The origin of sexually reproducing organisms from asexually reproducing ancestors is a profound mystery which has baffled many an evolutionary biologist. The origin and subsequent maintenance of sex and recombination is a phenomenon not easily explained by Darwinian evolution. Indeed, there are several substantive, well-known reasons why the origin of sex presents a serious problem for conventional evolutionary explanations.Since evolution is theorized to proceed as genetic mutations occuring over vast stretches of time confer some sort of advantage on a population of organisms, it's a mystery as to how sexual reproduction would've ever arisen from asexually reproducing organisms. But the problems extend even deeper than this.
There are several reasons why the origin of sex presents a problem. For starters, there is the waste of resources in producing males. Assuming a sexually-reproducing female gives birth to an equal number of male and female offspring, only half of the progeny will be able to go on to have more offspring (in contrast to the asexually reproducing species, all the offspring of which can subsequently reproduce).
Thus, it is to be expected that the asexual female will proliferate, on average, at twice the rate of the sexual species. Given the disadvantage thereby confronting the sexually-reproducing species, one would expect them to be quickly outcompeted by the asexual species.
Moreover, it must be borne in mind that, in contrast to the asexual species, the females of the sexually-reproducing species perpetuate only half of their successful genotype. To transition, therefore, from a state of asexuality to sexual reproduction is, in effect, to gamble with 50% of one’s successful genotype.
Given that the whole purpose of natural selection is the preservation of those organisms which pass on their successful genes, this strikes at the heart of evolutionary rationale.
There is, of course, the additional conundrum related to the fact that gametes (i.e. sex cells) undergo a fundamentally different type of cell division (i.e. meiosis rather than mitosis). Meiosis entails the copying of only half of the chromosomal material. In similar fashion to mitosis (which occurs in somatic cells), each chromosome is duplicated to yield two chromatids.For those who'd like a refresher of their high school biology on cell reproduction here's a relatively brief video on the difference between meiosis and mitosis. And then there is the added problem of male and female complementarity. Many physical and physiological structures as well as many chemical reactions that enable the whole process to work must develop in male and female virtually simultaneously, even though these structures and reactions are completely different in the two sexes.
In contrast to mitosis, however, the homologous chromosomes are also associated. So, at the start of meiosis, each visible ‘chromosome’ possesses four chromatids. At the first division, these homologous chromosomes are separated such that each daughter nucleus has exactly half the chromosome number.
At this stage, each is present as two copies (chromatids). These chromatids are hence separated at the second division such that each new nucleus only has a single copy.
In order for sexual reproduction to work, it is essential that the process of meiosis evolve to halve the chromosome number. And this ability must also only occur in the gametes and not in the somatic cells. This difficulty is accentuated by the multitude of novel elements which are found in meiosis, rendering it unlikely to be explicable in terms of single mutational steps.
One example is sperm capacitation. Chemicals in the head of the sperm have to be modified while on the way through the female reproductive tract in order to prepare the sperm for penetration of the ovum.
There are numerous such chemical reactions that occur in the process of sexual reproduction that occur in no other bodily process and which must have all evolved almost simultaneously and in both males and females for sexual reproduction to work.
This video illustrates just a few of them: One wonders whether Darwin, if he had been aware of all the problems that sexual reproduction entails, would have ever gone ahead with his theory of natural selection as the engine of evolution.
