Reply from Professor Gage:
I was delighted to receive the following letter from Professor Gage.  This was not the end of our correspondence but he made his points so well I thought you might be interested.  He has given me permission to post them without comment, as I am doing.  I think it fair to say that we may hear from him again some day but probably not soon.


Dear Linton,
Many thanks for your interest in our inbreeding paper. Unfortunately, and like too many people who have provided views, you obviously believe the work provides insight into human mating behaviour, and all the prejudices attached to that. It does not. Humans are but one of several million species on our planet that reproduce sexually, and the study was designed to investigate why polyandry occurs in the majority of these species (not necessarily Homo sapiens), when we know that it is costly for females.

The results show that polyandry can be beneficial, and selected for, when risks of genetic incompatibility between reproducing males and females are high. Risks of genetic incompatibility between males and females are especially high under inbreeding, and there is a huge body of work in the lab and field showing that inbreeding results in a reduction in reproductive success (including for humans), and that evolutionary selection has provided plenty of mechanisms to avoid it. We demonstrate that females possess such mechanisms here, and that to use them they must sample a diversity of males, and hence positive selection for female promiscuity in bottlenecked populations. Likewise, outbreeding or hybridization is also damaging to reproductive success, and selection has generated mechanisms to avoid this through pre- or post-mating isolation.

Mechanistically, deleterious recessive alleles are known to be a major driver of inbreeding depression, because they can be carried in the genome without harm, but become expressed under inbreeding. If you re-read our paper you will see that we also refer to the possibility of epigenetic signals. There isn’t much logic to your argument that evolution shouldn’t allow such a process to operate, so it must be a ‘speciation effect’. There isn’t much logic to evolution allowing cancer to exist and proliferate either, but the system isn’t 100% perfect, especially when the system becomes unnaturally perturbed by, for example, inbreeding.

The study also had nothing whatsoever to do with human reproductive output, which you seem prepossessed about in your message to me. There is no doubt that our study showed reduced reproductive output under inbreeding, and that this inbreeding depression of output could be avoided by female polyandry. I wholeheartedly disagree with your assertion that we have a human fertility problem. Quite the opposite in fact: the world’s human population is increasing at a frighteningly exponential rate, which can only mean that our reproductive output is constantly growing, not slowing. You are concerned that this population growth is not high enough. I am of the opposite view: we live and depend upon on a planet which has a finite set of resources. Looking at a bigger picture, it’s clear that his growth cannot be sustainable, so how will we resolve who has rights over the unsustainable demands for food or water in the future?? I’d wager it will be decided by wars. It’s already obvious what a detrimental effect this human population growth is having on the planet: our energy demands are warming the climate and our consumption demands are creating a mass biodiversity extinction. The future looks far bleaker because of unsustainable demands by a burgeoning human population on the planet that created us, not because of your worries about low ‘fertility’.  Please consider this broader view.

Matt Gage
Professor Matthew JG Gage
School of Biological Sciences
University of East Anglia
Norwich Research Park


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