Sexual Selection, Sperm Competition and Sexual Conflict - An Introduction to Behavioural Ecology - page 181

isogamy anisogamy

Fig. 7.2 Parker
et al . (1972)
proposed that
anisogamy evolved
from isogamy by
disruptive selection
for two gamete
sizes: large gametes
with food reserves
(eggs) and small
gametes (sperm)
which parasitize the
investment of the
large gametes.

Gamete size Gamete size

Now, once larger gametes evolve there is immediately selection on smaller gametes to
seek out larger gametes to fuse with, in order to parasitize their food reserves. At the
same time, larger gametes will be selected to resist the small ones since the most viable
zygote of all will be one resulting from the fusion of two large gametes. However, fusion
between small and large gametes will predominate, simply because small gametes are
produced in larger numbers. Furthermore, the penalty for a large gamete failing to
resist a small gamete will not be so severe as that paid by a small gamete if it fails to find
a large partner; a medium sized zygote has a chance of survival, a small one has none.
This means that selection acts more strongly on the small gametes. Also, because
smaller gametes are made in larger numbers, they are likely to occur in a greater variety
of genotypes, and suffer a higher rate of mortality (both a consequence of numerical
superiority). Therefore, they will evolve faster and during evolutionary time will 'outwit'
the defences of the larger gametes.
Parker et al . show that, in theory, the end result of this ancient evolutionary arms race
will be individuals specializing on either the production of many, small gametes (males)
or the production of fewer, large gametes (females). Producers of intermediate sized
gametes lost out because they enjoyed neither the advantage of great numbers nor the
benefits of large food reserves for the zygote (Fig. 7.2).
One of the basic assumptions of Parker et al .'s model is supported by a comparative
survey of the family of algae called Volvocales. Unicellular genera, in which food reserves
for zygote growth are relatively unimportant, tend to be isogamous, while multicellular
genera, where food reserves play a role in zygote survival, are mostly anisogamous
(Knowlton, 1974); other families of algae show similar, but less clear trends (Bell, 1978).
A primordial form of gamete competition may, therefore, account for why there are
two gamete-producing morphs, namely males and females. Gamete competition can
also explain why the two sexes are maintained (Parker, 1982). Consider a cow's ovum,
which is roughly 20 000 times the size of a bull's sperm. Now imagine a mutant bull
which doubles its sperm size but with the consequence that its sperm numbers are
halved (because resources are limited). The larger sperm would increase the cytoplasmic
reserves of the zygote by just one unit in 20 000, which is a trivial benefit. There would,
however, be a huge cost whenever two bulls mated with the same cow because the

Small gametes
parasitize the
investment of
large gametes

Sperm
competition
maintains
anisogamy