The following email was sent to Dr. Andreas Wagner, the author of the book “Arrival of the Fittest” on 2017-1-7:
Dear Dr. Wagner,
I was attempting to explain your theory to a friend recently, and thought that it would it be easier if I re-read your excellent book first. After doing so, I have a couple of comments which I wanted to share with you.
In order to explore a genotype neighbourhood, mutations are necessary. More specifically, germline cell mutations in the case of multicellular organisms which reproduce sexually. I hypothesize that sexual reproduction evolved and became the dominant mode of reproduction, specifically because it greatly speeds up the process of exploring the genotype neighbourhood.
Two of the characteristics of sexual reproduction which support this hypothesis, are:
- The vast quantities of male germline cells which are produced (in humans, well over 300 million sperm per day; plant pollen is also produced in large quantities, perhaps a billion over a season for some ragweed species). Each male germline cell is potentially subject to mutation, and mutations which are deleterious to the survival or performance of a particular cell will be weeded out in the race to fertilize a female germline cell. This early selection process occurs at very little energy cost to the parent organism. The competition between male germline cells occurs not only between cells from the same parent to select either a beneficial mutation, but certainly in pollinating plants, and also in many animal species, there is competition between male germline cells from different parents.
- The adaptations that have evolved for controlling the rate of mutations in germline cells. I am thinking specifically of mutations in male germline cells induced by UVB radiation. Plant pollen, for example, is often carried by the wind, and thus is exposed to insolation possibly for long periods of time. It has been suggested that the yellow colour of many pollens is there specifically to decrease UVB penetration into the pollen core (Flenley, 2011). For many male mammals, spermatozoa are produced and stored in a scrotum. The current theory to explain scrotums is that they provide temperature control for the developing sperm. A better theory, in my opinion, is that the scrotum with its thin skin, lack of hair, and highly exposed location, has evolved to subject sperm to large amounts of UVB to cause many mutations so as to facilitate exploring the genotype neighbourhood.
I would suspect that some species might also exert control over mutation rates in germline cells by adaptations dealing with other mutagens, for example, high temperatures or mutagenic chemicals in the environment. Finally, regulatory control over DNA repair mechanisms in the germline cell and later on in the zygote, would also enable a fine balance between an excessive mutation rate which might lead to extinction of a lineage, or a too-low mutation rate which would render a lineage unable to respond to environmental change.
This hypothesis is, I believe, a good candidate for exploration by the methods of computational biology. As a computer systems engineer as well as a physician, I would be most interested in collaborating with you on this.
Henry Olders, P. Eng, MD, FRCPC
Assistant Professor, Dept of Psychiatry, McGill University
Flenley JR. Why is pollen yellow? And why are there so many species in the tropical rain forest?: Why is pollen yellow. Journal of Biogeography. 2011;38:809-816.
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