Many microorganisms practice sex. Some algae and fungi reproduce sexually, including the mold that produces penicillin. Yet asexual reproduction works exceedingly well for many microbes. The littlest ones possess rather astonishing adaptability thanks to their ability to selectively pick up genic bits from the environment and each other (transformation and horizontal gene transfer, respectively).
Given that complex life forms are chock full of rapidly adapting commensal microbes which facilitate digestion and help fight disease, sexual reproduction is not a convincing mechanism for optimizing adaptability, especially considering the energy cost involved. Whatever selective advantage may be conferred by sexual reproduction, its results have often been emergence of 2 sexes so different in appearance and behaviors as to easily mistake sexes for species.
Sex-biased genes are those that have different expression depending upon sex. Both sexes have the same genes, but sex-limited ones express only in 1 sex, often at a specific period of development. Sex-biased genes are responsible for sexual dimorphism.
The unique properties of sex-limited genes led to the widespread assumption that they are the product of sexual selection and sexual conflict. Darwinists took for granted – as lynchpin proof of “survival of the fittest” – that sexual selection via male mating rights contests provides a sure means for delivering the best male genomes to females. Actuality is more complicated.
Promiscuous birds that must fight for mating rights have genomes that evolve faster than monogamous birds which pair for life. Adaptive rapidity accounts for the supernormality in males that may arise quickly, and so is instrumental in conflict-based sexual selection.
The males that win the mating battles may not carry the best genome. Sex-biased genes geared to winning fights are not necessarily best for survival in other ways, which is why females of such sex-selective species often have extra-pair matings with also-rans. Instead of proving Darwinism, sexual conflict illustrates how deviously convoluted Nature is.
A male may be attractive to a female and fight hard to mate with her, but he doesn’t deliver at the genetic level. As a result, his descendants will be less fit. ~ English evolutionary geneticist Judith Mank
Polyandry is a mating system of 1 female and multiple males. Polygyny is the converse (1 male, multiple females).
Polyandry is common among mice. Females can tell the difference between a healthy male and an unhealthy one by their scents. Yet, while preferring healthy mice for their primary sex partners, they still participate in extra-pair mating with those not so robust.
Females living under a conflict-based sexual selection mating system hedge their bets when they can, as a mating system rigged to competitive contests is no guarantee of overall genomic quality in the winners.
Mating-competitive males seldom contribute much to raising offspring. Consequently, species with such males compete for the available females, while females exercise discrimination because offspring are a considerable investment which limit a female’s reproductive output.
Getting worked up over sex takes its toll. The mate competition that males undergo lessens lifespan, whether vinegar fly or furless primate. On top of that, any physical dominance afforded by sexual dimorphism carries a cost in longevity. Sex extracts a price.