Variation and Selection

Variation: differences between individuals of the same species

Variation means the differences between individuals of the same species. Look at any group of people and you see it at once: they differ in height, in mass, in eye colour and in blood group. The same is true of a field of wheat or a litter of puppies.

Variation matters because it is the raw material of natural selection. If every individual of a species were identical, no individual could be better adapted than another, nothing could be selected for or against, and populations could never change over time. The whole of evolution depends on the fact that individuals differ.

• Variation is measured between individuals of the same species - comparing a human with a chimpanzee is classification, not variation.
• The only humans with identical genes are identical (monozygotic) twins, and even they differ because of their environment.
• Variation within a species is the starting point for every question on natural selection, evolution and selective breeding.

Continuous and discontinuous variation

Continuous variation results in a range of phenotypes between two extremes. There are no distinct categories, only measurements. Height and body mass are the standard examples: you cannot sort a class into 'tall' and 'short', you can only measure each person, and most values cluster around the middle.

Discontinuous variation results in a limited number of phenotypes with no intermediates. Each individual falls into one clear group. ABO blood groups, seed shape in peas and seed colour in peas are the syllabus examples: a person is group A, B, AB or O, and there is nothing in between.

The two types also differ in their cause. Discontinuous variation is usually caused by genes only, because it depends on a small number of alleles. Continuous variation is caused by both genes and the environment, so two people with similar genes can still end up different heights if they have different diets.

The causes of variation: genes and the environment

Genetic variation comes from four sources. Mutation creates brand-new alleles. Meiosis shuffles the existing alleles into new combinations when gametes are formed. Random mating means any two individuals may breed, so many combinations of alleles meet. Random fertilisation means any one of the many gametes may fuse, so which alleles end up together is a matter of chance.

Environmental variation comes from the conditions an organism lives in. Diet, exercise, light and temperature all change how an organism develops. A plant grown in a shaded corner stays short and pale even though its genes are identical to a plant grown in full sun; a person who trains hard builds more muscle mass than one who does not.

This is why the two types of variation have different causes. Discontinuous features such as blood group are fixed by genes alone and cannot be changed by the environment. Continuous features such as mass are pulled up or down by both, so identical twins can end up with different masses as adults.

Mutation: the source of new alleles

A mutation is a genetic change. In its most important form, a gene mutation is a random change in the base sequence of DNA. Because the base sequence is the instruction for a protein, changing it can change the protein the gene codes for, and so change the phenotype.

Mutation is the only way new alleles are formed. Meiosis, random mating and random fertilisation only reshuffle alleles that already exist; if a population is to gain a genuinely new version of a gene, a mutation must create it. This is why mutation is the ultimate source of all genetic variation.

Mutations occur naturally at a low rate, but ionising radiation (such as X-rays and gamma rays) and some chemicals (called mutagens) increase the rate at which they happen. Most mutations are neutral or harmful, but very occasionally one gives its owner an advantage - and that is the one natural selection can act on.

Natural selection: the five steps

Natural selection is the process by which the best-adapted individuals survive and reproduce, passing on their advantageous alleles, so that over many generations the allele frequencies in the population change. Cambridge lists the process as five steps, and you should be able to write them in order.

The key idea to hold on to is that selection acts on individuals but its effect is seen in the population. No individual evolves during its own lifetime; what changes is the proportion of alleles in the next generation.

• 1. Genetic variation within populations - individuals differ because of mutation and the shuffling of alleles during meiosis and fertilisation.
• 2. Production of many offspring - organisms produce far more young than can possibly survive.
• 3. Struggle for survival, including competition for resources - food, water, space and mates are limited, so most offspring die.
• 4. A greater chance of reproduction by individuals that are better adapted to the environment than others - the survivors pass their alleles on.
• 5. These individuals pass on their alleles to the next generation, so the advantageous alleles become more common in the population over many generations.

Adaptation, evolution and natural selection in action

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Notice that adaptation happens to a population, not to an individual.

Over many generations, natural selection changes the allele frequencies in a population so much that the population becomes a new species: it becomes so different from the original population that its members can no longer interbreed with them to produce fertile offspring. This is evolution, and natural selection is the mechanism that drives it.

You do not have to wait millions of years to see natural selection. It happens now, and two examples are on the syllabus. When a patient takes an antibiotic, bacteria with a mutation that makes them resistant survive, reproduce and pass the resistance allele on, so the resistant strain becomes common. The same thing happens when a pesticide is sprayed on insects: the few insects that are naturally resistant survive, breed, and their offspring inherit the resistance, so the pesticide stops working.

Selective breeding and genetic engineering

Selective breeding, also called artificial selection, is the process by which humans choose individuals with desirable characteristics to breed. It follows three steps: humans select individuals with the desirable features, cross them to produce the next generation, and then select the offspring that show the desirable features. Repeating this over many generations improves crop plants and domesticated animals.

The benefits are clear: higher yield, resistance to disease, and better quality - a wheat variety that produces more grain, a cow that gives more milk, a chicken that grows faster. The risks matter just as much. Because only a few individuals are bred from, genetic variation in the population is reduced, and closely related animals may be bred together (inbreeding). A population with little genetic variation is more vulnerable: if a new disease appears, there may be no resistant individuals left to survive it.

Genetic engineering is an alternative. Instead of waiting for the right allele to appear by chance, a gene for the desired feature is taken from one organism and inserted into another, so the feature appears in a single generation. It is faster and more precise than selective breeding, but it raises its own ethical questions.

The main difference from natural selection is the selecting agent. In natural selection the environment decides which individuals survive and reproduce, and it takes many generations. In artificial selection humans decide, choosing the features that benefit us rather than the features that suit the organism's environment.