Question

10. What is directional, stabilizing and disruptive selection and what roles do they play in adaptation and speciation? 

11. What is genetic drift? What role does it play in moving a population from one adaptive peak to a more fit adaptive peak?

Answer 1

Stabilizing selection is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation. Examples of this include the evolution of industrial melanism in moths. Disruptive selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection. Therefore these types of natural selection influence the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes displayed can become more similar or more disparate. So an adaptation of the phenotype suited for that particular environment occurs leading to speciation.

Although Genetic drift is a mechanism that does lead to evolution yet it cannot induce adaptations in a species. Adaption is defined as, "a change or the process of change by which an organism or species becomes better suited to its environment." Genetic drift, on the other hand, refers to the variation of alleles in a gene pool over time and is based solely on random chance events. If this change were to be considered an adaptation it could not be considered a result of genetic drift alone as the fact that this mutation made the organism better suited for its environment means that there would have been some sort of selective pressure. Essentially, if it can be considered an adaptation it means that the loss of the less desirable allele could not happen out of chance alone as some of these alleles would be removed via natural selection as well. Genetic drift can definitely move a species from one adaptive peak to another by chance events due to the accumulation of random mutations in the genes.