Question

(a) Why natural selection would favor females who become infertile to live many years before death? Any fitness benefit? (b) How could the existence of grandma possibly change life history traits of human beings? (c) How grandma obtained longevity genes and passed on to the next generation? (d) Please list all the fitness benefits from the existence of grandma:

Answer 1

a) Stopping reproduction and the age at which a woman last gives birth have important implications, both theoretical and empirical, for a variety of academic fields. For human reproductive biology and a medical perspective, the proximate constraints on fertility and the probability of conception are of huge social relevance as women try to conceive ‘late’ in life and experience fertility problems . The age at last pregnancy also has important health consequences for both the mother and developing fetus, as such pregnancies are correlated with higher risks. From an evolutionary perspective, the issues go beyond the questions of menopause and the post-reproductive lifespan, and include implications for population growth and evolutionary models of fitness and generation times.

b) The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking. It builds on the previously postulated "mother hypothesis" which states that as mothers age, the costs of reproducing become greater, and energy devoted to those activities would be better spent helping her offspring in their reproductive efforts. It suggests that by redirecting their energy onto those of their offspring, grandmothers can better ensure the survival of their genes through younger generations. By providing sustenance and support to their kin, grandmothers not only ensure that their genetic interests are met, but they also enhance their social networks which could translate into better immediate resource acquisition. This effect could extend past kin into larger community networks and benefit wider group fitness.

c) According to the ‘grandmother hypothesis’, post-menopausal women can increase their genetic contribution to future generations by increasing the survivorship of their grandchildren. While some demographic studies have found evidence for this, others have found little support for it. Here, we re-model the predictions of the grandmother hypothesis by examining the genetic relatedness between grandmothers and grandchildren. We use this new model to re-evaluate the grandmother effect in seven previously studied human populations. Boys and girls differ in the per cent of genes they share with maternal versus paternal grandmothers because of differences in X-chromosome inheritance. Here, we demonstrate a relationship between X-chromosome inheritance and grandchild mortality in the presence of a grandmother. With this sex-specific and X-chromosome approach to interpreting mortality rates, we provide a new perspective on the prevailing theory for the evolution of human female longevity. This approach yields more consistent support for the grandmother hypothesis, and has implications for the study of human evolution.

d)