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Author Topic:   Asexual to sexual reproduction? How?
EZscience
Member (Idle past 5153 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 14 of 78 (245744)
09-22-2005 1:48 PM


Evolution of sexuality
There have been a number of major books written on the evolution of sexuality that anyone truly interested in understanding the problem should read. You might begin with GC Williams "Sex and Evolution". Another, more advanced and formal treatment is Graham Bell's "The Masterpiece of Nature". Essentially, sexuality must provide a fitness advantage to individuals over asexuality to evolve and become fixed within a population. We can ignore the mechanics for a moment, some primitive forms of which have already been discussed below. It is much more important to understand the 'why' part of the question. Many hypothetical advantages have been proposed for evolution of sexuality and they are not mutually exclusive. We might touch on them one at a time, but there are far too many to deal with in a single post.
Let's review some facts first so there is stable ground for a discussion.
Asexuality is the primitive condition.
However, the vast majority of higher organisms are obligately sexual.
Sexual reproduction leads to more genetic variation in progeny than does asexual reproduction.
Sexual reproduction exacts a significant fitness cost for an individual compared to asexual reproduction, known as the 'cost of meiosis' (Maynard Smith). This arises because females only obtain 50% genetic representation in each offspring instead of the 100 % they would obtain if they produced asexually (all daughters the same as themselves). This means we must show how a sexual female can gain more fitness (reproductive success) than an asexual female.
Thus, the advantages of genetic diversity in one's progeny must somehow more than compensate for this cost in order for sexuallity to evolve.
I think there is great insight to be gleaned from organisms such as aphids (that I happen to study for a living, among others) that employ BOTH modes of reproduction. We can see that when environment is favorable (and food supply abundant and predictable) all reproduction is asexual. Males and sexual females are only produced at the end of the season when unpredictable conditions are faced (overwintering etc.) and a diverse range of progeny increases ones chances that some will survive. Williams calls this the 'lottery strategy'. Would you want to hold a thousand lottery tickets all with the same number, or a thousand with different numbers? However, this works only for organisms producing large numbers of offspring and investing very little in each.
There are many other scenarios where the cost of meiosis can be overcome. Perhaps I will have time to cover some others later.
Is anyone familiar with the 'Red Queen' theory? Named by Bell for the red queen in Alice in Wonderland who stated: "sometimes it takes all the running you can do to stay in the same place". It hinges on frequency dependent selection operating on populations inhabiting unpredictable or capricious environments.
Sex seems to pay off whenever uncertainty prevails for the next generation.
This message has been edited by EZscience, 09-22-2005 12:55 PM

Replies to this message:
 Message 19 by Cal, posted 09-22-2005 3:37 PM EZscience has replied

  
EZscience
Member (Idle past 5153 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 17 of 78 (245760)
09-22-2005 3:24 PM
Reply to: Message 15 by clpMINI
09-22-2005 2:18 PM


Re: Present day critters
clpMINI writes:
Zebra mussels are hermaphrodites...only takes one to tango.
Not exactly. A simultaneous herm cannot mate with itself - it needs a second individual. However, sperm are exchanged in both directions and eggs are fertilized in both individuals.
Simultaneous hermaphroditism seems to have evolved in species with very limited mobility in the adult stage. The advantage is that even if you only encounter one conspecific in your lifetime, sex will still be possible.
Hermaphroditism is a special case and really just one of the epiphenomena of sexuality.
Note also that sequential hermaphroditism (individuals changing from one sex to the other in their lifetime) is another special case and quite distinct from simultaneous hermaphroditism.
I do agree that extant examples of facultative sexuality can provide valuable insights into how and when sex is advantageous.

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 Message 15 by clpMINI, posted 09-22-2005 2:18 PM clpMINI has not replied

Replies to this message:
 Message 23 by Dr Jack, posted 09-23-2005 4:35 AM EZscience has not replied

  
EZscience
Member (Idle past 5153 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 18 of 78 (245762)
09-22-2005 3:34 PM
Reply to: Message 8 by Eledhan
09-22-2005 9:37 AM


Re: What?
I don't think you really understand how natural selection can work.
For example, imagine a simple aquatic organism that produces propagules capable of parthenogenetic development (no sex). In some cases these propagules might fuse with otherS OF the same species prior to initiating development. Assuming the diploid offspring SO produced had higher fitness than the haploid (parthenogentically produced) offspring, you could have selection for sexual reproduction. Initially, organisms would not have to abandon asexual reproduction and the population would consist of a mixture of both haploid and diploid individuals, although only the haploid ones would be able to reproduce sexually. The tricky part is the subsequent evolution of a mechanism to restore the haploid state to germ line cells so that diploid individuals could then also reproduce sexually. This is what meiosis does in all sexual individuals today.

This message is a reply to:
 Message 8 by Eledhan, posted 09-22-2005 9:37 AM Eledhan has not replied

  
EZscience
Member (Idle past 5153 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 20 of 78 (245773)
09-22-2005 4:38 PM
Reply to: Message 19 by Cal
09-22-2005 3:37 PM


Re: Red Queen
Cal writes:
I thought it was the "Red Queen Hypothesis"
Yes, quite correct. My carelessness.
Cal writes:
But I'm still fuzzy on who exactly it was that first invoked the Red Queen metaphor
It was Graham Bell in his 1982 book.
I actually had him as a supervisor of an undergrad thesis project I did at McGill Univ. back in the 70's.
Cal writes:
"ready to breed" would have been better than "pregnant", I suppose
Actually, that's exactly what I call sometimes it when I lecture on aphids. Tony Dixon would call it the 'telescoping of generations'. Each nymph actually has embryos developing within it at birth, so you essentially have 3 generations represented in a single individual !

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