What Are The Costs of Sexual Reproduction?
The Costs of Sexual Reproduction
At first, it seems unclear why sexual reproduction evolved in the first place as it incurs a higher fitness cost than asexual reproduction. For example:
Sexual reproduction takes more time at the cellular level and requires a greater amount of resources to occur. The process of meiosis that occurs during sexual reproduction takes a good deal of time longer than the mitosis in asexual species and an asexual individual could produce twice as many offspring through mitosis in the time it takes to reproduce through meiosis.
At a larger level, sexual species expend considerable time and energy attracting and seeking mates. An asexual individual does not need to expend this time and energy and thus has higher fitness in this regard as well.
What is Recombinational Load?
The term recombinational load refers to a decrease in fitness that results from the separation of epistatic (co-adapted) gene combinations during recombination. This can undue the effects of generations of selection.
What is the Cost of Males?
The term the "Cost of Males" refers to the mean fitness cost that having males incurs on a population. Males do not directly produce offspring and any resources that go towards producing and raising males detracts from any resources that may have gone towards any offspring producing females. A species that only produces offspring-producing females can increase twice as fast as a species that features both males and females.
How Did Sexual Reproduction Evolve?
So if sexual reproduction is so costly then how did it evolve? Well it must increase fitness in some manner for natural selection to have selected for it. Here are several possible explanation as to how this could be:
First we'll look at a hypothesis known as Muller's Ratchet. Muller proposed that asexual lineages may go extinct due to the accumulation of deleterious mutations in their genomes. He believed that, over time, asexual lineages that had few mutations would decrease as they gradually accumulated more and more mutations over each reproductive cycle. Since a harmful mutation could only be reversed by an additional mutation, it was deemed unlikely that mutations in these lineages would reverse. This would eventually result in extinction of the lineage.
Sexual reproduction avoids this problem due to recombination of chromosomes and random mating. Any genomes that featured a high level of mutation would eventually be eliminated by selection.
The Red Queen Hypothesis
"What is the Red Queen Hypothesis?" one might ask. Indeed, it does sound crazier than even Muller's Ratchet. Well the name itself is taken from Lewis Caroll's famous book Through the Looking Glass. There is a character in this book, the Red Queen, who said, "It takes all the running you can do, to keep in the same place." In an evolutionary context, this simply means that continual adaptation is necessary to maintain relevant fitness in a co-evolving system.
Here's an explanation for that explanation. It means that in an environment where all species are co-evolving at the same time, it is necessary for a species to continue evolving just to maintain its position otherwise it will fall behind.
Sexually reproducing species are able to adapt more quickly to novel circumstances and thus would be preferred over asexual species in this system.
Frequency-Dependent Selection and the Red Queen Hypothesis
Under the Red Queen Hypothesis, when parasites evolve to favor their virulence against the most common members of a host species frequency-dependent selection occurs on this host species. In this case it is negative frequency-dependent selection since the the fitness of rarer phenotypes increases.
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