Variation and Survival

In Mozambique’s Gorongosa National Park, ivory poachers hunted elephants during a civil war that lasted from 1977 to 1992. Before the war, about 1 in 5 of the females researchers sampled had no tusks; among the females that survived the war, about half had none. No elephant lost her tusks. So how did the population change? The answer depends on two facts: individuals differ, and not all of them survive.

Explore: put a population under pressure

In this model you watch a population of animals with an inherited trait, such as having tusks or not, over many generations. You set how strong a pressure is, such as how often hunters take tusked animals, and whether new variations can arise by mutation. A graph tracks what share of the population has each trait.

Try this.

1. Set the pressure to zero. Predict whether the share of tuskless animals will rise, fall or wobble. Run it several times.
2. Turn the pressure up to strong. Predict how many generations it takes for the share to change a lot. Then try a weak pressure and compare.
3. Start with no tuskless animals and new variation switched off. Predict what strong pressure does. Then switch new variation on.
4. Follow one animal through its life. Predict whether its trait changes before it dies.

Explain: where variation comes from

Look around a classroom. People differ in height, hair, voice and much more. Some of those differences are inherited, and some come from the environment, such as food or exercise. Only inherited differences pass to the next generation.

Variation is the differences among individuals in a population. An allele is one version of a gene. The gene pool is all the alleles of all the individuals in a population. Three processes add to or reshape it.

• Mutation is a change in an organism’s DNA. It is the original source of new alleles. Many mutations have no effect, some harm, and a few help, depending on the environment. In animals, only mutations in cells that make eggs or sperm pass to offspring.
• Gene flow is the movement of alleles from one population to another, when individuals move and breed or when pollen blows to another field. It can bring in an allele the population did not have.
• Sexual reproduction joins an egg and a sperm from two parents. You saw in Meiosis and Variation that meiosis shuffles chromosome pairs and swaps pieces between them. This makes new combinations of alleles that already exist, so each offspring is a new mix.

Over many generations, mutations build up. As they do, and as gene flow adds and removes alleles, the gene pool changes.

A bead jar is a helpful picture of a gene pool. The picture has a limit, because real alleles are carried inside living animals, so they leave the jar mainly when those animals die, fail to reproduce, or move to another population.

Explain: more young than can survive

Populations tend to make more young than the environment can support. A female blue crab in the Chesapeake Bay may carry an egg mass of about 750,000 to 8 million eggs, according to the Virginia Institute of Marine Science. Only a small fraction of those eggs become adult crabs. Most die young.

Natural selection is the process by which individuals with certain inherited variations survive and reproduce more than others, so those variations become more common in later generations. Strength alone does not decide it. What counts is how many offspring an individual leaves.

A selective pressure is anything in the environment that makes some variants more likely to survive than others: predators, disease, drought or human hunting. In Gorongosa, poachers wanted ivory, so tusked elephants were killed more often. Tuskless females already existed, and they survived more often. The researchers concluded that poaching most likely caused strong selection for tusklessness.

Pressure can be strong or weak. A strong pressure, like heavy poaching, can change a trait’s frequency within a few decades. A weak pressure changes it slowly over many generations. Biologists argue about how often evolution happens in quick bursts and how often it creeps along; you will meet those two ideas in Natural Selection and New Species.

A reproductive strategy is a species’ pattern of how many young it makes and how much it invests in each. The table compares two endpoints.

Many species fall between these endpoints, so treat them as the ends of a range rather than two boxes.

Elaborate: read the elephant and crab data

Sources: tusk percentages and park counts from Campbell-Staton and colleagues, Science (2021), Figure 1 (the paper’s text gives 52 prewar females, its figure 54). Amboseli births from Moss, Journal of Zoology (2001), a study begun in 1972. Gorongosa calf intervals from Poole and Granli, Pachyderm (2022), field trips 2011 to 2019. Egg mass range from the Virginia Institute of Marine Science, a summary range, not one dated sample.

1. By how many percentage points did the share of tuskless females rise from before the war to the survivors?
2. Why might the daughters’ share (33 percent) be lower than their mothers’ generation (50.9 percent)?
3. Compare the two elephant interval values. Why might they differ?

Design problem

Plan an ethical study to test whether hunting, not chance, explains the rise in tuskless females in a park. You cannot assign hunting to elephants. What groups would you compare, what would you count, and what could mislead you?

Evaluate: check your thinking

No Gorongosa elephant lost her tusks. Tuskless females were already part of the population, and poaching killed tusked elephants more often, so tuskless survivors left more of the next generation. The population changed because some individuals survived and reproduced while others did not.

Science practices you used: making predictions and testing them with a model, reading a table of percentages and sample sizes, calculating a change in percentage points, planning an ethical observational study, and naming the limits of small samples and of a model.