Source: here.
We flew up again with the yakShini. There we stood in the mid-welkin and
bewitching yakShini whispered in my ear:
“We need to talk about about a profound question”.
Me: What is it?
Y: Of all those sexually reproducing beings out there, the majority of
them are heterogamous There are some like Chlamydomonas that produce
similar-looking gametes- the male and female gametes are the same. But
the majority of cases have converged on clear anisogamy or heterogamy.
Me: That is indeed so. Why so ?
Y: Some have claimed that it is due to disruptive selection: The
selection favors bigger gametes on one end of the spectrum because they
can deliver more resource to the zygote and there by improve its
survival (and there by their own fitness). Thus you get the egg. On the
other end the more gametes you produce the more chance of propagating
your genes. So your produce many little gametes- the sperm. Naturally
you have a disruptive selection that favors both extremes and nothing in
between because they will be less efficient in their chosen tasks. Thus,
we get anisogamy.
Y: But now tell me the problem with this reasoning.
Me: Since most organisms start of isogamous the first guy who reduces
his gamete size in the quest for making more gametes will deliver less
resource to the zygote than an isogamous member. So reduction will be
disfavored. This will kill the very emergence of anisogamy. So anisogamy
will take place only if some way simultaneous you have guys going small
and other correspondingly going big.
Y: See these centric diatoms- they are oogamous- anisogamous to the
extreme. How do you think this evolved.
Y: Do you think we know the answer to this issue? Think on oh mAnava.