sex and evolution

[Gian]

There may already be an answer here to this question, still looking through all the topics.

 

Why is there male and female, and why is there sexual reproduction? Presumably there must have been some evolutionary advantage over asexual organisms. Any ideas what those advantages may be?

 

thanks

 

GIAN

 

[CharonY]

We have a few threads about it somewhere. But bottom line is that we do not have a definite answer but a number of hypotheses. One "textbook hypothesis" is that it results in larger genetic variance. However most models do not explain how that can overcome the two-fold cost of sexual reproduction (as only half of the genome would be inherited).

Other models explain it in a more mechanistic way by genetic elements that evolved from nuclear elements that coordinate inheritance. And by "enforcing" sexual reproduction, they make sure that they also get inherited (as seen in the context of selfish genes).

 

But currently there are no definite answers.

 

Also it is clear that benefits of sexual reproduction will not in every case outweigh the disadvantages, considering that we still got plenty of asexually reproducing organisms.

Edited July 3, 2013 by CharonY

[overtone]

Sexual reproduction, in the sense of exchange of genetic information between individuals with different code and using it to creating progeny with novel code, is found in almost all taxa at some stage of their life cycles or ecological reponses - it clearly has value even to organisms capable of reproducing without it.

 

There are many obligate sexual reproducers, especially in large organisms with longer life spans whose progeny are likely to face an accumulation of specifically adapted disease threats amid environments depleted or otherwise altered on a comparatively large scale over time by their large and long lived parent(s). There are very few obligate parthenogenetic organisms - one take is that an obligate clone producer will eventually produce progeny surrounded by specifically adapted enemies and specifically used up resource pools, in direct and specific competition with their parent without the parent's acquired defenses. The sequence of clones runs the genetic pool into a cul de sac of a disappearing niche with no exit, so to speak, and their advantage in numbers is obviated by their shared vulnerabiiities. This take is supported by the observation that many normally cloning organisms switch to sex when stressed and threatened.

 

Another take: one of the functions of sex is to vet and possibly repair the proposed string of progeny code before investing parental resources in it - reproduced code usually contains errors, and a series of clones suffers generations of accumulated copy mistakes. Sexual recombination can reveal and discard the worst of them immediately, before parental resources are wasted in attempted nurturing or development (the code has to successfully match the partner's, and function at least well enough to carry out the combination procedure, to produce progeny). The sexual combination can also replace or cover for bad stretches of code (the two combinitorial genomes are unlikely to have the same errors). Supporting this take is the observation that many incipient fertilizations fail in sexual reproducers, and cloning organisms such as bacteria in test tubes tend to produce noticeable percentages of crippled offspring, again partially obviating the numerical advantage of cloning's efficiency.

 

just a couple of thoughts.

[Gian]

Sexual reproduction, in the sense of exchange of genetic information between individuals with different code and using it to creating progeny with novel code, is found in almost all taxa at some stage of their life cycles or ecological reponses - it clearly has value even to organisms capable of reproducing without it.

 

There are many obligate sexual reproducers, especially in large organisms with longer life spans whose progeny are likely to face an accumulation of specifically adapted disease threats amid environments depleted or otherwise altered on a comparatively large scale over time by their large and long lived parent(s). There are very few obligate parthenogenetic organisms - one take is that an obligate clone producer will eventually produce progeny surrounded by specifically adapted enemies and specifically used up resource pools, in direct and specific competition with their parent without the parent's acquired defenses. The sequence of clones runs the genetic pool into a cul de sac of a disappearing niche with no exit, so to speak, and their advantage in numbers is obviated by their shared vulnerabiiities. This take is supported by the observation that many normally cloning organisms switch to sex when stressed and threatened.

 

Another take: one of the functions of sex is to vet and possibly repair the proposed string of progeny code before investing parental resources in it - reproduced code usually contains errors, and a series of clones suffers generations of accumulated copy mistakes. Sexual recombination can reveal and discard the worst of them immediately, before parental resources are wasted in attempted nurturing or development (the code has to successfully match the partner's, and function at least well enough to carry out the combination procedure, to produce progeny). The sexual combination can also replace or cover for bad stretches of code (the two combinitorial genomes are unlikely to have the same errors). Supporting this take is the observation that many incipient fertilizations fail in sexual reproducers, and cloning organisms such as bacteria in test tubes tend to produce noticeable percentages of crippled offspring, again partially obviating the numerical advantage of cloning's efficiency.

 

just a couple of thoughts.

Thanks. Will have to follow this one up

GIAN

PS any replies please keep as simple as possible. I aint a scientist (yet!)

GIAN

[Shinigami4]

Try to read: "Advantages of sexual reproduction" available on pubmed - Dev Genet. 1994;15(3):205-13.

 

You might get a better idea on the topic.

[Delbert]

I understand there is at least one amphibian that can be either asexual or non asexual depending on whereabouts it is in its environment. For example, the individuals at the boundary, where survival is more demanding, are non asexual. Whereas in the centre, where life is easier, they are asexual.

 

Indeed, I further understand the aphids on my runner beans are asexual during the peak munching time in summer!!

 

From the above it appears that sexual activity is helpful where survival isn't a stroll in the park.

 

So, when you're scraping along trying to hold a job down, struggling to work each day, paying bills, paying taxes, feeling exhausted at the end of the day, sex helps us to grin and bear it all!!!