1Variation, selection and adaptation

An adaptation is a genetic variation that is favoured by selection and shows up as a trait that gives an advantage in a particular environment. "Particular" matters: thick fur is an adaptation in the Arctic and a burden in a desert.

Mutations are not directed by the environment. A population of insects does not produce pesticide-resistance mutations because it is sprayed. Resistant alleles arise by chance, at a low rate, whether the pesticide is present or not. When spraying begins, the environment changes, insects with the resistant allele survive and reproduce, and the allele becomes common. The spray selected the variation; it did not create it.

That is the interaction the CED wants you to explain: random or preexisting variation plus a changed environment gives a changed population.

Heterozygote advantage is when the heterozygous genotype has a higher relative fitness than either homozygote. The classic case is the sickle-cell allele in regions where malaria is common.

• People homozygous for the normal haemoglobin allele have no sickle-cell disease but are more likely to die of malaria.
• People homozygous for the sickle-cell allele have sickle-cell disease.
• Heterozygotes have no serious disease and are more resistant to malaria.

Because heterozygotes leave the most offspring, both alleles stay in the population, even though one of them causes disease in homozygotes. Where malaria is absent, the advantage disappears and the sickle-cell allele is selected against.

Worked example (invented relative fitness values in a malaria region).

The heterozygote's fitness is highest, which is heterozygote advantage by definition. Selection keeps removing both alleles in homozygotes, so neither is lost.

2Invasive species

An invasive species is one introduced, on purpose or by accident, to an area outside its native range, where it spreads and harms the ecosystem. Invasives succeed for three reasons:

• they exploit a new niche free of predators, parasites or diseases that held them back at home;
• they meet no competitors for some resource;
• they outcompete native species for resources such as light, space or food.

Two examples the CED names:

• Kudzu, a climbing vine from East Asia, was planted in the southeastern United States to control soil erosion. It grows very fast, climbs over trees and shrubs, and shades them out.
• Zebra mussels, from the region of the Black and Caspian Seas, reached the Great Lakes of North America in ships' ballast water in the 1980s. They filter enormous volumes of water, remove the plankton that native species feed on, smother native mussels and clog water pipes.

Introduced pathogens act the same way. Dutch elm disease, a fungus spread by bark beetles, killed many millions of elms across Europe and North America in the twentieth century. Potato blight, a water mould, destroyed the potato crop in Ireland in the 1840s, where most farmers grew one potato variety: a monoculture (8.6) with no resistance.

A disruption does not create the variation a population needs; it selects among the variation that is already there.

3Human activity: faster change, more extinctions

Human activity speeds up ecosystem change at local and global scales, and the CED names two processes in particular.

Biomagnification. Some pollutants, such as the insecticide DDT and mercury, are not broken down and are stored in fat. A consumer eats many prey over its life and keeps the pollutant from all of them, so the concentration rises at each trophic level.

Worked example (invented). DDT concentrations in a lake food chain:

From water to bird the concentration rises by a factor of $\frac{25}{0.00005} = 500{,}000$. The top predators carry the highest load. In real birds of prey, DDT thinned eggshells so that eggs broke during incubation, and populations crashed until the chemical was banned in many countries.

Eutrophication. Fertiliser runoff or sewage adds nitrate and phosphate to a lake or coastal water. The steps:

1. Extra nutrients let algae grow very fast: an algal bloom.
2. The bloom shades plants below the surface, which die.
3. When the algae die, decomposers (bacteria) break them down, using oxygen in cellular respiration.
4. Dissolved oxygen falls; fish and other animals suffocate, and more dead matter feeds more decomposers.

Other human activities on the CED's list: global climate change (shifting ranges, coral bleaching, earlier springs), logging and urbanisation (habitat loss and fragmentation), and monocropping (low diversity, as in 8.6). Each can push species to extinction faster than they can adapt.

4Geological and meteorological events

Natural events change habitats and where ecosystems are found. Biogeography, the study of where species live, records these changes.

• El Niño is a periodic warming of surface water in the eastern tropical Pacific. Warm water weakens the upwelling of cold, nutrient-rich water, so plankton decline, fish stocks collapse and seabirds that eat the fish starve; rainfall patterns shift on land.
• Continental drift has moved landmasses apart over millions of years, isolating populations that then evolved separately. That is why many groups are found on only one continent.
• Meteor (asteroid) impact: an impact about 66 million years ago changed the global climate and caused a mass extinction that included the non-avian dinosaurs, opening niches that mammals later filled.
• Volcanic eruptions, droughts and floods disrupt ecosystems on smaller scales.

5Evaluating a hypothesis with data

Skill 5.D asks you to use data to decide whether to reject or fail to reject a null hypothesis.

Worked example (invented). A student tests whether soil from under an invasive shrub reduces the germination of a native wildflower. She plants 100 seeds in soil from under the shrub and 100 in soil from nearby uninvaded ground. 32 germinate in shrub soil and 48 in uninvaded soil, 80 in all.

Null hypothesis: the soil type has no effect, so the 80 germinated seeds are expected to be split equally, 40 and 40.

$$\chi^2 = \frac{(32-40)^2}{40} + \frac{(48-40)^2}{40} = 1.6 + 1.6 = 3.2$$

Degrees of freedom $= 2 - 1 = 1$; critical value at $p = 0.05$ is 3.84. Since $3.2 < 3.84$, fail to reject the null hypothesis. The data suggest an effect, but it is not statistically significant with this sample. A sensible next step is a larger sample, not a firmer claim.