Variation and evolution

Differences in the characteristics of individuals in a population is called variation, and it may be due to:

• the genes they have inherited — genetic causes • the conditions in which they have developed — environmental causes • a combination of genes and the environment.

The third is the commonest and the one students leave out. Height is a good example: your genes set a range, and diet and health decide where in that range you end up. The genome and its interaction with the environment together influence the development of the phenotype.

There is usually extensive genetic variation within a population of a species, and all variants arise from mutations. Of those mutations:

• most have no effect on the phenotype • some influence the phenotype • very few determine the phenotype.

Mutations occur continuously. Very rarely a mutation will lead to a new phenotype, and if that new phenotype is suited to an environmental change, it can lead to a relatively rapid change in the species.

Evolution is a change in the inherited characteristics of a population over time, through a process of natural selection, which may result in the formation of a new species.

The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

Explaining how evolution occurs is a four-step answer, and questions are marked on having all four:

1. There is variation within the population, caused by mutations. 2. Individuals with the phenotypes best suited to their environment are more likely to survive. 3. Those survivors are more likely to breed successfully and pass on the alleles for that phenotype. 4. Over many generations, the proportion of the population with that phenotype increases.

Write it in that order for any example you are given — antibiotic resistance, moth colour, beak shape — and the marks follow.

If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species. That is the definition of speciation, and the test in it is fertile offspring — not merely whether they can mate.

Selective breeding, or artificial selection, is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing it for thousands of years, since they first bred food crops from wild plants and domesticated animals.

The process is the same every time, and it is worth learning as four steps:

1. Choose parents with the desired characteristic from a mixed population. 2. Breed them together. 3. From the offspring, choose those with the desired characteristic and breed them together. 4. Repeat over many generations until all the offspring show the characteristic.

The characteristic can be chosen for usefulness or appearance: disease resistance in food crops, animals which produce more meat or milk, domestic dogs with a gentle nature, or large or unusual flowers.

The cost is inbreeding. Breeding closely related individuals over many generations reduces the variation in the population, so some breeds are particularly prone to disease or to inherited defects — and with little variation left, the whole breed can be vulnerable to the same problem at once.

Genetic engineering is a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic. Genes from the chromosomes of humans and other organisms can be 'cut out' and transferred to cells of other organisms.

Two examples are named. Plant crops have been genetically engineered to be resistant to diseases or to produce bigger, better fruits. Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.

An evaluation question wants you to weigh those against each other, and to notice that the answer differs by context: engineering a bacterium in a sealed vessel to make insulin worries almost nobody, while planting a herbicide-resistant crop across a landscape affects the wild plants and insects living in it. Same technique, very different balance.

Adult cell cloning has five steps, and the order is examined:

1. The nucleus is removed from an unfertilised egg cell. 2. The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell. 3. An electric shock stimulates the egg cell to divide to form an embryo. 4. These embryo cells contain the same genetic information as the adult skin cell. 5. When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

Cloning has benefits and risks in agriculture and in medicine, and some people have ethical objections. In favour: useful characteristics are preserved exactly, rare species can be saved from extinction, and cloned embryos may one day supply cells for treating disease. Against: a cloned population has almost no genetic variation, so a single disease could destroy all of it; adult cell cloning has a low success rate; and there are objections to cloning animals for human use, and stronger ones to any suggestion of cloning humans.