Question

Explain and contrast two explanations for the evolution of senescence: Antagonistic Pleiotropy Theory and Mutation/Selection Balance Theory. 12 points for explaining each theory and 6 points for contrasting (comparing) them.

Answer 1

ANSWER )

Antagonistic pleiotropy may be particularly relevant in plant viruses as an unavoidable consequence of their small genomes, which are highly compact, and encode multifunctional proteins.Antagonistic pleiotropy suggests that any mutations that confer manipulative ability to a virus in one host have the potential to confer maladaptive effects in a second host, with such effects potentially depending on the phylogenetic distance between host species.Antagonistic pleiotropy is the simplest and most intuitive one mutations that have a positive fitness effect on a given host are deleterious in an alternative one. According to AP theory, aging is a side effect of genes that are selected for their contribution to fertility and other essential components of individual fitness. In this standard view, aging exists because the benefits of enhanced fertility early in life are linked logically or physically to the long-term deterioration of the body, and evolution has been compelled to accept the latter as a cost of the former.  An important aspect of the AP model is that it is an adaptive theory of senescence, involving the action of positive selection on variants that increase survival or fertility at early ages. This immediately suggests using molecular evolution techniques to check for the signature of natural selection in genes and genomic regions involved in early–late antagonistic pleiotropies.

Mutation–selection balance is an equilibrium in the number of deleterious alleles in a population that occurs when the rate at which deleterious alleles are created by mutation equals the rate at which deleterious alleles are eliminated by selection.The majority of genetic mutations are neutral or deleterious; beneficial mutations are relatively rare. The resulting influx of deleterious mutations into a population over time is counteracted by negative selection, which acts to purge deleterious mutations. Setting aside other factors (e.g., balancing selection, and genetic drift), the equilibrium number of deleterious alleles is then determined by a balance between the deleterious mutation rate and the rate at which selection purges those mutations. The mutation-accumulation model explains the evolution of senescence by the accumulation of late-acting deleterious alleles. Any stable evolutionary state of a population in a static environment must involve a dynamic mutation–selection balance, where accumulation of deleterious mutations is on average offset by the influx of beneficial mutations.