Quote:
Originally Posted by dimebar
Also what about animals in nature? I know female felines harass there male offspring to stop interbreeding but are there any more examples of this?
As far as I know, usually inbreeding is avoided but in some species it is pretty standard.
For a quick description of the mechanic for anyone who might not be familiar with the topic, it goes like this. In most species, each individual has 2 copies of each chromosome (and therefore each gene) - one from each parent. Mutations happen. Most gene mutations simply mean that the protein that the gene codes will simply be useless. Not
harmful, just useless. Well, oftentimes when such a mutation happens in one chromosome, the cells will be able to satisfy their need for the particular protein by using the other chromosome, and nothing too bad happens. It is
astronomically unlikely that two chromosomes both just happen to develop a mutation in the exact same gene. So most of us walk around with many such mutations and they don't bother us in any way.
But when you start inbreeding, they will get in your way. For the worst possible scenario, imagine that Cornelius knocks up his own daughter. He himself will contribute half of his genes to the child and his daughter has half of his own chromosomes so she will on average contribute a quarter of his genes to the child, so for every chromosome a child with her has a 1/2*1/4=1/8 probability of getting two identical copies. If there are any mutations in that chromosome that cause a protein to be useless... Now there's no way to get that protein, because both copies are messed up. Compare this with a like 1/10000 or whatever probability of the child getting two mutations of the same kind when Cornelius knocks up a completely random hooker off the street, and you will notice that
if inbreeding is rare people
will accumulate tons of these kinds of mutations.
When you arrive at such a situation where recessive mutations are common it becomes obvious why many species avoid inbreeding though it doesn't necessarily answer why they started avoiding it and ended up in such a situation in the first place. You could claim that even if recessive mutations are very rare in the population, the effect remains that it is still drastically more likely to meet the same mutation that you have in your relatives than in a stranger. But IIRC, if the frequency of recessive mutations is rare enough, this effect is actually not strong enough to generate enough selective pressure to prevent inbreeding. IIRC there is some proven benefit ("gene dispersal"? or something) to breeding with nonrelatives regardless of the recessive mutation deal. So probably that thing discourages inbreeding, and once inbreeding has been discouraged for a long enough time, the population will accumulate enough recessive mutations that they alone will generate a huge pressure against inbreeding.
On the other hand if for one reason or another inbreeding slowly starts happening (if it happens suddenly for such a population, then it has disastrous results), so that there will be mild selective pressure against recessive mutations long enough that it will clear the population of them, then the genetic quality of the offspring will stop being an obstacle to inbreeding. In fact, you could say that in such a situation, if your genes are good in a sense you should
prefer to breed with as close relatives as possible because they are also likely to have good genes as you do. Unfortunately I'm not so good examples. In some parasitic wasps that lay their eggs in caterpillars, it can be exclusive that brothers fertilise the sisters (I think it's the difficulty of finding a mate that has led to inbreeding here). For someone closer to humans, I think someone in a similar discussion once mentioned that for mustangs it was common for the pack leader to knock up her own daughters with no ill effects. I guess it must be domestication effects that has led to loss of recessive mutations in this case.
Quote:
Originally Posted by dimebar
I still think the idea is disgusting but am unsure why. I don't know if its something that has been put into my mind by society or if its instinct.
Not being sexually attracted to people you grew up with in the same family actually seems to be instinctive. The instincts of course can't rely on the logical reasoning of who are your "blood relatives", they need something simpler such as "the people who were in your face every godforsaken day of your childhood". The fact that people are generally not actively disgusted by the idea of having sex with nonrelatives they grew up in the same family with however illustrates that
active disgust towards incest is a social rule. Not that it needed much illustration in my mind, seems blatantly obvious to me.
Quote:
Originally Posted by dimebar
When did the idea come about? has it always been looked down upon?
It looks like you have a very small timescale in mind. Incest must have been a near-universal taboo for tens of thousands of years if not hundreds. Because the current situation is that parent-child and brother-sister incest are pretty much guaranteed to produce diseased children. The only way to reach such a situation is if incest has been rare for a long time so that there has been no selective pressure to get rid of recessive mutations that would result in diseased children from inbreeding. Well, as far as I can tell the only way for incest to be so rare since tens of thousands of years ago is social rules.
Last edited by Vantek; 09-06-2010 at 06:36 PM.
