zlraa@marlin.jcu.edu.au (Ross Alford) (03/02/91)
I wonder why there seldom seems to be much discussion in this group? I know
for a fact that most population biologists have plenty of ideas circulating
in their heads much of the time. I suppose perhaps people hate to let go of
what they think are nice ideas in such a public forum? I thought I'd try to
stimulate a bit of discussion, anyway. Prepare for a moderately long
article.
First, some background: Twice a year, our current group of honours students
have to write an essay. They all get the same topic, and an initial
reference list. The latest group was given the topic "Why have sex?", and a
list of books to start their reference hunting with. These included some
moderately recent ones, such as Stearns ca. 1987, and a few others which
escape me at the moment.
I have been an interested spectator on this topic for some time. I did my
Ph.D. at Duke, where Marcy Uyenoyama has some interest in the evolution of
sex, and participated in a weekly discussion group where such things came up
from time to time. I haven't really concentrated on it though, so may come
up with something really naive (such fears are probably one reason for the
lack of discussion on this group, actually...)
Anyway, several things struck me in reading the student essays, and in light
of my sketchy background info on the topic. First, there still seem to be
some fundamental confusions in the literature on both the evolutionary
origin and the evolutionary maintenance of sex. For one, many animal-
oriented authors seem to treat sexual reproduction as something that
*requires* dioecy, and thus the production of male individuals who do not
directly contribute to increasing the number of offspring in the next
generation. It struck me that the field might best be treated on more
levels than usually seen:
1) Evolutionary origin and maintenance (EO&M) of increased recombination
2) EO&M of anisogamy
3) EO&M of dioecy
I am not going to say anything about level (1), except that I would
speculate that this ordering isn't entirely arbitrary, in that it
probably reflects the historical order of appearance of the phenomena. This
classification suggested a couple of interesting ideas to me, which
have not appeared in any of my limited reading in the field. First, I
wondered whether anisogamy may not be the almost inevitable outcome of life-
cycles involving haploid and diploid phases because it is the only ESS? To
elaborate slightly:
Imagine a multicellular organism which reproduces via meiosis followed
by gametic fusion, with outcrossing the norm. Say it is isogametic.
Being rather pan-selectionist for the moment, imagine that the size of
gametes produced will be the outcome of selection over a long period
for zygotes carrying sufficient nutrients to have a reasonably high
rate of survival through many initial divisions, until the organism can
begin acquiring nutrients from an outside source. It is likely that in
the real world, anyway, the size of the zygote will be > the minimum
size necessary for survival, since it must cope with fluctuations in
temperature, nutrient availability, etc. This means that there is
usually room for cheating: some individuals can produce slightly
smaller gametes, which allows them to produce slightly more of them.
This of course might reduce the long-term mean fitness of the
population, while increasing the short-term fitness of the cheaters.
One (perhaps the only?) way out of the problem is to produce two
specialised types of gametes: ones with the absolute minimum invested,
to be broadcast as widely as practical in as large numbers as
practical, and another sort which are themselves sufficient to give the
zygote a reasonable chance of survival.
Right. There's an explanation for anisogamy. Not necessarily for dioecy,
though. Individuals could still produce both sorts of gametes. Next, the
ESS argument for evolution of dioecy:
Now, say we've got a population that's passed through the above trauma,
and emerged as monoecious, simultaneous hermaphrodites. At mating
time, individuals come together, provide microgametes to each other,
use the supplied microgametes to fertilise their macrogametes, and all
s well. Until, that is, cheating once again rears its ugly head. Some
individuals provide microgamates, all right, and initially anyway are
happy to take in the microgametes of others, but rather than expend
time and energy on macrogamete production, they just dump the
microgametes of their mates and go off looking for more individuals to
fertilise. Macrogamete producers who recognise and mate with such
cheaters gain in inclusive fitness, since they produce cheating
microgamete-producing offspring as well as macrogamete producers, and
end up leaving more grandchildren. Fisherian sex-ratio theory comes
into play, leading to populations of half microgamete-producing males
and half macrogamete-producing females.
Aha, you say, what about plants? There are plenty of monoecious
plants. I suspect the critical thing here is behaviour. It may be
easier and a better bet for an animal to pursue the parasitic-
microgamete-producing-cheater lifestyle in a largely monoecious
population than for a plant under similar circumstances. Animals can
move about seeking mates, engage in aggression and resource defence,
and so on. Plants have more limited options. This may account for the
differing rates of occurrence of monoecy in the two groups.
So, perhaps both the origin of anisogamy and the origin of dioecy are due to
similar evolutionary processes operating at two levels, with anisogamy
preceding and providing the necessary climate for dioecy?
I look forward to seeing some discussion! If this goes well, I have at
least one or two other half (or less) baked ideas to try on.
Ross
--
Ross A. Alford zlraa@marlin.jcu.edu.au
Department of Zoology +61 77 81 4732
James Cook University
Townsville, Queensland 4811 Australia