May 09, 2009

Gender differences in reproductive success (Brown et al. 2009)

This is a wonderful paper which gathers data to address the issue of how sexual selection operates in men and women. Bateman's principles as presented here are:
  1. greater mating variance in men than women;
  2. greater reproductive variance in men than women;
  3. correlation between mating and reproductive success.

Each man and woman has a certain number of lifetimes sexual mates and a certain number of offspring. While women are more similar to each other, with relatively fewer having too few (or too many) partners/offspring compared to the average, men are more variable, with a few of them having no or many offspring/partners.

The authors bring up the interesting point that greater male variance does not -in itself- substantiate sexual selection as it is often assumed. This is because variance can be either due to selection or to random genetic drift.

A good way to see this (not found in the paper), is to imagine the same set of people living their lives either (a) in the peaceful countryside, or (b) in a big city during a series of air raids. In case (b) variance will be greater, as those killed or maimed by the raids will not mate or reproduce, and the survivors will, whereas in case (a) everyone will have the same a priori opportunities.

So, if everyone has the same number of offspring as everyone else does imply a lack of sexual selection; but, variability in reproductive success does not in itself imply selection. Only when mating and reproductive success (Bateman's third rule) are correlated do we have a good case for sexual selection.

The authors collect data on the male- and female- specific variance in mating and reproductive success, although they note a dearth of data in favor of the third principle. One can't disagree with their call for the collection of relevant data to investigate whether the three principles apply in humans, nor with their observation that what is applicable to fruit flies (the subject of Bateman's original research) does not necessarily apply to humans, and certainly not to all societies (*)

(*) An interesting observation from the paper is that although monogamous societies are a minority of human societies, they tend to encompass the largest number of people. Moreover, in about half of nominally polygamous societies, in practice monogamy is practiced by the great majority of the population.

Below is Table 1 from the paper.


Not related to the subject of this paper, but this gives us the opportunity to examine realistic demographic parameters in simulations such as these, where an assumption of Poisson distributed number of offspring (with mean m) is used. In the Poisson distribution, the variance is also m. As the table above shows, the variance is almost equal to the mean in some populations (e.g., USA), but quite different in others (e.g., 19th c. Sweden); indeed the latter seems more common.

Departure from the Poisson assumption in the direction of greater reproductive variance is entirely consistent with my observations in the above-linked post on the importance of reproductive inequality.

Trends in Ecology & Evolution doi:10.1016/j.tree.2009.02.005

Bateman’s principles and human sex roles

Gillian R. Brown et al.

Abstract

In 1948, Angus J. Bateman reported a stronger relationship between mating and reproductive success in male fruit flies compared with females, and concluded that selection should universally favour ‘an undiscriminating eagerness in the males and a discriminating passivity in the females’ to obtain mates. The conventional view of promiscuous, undiscriminating males and coy, choosy females has also been applied to our own species. Here, we challenge the view that evolutionary theory prescribes stereotyped sex roles in human beings, firstly by reviewing Bateman's principles and recent sexual selection theory and, secondly, by examining data on mating behaviour and reproductive success in current and historic human populations. We argue that human mating strategies are unlikely to conform to a single universal pattern.

Link

2 comments:

Maju said...

One can't disagree with their call for the collection of relevant data to investigate whether the three principles apply in humans, nor with their observation that what is applicable to fruit flies (the subject of Bateman's original research) does not necessarily apply to humans, and certainly not to all societies.

Very true.

Importantly it would be decissive to probe if sexual selection (apparently somewhat stronger among males than females) is effectively inherited or not and, if it is, how strongly. If a particular man is very succesful reproductively, this may or not affect his offspring and the next generation would get a different Don Juan from a completely different lineage, balancing the effect of sexually-biased drift in the long run to a great extent.

This issue is crucial when debating hyped single-founder lineages as the alleged Chingis Khan lineage but also when looking at the whole Y-DNA patterns in humankind. If Chingis Khan was an exceptionally succesful father, he would still need his sons, grandsons, etc. to be similarly succesful in terms of reproductive success in order for his Y-DNA lineage to be actually so widespread. This IMO is not likely to happen, at least not with enough force as to cause such alleged massive founder effects in an already crowded post-Neolithic World.

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