Difference in characteristics between individuals from the same species .

Variation

ImportanceProvides raw material for evolution

Increase ability of species to survive in a changing environment (adaptation )

Types of Variation

Environmental factorsClimate e.g. light, temperature, humidityFood & nutrientWaterSoil condition

Stabilising selectionA type of natural selection in which genetic

diversity decreases as the population stabilies on a particular trait.

Favours individuals with intermediate phenotypes

Extreme phenotype are selected against

Operates most of the time

frequency of alleles which have deleterious effect on phenotype

Disruptive selectionA type of natural selection that favours

individuals at both extremes of a characteristic range

Favours both extreme

Intermediate phenotype is selected against

The disruptive selection pressure increases the chances of the advantageous alleles to be passed on to the next generation

After many generations formation of 2 separate gene pools & new species

Opposite of stabilising selection

Directional selection

Directional selection. This occurs when members of a population at one end of a spectrum are selected against, while those at the other end are selected for. For example, imagine a population of elephants with various sized trunks. In this particular environment, much more food is available in the very tall trees than in the shorter trees. Elephants with what length trunk will survive and reproduce the most successfully? Those with the longest trunks. Those with shorter trunks will be strongly selected against (and those in the middle will also be in the middle in terms of success). Over time we expect to see an increasing percentage of elephants with long trunks (how quickly this change occurs depends on the strength of selection—if all the short-trunked elephants die, we can imagine that the allele frequencies will change very quickly) (See Figure 12.1.)

• Snail shell color is an example of disruptive selection. Imagine an environment in which snails with very dark shells and those with very light shells are best able to hide from predators. Those with an in-between shell color are gulped up like escargot at a cocktail party, creating the double-hump curve seen in Figure 12.1.