IGCSE Biology Variation & selection notes

Variation is the presence of differences between individuals of the same species. It comes in two shapes. Continuous variation results in a range of phenotypes between two extremes — height, body mass, body length, hand span: everyone falls somewhere along a smooth spectrum, with most people near the middle. Discontinuous variation results in a limited number of phenotypes with no intermediates — the ABO blood groups, pea seed shape (round or wrinkled) and pea seed colour (yellow or green): you are one thing or the other.

The causes differ too: discontinuous variation is usually caused by genes only, while continuous variation is caused by both genes and the environment — your genes set your potential height, but nutrition decides how much of it you reach. You can investigate both: measure classmates’ hand spans (continuous — a bell-shaped spread) and tally tongue-rolling or blood groups (discontinuous — distinct categories).

A mutation is a genetic change — a change in the DNA of a cell — and it is the way in which new alleles are formed. Without mutation there would be no new versions of genes to sort between. Ionising radiation (UV, X-rays, gamma rays, radioactive emissions) and some chemicals (mutagens, including chemicals in tobacco smoke) increase the rate of mutation.

Supplement: at the molecular level, a gene mutation is a random change in the base sequence of DNA — bases swapped, inserted or deleted — which may change the protein the gene codes for. And mutation is not the only mixer: mutation, meiosis, random mating and random fertilisation are the four sources of genetic variation in populations — mutation makes new alleles; the other three recombine existing ones into new combinations.

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. Exam questions give you a picture or data about a species and ask you to describe adaptive features — the skill is naming the feature and linking it to survival: a fox’s thick fur *helps it survive* by reducing heat loss in a cold climate; a cactus’s spines *help it survive* by reducing water loss and deterring herbivores.

Supplement — the plant specialists: xerophytes live where water is scarce (deserts, cold dry winds). Their adaptive features reduce water loss and store what they have: a thick waxy cuticle, rolled leaves that trap humid air, sunken stomata sheltered in pits, spines instead of broad leaves, deep or widespread root systems and fleshy water-storing tissues. Hydrophytes live in or on water, where the problems are too much water, little light below the surface, and weak support: thin or absent cuticle (no water-loss risk), stomata mostly on the upper surface of floating leaves (in contact with air), large air spaces for buoyancy and gas exchange, and reduced xylem (the water needs no pumping).

Natural selection has a five-step logic the syllabus wants reproduced exactly: (1) genetic variation exists within populations (mutation, meiosis, random mating, random fertilisation); (2) organisms produce many offspring — more than the environment can support; (3) there is a struggle for survival, including competition for resources — food, water, space, mates; (4) individuals better adapted to the environment have a greater chance of reproducing; (5) these individuals pass on their alleles to the next generation — so the helpful alleles become more common.

Supplement: over many generations this process produces adaptation — the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Note the direction: individuals do not adapt (their features are inherited and fixed); the population’s allele frequencies shift as selection sorts the variants.

Supplement. Strains of antibiotic-resistant bacteria develop by natural selection, exactly as the five steps predict: within a bacterial population there is genetic variation — a few cells carry resistance mutations; bacteria reproduce rapidly in enormous numbers; an antibiotic is the selection pressure — it kills non-resistant cells in a struggle for survival; resistant cells survive and reproduce; and they pass on their resistance alleles — soon most of the population is resistant, and the drug no longer works. MRSA is the famous result. The same logic explains pesticide-resistant insects and herbicide-resistant weeds.

Selective breeding is selection by humans: (1) individuals with desirable features are chosen from a population; (2) these are crossed to produce the next generation; (3) the offspring showing the desirable features are selected and crossed again — repeated over many generations, each round concentrating the alleles for the wanted feature.

Applications: crop plants bred for higher yield, disease resistance or drought tolerance; dairy cattle bred for milk yield; meat animals for growth rate and lean meat; dogs for behaviour or appearance. The feature must be heritable — selection only works on genetic variation.