Many people believe that recombination evolved because it increases genetic variation in a population and this provided a selective advantage over species that didn’t have recombination. Do you agree with this explanation for the evolution of recombination? Why, or why not? What are the other possibilities?
Question #1, Question #2, Question #3, Question #4, Question #5, Question #6
It evolved because the dumb luck god has magical mystery powers and it knew in its 100% lack of wisdom that populations would die out long before Lady Luck could poof out a sufficient number of copying errors to make good things happen. See dumb luck god made a kind of crutch for naturalism by inserting all kinds of dynamic, non-random, regulated and otherwise intelligent mechanisms within individuals that, if not known about or admitted to (think hgt/epigenetics etc) could be misconstrued and lied about by scientists and then passed off as "evolution" This comes in handy when individual organisms in the field seem to change adaptively within their surroundings. Dumb luck god also knew how many people could be tricked into believing that "only populations evolve" when in reality individuals have a suite of mechanisms, including recombination, to help them adapt to the environment. Dumb luck god is still tricking people today that RMNS does anything in the way of animal-building.
ReplyDelete... so instead a wizard did it.
DeleteIt's more feasible that intelligence and intelligent things were brought about by intelligence than by "just for the helluvit." The latter is far less scientific. Juvenile actually.
ReplyDeleteSo, a wizard.
Delete"Genetic variation" seems to be a poorly defined term in the question. Does genetic variation refer to the mixing of alleles from the two haploid copies from both paretns? Does it refer to the new mixtures of alleles in germline cells produced during meiosis?
ReplyDeleteI would say that sexual recombination has the advantage of separating detrimental alleles from beneficial alleles. This reduces the accumulation of deleterious mutations, often called Muller's Ratchet. No longer does natural selection work at the level of the whole genome. Rather, natural selection can work at the level of the allele.
None of my students were confused about the meaning, and the potential causes, of genetic variation. They also know that diploidy and meiosis are effective ways of reducing the effects of Mueller's Ratchet. They also know that simply increasing the number of chromosomes can work almost as well as recombination.
DeleteYou need to be precise in your definition of recombination in the exam question if what you mean is crossing over between parental alleles in meiosis. If you mean recombination in the sense of the molecular activity of RecA/Rad51 and similar molecules, then there are no species that do not have recombination. The distinction matters in this context if the fundamental role of recombination is to restore the correct topology to collapsed replication forks, with this biochemical activity later exploited to identify parental alleles thereby allowing reductive meiotic division and enabling sexual reproduction.
ReplyDeleteNone of my students are confused about the meaning of "recombination."
DeleteI think recombination main benefit is to mix variants by breaking linkage between alleles. This is very powerful.
ReplyDeleteYou would fail if that was your answer.
DeleteRecombination makes sense after selection. Since advantageous traits can come from different kinds of mutations, it makes sense that independent mutations would be recombined. Some experiments in directed evolution (90s), showed that the combination of selection, recombination speeded up, several-fold, the evolution of new specificity (from beta-lactams to cefotaxime, I think).
DeleteBut without selection it doesn't seem to make a lot of sense.
(I know, parcial marks, if any). :)
While there is no doubt regarding the impact of recombination on selection, I believe the question is directed more towards the origination of recombination. Professor Moran, what is your opinion regarding Cavalier-smith's review on this topic titled "Origins of the machinery of recombination and sex"?
DeleteWhat's the correct answer?
DeleteNo, I don't think that increasing genetic variation in a population and thus providing a selective advantage over species that don’t have recombination could be the be the explanation for the original evolution of recombination. That explanation is too far "downstream." Evolution happens generation by generation. At least at the start, the process has to provide an advantage (in survival or reproduction) to the individuals that have it.
ReplyDeleteI hypothesize that it could have started as a DNA repair mechanism. (Answer is very generalized; it's been a long time since I studied DNA in much detail.) DNA is long and can break. The broken ends are active and can reattach, but may not attach in the right place. Anything that would tend to control where reattachment happens would help the cell survive and reproduce. Mechanisms to break and reseal DNA help untwist it when it's in knots, and at this point are necessary for DNA replication.
The repair can happen at another level. Mutated, poorly functioning DNA can be replaced by a better copy if two similar DNA strands line up and are cut and the pieces are exchanged. (Of course, one strand will end up worse than before, too.) That's an advantage that would function at the level of the individual and its offspring.
Recombination could begin as a way to incorporate new DNA sequences (from food) into the genome. I suppose it's even possible that it got started inserting virus-like sequences into genomes. Lots of speculation here.