The how and why in the origins of sexual reproduction
Abstract
The origin of sexual reproduction, how it evolved, and its benefits have been topics of active research. Many different theories have been proposed and various models have been made to explain sexual reproduction in species. There is an inherent problem of which proponent of the evolution of sex is correct or whether it is a combination of theories that is correct. Although not exhaustive, the current review aims to synthesize and explore this issue and ideas based on some of its arguments including the generation of sex differences, the molecular basis for sex, and why it may have been selected for and kept in certain species. By providing a baseline for the discussion, further research may be executed in deriving the reasoning behind the evolution of sex and its pros and cons compared to asexual reproduction.
What is obligatory sex and who has it?
The definition of sex has generally been accepted as a means of reproduction that requires the genetic material of two parents combining (via gametes) to form a zygote (Bai 2015). However, this is not the only means of reproduction as organisms can also pass on their genetic material through asexual reproduction where the parent clones its genes for the offspring. It is important to note that about only 0.1% of animal species reproduce asexually (Phillips et al. 1990). An even more curious and interesting note is most asexual lineages have come from sexually reproducing ancestors which may indicate that asexual reproduction is probably a niche tool for evolution in certain evolutionary situations (Geodakyan 1991). Both sexual reproduction and asexual reproduction have a facultative side and an obligate side that can be observed in species. Obligate asexual reproduction has been thought to arise because of inbreeding or mutation in large populations (Scheuerl et al. 2011). Facultative sexual reproduction is relatively rare, being only observed in a few animal species. The reason for this seems to be a response from species for lack of viable mates in certain environments. On the other hand, obligate sexual reproduction is when species reproduce exclusively via sexual reproduction and is the reproductive method most observed in the animal kingdom.
It may initially appear evident and obvious as to why and how sexual reproduction is advantageous and thus so prevalent. Unfortunately, this is not the case, as the origin and maintenance of sexual reproduction is still one of the most elusive and hotly debated topics in evolutionary biology. The reasons behind this are because sexual reproduction requires more resources and has more costs associated with it compared to asexual reproduction, and yet it is still prevalent. Therefore, there must be a greater payoff to the risks and costs that keeps sexual reproduction the main mode of propagation in species. In face of this logic, no concrete and agreed upon theory has been presented which explains how sexual reproduction came to be and how it is maintained. In the current review, some major theories and hypotheses will be discussed to provide a foundation for future research.