Variation and natural selection (CCEA)

This is a gradual change in a characteristic across a population, it could be human height, mass or shoe size.

Continuous variation is represented as a histogram (no gaps between bars).

A histogram should show normal distribution, with most individuals around the average value and a few at the extremes.

A characteristic of any species with only a limited number of possible values shows discontinuous variation. For example:

• gender - male or female

• blood group - A, B, AB or O

• tongue rolling – can or can’t roll your tongue

• hand dominance – left, right or both

There are no values in between (intermediate values), so this shows discontinuous variation.

Discontinuous variation is represented as a bar chart.

Caused by changes to chromosomes or genes (DNA).

Mutations are random changes in:

• number of chromosomes (eg Down's syndrome)
• gene structure (eg cystic fibrosis)

Sexual reproduction:

• independent assortment during meiosis creates genetically different gametes.
• random fusion of gametes leads to different phenotypes in a population.

Environmental basis:

• an individual’s environment can also cause variation.
• for example, human height is genetically determined but affected by diet and health.

Living organisms are adapted to their environments in order to survive.

Competition makes these adaptations for survival very important. Only the best adapted will survive.

The theory of natural selection

• There is variation among the phenotypes of individuals in a population.

• Organisms compete for resources (for food, mates, habitat etc).

• The best-adapted individuals survive, and the less well-adapted individuals do not survive – this is differential survival.

• The surviving organism with advantageous phenotypes reproduce.

• They pass on their beneficial genes to the next generation.

There is variation within bacterial phenotypes. Some are resistant to antibiotics and some are not.

Treating bacteria with antibiotics provides competition.

The resistant individuals survive and the non-resistant individuals do not survive – this is differential survival.

The resistant bacteria are able to reproduce.

The resistant bacteria pass on the resistance gene to the next generation.

Number of resistant bacteria increase and non-resistant bacteria decrease.

The gene for antibiotic resistance is usually caused by a beneficial mutation in the bacterial cells.

Charles Darwin used the theory of natural selection to explain the process of evolution.

Evolution is a continuing process of natural selection that leads to the gradual change of an organism over time.

It may result in the formation of a new species.

Fossils are the remains of living organisms that have been preserved in rock for millions of years.

They provide evidence for evolution by showing how an organism looked millions of years ago and how it has changed over time.

Species are extinct if there are no living individuals left.

Extinction happens if a species fails to adapt to changes in its environment.

Fossils have provided us with information on some extinct species.

Species that are at risk of becoming extinct are known as endangered.

A species can become extinct for many reasons, including:

• hunted by humans

• disease

• loss of habitat (eg deforestation)

Extinction can be avoided by:

• laws preventing the hunting of endangered species

• special programmes such as creating nature reserves to protect habitats

• education that encourages people to do their part in protecting the environment

Natural selection can be manipulated by selecting desirable characteristics in crop plants or domesticated animals that are of use to humans.

This is known as selective breeding or artificial selection.

Humans select individual plants or animals with desirable characteristics (such as increased crop yield, disease resistance and longer shelf life). These are then bred to produce offspring.

Repeated selection and breeding over many generations results in all offspring containing the desirable characteristic.

Wheat has been bred over many years to produce the following desirable characteristics:

• a shorter stalk length less likely to suffer wind damage
• a uniform size that’s easier to harvest
• a larger yield