Why Evolution Is True

by Jerry A. Coyne

Flatfish are poorly designed.

wouldn’t you have rerouted the female reproductive tract so it exited through the lower abdomen instead of the pelvis?

The Fallopian tube is an imperfect connection because it evolved later as an add-on in mammals.

the particular bad designs that we see make sense only if they evolved from features of earlier ancestors. If a designer did have discernible motives when creating species, one of them must surely have been to fool biologists by making organisms look as though they evolved.

Alexander Selkirk, the real-life Robinson Crusoe, lived out his solitary tenure as a castaway.

Through his own youthful travels on the HMS Beagle and his voluminous correspondence with scientists and naturalists, he realized that evolution was necessary to explain not just the origins and forms of plants and animals but also their distributions across the globe.

if species were created, why did the creator stock distant areas having similar terrain and climate, like the deserts of Africa and of the Americas, with species that were superficially similar in form but showed other, more fundamental differences?

the field of biogeography—the study of the distribution of species on earth.

The biogeographic evidence for evolution is now so powerful that I have never seen a creationist book, article, or lecture that has tried to refute it. Creationists simply pretend that the evidence doesn’t exist.

Evolutionary theory predicts, and data support, the notion that as species diverge from their common ancestors, their DNA sequences change in roughly a straight-line fashion with time. We can use this “molecular clock,” calibrated with fossil ancestors of living species, to estimate the divergence times of species that have poor fossil records.

Every bit of biogeographic detective work turns out to support the fact of evolution.

The plant was finally controlled in 1926 by introducing the cactoblastis moth, whose caterpillars devour the cacti: one of the first and most successful examples of biological control.

If animals were specially created, why would the creator produce on different continents fundamentally different animals that nevertheless look and act so much alike?

No creationist, whether of the Noah’s Ark variety or otherwise, has offered a credible explanation for why different types of animals have similar forms in different places. All they can do is invoke the inscrutable whims of the creator. But evolution does explain the pattern by invoking a well-known process called convergent evolution. It’s really quite simple. Species that live in similar habitats will experience similar selection pressures from their environment, so they may evolve similar adaptations, or converge, coming to look and behave very much alike even though they are unrelated. But these species still retain key differences that give clues to their distant ancestry.

Convergent evolution demonstrates three parts of evolutionary theory working together: common ancestry, speciation, and natural selection.

At the time of the marsupial invasion, South America and Australia were joined as part of the southern supercontinent of Gondwana.

Asia and North America were once well connected via the Bering land bridge, over which plants and mammals (including humans) colonized North America. And South America and Africa were once part of Gondwana.

Creationism is hard-pressed to explain these patterns: to do so, it would have to propose that there were an endless number of successive extinctions and creations all over the world, and that each set of newly created species were made to resemble older ones that lived in the same place. We’ve come a long way from Noah’s Ark.

The profusion of ape-human transitional fossils unearthed since then, with the earliest ones always African, leaves no doubt that Darwin’s prediction was right.

oceanic islands are missing many types of native species that we see on both continents and continental islands.

While the islands are well stocked with native birds, plants, and insects, they completely lack native freshwater fish, amphibians, reptiles, and land mammals.

Take the Galapagos. Among its thirteen islands there are twenty-eight species of birds found nowhere else.

Around sixty of these species—nearly half the bird fauna—were honeycreepers, all descended from a single ancestral finch that arrived on the islands about four million years ago.

Although the Hawaiian Islands make up only 0.004 percent of earth’s land, they contain nearly half of the world’s two thousand species of Drosophila.

Species in the first column can colonize an oceanic island through long-distance dispersal; species in the second column lack this ability.

“visitor” birds found thousands of miles from their normal habitat, the victim of winds or faulty navigation.

some plant species could still germinate after prolonged immersion in seawater.

Many spiders disperse as juveniles by “ballooning” with parachutes of silk; these wanderers have been found several hundred miles from land.

Animals and plants can also hitch rides to islands on “rafts”—logs or masses of vegetation that float away from continents, usually from the mouths of rivers.

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.

Terrestrial mammals are not good colonizers, and that’s why oceanic islands lack them.

New Zealand possesses two bats found nowhere else in the world:

And if dispersal were common, life on oceanic islands would be quite similar to that of continents and continental islands.

As the zoologist George Gaylord Simpson remarked, “Any event that is not absolutely impossible ... becomes probable if enough time passes.” To take a hypothetical example, suppose that a given species has only one chance in a million of colonizing an island each year. It’s easy to show that after a million years have passed, there is a large probability that the island would have been colonized at least once: 63 percent, to be exact.

with few exceptions, the animals and plants on oceanic islands are most similar to species found on the nearest mainland.

Oceanic islands have unbalanced biotas— they are missing major groups of organisms, and the same ones are missing on different islands. But the types of organisms that are there often comprise many similar species—a radiation—and they are the types of species, like birds and insects, that can disperse most easily over large stretches of ocean. And the species most similar to those inhabiting oceanic islands are usually found on the nearest mainland, even though their habitats are different.

the inhabitants of oceanic islands descended from earlier species that colonized the islands, usually from nearby continents, in rare events of long-distance dispersal.

In short, oceanic islands demonstrate every tenet of evolutionary theory.

Unlike oceanic islands, continental islands were cut adrift with most of their species already in place.

Why would a creator happen to leave amphibians, mammals, fish, and reptiles off oceanic islands, but not continental ones? Why did a creator produce radiations of similar species on oceanic islands, but not continental ones? And why were species on oceanic islands created to resemble those from the nearest mainland? There are no good answers—unless, of course, you presume that the goal of a creator was to make species look as though they evolved on islands. Nobody is keen to embrace that answer, which explains why creationists simply shy away from island biogeography.

Very old continental islands, which separated from the mainland eons ago, should show evolutionary patterns that fall between those of young continental islands and oceanic islands. Old continental islands such as Madagascar and New Zealand, cut off from their continents 160 million and 85 million years ago, respectively, will have been isolated before many groups like primates and modern plants had evolved.

lemurs—the most primitive of the primates—

Some orchids further seduce their pollinators by producing chemicals that smell like the sex pheromones of bees.

In fact, we know exactly which two genes have the largest effect on the dark/light color difference. One of them is called Agouti, the same gene whose mutations produce black color in domestic cats. The other is called Mc1r, and one of its mutant forms in humans, especially common in Irish populations, produces freckles and red hair.

Genetic variation generated by mutation is widespread: mutant forms of genes, for example, explain variation in human eye color, blood type, and much of our—and other species’—variation in height, weight, biochemistry, and innumerable other traits.

On the basis of many laboratory experiments, scientists have concluded that mutations occur randomly. The term “random” here has a specific meaning that is often misunderstood, even by biologists. What this means is that mutations occur regardless of whether they would be useful to the individual. Mutations are simply errors in DNA replication. Most of them are harmful or neutral, but a few can turn out to be useful.

The legs of tetrapods, for example, are simply modified fins.

An adaptation must evolve by increasing the reproductive output of its possessor. For it is reproduction, not survival, that determines which genes make it to the next generation and cause evolution.

Of course, passing on a gene requires that you first survive to the age at which you can have offspring. On the other hand, a gene that knocks you off after reproductive age incurs no evolutionary disadvantage. It will remain in the gene pool. It follows that a gene will actually be favored if it helps you reproduce in your youth but kills you in your old age.

In every case, when one species does something to help another, it always helps itself.