Natural Selection and New Species

In 1977 a severe drought struck Daphne Major, a small island in the Galápagos, and most of its medium ground finches died. The birds that were left had, on average, deeper beaks than the birds that had lived there before.

Researchers had measured hundreds of these birds before the drought, and they knew which lived. So you can ask a real question: did the drought change the birds, or change which birds were left?

Explore: send a finch population through a drought

In the model below, finches have a spread of beak depths. You set the seed supply: mostly small soft seeds or mostly large hard ones. Survival depends on beak depth, survivors breed, and their young inherit beak depth with some variation, shown as a histogram each generation. A second tab splits one population with a river and lets the two sides change on their own.

Try this.

1. Predict where the mean beak depth will move if the island has only large hard seeds. Run five generations and check.
2. Switch back to small soft seeds. Predict first: will the mean move back, stay, or keep rising?
3. Set the starting population so every bird has the same beak depth. Predict what drought does now, then test it.
4. On the river tab, run both sides for many generations. Watch when the two groups can no longer interbreed.

Explain: how natural selection works

Natural selection is the process in which individuals with some inherited traits survive and reproduce more than others, so those traits become more common in a population. It works only when three things are true at once.

• Variation: individuals differ in a trait. Variation means the differences among individuals in a population.
• Inheritance: the differences are passed to offspring. An inherited trait is a trait carried in genes from parent to young.
• Unequal success: the differences change how many offspring an individual leaves.

Where does variation come from? A mutation is a change in DNA, and it is the source of new versions of genes. Sexual reproduction then shuffles those versions into new combinations, as you saw in that lesson. Selection does not make variation. It sorts what is already there.

Fitness is how many offspring an individual leaves that survive to reproduce. An adaptation is an inherited trait that helps an organism survive and reproduce in its present environment. On Daphne Major in 1977, a deep beak was an adaptation. The study that measured the birds found that large birds, especially males with large beaks, survived best, because they could crack the large, hard seeds left after the drought.

No bird’s beak grew because of the drought. The birds that died were, on average, different from the birds that lived. Studies of parents and young showed that beak size is strongly inherited, so the next generation’s beaks were deeper on average too. Over many generations, unequal survival like this leaves a population made up mostly of well-suited individuals, and the inherited change tends to last unless the environment later favours something else.

Explain: fast change, slow change and new species

Selection can be fast. Bacteria divide often, so an antibiotic can sort a population within days. In one laboratory study, E. coli grew resistant to a very high dose of an antibiotic in about 10 days. You can read more in that lesson. Large animals can change in decades: poaching for ivory in Mozambique favoured female elephants with no tusks. Other changes build up over thousands or millions of years.

Biologists describe two patterns in the history of life.

Gradualism is the idea that change builds up in small steps over long periods. Punctuated equilibrium is the idea that species stay much the same for long periods, then change in short bursts, which can still last thousands of years. The two can occur together in one lineage at different times.

A species that cannot adapt to a change may suffer extinction, the death of every member of a species. Adapting does not guarantee rescue, though. A population can become better suited and still shrink to nothing if the environment worsens faster than it changes.

A species is a group of organisms that can interbreed and produce fertile offspring. This definition works for organisms that reproduce sexually. Speciation is the process in which one species splits into two or more groups that can no longer interbreed and produce fertile offspring. Speciation often starts with geographic isolation, the separation of groups of one species by a barrier such as a river, a mountain range or a canyon. On each side, different mutations arise and different traits are favoured, so the groups change in different ways.

Separation alone is not speciation. The Kaibab squirrel on the north side of the Grand Canyon looks different from the Abert’s squirrels on the south side, yet biologists usually treat it as a subspecies of Abert’s squirrel, so as one species. They show a split that may or may not be finished. Speciation can also happen without a barrier. It can also begin fast: on Daphne Major, a new finch lineage that started with one immigrant male stopped breeding with the other finches within three generations.

Elaborate: read the drought data

The table uses a real classroom sample of 100 medium ground finches. Each beak was measured before the drought, and each bird is labelled by whether it survived. The sample was chosen to hold 50 survivors and 50 nonsurvivors.

Counts are calculated from HHMI BioInteractive’s “Finches in the Galápagos” dataset, data from Peter and Rosemary Grant, birds of Daphne Major measured about 1976, sorted by survival of the 1977 drought.

A separate summary of the Grants’ figures, from the Open University, gives a mean beak depth of about 9.2 mm for adults before the drought, about 9.9 mm for the survivors that bred, and about 9.7 mm for their young hatched in 1978. Those are different groups of birds from the table, so do not mix the two sets.

1. Find the beak depth range where survivors first outnumber nonsurvivors. What happens in the ranges above it?
2. Subtract the two means. How much deeper, on average, were the survivors’ beaks?
3. The sample is half survivors on purpose. Why can it not tell you what share of the island’s finches survived?

Design problem

A different island has a population of seed-eating finches that has never been studied. Plan a study that could show whether the next drought selects on beak depth.

Evaluate: check your thinking

The drought did not reshape any finch’s beak. Birds with deeper beaks were already in the population, they cracked the hard seeds that were left, and more of them lived to breed. Because beak depth is inherited, their young had deeper beaks too, so the population changed between generations.

Science practices you used: planning a fair study with variables and a comparison, reading a data table with counts and means, using a model and naming its limits, and making a claim from evidence.