that look almost exactly alike, but differ in where they live and which hosts they bite. Some do not prey on humans and so carry no danger of malaria. If we are to combat the disease effectively, it is critical to be able to tell these species apart. Further, because humans are visual animals, we tend to overlook traits that can’t easily be seen, like differences in pheromones that often distinguish species of similar-looking insects.

You might have asked yourself why, if these cryptic forms look so similar, we think that they’re actually different species. The answer is that they coexist in the same location and yet never exchange genes: the members of one species simply don’t hybridize with members of another. (You can test this in the laboratory by doing breeding experiments, or by looking at the genes directly to see if the groups are exchanging them.) The groups are thus
reproductively isolated
from one another: they constitute distinct “gene pools” that don’t intermingle. It seems reasonable to assume that under any realistic view of what makes a group distinct in nature, these cryptic forms
are
distinct.

 

And when we think of why we feel that brown-eyed and blue-eyed humans, or Inuit and !Kung, are members of the same species, we realize that it’s because they can mate with each other and produce offspring that contain combinations of their genes. In other words, they belong to the
same gene pool.
When you ponder cryptic species, and variation within humans, you arrive at the notion that species are distinct not merely because they
look
different, but because there are barriers between them that prevent interbreeding.

Ernst Mayr and the Russian geneticist Theodosius Dobzhansky were the first to realize this, and in 1942 Mayr proposed a definition of species that has become the gold standard for evolutionary biology. Using the reproductive criterion for species status, Mayr defined a species as
a group of interbreeding natural populations that are reproductively isolated from other such groups.
This definition is known as the biological species concept, or BSC. “Reproductively isolated” simply means that members of different species have traits—differences in appearance, behavior, or physiology—that prevent them from successfully interbreeding, while members of the same species can interbreed readily.

What keeps members of two related species from mating with each other? There are many different reproductive barriers. Species might not interbreed simply because their mating or flowering seasons don’t overlap. Some corals, for example, reproduce only one night a year, spewing out masses of eggs and sperm into the sea over a several-hour period. Closely related species living in the same area remain distinct because their peak spawning periods are several hours apart, preventing eggs of one species from meeting sperm from another. Animal species often have different mating displays or pheromones, and don’t find one another sexually attractive. Females in my
Drosophila
species have chemicals on their abdomens that males of other species find unappealing. Species can also be isolated by preferring different habitats, so they simply don’t encounter one another. Many insects can feed and reproduce on only one single species of plant, and different species of insects are restricted to different species of plants. This keeps them from meeting others at mating time. Closely related species of plants can be kept apart because they use different pollinators. Two species of the monkeyflower
Mimulus
, for example, live in the same area of the Sierra Nevada, but rarely interbreed because one species is pollinated by bumblebees and the other by hummingbirds.

Isolating barriers can also act after mating. Pollen from one plant species might fail to germinate on the pistil of another. If fetuses are formed, they might die before birth; this is what happens when you cross a sheep with a goat. Or even if hybrids survive, they may be sterile: the classic example is the vigorous but sterile mule, the offspring of a female horse and a male donkey. Species that produce sterile hybrids certainly can’t exchange genes.

And of course several of these barriers can act together. For much of the last ten years I’ve studied two species of fruit fly that live on the tropical volcanic island of São Tome, off the west coast of Africa. The species are somewhat isolated by habitat: one lives on the upper part of the volcano, the other at the bottom, though there is some overlap in their distributions. But they also differ in courtship displays, so even when they do meet, members of the two species rarely mate. When they do succeed at mating, the sperm of one species is poor at fertilizing the eggs of the other, so that relatively few offspring are produced. And half of these hybrid offspring—all of the males—are sterile. Putting all these barriers together, we conclude that the species exchange virtually no genes in nature, and we have confirmed this result by sequencing their DNA. These, then, can be considered good biological species.

The advantage of the BSC is that it takes care of many problems that appearance-based species concepts can’t handle. What are those cryptic groups of mosquitoes? They are different species because they don’t exchange genes. What about Inuit and !Kung? These populations may not mate directly with each other (I doubt that such a union has ever occurred), but there is
potential
gene flow from one population to the other through intermediate geographical areas, and little doubt that if they did mate they’d produce fertile offspring. And males and females are members of the same species because their genes unite at reproduction.

According to the BSC, then, a species is a reproductive community—a gene pool. And this means that a species is also an
evolutionary
community. If a “good mutation” crops up within a species, say a mutation in tigers that boosts a female’s output of cubs by 10 percent, then the gene containing that mutation will spread throughout the tiger species. But it won’t go any further, for tigers don’t exchange genes with other species. The biological species, then, is the unit of evolution—it is, to a large extent,
the thing that evolves.
This is why members of all species generally look and behave pretty much alike: because they all share genes, they respond in the same way to evolutionary forces. And it is the lack of interbreeding between species living in the same area that not only maintains species’ differences in appearance and behavior, but also allows them to continue diverging without limits.

But the BSC isn’t a foolproof concept. What about organisms that are extinct? They can hardly be tested for reproductive compatibility. So museum curators and paleontologists must resort to traditional appearance-based species concepts, and classify fossils and specimens by their overall similarity. And organisms that don’t reproduce sexually, such as bacteria and some fungi, don’t fit the criteria of the BSC either. The question of what constitutes a species in such groups is complicated, and we’re not even sure that asexual organisms form discrete clusters in the way that sexual ones do.

But despite these problems, the biological species concept is still the one that evolutionists prefer when studying speciation, because it gets to the heart of the evolutionary question. Under the BSC, if you can explain how reproductive barriers evolve, you’ve explained the origin of species.