The question of how sexual organisms evolved from asexual
ones was recently raised. This is one the most puzzling questions
in evolutionary biology today.
Sex is thought to be maintained because of recombination.
Imagine a hypothetical asexual organism with two genes, A and B.
Further imagine that, at time zero, the organism has allele A1 at
locus A and allele B1 at locus B. Now, if a mutation to from allele
A1 to A2 was beneficial and a mutation from B1 to B2 was also
beneficial, the hypothetical organism would have to have two
independent mutation events in it's lineage to reach the A2/B2
genotype. One mutation would occur (say A1 to A2) then this
resulting lineage would have to wait for the B2 mutation to
occur. But, if the organism were sexual, the A1 to A2 mutation
could occur in one organism and the B1 to B2 mutation could occur
in another and these alleles could be brought togethor by recombination.
In other words, populations can evolve faster if they are sexual.
This is especially true if the mutation rate is high and/or the
environment is unstable. It also saves lineages from Mullers ratchet
(asexual lineages can't "lose" deleterious mutations, they just
accumulate and fitness constantly decreases).
Note that this is a group selectionist argument. The
populations fitness is increased at the expense of the individuals.
Individuals, instead of making offspring that are 100% identical
to themselves, produce offspring with only half their genes. This
is often called the 50% cost of sex (of course this number actually
gets lower as the inbreeding coefficient gets higher).
In stable environments, lower recombination rates are
selected for (because recombination may just break up co-adapted
sets of alleles). Even more striking, some organisms revert to
asexual reproduction (or parthenogenesis) when the environment is
stable and undergo sexual reproduction in a changing environment.
Two examples are yeast and rotifers. The rotifers are a particularly
interesting example in that, when males are produced, they are
degenerate -- unable to feed themselves. They simply expend all their
stored energy to make sperm. Other organisms have reverted to
parthenogenesis altogethor. Two examples are the dandelion
_Taraxacum officinale_ and lizards in the genus _Cnimidophorus_.
The lizards are neat because in order for the female to repro-
duce, another female has to "mount" her to simulate sex and get
the right hormones flowing -- if that doesn't scream modification
with descent, I don't know what does!
All the above examples, and there are more partheno's
out there, have recently evolved from sexual species(*). The only
group of ancient, parthenogenetic organisms known is the
rotifer genus _Bdelloidea_ (not the same as the rotifers mention-
ed above). Most asexual lineages seem to go extinct quickly on a
geologic scale. Before anyone mentions bacteria, be aware that they
do indeed have sex -- much wilder sex than (from a phylogenetic pers-
pective) any animal, plant, fungus or protist could imagine. They
swap DNA (the essential process behind sex) with almost anything.
So, over the long term, sex seem to be an advantage (to groups if
Of course, proving something is adaptive now is not the
same as showing how it arose. This is, at the current time, a
*** Begin rabid, iconoclastic speculation ***
(take the following with one salt lick)
I happen to think that in the vast majority of sexual
species, sex is not adaptive; I believe it is retained as a
historical constraint. I think the mechanisms of sexual repro-
duction in most species have been fine tuned so much that it is
impossible to reverse them and organisms are just stuck with it.
"Why do think anything so gawd-awful silly?" Well, I'll give you
two reasons. One, a few species do revert to parthenogenesis (and one
author claims that these species then outcompete their sexual
rivals; I've got to check the evidence before I believe this, though.)
Two, I don't think genetic variation is always available for organisms
to make transitions -- even to simple adaptive solutions. I think most
sexual species are just stuck with a way of life that benefited their
ancestors. (Gee, and just about a year ago I was almost a caricature
of a pan-selectionist.) The ultimate adaptive strategy (IMHO) would be
being able (like the rotifers) to be asexual in a stable environment
why mix your successful genes with someone elses?) and sexual in an
unstable one (maybe your mate has an allele or two that would
compliment some of yours).
Of course, sex might return to being adaptive. In fact, it
may right now be returning to be adaptive. With the rate that
humans are destroying the biosphere, there are probably numerous
niches being opened up for some nice, sexual generalist to radiate
into. Who knows.
*** end mouth frothing speculation ***
(Now have a few beers to go with that salt lick -- try a Harpoon Ale
if you can find it in your neck of the woods. Else try Dock Street;
it's great tasting ale with a massive hoppy flavor -- but I digress.
In fact, I soon hope to digress to the local pub -- but that is
another digression. And digressions, like if I were to out of
the blue call Lionel Tun, "Looney Toon", should be frowned upon
as they distract the readers -- but I digress. I could attempt
to explain this stupid digression in terms of how lousy my day
has been, but alas, that would cause me to digress further.)
Here's a book that reviews recent thought on the evolution
of sex. I'm posting this instead of the reams of references I have
because you can find them all in this book.
Michod and Levins, 1988, The evolution of sex, an examination of
current ideas, Sinauer, Sunderland, Mass
Chris Colby ## email: firstname.lastname@example.org ##
"The college idealists who fill the ranks of the environmental
movement seem willing to do absolutely anything to save the
biosphere, except take science courses and learn something about
it." - P.J. O'Rourke "Parliament of Whores"