Assertion #4.21 Sexual reproduction could not have come about through evolution.
Analysis:
Sexual reproduction and "junk DNA" are both considered paradoxes under the traditional view of evolution. However, both can be resolved or at least explained by the replicator-level selection.
At first glance, sexual reproduction appears to be a paradox if viewed in the organism-level interpretation. An organism that simply "buds" or otherwise reproduces asexually will pass on all of its genes to the next generation. By contrast, an organism that reproduces sexually only passes on ½ of its genes to its offspring. This once led many theorists into a group-selectionist interpretation of sex; the beneficial effects of genetic recombination on the success of the species as a whole are easily seen. However, group selection as a theory is generally lacking in supportive evidence, and no workable proposal of its mechanism has ever been introduced.
This apparent paradox is much more easily understood if viewed from the point of view of a replicator. The organism's efficiency in reproducing its own genetic makeup is irrelevant. If one views sexual and asexual reproduction as a simple genetic alternative, then the paradox recedes: sexual reproduction obviously is a benefit to a gene for sexual reproduction, and therefore sexual reproduction exists. As Richard Dawkins put it in The Selfish Gene, "A gene 'for' sexuality manipulates all the other genes for its own selfish ends." (p.44).
This argument applies equally well to other genes that appear to be detrimental to the organism's maximal efficiency of reproduction. Genes that increase the rates of copying errors made in the transcription of other genes obviously do not benefit the genes they impede, or the organism as a whole. But if their activities benefit themselves, these "mutators" will spread through the gene pool. (The same applies to genes for crossing-over during meiosis.)
This argument also works quite well in explaining the existence of introns, or "junk DNA" that is never translated into protein and thus has no effect on the phenotype of an organism. These DNA sequences are examples of replicators that take advantage of other replicators' mechanisms of survival and reproduction (organisms) to their own benefit. Again as Dawkins put it, "If the 'purpose' of DNA is to supervise the building of bodies, it is surprising to find a large quantity of DNA which does no such thing. . . . [but] the true 'purpose' of DNA is to survive, no more and no less. The simplest way to explain the surplus DNA is to suppose that it is a parasite, or at best a harmless but useless passenger, hitching a ride in the survival machines [organisms] created by other DNA." (p.45).