Genetic Variation, Natural Selection and Selective Breeding

Genetic differences within a population

A population is a group of organisms of the same species living in the same area. There is usually extensive genetic variation within a population: its individuals have differences in their DNA. They are not all genetically identical, even though they belong to the same species.

A gene is a section of DNA, and alternative versions of a gene are called alleles. Differences in alleles can help explain why individuals have different characteristics. The observable characteristics of an organism make up its phenotype. Phenotype depends on both genetic information and the environment: a plant’s growth, for example, depends on its inherited characteristics as well as the light and water available to it.

Mutations create genetic variants

A mutation is a change in the DNA base sequence. Mutations create new genetic variants, some of which can be inherited by offspring.

Most genetic variants have no effect on phenotype, some influence phenotype, and a very few determine phenotype. A DNA change does not necessarily produce a noticeable difference: it may leave the resulting protein unchanged, or change the protein without affecting its function. Other changes affect protein function enough to alter a characteristic.

A variant that influences a characteristic contributes to it alongside other genetic or environmental factors. A variant that determines a characteristic has a decisive effect. These distinctions matter because natural selection acts on differences in phenotype that affect survival and reproduction, rather than automatically favouring every new DNA variant.

Competition makes differences matter

Organisms often produce more offspring than their environment can support. Food, water, space and other resources are limited, so individuals compete for them.

A community contains the populations of different species living in an area. Competition can occur both within a species and between species. For example, plants in a community compete for light, water and mineral ions. Animals may compete for food and territory; individuals of the same species may also compete for mates.

Individuals do not all have an equal chance of surviving and reproducing. An inherited characteristic that helps an organism obtain a limited resource can give it an advantage. Which characteristics are advantageous depends on the environment.

How natural selection changes a population

Natural selection occurs when individuals with inherited phenotypes better suited to their environment are more likely to survive and reproduce. They are therefore more likely to pass the alleles responsible for those characteristics to their offspring.

Suppose a population of plants contains inherited variation in drought tolerance. During a prolonged dry period, water becomes scarce. Plants whose characteristics allow them to survive with less water are more likely to live long enough to reproduce. Their offspring can inherit the alleles contributing to drought tolerance. Over many generations in these conditions, those alleles can become more common in the population.

The dry conditions do not create a mutation specifically because the plants need it. Instead, the conditions favour plants with advantageous inherited variation. If conditions change, a different phenotype may become advantageous.

Survival is only part of the explanation. A characteristic spreads through natural selection when it increases an organism’s chance of leaving offspring that inherit it. This is why reproduction and inheritance are essential links in the process.

Evolution happens across generations

Evolution is a change in the inherited characteristics of a population over a number of generations. Through natural selection, advantageous alleles become more common, so the population’s characteristics gradually change. An individual does not evolve during its lifetime; the change occurs across generations of a population.

Over time, populations of one species may become so different that they can no longer interbreed to produce fertile offspring. They have then become separate species. The formation of new species is called speciation; it is a possible outcome of evolution, not the outcome of every evolutionary change.

Charles Darwin and Alfred Russel Wallace independently developed explanations of evolution by natural selection. Their ideas were presented together in 1858, and Darwin published On the Origin of Species in 1859. The modern explanation connects their ideas about variation and competition with our understanding of mutations, genes and inheritance.

Selective breeding puts humans in charge of selection

In selective breeding, also called artificial selection, humans choose which organisms reproduce because they have desirable inherited characteristics. The basic process is to:

1. Choose parents with the desired characteristics and breed them together.
2. Select offspring that show those characteristics.
3. Breed the selected offspring and repeat the selection over many generations.

Not all offspring have the desired combination of characteristics, so selection must continue. Both natural selection and selective breeding change inherited characteristics over generations. The difference is what determines reproductive success: environmental conditions in natural selection, and human choices in selective breeding.

Benefits for food production and domesticated animals

Food plants can be selectively bred for increased yield, disease resistance, drought tolerance or better-tasting fruit. Higher yields provide more food from a crop, while disease resistance and drought tolerance can reduce losses. Selective breeding of wild brassica has produced familiar vegetables including cabbage, cauliflower, broccoli and Brussels sprouts.

Domesticated animals can be bred for characteristics useful to people. Examples include cows that produce more milk, animals that produce more meat, chickens that lay larger eggs and sheep with good-quality wool. Dogs can be bred for a gentle temperament. Selective breeding has produced many different domestic dog breeds from wolf ancestors.

Why selective breeding can cause problems

Repeatedly breeding only a small number of selected individuals can reduce genetic variation: fewer different alleles remain in the breeding population. It can also lead to inbreeding, where closely related organisms breed together.

Inbreeding increases the risk of offspring inheriting harmful genetic defects. Reduced genetic variation also leaves a population more vulnerable to new diseases because there is less chance that some individuals carry alleles giving resistance. Choosing several suitable, less closely related parents can help reduce the risk of inbreeding.

Selective breeding therefore brings valuable improvements, but concentrating on desirable characteristics can also reduce the genetic diversity needed to cope with future environmental changes.

Variation and mutation

• Populations usually contain extensive genetic variation.
• Mutations create genetic variants: most have no effect on phenotype, some influence it, and a very few determine it.
• Phenotype depends on genes and environmental interactions.

Natural selection

Inherited variation → competition for limited resources → better-adapted individuals more likely to survive and reproduce → advantageous alleles passed on → alleles become more common over generations.