(2) WJEC A2 Biology 4.4 Competition for breeding success and survival

Intraspecific competition

Intraspecific competition

Between members of the same species

Interspecific competition

Between members of different species

Selective agent/selection pressure

An environmental factor that, when it is a limiting factor, can alter the frequency of alleles in a population.

Selection pressure: overcrowding

Allows diseases to spread

Selection pressure: predation

Population grows = more food for predators therefore their numbers increase also. Some individuals in the prey population are more likely to survive thus more likely to reproduce, passing on the alleles that made them successful like:

• Some are better camouflaged

• Mimic species → have characteristics of other species that ARE toxic but are actually harmless themselves!

Selection pressure: availability of nesting sites

Some animals raise young in very specific locations e.g crows’ nests between 10m and 20m high.

Selection pressure: day length

For example, male rabbit testes begin to enlarge in November in response to the shorter days. Results in most female rabbit pregnancies being in May.

Selection pressure: temperature

For example

• Mosquito larvae → best survive in 24-28 degrees C water

• Wolverines can only reproduce if heavy snowfall (very very cold). They dig their dens under the snow.

Selection pressure: human impact (duh!)

Habitat loss destroys breeding grounds eg cyanide fishing (sodium cyanide squirted into water for easier catch of fish HOWEVER this kills coral and algae!).

What does the value of an allele depend on?

The environment:

• Phenotype is determined, in part, by the genotype, yes.

• However, consider a small number of rabbits are homozygous for a recessive allele for a white coat.

• In rabbits’ usual habitats, this would mean more likely to be caught and killed by a predator thus chances of it reproducing and passing on its white fur allele are very small = the allele remains rare in population.

• However, for an arctic animal, white fur WOULD be advantageous due to camouflage with the snow! Animal more likely to survive thus pass on alleles. Therefore in this environment the white fur allele is more common.