Why is the number of sexes not 3 or more?

- a study on the distribution of the number of sexes

Yilong HAN
A working paper in Prof. Philippe Cluzel's biophysics course 491 PDF
Department of Physics, University of Chicago (2002)


There is little research about this topic. Although my paper mainly focused on DNA-based lives, the heredity materials are not limited to DNA and most conclusions should be applicable to most aliens on other planets. (Anyway, I think natural selection is a logical truth which should not depend on observation.)

Abstract

What is the best way of self-reproduction for life? 2-sex is much superior than 1-sex because good mutations can propagate and meet in the population instead of limited in some individuals. Similarly 3-sex is much superior than 2-sex. For example, in our gene pool, say genes A, B, C are good; AB, AC and BC are fatal combinations; while ABC is a superman. Then it is almost impossible to get ABC from 2-sex reproduction, but very easy from 3-sex reproduction. N-sex is much superior to explore more DNA sequence space than (N-1)-sex. At equilibrium, N-sex population should have 1 type of female (big gametes provider) and N-1 different types of male (small gametes provider) and each type occupies 1/N population. At very high natural selection pressure, I estimated that the efficiency is mainly proportional to lnN/N, which reaches the maximum at sex number N=3. Thus it appears that the natural selection pressure on our planet was not extremely high so that 2-sex could wipe out 3-sex at its baby stage; the selection pressure was not very low so that 2-sex were not wiped out by 1-sex. Multi-sex is better for the parent-competing-resource mode than the offspring-competing mode.

"It should be possible by asexual means to produce an oak tree from a bacterium, but not, probably, within the total lifetime of the universe [5]." Fortunately the complexity of intelligence for developing science is below the upper limit of the 2-sex complexity, so that those 2-sex scientific organisms can shuffle DNA between many parents or even engineer DNA in the lab to break the upper limit of 2-sex. "Human" with wings and chlorophyll is just a very conservative guess.

I think it would be the best to have flexible number of sexes in one species. It has advantages of both low sex number and high sex number. It is not clear why this did not happen.