Question

There are three modes of natural selection: stabilizing selection, directional selection and diversifying/ disruptive selection. Describe each mode and explain how each mode affects the evolution of a population.

Answer 1

Stablilzation selection :-Stablizing selection is one of the type of natural selection in which the mean population stabilizes on a particular non-extreme value .This is thought to be the most common mechanism of action for natural selection because of most of the traits donot appear to change drastically over time.Stabilizing selection commonly uses negative selection to select against extreme values of the character .It is opposite of disruptive selection which instead of favoring individuals with extreme phenotype ,it favors the intermediate variants.Stabilizing selection is based on phenotypes of a population .In phenotype based stabilization selection ,the mean value of phenotype is selected for resulting a decrease in the phenotypic variation found in a population.

Examples:-Stabilizing selection is the most common form of nonlinear selection in humans.There are few examples of genes with direct evidences of stabilizing selections in humans..However most quantitative traits are thought to be under stabilizing selection ,due to their polygenecity  and the distribution of the phenotypes throughout human population.

Directional selection:- Directional selection or positive selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes ,causing the allele frequency to shift over time in the direction of that phenotype.It was first described by Charles Darwin in the book On the Origin of species.Under directional selection , the advantages allele increases as a consequence of difference in survival and reproduction among different phenotypes.This increases are independent of the dominance of the allele and even if the allele is recessive ,it will eventually become fixed

Example:- An example of directional selection is fossil record that shows that the size of the black bears in Europe decreased during interglacial periods of the ages ,but increases during each glacial period.Another example is the beak size in the population of finches.

Disruptive / diversifying selection:-Disruptive selection is a type of natural selection that selects against the average individual in a population.The make up of this type of population would show phenotype of both extreme but have very few individuals in the middle .It is the rarest of the three types of natural selection and can lead to deviation in species line.It comes down to the individual in the group who survive the best ..They are the ones who have traitson the extreme ends of the spectrum.Disruptive selection occurs in times of change such as habitat change or change in resource availabilty.

Example:-

Color-In regards to camouflage ,color serves as a useful example in many different kinds of species because those individuals that can hide from predators the most will effectively live the longest.If an environment has extremes ,those who don't blend with the environment  will be eaten the most quickly.

Diversive selection occurs when extreme values of a trait are favored over the intermediate.This type of selection often drives speciation.The result of this type of selection is increased genetic varaiance as the population became more diverse.Stabilzing selection results in a decrease of a population's genetic varaiance .In directional selection occuurs when a single phenotype is favored ,causing the allele frequency to continuously shift in one direction ; so a population's genetic variance shifts towards a new phenotype when exposed to environmental changes .Thus how this three modes of evolution affects the evolution population.