|Medical News News, information and discussions about health issues, medicine and biotechnology.
02-15-2009, 08:36 AM
Senior Level 6
Join Date: Aug 2008
Thanked 456 Times in 319 Posts
The evolution of sex
||Last year, Swedish researchers generated headlines around the world with the news a man's reluctance to settle down might be due to his genes.
Researcher Hasse Walum and his colleagues studied sets of male twins, and found that those with a particular form of a hormone called vasopressin were less likely to be married. Married men who carried the variant were also more likely to have had some kind of marital strife in the past year.
The Swedish findings, published in the Proceedings of the National Academy of Sciences, are just the latest addition to a substantial body of evidence that suggests the processes of natural selection that Charles Darwin wrote about 150 years ago have helped shape human sexual behaviour. And it seems those same effects are still moulding our choices today.
"There's no doubt whatever that our sexual behaviour is strongly influenced by evolution," says psychologist Professor David Barash from the University of Washington.
Evolutionary forces are not the only factor driving our behaviour, he notes, but the fact that people around the world behave in similar ways when it comes to sexual relationships strongly suggests it's one of them.
"Given that these universals are consistent with evolutionary predictions, the role of natural selection in colouring our sexual behaviour cannot be seriously doubted," he says.
Not like Arnold Schwarzenegger
One of the most striking pieces of evidence in support of evolution's role is that wherever you go in the world, women and men prefer similar body shapes in their partners.
"I'm not talking overblown muscular like Arnold Schwarzenegger in his heyday. I'm talking Olympic gymnasts very fit looking, well built, muscular... or average."
More heavily built men, or those who are slim and wiry tend to rate as somewhat less attractive, Dixson says, and there's likely a very good evolutionary explanation.
"When you look at very early human precursors, which almost certainly showed some form of hunting and gathering community, then men who were muscular were able to run faster, engage in endurance running and persistent hunting.
"They probably had a huge advantage in getting protein and in inter-male competition, so female preferences for these physiques may be sexually selected. To some degree those preferences are genetically encoded."
Similarly, when men are asked to rate pictures of women, they also tend to make similar choices.
Generally speaking, they tend to chose women with more fat in their hips, thighs and breasts, Dixson notes. This classic hourglass figure seems to be related to reproductive success as fat deposits in these areas help fuel the physiological needs of pregnancy and lactation.
Nobody is suggesting that body shape is the only consideration when choosing a partner, but it might serve as a kind of initial screening tool when we first meet or get to know another person, researchers say.
Born to cheat?
One of the most socially contentious ideas about the impact of evolution on our sexual behaviour is the idea that polygyny where some males dominate reproduction by fathering children with several women has been the norm throughout human history and prehistory.
Last year, US researchers reported genetic evidence supporting this idea. They gathered DNA from people in the Central African Republic, Senegal, Namibia, France, China and Papua New Guinea, and compared regions that don't include known genes in the X chromosome, and some of the other 22 chromosomes.
If an equal number of males and females had been reproducing effectively through human evolution, then genetic variation in the two kinds of chromosomes should be roughly equal, the researchers explained. Instead, they found that there was much more variation on the X chromosomes, suggesting polygyny was the likely cause.
In other words, women have been more successful on average in passing their genes on to the next generation.
"This is because a few males have fathered children with multiple females, which occurs at the expense of other less successful males", says lead researcher Dr Michael Hammer, from the University of Arizona.
There are plausible evolutionary reasons to explain why this pattern might offer benefits in terms of evolutionary 'fitness', explains David Barasch.
"In broad strokes, males derive fitness benefits from having multiple partners because doing so increases the chances that they will father additional children as a result."
For females, the situation is more complex, especially since they are unlikely to produce additional children as a consequence of having more than one partner.
"The payoffs to women of multiple sexual partners largely involve some combination of accruing better genes to combine with their own better than those on offer from their designated partner obtaining additional resources, and/or prospecting for a better relationship," he says.
Researchers trying to study the evolution of human sexual behaviour face a major problem: although there are some fossils of our ancestors, they offer few clues about what reproductive antics they may have got up to.
So researchers have turned instead to comparing us to our closest evolutionary cousins, non-human primates such as chimpanzees, apes and orangutans. Doing so, they have found that there are some correlations between mating systems and the structure of the male reproductive system.
Those correlations largely have to do with the fact that if females have multiple partners, then each male's sperm is effectively competing with the sperm of other males to fertilize an ovum. In species where there is more sperm competition going on such as chimpanzees testes tend to be bigger, and sperm tend to have bigger mid-sections to boost motility, for example.
The conclusion from these studies, Dixson says, is that our ancestors probably did not have the kind of multi-male, multi-female mating system the chimpanzee does.
"We fit with the group that has either a polygynous or monogamous background," he says. "That much you can get from the reproductive biology. But it's very ancient stuff, not what people are doing in New York or Paris now."
And in the end, this is the message, adds Barasch.
"To some extent, yes, we are 'born to cheat,' in that we are predisposed to do so if, as a result, we enhance our biological success. But this doesn't mean that we are obliged to do so! More than any other species, human beings are endowed with the capacity to say 'No' to our genetically given inclinations."
Full article here:
There are always dozens of reasons why something "can't" be done. That's no excuse in my book. If you want it bad enough, you find a way. That's how life works for grown ups. -- Booger
Don't be afraid to be open-minded. Your brain won't fall out.
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
All times are GMT -4. The time now is 04:52 PM.