Sex and Sexual Selection

Why bother with sex?

What’s the point of sperm and egg (gamete) production, and the recombination of two parent’s chromosomes into their offspring, when we could pass on our genome directly to the next generation in an asexual manner?

Despite the high social value humans place on sex, there are a number of reasons that would suggest this pleasure is not worth its costs.

  1. Search costs – finding a mate out in the reproductive wilderness can cost time and money (ie. Tinder Gold). Or more accurately within nature, it will cost metabolic energy and the risk of predation.
  2. Disease costs – interaction with a mate can lead to the contraction of STIs or other contact-driven diseases.
  3. Mating risks – while in humans, women are most at risk of domestic violence, roles are typically reversed in nature, with the females of a number of species consuming the unsuspecting male post-copulation (eg. black widow spider, praying mantis).
  4. Reduced relatedness – the parent’s genotype becomes ‘diluted’ through only passing (on average) half of the genome to their offspring.
  5. Recombination – shuffling of gene through recombination can potentially break up favourable gene combinations (haplotypes) that could result in reduced fitness.
  6.  The Twofold Cost of Sex – asexual populations will propagate twice as fast with both males and females being able to produce offspring, while males within a sexually reproducing population are unable to bear offspring.

Despite these costs, only ~0.2% of vertebrates are asexual. So what has promoted this imbalance between the number of sexually and asexually reproducing species?

  1. Sex purges deleterious mutations – For asexual species, a lack of recombination prevents removal of deleterious mutations once they arise. Muller’s Ratchet ‘turns’ when a new mutation, or drift, increases the minimum number of deleterious mutations found in every individual in the population. This is due to genotypes (and mutations) being identically copied into the next generation, with back mutations (reversion of deleterious mutations to the original state) being so rare that they can be excluded.
  2. Sex speeds up the rate of adaptation – by creating favourable gene combinations through recombination during sexual reproduction, beneficial mutations are brought together faster than asexual species that derive mutations in series.
recombination

Bergstrom, C.T., Dugatkin, L.A., 2012. Evolution. Norton.

3. Arms races between pathogen and host – the presence of dangerous pathogens require species to reproduce sexually. Pathogens evolve faster, with their higher population sizes and significantly faster generation times, which requires faster reciprocal adaptation by the host (as mentioned in #2).

4. Unpredictable offspring environments – sexual reproduction increases genetic variation in the population, increasing the chance that a proportion of the population will survive in the future if the environment changes. In addition, different phenotypes can fill ecological niches as opposed to directly competing with each other for resources. The pathogen Daphne Magma has been able to utilise the benefits of both reproductive strategies, usually being asexual but switching strategies to sexual reproduction when environmental cues (ie. predation, food shortage) necessitate it.

 

Why are there such large differences between genders within the same species?

What drives the sometimes extreme physical differences between males and females – think male peacock’s tails, female pipefish and more colourful male birds – even though the genders share ~90% of their genomes?

c18fata-Rainbow-Belly-Pipefish-Female-Sail-600x347

The female Rainbow Belly Pipefish uses her colourful sail for courtship

While there are differences between males and females, in terms of which genders are capable of undergoing asexual reproduction or bearing offspring, there are also widespread differences in morphology, physiology and behaviour between the genders.

A fundamental form of sexual dimorphism is anisogamy: dissimilar gametes produced by each gender that fuse to form the zygote. For females, eggs are costly; they require all the resources to nourish the zygote as it develops, with larger original zygote size conferring higher viability post-fertilisation. For males, sperm are cheap, only needing to reach the egg and transmit its genetic information to succeed. The functional differences between the gametes of the sexes have favoured the evolution of stable optima for differing gamete sizes.

But gamete production is only the beginning for differences between the sexes, as Darwin writes in his other major book:

“The female has to expend much organic matter in the formation of her ova, whereas the male expends much force in fierce contests with his rivals, in wandering about in search of the female, in exerting his voice, pouring out odoriferous secretions, etc.” –  Charles Darwin, The Descent of Man and Selection in Relation to Sex (1871)

Ultimately, sexual selection is the selection on traits that confer a fitness advantage over others of the same sex within the same species. This phenomenon comes in two forms. The easily discernible form is intrasexual selection, where traits are selected based on enhancing fitness during male-male competition (ie. male deer antlers, crab claws).

Capture

Male deer fight. CC0

The second less obvious form is intersexual selection. This is dependent on which traits females find desirable, which is selected up when females choose their mate for reproduction. The four aspects of this concept include:

  1. Direct benefits – Stronger, smarter or faster males will preferentially be able to provide accompanying females with food, protection and territory.
  2. Indirect benefits – Shopping for ‘good genes’, with an individual’s strong genetic background being presented through the proxy of a visual trait (ie. peacocks with extravagant tails. The number of eyespots in the male’s tail correlates with the viability of his offspring. In addition, males can’t cheat this trait, because excessive resource allocation to this trait will result in suffering due to disregarding alternative survival-based traits).
  3. Arbitrary choice & Fisher’s Runaway – Based on non-random mating (and Linkage Disequilibrium), where females begin to favour a derived male trait, which increases its frequency and intensity in the male population over time. The runaway process is halted when the trait becomes so ridiculous as to induce strong predation pressure.
  4. Sensory bias – Females are predisposed to prefer a trait (through association with a food source or shelter), to which males take advantage of and derive a visually similar trait on their body. If males present an image or colour on their coat that mimics the females food source, they will be attracted to this and the male will have a higher chance for reproductive success.
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