Evolution and Adaptation

What is evolution?

Evolution is change in the inherited traits of a population over many generations. A population is a group of the same species living in the same place, such as all the peppered moths in one forest.

Evolution starts with variation: the differences between members of a population. Look closely at any group of living things and no two are exactly alike. Some moths are paler and some darker. Some finches have deeper beaks than others. Some bacteria can survive a drug that kills their neighbors.

Some variation is inherited, passed from parents to offspring in their genes. Some is not. A dog that loses its tail in an accident still has puppies with tails. Only inherited variation can be passed on, so only inherited variation matters for evolution. New inherited variation comes from mutations, changes in DNA, and from the way parents' genes are shuffled together in their offspring. (You met genes and alleles in Heredity and Genetics.)

Adaptations

An adaptation is an inherited trait that helps a living thing survive and reproduce in its environment. Scientists often sort adaptations into three kinds:

An adaptation fits a particular environment. The arctic fox's white winter coat hides it in snow, but the same coat would make it easy to spot in a summer meadow. That is why a trait that helps in one place can be useless, or even harmful, in another.

Natural selection

Natural selection is the main way that populations become adapted to their environment. It works when four things are true:

1. Variation. Members of a population differ in their traits.
2. Inheritance. Some of those traits are passed from parents to offspring.
3. Overproduction. Living things produce more offspring than can survive. There isn't enough food, space, or shelter for all of them, so they compete.
4. Differential survival. Individuals with traits that suit their environment are more likely to survive and reproduce. ("Differential" means "different for different individuals.")

Put the four together and the result follows. The helpful traits get passed on more often, so over many generations they become more common in the population.

Notice what natural selection does not do. It doesn't create the brown color. The brown beetles were already there. Selection only changes how common each form becomes. Notice too that "fit" in the phrase "survival of the fittest" doesn't mean strongest. It means best suited to the environment, measured by how many offspring survive.

Darwin and Wallace

Two British naturalists came up with the idea of natural selection separately. Charles Darwin spent more than twenty years gathering evidence for it. Alfred Russel Wallace worked it out while collecting animals in what is now Indonesia. Both had read an essay by the economist Thomas Malthus about how populations grow faster than their food supply.

The evidence

Evolution is supported by many separate kinds of evidence. Each kind could have turned out to disagree with the others. Instead, they fit together.

Fossils

Fossils are the remains or traces of living things preserved in rock. Rock forms in layers. Where the layers haven't been disturbed, the lower layers formed first, so the fossils in them are older. Across the world, fossils show up in a consistent order. Simple sea creatures appear in older rock, and animals like mammals appear only in much younger rock.

Fossils can also catch in-between forms. In 2004, Neil Shubin and his colleagues found fossils of Tiktaalik roseae in rock about 375 million years old on Ellesmere Island in Arctic Canada. Tiktaalik was a fish with gills and scales, but it also had a flat head, a neck, and sturdy fins with bones like the start of a limb. It is part of the evidence for how fish with limb-like fins gave rise to the first four-legged animals. The team searched rock of that age on purpose because they predicted such a fossil should be there.

Homologous structures

Homologous structures are body parts that are built the same way in different species because they were inherited from a common ancestor. Your arm, a cat's front leg, a whale's flipper, and a bat's wing do very different jobs. Yet all four have the same set of bones: one upper arm bone, two forearm bones, then wrist bones and finger bones.

Some homologous structures no longer have much of a job. These are called vestigial structures. Whales, for example, have small hip and leg bones inside their bodies, left over from four-legged ancestors that lived on land.

DNA

All living things use DNA to carry their genes, and they use the same genetic code to read it. Comparing DNA lets scientists measure how closely species are related: the more recently two species shared an ancestor, the more alike their DNA. Where human and chimpanzee DNA can be lined up base by base, about 98.8% of it is the same. DNA comparisons and fossils usually point to the same family tree, two separate lines of evidence agreeing.

Evolution we can watch

Most evolution is far too slow to see in a lifetime. But when the environment changes quickly, or when living things reproduce quickly, scientists can measure natural selection as it happens.

Darwin's finches on Daphne Major

Since 1973, Peter and Rosemary Grant and their team have studied the finches of Daphne Major, a tiny island in the Galápagos. They caught, measured, and banded the birds year after year, so they knew each finch's beak size and who its parents were.

In 1977, a severe drought hit the island. The small, soft seeds ran out. What was left were mostly the large, hard, spiny seeds of a plant called Tribulus. Medium ground finches with deeper beaks could crack them, and finches with smaller beaks struggled. Of about 1,200 medium ground finches on the island, fewer than 20% survived. On average, the survivors' beaks were about half a millimeter deeper than the beaks of the birds that died. Their offspring also had deeper beaks than the population before the drought, because beak depth is inherited.

Selection can also switch direction. During another drought in 2004 and 2005, a larger finch species that had arrived on the island ate most of the big seeds. This time, medium ground finches with smaller beaks, which could eat small seeds, did better.

Peppered moths

Peppered moths rest on trees by day, and birds eat them. The usual form is pale with dark speckles. A dark form is caused by a single gene change that arose around 1819. In Manchester, the dark form was first recorded in 1848. By 1895, about 98% of the peppered moths there were dark. Since about 1970, as air pollution was cut and tree bark became less sooty, the dark form has become rare again.

The usual explanation is that birds more easily spotted moths that didn't match the bark. Bernard Kettlewell tested this idea with experiments in the 1950s. Later scientists criticized parts of his method, such as releasing large numbers of moths and placing them on tree trunks in daylight. Some people used those criticisms to claim the whole story was wrong. To settle it, the biologist Michael Majerus ran a much larger and more careful experiment from 2001 to 2007, releasing 4,864 moths near Cambridge, England. He found that birds did eat more of the dark moths on today's cleaner trees, enough to explain why the dark form is disappearing. Scientists continue to study the details, but the main story has held up.

Bacteria and antibiotics

Antibiotics are medicines that kill bacteria. When a population of bacteria is treated with an antibiotic, a few may already carry a gene that lets them survive. Those survivors multiply, and the next generation is mostly resistant. The drug doesn't cause the resistance. It selects for bacteria that already had it. Because bacteria reproduce so quickly, this can happen in a short time, which is why antibiotic resistance is a serious health problem.

Artificial selection

People have been breeding plants and animals for thousands of years by choosing which ones to save seeds from or breed. This is artificial selection. It works like natural selection, except that people, not the environment, decide which individuals reproduce. Darwin studied pigeon breeders closely and used their results as evidence in the Origin.

Dogs are another example. Scientists think dogs were domesticated from wolves roughly 20,000 to 40,000 years ago, probably from more than one group of wolves. Many of today's breeds were developed in just the last 200 years.

Extinction

A species is extinct when its last member dies. Extinction is a normal part of the history of life. More than 99% of all the species that have ever lived are now extinct.

Species can go extinct when their environment changes faster than they can adapt, when a new predator or competitor arrives, or when they lose their habitat. Natural selection can only work with the variation a population already has. If no individuals have traits that let them survive a change, the population can't adapt.

Sometimes huge numbers of species die out in a short time. This is a mass extinction. There have been five in the last 500 million years or so. The most recent, 66 million years ago, killed about 78% of species, including all the dinosaurs except the ones that became birds.

People cause extinctions too. The dodo, a large flightless bird, lived only on the island of Mauritius. After sailors and settlers arrived with the animals they brought, it was gone. The last widely accepted sighting was in 1662, though it may have survived into the 1680s or 1690s.

Common misconceptions

• "Individuals evolve."They don't. A single moth can't change its wing color, and a giraffe can't stretch its neck and pass a longer neck on. Populations evolve, because the mix of traits changes from one generation to the next.
• "Evolution is trying to make something."Evolution has no goal and doesn't plan ahead. Living things don't get the traits they need because they need them. Natural selection can only favor variation that already happens to exist, which is why many species go extinct instead.
• "Evolution means always getting better."A trait is only "better" in a particular environment. When the environment changes, the same trait can become a problem, as the dark peppered moths found when the trees got cleaner.
• "It's just a theory."In everyday talk, a theory is a guess. In science, a theory is much stronger. The U.S. National Academy of Sciences defines it as "a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses." The theory of evolution, like the theory that germs cause disease, is supported by a huge amount of evidence.
• "Humans evolved from chimpanzees."Humans and chimpanzees both evolved from a common ancestor that lived millions of years ago. Chimps are our relatives, like cousins, not our ancestors.

You will follow six populations of 20 living things. Each population has two forms of a trait. Read about the environment, then predict whether one form will become more common, less common, or stay about the same after five generations. Then choose the best explanation for what happened.

How scoring works: each correct prediction is worth 1 point and each correct explanation is worth 1 more, so each population is worth 2 points and a perfect round is 12. Your best round is saved in this browser only.

The game uses a simple model. In each generation, the better-suited form survives more often, and the survivors have young until the population is back to 20. Real populations are messier, and chance plays a part too.