U4 AOS2 Topic 2: Selection Pressures

A factor that can select only those Individuals that is able to adapt and survive in a specific environment. Environmental selection pressure involves:

·        Climate change

·        Disease

·        Competition

·        Predation

The individuals that can overcome these environmental selection pressures can be naturally selected, and their alleles can be passed on to the next generation and therefore the allele frequency of these alleles can be increased. These individuals are said to be “genetically fit” because they show advantageous phenotypes coded by the naturally selected alleles.

Conditions that facilitate natural selection:

1.   Variation: individuals of the same species in a population can vary genetically and show different phenotypes. For example, black and white colored moths.

2.   Selection pressures: environmental selection pressure allows only genetically fit individuals to survive and reproduce in a population. For example, increased soot production by industries coated the trees black and caused white colored moths easily visible and attacked by birds. Soot production and birds act as selection pressures.

3.   Selective advantage: individuals with advantageous phenotypes are conferred as selective advantage. For example, black colored moths were able to camouflage within the soot covered trees and were protected from attacks of birds.

4.   Heritability: the advantageous alleles are heritable. For example, allele for black body color of moths was heritable and allele frequency of these alleles increased with time.

                                                                

Relation of selection pressure with genetic diversity:

Environmental selection pressure selects the advantageous alleles to pass on to the next generation increasing its allelic frequency in that population. Only fitter individuals survive and adapt to the new environment causing the genetic diversity to decrease. Small populations have lower genetic diversity compared to large populations and therefore are at the risk of extinction.  For example, when plants are exposed to limited light, the plants with larger leaves are naturally selected and are more common in a population and the allelic frequency of alleles with larger leaves increases causing the genetic diversity to decrease. Due to this decreased genetic diversity, the population will now struggle to adapt to new environmental pressure such as drought or limited water as plants with larger leaves are more susceptible to lose water and therefore are at greater risk of extinction.