Evolution and speciation definitions

Natural selection: variation, selection and allele frequency

Mutation

A change in the base sequence of DNA. Mutation creates new alleles.

Genetic variation

Differences between individuals in a population that are caused by differences in their alleles, and can therefore be inherited.

Environmental variation

Differences in phenotype between organisms caused by the conditions they grew in rather than by their alleles, and therefore not passed to offspring.

Gene pool

All the alleles of all the genes in a population at a given time.

Selection pressure

An environmental factor that causes individuals with certain phenotypes to survive and reproduce more successfully than others.

Fitness

The contribution an individual makes to the gene pool of the next generation, relative to other individuals in the population, usually counted as offspring that themselves survive to reproduce.

Adaptation

A characteristic that increases an organism's chance of surviving and reproducing in its environment. Adaptations may be anatomical, physiological or behavioural.

Natural selection

The process by which individuals with phenotypes better suited to the environment reproduce more successfully, so the alleles responsible become more frequent.

Stabilising selection

Selection that favours individuals near the mean and acts against both extremes, so the mean is unchanged and the variation falls.

Directional selection

Selection that favours individuals at one extreme of the range, so the mean of the population shifts in that direction over generations.

Disruptive selection

Selection that favours both extremes and acts against values near the mean, so the variation increases and the distribution can split into two peaks.

Evidence for evolution, resistance and genetic drift

Antibiotic

A substance produced by a microorganism, or a synthetic derivative of one, that kills or inhibits the growth of bacteria.

Antibiotic resistance

The ability of a bacterium to survive and reproduce in a concentration of an antibiotic that would kill or inhibit others of its species.

Vertical gene transfer

Passing genes from a parent cell to its daughter cells by binary fission.

Horizontal gene transfer

Passing genes between bacteria that are not parent and offspring, most often on a plasmid transferred by conjugation.

Bactericidal antibiotic

An antibiotic that kills bacteria, so the number of living cells falls while it is present. Penicillin is an example.

Bacteriostatic antibiotic

An antibiotic that stops bacteria growing and dividing without killing them, leaving the immune system to remove them. Tetracycline is an example.

Hospital-acquired infection

An infection that a patient acquires while in hospital, which was not present or incubating on admission. MRSA and Clostridioides difficile are examples.

Infection prevention and control

The hospital practices that stop pathogens spreading between patients, such as hand hygiene, screening, isolation and cleaning.

Genetic drift

A change in allele frequencies from one generation to the next caused by chance sampling of gametes, rather than by selection. Its effect is greatest in small populations.

Genetic bottleneck

A sharp reduction in population size that leaves the survivors carrying only a fraction of the original genetic diversity.

Founder effect

The reduced and unrepresentative genetic diversity of a population established by a small number of individuals separated from a larger population.

Zone of inhibition

The clear area round a disc on a lawn of bacteria, where a substance diffusing from the disc has stopped the bacteria growing.

Antibiotic disc

A small disc of absorbent paper carrying a fixed amount of one antibiotic, laid on a lawn of bacteria to test whether that antibiotic stops them growing.

Speciation and reproductive isolation

Species

A group of organisms with similar characteristics that can interbreed to produce fertile offspring, and which are reproductively isolated from other such groups.

Gene flow

The movement of alleles between populations, by migration of individuals or of gametes, which keeps their gene pools similar.

Reproductive isolation

The state in which two populations cannot exchange alleles because of features of the organisms themselves, such as courtship, breeding time or place, gamete compatibility or hybrid fertility, so they stay separate even where they meet.

Speciation

The formation of a new species from an existing one, in which gene flow between populations is reduced or stopped, their gene pools diverge, and they become reproductively isolated.

Allopatric speciation

The formation of new species from populations separated by a geographical barrier, which stops gene flow while their allele frequencies diverge.

Sympatric speciation

The formation of new species from populations living in the same area, where gene flow is reduced by ecological or behavioural separation, or by polyploidy.

Continuous and discontinuous variation

Continuous variation

Variation in which the individuals of a population differ by small amounts over an unbroken range, with any value between the extremes possible; height is the standard example.

Discontinuous variation

Variation in which the individuals of a population fall into a small number of distinct categories with no intermediates; ABO blood group is the standard example.

Polygenes

A group of different genes, each with a small additive effect, that together control one characteristic and give it a continuous distribution.

Phenotype

The observable characteristics of an organism, produced by its genotype developing under a particular set of environmental conditions.

Artificial selection and selective breeding

Artificial selection

The breeding of organisms chosen by people for characteristics they want, so that the alleles for those characteristics become more common in the next generation. Also called selective breeding.

Progeny testing

Judging a breeding animal by measuring the performance of its offspring, used where the characteristic cannot be measured in the animal itself.

Inbreeding depression

The reduction in vigour, fertility and disease resistance that follows repeated breeding between close relatives, caused by harmful recessive alleles becoming homozygous.

Hybrid vigour

The greater size, yield or vigour of the offspring of a cross between two inbred lines, compared with either parent line.