Evolution and inheritance, Year 11: variation, natural selection and selective breeding

Mutations

提示They are changes in DNA, and they happen continuously.

为什么Every variant that exists in a population began as a mutation, a change in the DNA, in some individual in the past. Sexual reproduction then shuffles the existing variants into new combinations.

no

提示Think how rarely one mistyped letter in a long book changes its meaning.

为什么Of all mutations, most do not change the phenotype at all, some influence it, and very few determine it. Only very rarely does a mutation lead to a new phenotype.

A great deal — genetic variation within a population is usually extensive

提示Looking alike from a distance is not the same as having identical DNA.

为什么Individuals in a population differ in many of their alleles, even when they look alike, because mutations have been building up for a very long time. This store of variation is what natural selection acts on.

A relatively rapid change in the species

提示Natural selection would favour the new form, and quickly.

为什么Individuals with the new phenotype are better suited to the changed conditions, so they are more likely to survive and breed. The new variant spreads through the population in few generations.

It depends on the genome interacting with the environment

提示The DNA is the same in both plants, so look at what differs around them.

为什么The genes give the plant the ability to make flower pigment, and the soil decides which colour appears. Many characteristics come about this way, through genes and conditions acting together.

Evolution

提示Darwin's theory explains how it happens.

为什么Evolution is change in a population across many generations, and it concerns inherited characteristics only. It happens by natural selection and may result in the formation of a new species.

More than three billion years ago

提示The answer is counted in thousands of millions, and the Earth is about 4.6 of those units old.

为什么The theory states that all species of living things have evolved from simple life forms that first developed more than three billion years ago. That enormous length of time is what allows small changes to add up to very large ones.

No — they cannot produce fertile offspring together

提示The test of a species is about the grandchildren, not the children.

为什么Members of one species can interbreed to produce fertile offspring. When two populations have become so different that they can no longer do this, they have formed two species. Being able to mate is not enough.

Individuals with it are more likely to survive and breed, passing it on

提示Ask who gets to leave the most offspring.

为什么Variants give rise to different phenotypes. Individuals whose phenotype best suits the environment are more likely to survive and reproduce, so their alleles are passed to more offspring. Repeated over generations, the allele spreads.

simple

提示The earliest living things were probably single cells.

为什么Every species alive today, however complex, is descended from simple life forms that developed more than three billion years ago. All living things are therefore related, some closely and some very distantly.

Selective breeding

提示It is also called artificial selection.

为什么People have done this for thousands of years, since they first grew food crops from wild plants and domesticated animals. Characteristics are chosen for usefulness or appearance, such as disease resistance in crops or large flowers.

Choose parents with the desired characteristic (high milk yield) from a mixed population and breed them together

提示Start by looking for the best individuals already in the herd.

为什么The method is: choose parents with the desired characteristic, breed them, pick the offspring that show the characteristic most strongly, and breed those. Animals that produce more milk or meat are one of the examples named at GCSE.

generations

提示Parents, then their young, then the young of those, and so on.

为什么One round of breeding is not enough, because the offspring still vary. Each time, only those with the desired characteristic are bred, so the characteristic becomes more common until every individual has it.

They are particularly prone to disease or inherited defects

提示Think of the health troubles of some pedigree dogs.

为什么Breeding closely related individuals again and again reduces the variety of alleles in the breed. Harmful recessive alleles are then more likely to come together in the same animal.

People choose

提示That is why its other name is artificial selection.

为什么Both processes work on the variation already present in a population, and both change it over many generations. The difference is what does the selecting: survival in the environment, or human choice for usefulness or appearance.