The starting point of Darwin's theory of evolution is precisely the existence of those differences between individual members of a race or species which morphologists for the most part rightly neglect. The first condition necessary, in order that any process of Natural Selection may begin among a race, or species, is the existence of differences among its members; and the first step in an enquiry into the possible effect of a selective process upon any character of a race must be an estimate of the frequency with which individuals, exhibiting any given degree of abnormality with respect to that, character, occur. The unit, with which such an enquiry must deal, is not an individual but a race, or a statistically representative sample of a race; and the result must take the form of a numerical statement, showing the relative frequency with which the various kinds of individuals composing the race occur.

As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.

Nature progresses by unknown gradations and consequently does not submit to our absolute division when passing by imperceptible nuances, from one species to another and often from one genus to another. Inevitably there are a great number of equivocal species and in-between specimens that one does not know where to place and which throw our general systems into turmoil.

In response to these distasteful episodes of racism, some scientists have overreacted, arguing that human races have no biological reality and are merely sociopolitical “constructs” that don’t merit scientific study. But to biologists, race—so long as it doesn’t apply to humans!— has always been a perfectly respectable term. Races (also called “subspecies” or “ecotypes”) are simply populations of a species that are both geographically separated and differ genetically in one or more traits. There are plenty of animal and plant races, including those mouse populations that differ only in coat color, sparrow populations that differ in size and song, and plant races that differ in the shape of their leaves. Following this definition, Homo sapiens clearly does have races. And the fact that we do is just another indication that humans don’t differ from other evolved species.

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As we would expect from evolution, human physical variation occurs in nested groups, and in spite of valiant efforts by some to create formal divisions of races, exactly where one draws the line to demarcate a particular race is completely arbitrary. There are no sharp boundaries: the number of races recognized by anthropologists has ranged from three to more than thirty. Looking at genes shows even more clearly the lack of sharp differences between races: virtually all the genetic variation uncovered by modern molecular techniques correlates only weakly with the classical combinations of physical traits such as skin color and hair type commonly used to determine race.

Direct genetic evidence, accumulated over the last three decades, shows that only about 10 to 15 percent of all genetic variation in humans is represented by differences between “races” that are recognized by difference in physical appearance. The remainder of the genetic variation, 85 to 90 percent, occurs among individuals within races.

But recent work shows that our genetic resemblance to our evolutionary cousins is not quite as close as we thought. Consider this. A 1.5 percent difference in protein sequence means that when we line up the same protein (say, hemoglobin) of humans and chimps, on average we’ll see a difference at just one out of every 100 amino acids. But proteins are typically composed of several hundred amino acids. So a 1.5 percent difference in a protein 300 amino acids long translates into about four differences in the total protein sequence. (To use an analogy, if you change only 1 percent of the letters on this page, you will alter far more than 1 percent of the sentences.) That oft-quoted 1.5 percent difference between ourselves and chimps, then, is really larger than it looks: a lot more than 1.5 percent of our proteins will differ by at least one amino acid from the sequence in chimps. And since proteins are essential for building and maintaining our bodies, a single difference can have substantial effects.

Now that we’ve finally sequenced the genomes of both chimp and human, we can see directly that more than 80 percent of all the proteins shared by the two species differ in at least one amino acid. Since our genomes have about 25,000 protein-making genes, that translates to a difference in the sequence of more than 20,000 of them. That’s not a trivial divergence. Obviously, more than a few genes distinguish us. And molecular evolutionists have recently found that humans and chimps differ not only in the sequence of genes, but also in the presence of genes. More than 6 percent of genes found in humans simply aren’t found in any form in chimpanzees. There are over 1,400 novel genes expressed in humans but not in chimps. We also differ from chimps in the number of copies of many genes that we do share. The salivary enzyme amylase, for example, acts in the mouth to break down starch into digestible sugar. Chimps have but a single copy of the gene, while individual humans have between two and sixteen, with an average of six copies. This difference probably resulted from natural selection to help us digest our food, as the ancestral human diet was probably much richer in starch than that of fruit-eating apes.

Putting this together, we see that the genetic divergence between ourselves and chimpanzees comes in several forms—changes not only in the proteins produced by genes, but also in the presence or absence of genes, the number of gene copies, and when and where genes are expressed during development. We can no longer claim that “humanness” rests on only one type of mutation, or changes in only a few key genes. But this is not really surprising if you think about the many traits that distinguish us from our closest relatives. There are differences not only in anatomy, but also in physiology (we are the sweatiest of apes, and the only ape whose females have concealed ovulation), behavior (humans pair-bond and other apes do not), language, and brain size and configuration (surely there must also be many differences in how the neurons in our brains are hooked up). Despite our general resemblance to our primate cousins, then, evolving a human from an ape-like ancestor probably required substantial genetic change.

The way we discovered how species arise resembles the way astronomers discovered how stars “evolve” over time. Both processes occur too slowly for us to see them happening over our lifetime. But we can still understand how they work by finding snapshots of the process at different evolutionary stages and putting these snapshots together into a conceptual movie. For stars, astronomers saw dispersed clouds of matter (“star nurseries”) in galaxies. Elsewhere they saw those clouds condensing into protostars. And in other places they saw protostars becoming full stars, condensing further and then generating light as their core temperature became high enough to fuse hydrogen atoms into helium. Other stars were large “red giants” like Betelgeuse; some showed signs of throwing off their outer layers into space; and others still were small, dense white dwarfs. By assembling all these stages into a logical sequence, based on what we know of their physical and chemical structure and behavior, we’ve been able to piece together how stars form, persist, and die. From this picture of stellar evolution, we can make predictions. We know, for example, that stars about the size of our Sun shine steadily for about ten billion years before bulging out to form red giants. Since the Sun is about . billion years old, we know that we’re roughly halfway through our tenure as a planet before we’ll finally be swallowed up by the Sun’s expansion.

And so it is with speciation. We see geographically isolated populations running the gamut from those showing no reproductive isolation, through those having increasing degrees of reproductive isolation (as the populations become isolated for longer periods), and, finally, complete speciation. We see young species, descended from a common ancestor, on either side of geographic barriers like rivers or the Isthmus of Panama, and on different islands of an archipelago. Putting all this together, we conclude that isolated populations diverge, and that when that divergence has gone on for a sufficiently long time, reproductive barriers develop as a by-product of evolution.

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 each other 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 each other. 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 each other 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 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  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.

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Further, when you look at the type of insects and plants native to oceanic islands, they are from groups that are the best colonizers. Most of the insects are small, precisely those that would be easily picked up by wind. Compared to weedy plants, trees are relatively rare on oceanic islands, almost certainly because many trees have heavy seeds that neither float nor are eaten by birds. (The coconut palm, with its large buoyant seeds, is a notable exception, occurring on almost all Pacific and Indian Ocean islands). The relative rarity of trees, in fact, explains why

Mayr lived exactly 100 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.