In terms of population growth speeds, asexual animals have a significant advantage: so why do so many species use sexual reproduction to propagate?
One of the basic evolutionary weapons in the arsenal of a species competing for dominance in an environment is the ability to grow and spread rapidly, like a thicket of brambles overtaking an ecosystem and blocking out the light for other vegetation. One way of gaining a head start in the race to produce the most offspring is for all members of a species to bear young: meaning that, at maximum capacity, all mature members of a species could theoretically be producing offspring simultaneously.
The perks of parthenogenesis
Asexual reproduction occurs in several successful animal species, such as aphids and some scorpions, typically through the process of parthenogenesis: where an embryo develops from an unfertilized egg cell. The females of some species, including the fearsome Komodo dragon, are even capable of swapping from sexual reproduction to parthenogenesis when there are no males in their environment.
With this in mind then, why is it that the vast majority of complex multicellular organisms embrace sexual reproduction, splitting their population in half, with 50% incapable of bearing young? Such an evolutionary decision cuts your species’ maximum output by half; figures any evolutionary industrialist would balk at.
What’s the point of papa?
There are some obvious reasons for adopting sexual over asexual reproduction; primarily that sexual reproduction allows for the maintenance of more diverse, healthy genomes within a species, as sections of two different genomes combine to make one. This provides one method of avoiding the issues that can arise with inbreeding. Not all species that opt for sexual reproduction have a 1:1 ratio for males and females; however, it is interesting to note that the majority do maintain a sex ratio close to 1:1.
Setting out to solve this conundrum, researchers from Uppsala University (Sweden) have designed a study to provide empirical evidence for the benefit of burdening a species with such a high portion of the unproductive male sex.
competition between males enables selection against individuals with detrimental genetic mutations. A small portion of males is capable of fertilizing all of the females, so this selection has little bearing on the population growth rate while helping to maintain a healthy gene pool. If this reproductive selection against deleterious mutations was observed in females; however, there would be a significant impact on the rate of population growth.
To test this theory, the team obtained 16 genetic strains of seed beetle, intensively inbreeding them to accelerate the acquisition of deleterious mutations. Selected progeny of this inbreeding from each strain were then crossbred with the inbred offspring of another strain, producing a hybrid beetle. The team then measured the extent of heterosis in these hybrids.
What the heck is heterosis?
When two varieties of a species, or two separate species, are successfully bred the resultant offspring can sometimes exhibit improved characteristics, such as greater biomass, speed of development or fertility than their parents: a trait referred to as positive heterosis. This phenomenon is observed in animals such as the liger, which towers over its lion and tiger parents.
When an offspring is outbred from two severely inbred varieties of a species, which will have obtained numerous recessive deleterious mutations, these differences can be amplified. While the reason for this is not entirely understood, many posit that this is due to the disparate, recessive-mutation riddled genomes of the parents combining, canceling out their respective recessive mutations to an extent, which leads to a more genetically healthy specimen.
Genetic health: the answer to man’s search for meaning
The team then compared the competitive lifetime reproductive success of the inbred strains to the outbred crosses, finding that the mutations damaged the inbred males and females equally. However, when comparing the outbred crosses to each other using the extent of heterosis as a proxy for the extent of the recessive deleterious mutations acquired within them, the team found that the negative impact on reproductive success was only observed in the males.
This demonstrates that in the more natural situation – the comparison of the more genetically varied outbred crosses – those deleterious recessive mutations would be effectively cleared by male- rather than female-specific selection. Commenting on the findings, the study’s lead author Karl Grieshop (University of Toronto, Canada) stated that:
“This indicates that although these mutations do have a detrimental effect on females’ reproduction, they are more effectively removed from the population by selection acting on male carriers than female carriers. Previous research from our group and others has succeeded in showing this effect by artificially inducing mutations, but this is the first direct evidence that it ensues for naturally occurring variants of genes.”
Ultimately, this research supports the theory that the evolutionary selection of sexual reproduction and the equal split of male and female members of a species, enables competition between males to reproduce. As the chance of success is influenced by the number of deleterious recessive mutations present, more of these mutations are filtered out of the population, without impacting the rate of population growth.