Natural selection describes what happens when carriers of some genes leave more offspring than others. The genes that enhance reproduction, often by making an animal or plant more adapted to the environment, thus come to predominate in populations. For example, the ancestor of the polar bear was brown, but when it came to occupy snowy areas, genes for white color were favored, as they camouflaged the bears from their prey, giving white bears a greater ability to get food—and hence leave more offspring than did brown bears. Over time, this form of natural selection genetically transformed the color of the species from brown to white.

All that is required for natural selection is that there is variation among the forms of genes (i.e., different “alleles,” like those producing brown versus blue eyes in humans, or our different blood types), and that some genes promote enhanced reproduction of their carriers than do alternative forms of those genes. The variant forms of genes arise by the process of mutation, which is “random” in the sense that errors in the genetic material occur regardless of whether they are useful. But then a deterministic process—natural selection—takes over and winnows out the bad variants, preserving the good. In general, this kind of change improves the adaptation of organisms to their environment, explaining why plants and animals are remarkably suited to their habitat.

Natural selection is often called “survival of the fittest”, but this is a misnomer in two ways. First, what is important is reproduction: carriers of some genes leave more copies of those genes, in their offspring, than do carriers of others. Survival is important only if it leads to greater reproduction. (Genes that kill organisms after they’ve stopped reproducing are not detrimental.) Second, natural selection does not produce absolute perfection, but only individuals better adapted than those of previous generations. Mutations making an animal perfectly adapted to its environment may simply not be possible. (If perfection were possible, species would not go extinct, for they’d be able to withstand any assault by the environment.)

Thus, a better characterization of natural selection would be “reproduction of the fitter.”

Finally, although natural selection is often described as a “force” or “agent” acting on organisms, that’s just shorthand for what is really happening: not some external force changing a species directly, but simply a process of differential sorting of genes that often depends on the environment in which those genes are expressed.

One of the most obvious observations about nature is that it is divided into discrete units. When we look at a bird, for instance, we can immediately identify it as a cardinal, a sparrow, a pigeon, a starling, and so on. These groups are not arbitrary, so that birds do not form an avian continuum in which individuals blur together insensibly. And so it is with nearly every group of animals and plants. These objectively distinct groups are called “species.”

To evolutionary biologists, a “species” consists of a group of organisms that can mate and produce offspring with each other, but are separated from members of other species by “reproductive isolating barriers”: those biological features that prevent different species from producing fertile offspring. The barriers are diverse, including those factors that adapt species to different environments, so that they never meet (e.g., polar bears vs. brown bears); those factors that affect the likelihood of mating, so that members of different species either attain reproductive condition at different times (e.g., different coral species) or simply do not find each other acceptable as mates (e.g., many insect species); and those factors that impair the viability or fertility of hybrids (e.g., the sterile mule, a hybrid between a donkey and a horse). All of these factors prevent genes from moving between different species and thus keep the spcies distinct.

The process of “speciation” occurs when one ancestral species branches into two or more reproductively isolated species. This usually occurs when a single species has populations that are separated by geographic barriers like oceans or mountain ranges. By preventing mating between populations of a species, these geographic barriers allow the populations to evolve independently, along different genetic pathways. If these pathways diverge to such an extent that the populations have evolved nearly complete reproductive barriers, then speciation has occurred. In other words, the geographic barriers, by allowing independent evolution, promote the evolution of the reproductive barriers. This process of repeated branching of lineages explains the “tree of life”: how, starting with one ancestral species about 3.7 billion years ago, we now have millions of branches: the living and extinct species.

Sexual selection is a subset of natural selection based not on adapting organisms to their environment, but on adapting each sex to the mating preferences of the other. If females have preferences for certain characteristics of males (in animals, it’s almost always females who choose males rather than the other way round), then males with those characteristics will leave more offspring than males without them, and so the population will evolve males with the attractive traits. In peacocks, for example, females prefer males with showier tail feathers and more “eyespots” on their feathers. Over time, this led to the evolution of males with the long, extravagant, and beautiful tails we so admire.

But this process can actually reduce the survival of males: male peacocks, weighted down by those tails, are not as good at flying or escaping predators as peacocks with shorter tails. But what is important in natural selection is reproduction, not just survival, and the mating advantage of long-tailed males more than compensates for their survival disadvantage.

Two other points. First, sexual selection, first proposed by Darwin in 1871, is the explanation for why, if one sex is showier than the other, or has brighter colors, louder calls, or fancier plumage, that sex is almost invariably the males. Many experiments have supported this idea. Second some mysteries remain: why do female peacocks, for example, prefer longer tails in males, and female house finches brighter colors in males? In some cases, as in the armaments of males like the longer tusks of male elephants, the larger body size of elephant seals, or the big jaws of stag beetles, we do have the answer: males need these weapons to fight for females. But an explanation for traits like bright male color or long male plumage still eludes us, and scientists are working on these problems now.

Opponents of evolution often say that we have no “missing links” between species showing that they had a common ancestor, so that evolution could not have occurred. But this claim isn’t really true. A “missing” link between two groups would represent a single fossil species that itself was the ancestor of two existing groups (i.e., of modern humans and modern chimpanzees); and finding a single species in the fossil record is very difficult: far less than 1% of all species that ever lived are known as fossils. But you can demonstrate common ancestry in an easier way: the existence of “transitional forms” that are on the line between a common ancestor and their modern descendants. So, if we go back in time through the fossils, we should find species resembling the common ancestor more and more.

And indeed, we have many examples of such transitional forms: early birds that resemble their theropod dinosaur ancestors, but with feathers. We have early horses that have four toes, which gradually disappear as horses fossils come closer and closer in time to the present day. Likewise, we have transitional forms between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and between modern humans and their more apelike ancestors. There are hundreds of these examples, decisively proving the idea of common ancestry. Further, the transitional forms occur in the right time in the fossil record. Feathered dinosaurs, for instance, are found about 150 million years ago, after the theropod ancestors already lived but before we see fossils of modern birds.

If you squeezed the 3.5-billion- year history of life into just a single minute, multicellular life would appear only after 50 seconds, the invasion of land by vertebrates after 54 seconds, modern angiosperms (flowering plants) after 58 seconds, and just in the last 0.04 seconds would modern humans (Homo sapiens) take the stage. Evolution by natural selection is a very slow process—taking thousands or millions of years—especially when it produces complex traits like animal metabolism and brains. We humans don’t appreciate the long time period over which evolution has occurred because, being on earth only for a few decades, we’re used to thinking in spans of less than a century.

Most people think of a “theory” as “a mere guess or speculation,” like “It’s my theory that Shirley will get a promotion at work this week.” But that’s not the way that scientists use the word. In science, a “theory” is a coherent explanation for a diverse set of observations, an explanation that accounts for all the observations. For instance, we still refer to the “theory of gravity” or “the atomic theory” or “the germ theory of disease,” yet each of these are well supported explanations for why objects fall the way they do, why matter behaves in certain ways, and why infectious disease involves viruses, protozoans, and bacteria.

Although evolution is often dismissed as “only a theory,” it is in fact as well supported as are the “theories” of gravity, atoms, and disease-causing microbes. (Nobody doubts, for instance, that smallpox is caused by a virus or that the formula for water is H 2 O.) There are literally thousands of observations that support evolution, and no observations that refute it. (There could have been such observations: for example, the discovery of mammalian fossils in 400- million-year old layers of the fossil record. But this kind of data simply hasn’t been seen.) And the observations that support evolution come from diverse areas of science: paleontology, embryology, morphology, the fossil record, molecular biology, biogeography, and so on. This gives us strong confidence that evolution is true.

Although no scientific theory can ever be proven correct—it’s always possible that some remarkable observations that haven’t yet been made could disprove evolution—evolution is about as certain to be true as any theory can be. The fact that, after 150 years, no observations have disproven it gives us confidence that the theory of evolution really is true. You could confidently bet your life savings on that!

The answer is a firm “yes”! Molecular data show unequivocally that all living species descended from a single species, called the “Last Universal Common Ancestor” that probably lived about 3.8 billion years ago. That single species then branched again and again, giving rise to not only the millions of species living today, but to the millions of species that went extinct without leaving descendants.

The relatedness of all species is shown not only through similarities and differences in their DNA, but in the retention of ancestral traits in some descendants, like the muscles that enable some humans to wiggle their ears: a nonfunctional remnant of the muscles that moved the ears of our mammalian ancestors in an adaptive way—to localize sound. We still share genes with many of our distant relatives, like bacteria, flies, worms, and even with the cotton plant that produced your shirts.

The story of human evolution began in Africa about six million years ago and involves the many changes in our bodies, brains, and physiology that our ancestors experienced on their way to modern humans. We’re not the endproduct of our lineage, of course, for Homo sapiens, like all species, continues to evolve. Both fossil and molecular evidence show that our common ancestor with our closest relatives (the two species of chimpanzees) lived 6-7 million years ago. We diverged from our next closest relative, the gorilla, via a common ancestor that lived about 9 million years ago.

In the past six million years, at least a dozen different species of “hominins” evolved —those species that sprouted from “our” branch that diverged from chimps—with perhaps three or four of them living at the same time, and perhaps in the same place! (Neandertals and “modern” humans are two of these; and some of them actually interbred.) Imagine these members of different human-like species encountering each other for the first time! Many of these species simply went extinct without leaving descendants, while others continued on the lineage that gave rise to us. We’re still not clear about the precise species on that lineage, but paleontologists and molecular biologists are constantly revising and clarifying the story of human evolution.

Born in 1809, Charles Darwin was only 22 years old when was named the companion of the captain of the HMS Beagle, a ship appointed by the British government to survey the coast of South America. It turned out that the voyage lasted five years, giving Darwin, an amateur naturalist, a chance to observe and collect plant, animal, and rock specimens from around the world.

After the Beagle returned to England in 1836, Darwin began pondering all these observations, and slowly realized that organisms were not created all at once and had been unchanged ever since—the “creationist” view of virtually all scientists of the time—but had evolved gradually over millions of years. Scrutinizing his data also led him to see that organisms hadn’t just changed over time, but that their lineages had split repeatedly, leading to the appearance of new species and to the branching bush of life. This, in turn, implied that all species had common ancestors. Finally, after pondering the remarkable ability of humans to create new varieties of plants and animals by controlled breeding (“artificial selection”), Darwin hit on his most novel idea: natural selection—the process explaining the remarkable adaptations of plants and animals to their environments. (See “What is natural selection?”)

The development of Darwin’s theory was very slow, for he worked it out over decades not only through careful thinking, but through corresponding with hundreds of people around the world and discussing his ideas with his colleagues. Finally, in 1859, he put it all together in a single world-changing book: On the Origin of Species by Means of Natural Selection, usually called simply The Origin. His ideas and the data he gave supporting them were so convincing that within a decade virtually every thinking person in Europe and the United States accepted evolution as true.The concepts of evolution and natural selection have been called “the best ideas anybody ever had”, for they not only changed our view of nature, but also of our own place in nature and our evolutionary origin from earlier species and relatedness to all living species. Scientific work since 1859 has shown Darwin’s major ideas to be correct, and so, in this single book, he laid out the true story of not just human origins, but the origins of every creature, living and extinct. No other biologist has ever changed our understanding of nature as much as Darwin. That is why he’s considered a hero to many scientists.

Here is the stirring last paragraph of The Origin:

“Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” 

― Charles Darwin, The Origin of Species