Natural Selection

Discuss asks you to identify issues and provide points for and against, or to examine a process from more than one side. Here it means showing how the three named factors work together, not defining them one after another.

6 marks, three named factors. Budget roughly two marks each, and make sure every factor is tied to the next one rather than listed on its own. Pick one example at the start and carry it the whole way through.

Consider a population of beetles living on grey tree bark, in which some individuals are pale and some are dark. Insect-eating birds hunt by sight and take the beetles they can see most easily against the bark.

Genetic variation is the existing difference in alleles between individuals in a population, arising ultimately from mutation and shuffled each generation by meiosis and sexual reproduction. In this population the pale and dark forms are a heritable difference the beetles are born with, not something they acquire. Without pre-existing variation there would be nothing for selection to act on and the population could not change.

A selective pressure is an environmental factor that causes differential survival and reproduction. Bird predation is the pressure here: dark beetles are more visible against grey bark, so they are taken more often, while pale beetles are more likely to survive to reproductive age. Fitness in this sense is not strength but the number of surviving offspring an individual leaves.

Because colour is determined by alleles passed from parents to offspring, the survivors transmit their alleles to the next generation. Each generation the surviving pale beetles contribute proportionally more alleles for pale colouration to the gene pool, so the frequency of that allele rises while the frequency of the allele for dark colouration falls.

Over many generations the repeated cycle of variation, differential survival under the selective pressure, and inheritance by the survivors shifts the proportion of alleles in the population. That measurable change in allele frequency over generations is evolution by natural selection. Note that no individual beetle changes colour: the individuals are fixed, and it is the population that changes.

The process depends on conditions that are not always met. If the population has no variation in the relevant trait, selection cannot act; if the environment changes faster than generations turn over, the population may not track it; and if the pressure removes too much of the population, the loss of variation itself limits future adaptation. Natural selection can only work on the variation that already exists, which is why it produces workable solutions rather than optimal ones.

Explain means cause and effect: say why, not just that. Stating that populations evolve and individuals do not simply restates the question.

5 marks. Establish what an individual can and cannot change, then show what a population can change that an individual cannot, then say why the distinction matters.

An individual organism inherits its alleles at fertilisation and carries the same genotype in its body cells for life. A beetle born pale cannot become dark because a bird is hunting, and a beetle born dark cannot become pale. The environment can change an individual, but through growth, learning, acclimatisation or damage, and none of those changes its alleles.

Even a change that clearly benefits an individual is not passed on unless it is in that individual's gametes. A plant grown in shade may produce larger leaves, and an animal that trains may build muscle, but neither alters the alleles in the gametes, so neither appears in the offspring. This is why the idea that organisms change because they need to, and pass that change on, is not how the process works.

A population, unlike an individual, is a collection of genotypes and therefore has a property no individual has: the relative frequency of each allele in its gene pool. That frequency can change between one generation and the next, because individuals carrying different alleles survive and reproduce at different rates. The composition of the group changes even though every member of it stays as it was born.

Natural selection therefore acts on individuals but produces its effect in populations: individuals are what live, die and reproduce, so they are what the pressure sorts, but the resulting change is a shift in allele frequency, which only a population can undergo. An individual cannot evolve during its lifetime because evolution is a change in the genetic composition of a group across generations, and an individual is neither a group nor able to alter the alleles it inherited.