: Why are males considered nonreproductive individuals? Oh, goody! This gets down to the c
: Why are males considered non-reproductive individuals?
This gets down to the crux of several biscuits:
--Why these are really the costs of anisogamy, rather than just sex.
--Why the _origin_ of sex is a different question from the
_maintenance_ of sex.
--What determines sex (i.e. gender) allocation ratio.
--Hurst's cool models for the evolution of sexes
--The basis for sexual selection (_real_ sexual selection)
--And much, much more!
(Don't worry, I'll only go into the first two in any detail).
First, let's define anisogamy. The simplest definition of anisogamy
is "the condition in which the gametes of some individuals are less
expensive than those of others." In the case of interest here, there
are two gamete types, and each can only fuse with the other type.
Hence, there are two sexes. [How this occurs is explained very well by
Hurst, particularly in combination with other models. But that's a
post for another day]. The cheap gametes are male gametes, and the
expensive gametes are female.
Now, since male gametes are cheap and female gametes are expensive, it
seems safe to make the following assumption: Success as a female is
determined by the resources available, whereas success as a male is
determined by resources *and*the*availability*of*females.* In other
words, male fitness is more variable than female fitness, and the
number of offspring from a mating is determined by the amount of
resources the *female* can allocate to reproductive function (i.e. the
number of eggs the female can make). This is (part of) what is known
as Bateman's Principle. [It is this greater variability in male
reproductive success which sets up the conditions for sexual
Note that the relative reproductive value of male function isn't
always zero, depending in part on sex (gender) ratio [though models
based on Bateman's principle predict that sex ratio will often be
50:50], which is why the cost of males isn't always twofold. Things
like male parental care, etc., also affect the cost.
So, why are they costs of anisogamy rather than sex per se? Because
without an asymmetry in gamete size, there's no "expensive" or "cheap"
gamete type, and therefore no Bateman's Principle.
And why are the _origins_ and the _maintenance_ of sex different
questions? Because when the mechanisms of sexual reproduction were
evolving, there was no anisogamy, and therefore no cost of sex. The
Hurst model and others argue pretty convincingly that anisogamy is
(almost) inevitable once sex is in place; once we have anisogamy there
is a _cost_ to sex, and the question becomes, "how do sexual
populations persist in the face of this cost and recurrent spinoffs of
Whew! I'm sure that wasn't very clear. Let me know where I can be
Oh, yeah - references. Again, I don't have a list with me, but here
are some author names & approximate dates, along with the subjects
they deal with (I'll get the full refs tomorrow):
Bateman's Principle/Reproductive Value of Males:
Fisher (1930,1958), _The Genetical Theory of Natural Selection_ (Book)
Evolution of anisogamy:
Hurst, Hurst & Hamilton (1992)
Fisher again, and Hamilton (1968?)
Cost of sex model:
Lively & Lloyd (1990)
Also see Maynard Smith, _The Evolution of Sex_ (1978, book) and
Williams, _Sex and Evolution_ (1975, book).
E-Mail Fredric L. Rice / The Skeptic Tank