As I posted, I was sure that a Darwinist would come up with that explanation–the only one possible. A shark kept in an aquarium is supposedly given birth via parthenogenesis. But then the question becomes: how did it evolve? When it evolves, sure, it’s better; but HOW did it evolve? What’s the mechanism?

As to “topping the top”, how does an organism that is the “fittest” become “more fit”? But lest you suggest that parthenogensis is such an example, please first explain the mechanism?

Well that’s sure a fine theory, b. But if that’s your story and you’re sticking with it you’re stuck explaining why, in the fossil record, species with obligatory sexual reproduction appear abruptly, hang around unchanged for an average of 10 million years, and then 999 times out of 1000 go extinct without splitting off any new species.

Sexual reproduction allows deleterious mutations to accumulate in individuals heterozygous for the mutation. That’s fine in a large population where the mutations aren’t likely to match up. But if the environment becomes adverse and reduces the population then all of a sudden the effects of inbreeding show up. Deleterious mutations start becoming homozygous and the small population quickly dies off. Parthenogenesis resulting in individuals homozygous at all loci would thus be rare; a last ditch effort at survival because in an old species it would bring out all the deleterious mutations previously hidden by heterogeneity. Parthenogenesis is worse than doing it with a cousin, it’s doing it with yourself, inbreeding on steroids so to speak. But hey, that’s just my theory. Don’t let its ability to explain the facts cause you to abandon any explanation that can’t. NDE couldn’t pass the giggle test if it had to fit the facts.

I wasn’t precise. What I meant is regarding two non-allelic mutations (not copies).

In other words, say the deleterious selection coefficients were designated s.

The fitness of an individual, in the scenario I mentioned, with 1 mutation would be (1-s). If the fitness of an individual with 2 mutations were less than (1-s)^2, then the individual with two mutations would be disporportionally less fit than individuals with just one mutation. Thus, sex may be favored. Of course whether selection acts like this is not known.

Another one of them is that harmful mutations are more than double harmful when in two copies.

That’s the wrong POV. Only one copy in a diploid organism is half as harmful as one copy in a haploid organism. Thus harmful mutations can accumulate until they eventually drive the species into extinction. Allelic mutation allows rapid adaptation but the eventual cost is extinction.

As you point out, the question of why sex is maintained is a big question. As Bob points out, there are many theories. Another one of them is that harmful mutations are more than double harmful when in two copies. Thus, when brought together by recombination (ie, sex) they are purged much faster in the population. Thus, sex may allow populations to avoid the slow decline of deleterious mutations due to Mullers Ratchet. This argument has many complications however – one being, as you point out, selection doesn’t act strongly on the “benefit to the population”.

However, it has been shown to be true that very few asexual lineages have lasted a long time. If one makes a phylogeny, one will see that most asexuals are at the “tips” of the tree – there are not a lot of deeply diverging asexual clades.

One major exception to this – the bdelloid rotifers (thus my handle !).

My question is, absent of such catastrophic changes in environment (which may or may not provide temporary greater than 50% increases in fitness) why don’t the animals with the ability to reproduce in both modes (such as these dragons) return to favoring asexual reproduction?

Again, if fractional gene passing can explain altruism in Kin Selection theory, why does it not apply here, when we see an obvious “selfish gene” benefit?

Go on, follow the link, you know you want to

Bob