Why SEX is not a regressive step in evolution

Post of the Month: December 2006

by

Subject:    | ASEXUAL ANTICS
Date:       | 17 Dec 2006
Message-ID: | 1hqihop.2b9n6k14ct02kN%j.wilkins1@uq.edu.au

Alabastar wrote:

> Am I right to suggest:
>
> Evolution supposes we were once asexual, i.e. self reproducing?
>
> Surely that is the ultimate condition, i.e. - not needing a partner to
> reproduce?
>
> Surely becoming reliant on another individual is an evolutionary regress?
>
> Does evolution sometimes take some backward steps?
>
> Can anyone explain that please.
>
> Please don't suggest we held onto the intercourse aspect, simply because of
> the enjoyment factor, since it (sexual intercourse), supposedly hadn't
> evolved at that point in time, i.e. while asexual reproduction was the norm.
>
> Thanks

Sex evolved, yes, but that doesn't mean that our ancestors never shared genes before it did. Genetic transfer has an advantage - if favourable mutations occur in two distinct lineages, such as a gene that permits the use of a novel food source or one that confers some immunity to parasites, then genetic exchange means that the chances of both favourable mutant genes ending up in a single lineage are increased dramatically. Most supposedly "asexual" organisms have mechanisms for the exchange of genes, including a process called transduction (uptake of free floating genes from a cell that has broken its cell wall) and conjugation (where some part of the genetic material is inserted into another cell).

At some point, conjugation evolved into strict mating types. Some organisms, such as the malarial parasite, have up to seven such types, each of which is triggered by one or more of the other types, and these are often linked to specific lifecycle stages. It is not hard to see one of these stages becoming more significant, and the associated mating types becoming "sexes". When cells become colonial, and then multicellular organismal, there would be immediate selection for compatibility of mating types. And two mating types is optimal when organisms have set sex cells (gametes) because otherwise there would be instability in the mating cycle.

There is never a stage in the evolution of sex (which has happened at least twice, once for animals and once for plants) where the evolving sexes had no counterparts. Each "step" in the evolution of sex occurred in a population, or collection of populations, in which the mating types were attuned to the other members of the population. So as we converged on two sexes, there were always breeding opportunities.

Gender, or differential mating types, is a bit more complex. It pays to be male, in the sense that all you are contributing is sperm, which is easy to make and relatively easy to distribute. Eggs cost more, because they also contain expensive resources for the development of the progeny. So in animals, eggs are surrounded by some material for the early, and in many cases, total development of the fertilised zygote to a free standing organism. In plants, this is done with seeds, tubers, and so on. You would expect that all organisms would end up male (and shortly afterwards, extinct), except that the question is not the cost to the individual organism, but to the lineage's overall fitness. So males, while cheaper for the individual, still have mothers and daughters, and the cost is shared equally over evolutionary time.

Once you have this scenario, then selection will occur on each gender's developmental process, so that depending on the nature of the lineage or species, sexes will diverge in form and size, or functionality. This leads to, as it has in our lineages, sexual dimorphism (difference in overall phenotype, or bodily form and function) between the two sexes, but each gender shares the same genes, except for the sex-determining genes, as both parents. This is, incidentally, why human males, indeed all mammalian males, have nipples.

So sex is not a "backward step" (although yes, evolution can cause regression; many males of some species are vestigial - little more body and brain than is needed to find and attach itself to a female and then become sperm-bearing parasites); it is instead the evolution of a system that has its own advantages and costs, and which drives further evolutionary changes.

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