A old thread was bumped recently in which posters from 9 years ago raised objections to evolution basesd on personal incredulity regarding sexual reproduction.
I remember an elder in one of my old congregations saying something to the effect that "how could evolution be true, if men evolved before women...blah blah" you get the picture.
I have refreshed my memory of a chapter in Nick Lane's excellent book "Life Arising - The Ten Greatest Inventions of Evolution" and put together a summary. It would be great if others would add to it - Cantleave, Adam, Tiktaalik, S&R, Bohm et al?
Here is my starter focusing on the problem with non-sexual selection...
If the driving force in evolution is the struggle of selfish genes to pass on copies of themselves to future generations, then sex seems counter-productive. Organisms that reproduce by cloning pass on 100% of their genes to their offspring but sex results in only 50% of genes making it to the next generation.
Sex also imposes the cost of finding a mate and for many species, fighting for a mate. In addition the process of producing sex cells opens up opportunities for parasitic genes to hitch a ride, some of which cause harm to their host. Half of the 3 billion base pairs in the human genome consists of such parasitic freeloaders.
Despite the disadvantages, sexual reproduction is almost universal among all forms of complex life. Species that revert to cloning are all relatively young. The bdelloid rotifer is a rare exception; most celibate eukaryote species just don't prosper in the long term. The reason why the bdelloid rotifer beats the odds is an interesting topic for later.
The big problem with non-sexual reproduction is known as "Muller's Ratchet" named after Nobel Prize winner Hermann Muller. He knew from experiments on fruit flies exposed to x-rays that most mutations are harmful. A clonal population has no means to rid itself of the gradual accumulation of these mutations. Chance alone is just as likely to eradicate a "clean" individual as one that contains a harmful mutation. The less fit individual passes on its genes despite the errors and the population becomes one notch less fit than before.
Now imagine an individual that experiences a beneficial mutation. Without sex that helpful mutation has no option but to share a chromosome with numerous other genes that have experienced detrimental mutations. Its potential benefit may be lost because of the lack of fitness of the rest of the genome in which it finds itself. Dubbed "selective interference" the beneficial mutation becomes invisible to natural selection.
On the other hand, if its positive effect is so strong that is gets selected for despite its bad neighbours, it displaces all the other variations of that gene in the population. But remember that in non-sexual species - whatever that word means in this context - all genes are stuck in place like beads on a string. So not only does the good mutation spread through the population but so too do all the other genes in that genome. As a result practically all genetic diversity is lost, a consequence known as "selective sweep".
Sex ingeniously solves the problem.
From a biological point of view the main point of sex is to reshuffle the genes in a process known as meiosis....
more to follow...