The Why Files The Why Files --

Evolution: The never-ending story

Evolution's big birthdays!

Later this year, the cardinal principle of biology -- evolution through natural selection -- reaches the august age of 150.

On Feb. 12, Charles Darwin, author of that theory, would turn 200.

We'll doff our hats to Alfred Russel Wallace, an English naturalist who published essentially the same theory on the same day as Darwin, and therefore deserves to be remembered as the co-discoverer of evolution.

Then we will plunge head-first into rising tide of Darwo-philia.

The coral appears to consist of flat stiff light orange cheese crossed with a shrub.
Photo courtesy Gordon Medaris, Jr.
This elkhorn coral, photographed in the Bahamas, is part of a coral reef, a hotbed of oceanic evolution and biodiversity.

What is evolution?

In biology, "evolution" typically means “evolution through natural selection,” a process whereby the best adapted organisms tend to have more offspring than those that are less adapted to their environment. Evolution, technically defined as a change in the genetic composition of a population, gradually changes biology by altering existing species, creating new species and deleting old species.

The result of evolution is a tree of life that traces back billions of years to our most primitive ancestors.

Natural selection emerges from the fact that organisms have too many offspring to survive, and those that cannot survive, generally because they are less adapted to their situation -- get weeded out, while the better adapted organisms survive and prosper.

In the sweepstakes called evolution, the essential measure of success is having more offspring that survive and reproduce. It's not just about survival, notes Armin Moczek, an assistant professor of biology at Indiana University. “Just because you live longer doesn't mean you have more offspring. In many cases, selection has favored shorter male life span because it facilitates greater reproductive success.” Some male spiders, he notes, are eaten by their mate just after mating. His genes get passed along, and she gets more energy to raise their offspring.

Evolution is a process that begins with random changes -- and then through an incessant trial-and-error testing -- crafts exquisitely complex solutions to the twin problems of survival and reproduction. To put it another way, evolution is a brainless process capable of forming brains -- and eyes.

Barebones evolutionary concepts

We don't have room to rehash the entire theory of evolution, but here are some basic terms and concepts:

• Speciation: Formation of a new species.

• Common ancestor: A species that gave rise to two or more descendants; a "fork in the road" of speciation.

• Natural selection: Selection for genes that can meet demands of current environment.

• Sexual selection: Selection for genes based on choices made (usually by the female) during the mating process.

• Adaptation: Genetic changes that improve chances for survival and reproduction.

• Differential reproduction: Changes in gene frequency based on superior survival of better adapted organisms.

Why evolution is for real

You may have noticed that some people reject evolution, a natural-science explanation for the breadth of living creatures, in favor of creationism, a religious-supernatural belief that God created all the living creatures.

What evidence supports evolution through natural selection?

1. Evolution is written in the leftovers. Why do ostriches have wings if they cannot fly? A gifted designer would have jettisoned the wings, but evolution's "use-it-if-you-didn't-lose-it" approach has instead repurposed them to be used in threat displays and balancing the bird as it sprints.

2. Evolution is written in development. During their early stages, animals often mimic the embryonic stages of common ancestors. In a dolphin fetus, buds for rear limbs develop and then disappear, showing that dolphins (like whales) descended from four-legged animals. (Illustrating the previous point, the bones in the dolphin’s front flippers reveal their evolutionary origin as front legs.) The study of evolution through development, called "evo-devo," reflects the fact that the genetic controls on development resist change. Thus evolution, instead of deleting structures, may change the purpose and/or alter the structure in a way that promotes survival and reproduction.

3. Evolution is inscribed in the rocks. Younger rocks overlie older ones, so fossils of younger descendants always appear above fossils of older ancestors, and living creatures resemble younger fossils more than older ones. While Darwin had little fossil evidence and relied largely on studies of living organisms to examine change through time, in the 150 years since "The Origin of Species," millions of fossil discoveries have proven him correct.

4. Evolution is written in the genes. As organisms divide into new species, they retain almost all of their genes, even as occasional changes help establish a new species. Because humans and chimps had a common ancestor about 7 million years ago, our genes are about 98.5 percent alike. Our last common ancestor with the fungi appeared more than 500 million years ago, yet about half of our genes are also found in fungi. Why? Because once nature solves a problem, it hoards the genes for that solution. Although our ancestors developed heads and limbs, they still needed -- and retained -- genes to solve many of the same basic biochemical problems that confront fungi.

Convergent evolution

A green cylinder has many small yellow-petal flowers with pink centers and long beige spikes.
Euphorbia actinoclada, Frank Vincentz
The Old World euphorbs (above) and the New World cacti (below) evolved water-retentive flesh and spines to cope with a desert environment. The groups may look alike, but genetically they are not closely related.
Cacti in Volunteer Park Conservatory, Seattle, Joe Mabel
A variety of cylindrical cacti with white fuzz, straight and star-shaped spines are pictured

5. The march of the look-alikes. The deserts of the new world contain spiny cacti, while those of the old world contain euphorbs. Both groups comprise tough, leafless plants with low surface area and a succulent tissue that holds water, but cacti contain a clear fluid, and euphorbs contain a milky, bitter fluid. This is convergent evolution: Similar environmental forces have shaped ancestors that lacked a close genetic relationship into modern organisms with a strong external resemblance.

6. Island idyll. Barren volcanic islands that rise from the ocean offer empty ecological niches to the first arriving organisms, which can then diversify through "adaptive radiation" into numerous, closely related groups. In the Galapagos Islands, which played a key role in spurring the theory of evolution, Darwin saw that one species of finch had arrived and then branched out to form a group of related species which could take advantage of the local seeds, nuts and fruit. "Darwin's" finches are unique to the Galapagos, because they evolved after they reached the islands.

Five cichlids with specific mouth adaptations are paired with descriptions like 'eats fish scales.'
Photo courtesy of Dr. Michael K. Oliver,

7. Island influence. Remote, oceanic islands are often missing big groups of organisms; Hawaii, for example, has abundant native plants, insects and birds but no native mammals, freshwater fish or reptiles. Why? Because plants, insects and birds could survive long journeys across or above the ocean, the other groups could not.

Making book on evolution

We condensed the above examples from "Why Evolution Is True," a new book that explains exactly what its title claims (see #1 in the bibliography). We phoned author Jerry Coyne, a University of Chicago evolutionary biologist, and asked why he'd written the book.

After all, we said, biologists embrace evolution, but creationists who hew to a spiritual explanation seem immune to scientific evidence. "I was trying to appeal to the open-minded," said Coyne. "There is no controversy about evolution, at least not a scientific one, although many of my colleagues did not know all the evidence. When you teach physics, you do not start by saying, 'This is the reason physics is true.' But evolution is different because of the tremendous social consequences behind the acceptance."

Evolution, Coyne continued, "is the ultimate explanation of all biological phenomenon. Why something is there in the first place is an evolutionary question. Why did this feature give an advantage to the animal? Why do animals behave as they do? Everything goes back ultimately to Darwin. Any feature, in some way, enhances our reproduction."

Chatting with Charles: What would you say to Darwin?

The Why Files asked evolutionary biologist Sean Carroll at the University of Wisconsin-Madison, who is sometimes described as "the person Charles Darwin would most like to have lunch with."

"I'd ask him where he felt the weakest evidence was at the time he wrote 'The Origin,' and would regale him with how those are not tough things anymore. I think Darwin would almost come to tears over the transitional fossils. From his geological knowledge, his understood that Earth was much older than people thought, that there should be extinct transitional forms that nobody had discovered in 1859. Now we have a bunch. The mechanism of inheritance is another crucial advance. There has been a century of genetics, it's rock solid. I could give him unlimited examples of where genetic features are shared in some groups and families but not by others. In the Southern Ocean, about 130 species of fish have 'antifreeze' genes. Why these 130 species, but not others? Creationists would say you have invent these genes 130 times, but evolution says they were invented once, and all descendants inherited them. We now have genetic fingerprints that link groups of species, genera, families, and that show exactly the pattern you would expect to see from a branching tree, a lá Darwin. Genetics is the overwhelming, smoking-gun evidence for how species descended from their ancestors."

What can we learn from a trilobite?

Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2023, University of Wisconsin, Board of Regents.