Correct option (a) Both A and R are true and R is the correct explanation of A
Explanation :
Process of ageing is called senescence. Ageing, is cumulative changes in an organism, organ, tissue, of cell leading to a decrease in functional capacity. In humans. ageing is associated with degenerative changes in the skin, bones, heart, blood vessels, lungs, nerves, and other organs and tissues.
It is genetically controlled programme regulated by regulatory genes. This view is suggested by the great range of life spans among different animal species - form a few days in the fruit fly to more than 100 years in some tortoises. Scientists have recently learned how to double the lifespans of such laboratory organisms as roundworms and fruit flies through genetic manipulation, and mutant genes in mice have been observed to have a comparable effect in postponing ageing.
Genes which have a deleterious effect on individual's fitness are selected against by natural selection, Mutations in these genes which postponed the deleterioous effect of the gene to a later time in individual's life history reduce the effect of natural selection to the gene, because the selection has less time to act on it . If the gene doesn't have a negative effect until after the individual has reproduced, the gene may escape natural selection altogether, because when selection starts to affect the gene, it has already propagated to the next generation. In a related fashion, some genes may help to ensure reproduction fashion, some genes may help to ensure related fashion, some genes may help to ensure reproduction at the cost of diseases later in life. For instance, our immune system generally helps to keep us healthy during reproductive age, but many adults acquire autoimmune diseases such as rheumatiod arthritis as they age.
