April 2015

Where's the evolution?

In fact, most species never do stop reproducing. Females of only three species — humans, killer whales, and short-finned pilot whales — spend a significant portion of their lives in menopause, a time when they are healthy, but cannot produce offspring. Most species breed and reproduce for the vast majority of their lives. The existence of menopause is surprising from an evolutionary point of view because natural selection works by favoring traits that get more offspring into the next generation, so gene versions that allow females to continue to reproduce until they are near death should be favored. How then would a trait that either cuts reproduction short or extends life spans beyond the limits of reproduction evolve?

Biologists hypothesize that, in certain species, menopause might be favored by increasing the reproductive success of close relatives, through the concept of inclusive fitness. In evolutionary terms, fitness is simply a measure of how successful an individual with a particular genotype or gene version is at reproducing; the more offspring an individual leaves behind, the higher its fitness. Inclusive fitness expands on this basic idea and suggests that getting offspring into the next generation isn’t the only thing that matters for natural selection: the reproductive success of any individual who carries the same gene versions that you do (i.e., your offspring and close relatives) also matters.

Inclusive fitness helps explain how traits like altruism evolve. Altruistic acts — such as making a loud call that warns others to run and hide from a predator but that also attracts the attention of the predator — tend to decrease the actor’s chances of surviving and reproducing while increasing other individuals’ chances. Normally, we would expect natural selection to weed such traits out of a population, since the individuals with the gene versions that predispose towards altruistic behavior don’t get to pass on those genes as often as selfish individuals get to pass on theirs. However, if the altruistic acts end up benefitting close relatives — individuals who carry many of the same gene versions as the actor, including the altruistic genes — then these gene versions may be favored by natural selection and spread. So for example, the death of a vervet monkey that spots a leopard, generates an alarm call, and then gets eaten by the leopard may save several relatives who also carry the genes that predispose them to sound the alarm.

The same process may help explain the existence of menopause in some species. If a non-breeding female is able to boost the survival and reproduction of her offspring (or grandchildren, siblings, or cousins), genes that cut her reproduction period short or extend her lifespan past reproductive age will be favored if the benefits to relatives outweigh the fitness cost of not reproducing oneself. Scientists hypothesize that menopause may be particularly likely to evolve in species where offspring do not disperse, but stay with their family units most of their lives. This makes it easier for the actions of a non-reproducing individual to benefit his or her family members. Resident killer whales generally stay in their family pods for their entire lives, and now it is clear that the matriarchs of these pods are able to help out their younger relatives with their decades of hunting experience.

In humans, menopause might be explained by a similar mechanism, known as the grandmother hypothesis. The idea is that humans experience menopause because, later in life, it can be advantageous to focus on improving the survival and reproduction of one’s existing offspring (and one’s grandchildren) instead of on producing new offspring. In support of this hypothesis, some studies have found that when grandmothers live longer (and so are able to provide resources like extra childcare and extra income), her children reproduce more and her grandchildren are more likely to survive. It may not be the free salmon dinners that killer whale grandmothers provide, but you can bet that human parents are grateful for the help!