Similarities and differences between organisms may be defined in terms of variation in DNA. Differences in DNA lead to genetic diversity. The influence of the following on genetic diversity • selection for high-yielding breeds of domesticated animals and strains of plants • the founder effect • genetic bottlenecks. Candidates should be able to discuss ethical issues involved in the selection of domesticated animals.

Genetic diversity is the level of variation between organisms, the more differences in DNA (nucleotides, genes of genomes) the larger genetic diversity. It can also be defined as the number of different allels present in a population.

Genetic variation is seen as a good thing because it reduces the inheritance of genetic diseases from recessive alleles. It also means that if a disaster occurred (e.g. a change of atmosphere or the introduction of a new predator) some of the species may be able to survive because of their differences.

Selective breeding
This is where humans choose animals or plants with desired characteristics and breed and nurture them. The selected ones will be more successful at reproducing as they are encouraged by the humans and therefore be a successful species.

A prime example is the breeding of dogs, which was occurring even in the Roman times when wolves with the ability to read human behaviour were selected to breed and keep as pets.

Another species that this can be seen in is the pig, where wild boar were captured and farmed. Characteristics such as being fat (more food) and having small legs (can't run away) meant the animals would be selected to have offspring to pass on the traits. Years of selective breeding have left us with the domestic pig, too fat and disproportionate to walk properly- it raises ethical issues about weather it is fair to manipulate the DNA of animals for domestic qualities as it does not benefit a species and can reduce their chances of survival in the wild.

The founder effect
When some of a population moves to start a new colony, only some of the alleles will be taken. This means that in the new community there a smaller variety of different DNA so genetic diversity is decreased. This can cause problems, for example in the Amish community, people suffer from many genetic disorders because they are more likely to have two parents with the same genotype who are both carriers.

Bottle necks
A similar thing can happen when the size of a population suddenly drops- some alleles may be completely lost (either by chance or because their characteristic was vulnerable), decreasing the genetic diversity. When cheetahs died in the ice age, the population sank to around just fifty, of the survivors a very high proportion had fertility issues, this meant the characteristic was passed on to a high proportion of the offspring and that even as the population began to expand again the percentage with fertility issues remained high.