For what the term 'gender' means in this essay, see the 'gender meaning' essay.
There are a number of theories about the origin of
- One theory suggests males differ from females primarily
as a result of disruptive selection acting on gamete size.
This theory originated with Parker, Baker, and Smith in 1972
Their theory suggests males differ from females primarily
according to whether they have small or large gametes.
It suggests isogamy (having gametes of equal size) is rare
because it is not a stable strategy.
Disruptive selection causes large nutrient-
containing gametes to evolve in conjunction with small
- Another theory proposes that gamete encounter rates
explain the difference in size - rather than nutrient
This theory seems to be due to D Dusenbery [2, 3].
- Another theory holds that males differ from females as a
result of selection against pathogens favouring a single
Reproduction via cell fusion has the potential to allow
pathogens from both parents into the offspring - increasing
its risk of disease.
An example of vertical pathogen transmission through egg
cytoplasm in mammals is the lymphocytic choriomeningitis
Strip searching one set of genes minimizes the chances of
- Another theory involves conflict between nuclear and cytoplasmic genes.
This theory is associated with Leda Cosmides and John Tooby .
Their theory suggests that males differ from females primarily
according to whether their offspring inherit their
organelles or not.
- Lastly, there is the theory that gender exists to
prevent inbreeding and selfing.
Gender exists to help prevent inbreeding - in many fungi.
These use 'mating types' to allow organisms to avoid mating
with themselves and their near relatives.
The genes responsible often involve multiple loci on
different chromosomes - and many alleles.
The result is a system which allows organisms to breed
with most of the rest of the population, except for
all but a small number of relatives.
While this indicates that gender exists to help prevent
inbreeding in fungi, it also indicates strongly that it does
not play that role very effectively in most animals
Having only two genders is a hopelessly inefficient
system if preventing inbreeding is the aim.
Under such a scheme, selfing is prevented - but you can
still breed with half of your closest relatives, and mate
choice in the general population is needlessly
A system with multiple mating types works much
better if preventing inbreeding is the goal. The two
genders of most plants and animals must exist for some other
Contingency vs inevitability
In the case of the cytoplasmic genes theory, gender
is a consequence of particular contingent events
during the early history of life, that resulted in living
organisms forming a symbiotic union.
The disruptive selection theory instead suggests
that gender is practically inevitable for complex
So: which is it, contingency or inevitability?
Inevitability gets my vote.
Even if organisms had no mitochondria, gender would
still exist, due to all the adaptive reasons
favouring it, as detailed in the list of theories of the
origin of anisogamy, above.
From this perspective, mitochondrial conflict is regarded as
being more likely to be to do with how gender exists in
living organisms than why it exists - since it would
still exist even if mitochondria did not.
Even our distant descendants - or advanced alien species -
will still make extensive use systems where there is a
division of labour between direct production of offspring,
and propagation of information about how
best to construct those offspring.
It is that division of labour which best characterises male
and female roles - not anything to do with organelles.
I think most biologists regard gamete size as the most
fundamental difference between males and females.
Richard Dawkins expresses this point of view in
The Selfish Gene:
One group of individuals has large sex cells, and it
is convenient to use the term female for them.
The other group, which is convenient to call male has small sex cells.
The difference is especially pronounced in reptiles and birds, where
a single egg cell is big enough and nutritious enough to feed a
developing embryo for several weeks.
Even in humans, where the egg is microscopic, it is still many
times larger than the sperm.
As we shall see, it is possible to regard all the other
differences between the sexes as stemming from
this one basic difference.
- Richard Dawkins, The Selfish Gene, 2nd ed. page 142.
He goes on to cite: "The evolution of anisogamy: a
game-theoretic approach"  - a paper about the
disruptive selection theory.
Gene conflict theory popularity
I note that many modern writers seem to have embraced Leda
Cosmides and John Tooby's more recent theory as though it is
the whole truth.
In The Red Queen  Matt Ridley asks:
To ask 'Why are there two genders?' is to ask 'Why are mitochondria inherited through the maternal line.'
Such a statement assumes the Cosmides/Tooby perspective - that gender
is all down to organelles.
He goes on to write:
Two genders have been invented, killer which provides the organelles, and victim which does not.
Similarly, in Mendel's Demon , Mark Ridley says:
The deep nature of maleness is to eject organelles from reproductive cells; The deep nature of femaleness is to keep them.
He goes on to make it clear that he regards gender as a contingency:
The theory implies that the existence of gender itself, in life on Earth, is an evolutionary fluke.
He says it depends on two accidents:
To be more exact, gender depends on two flukes, or events that look like flukes. One is that complex
life evolved via the merger; the second is that both DNA sets of the merger partners were retained in the
post-merger cell [...]
The comments above by modern authors ignore the adaptive
reasons for anisogamy and the existence of gender - and are
therefore seriously in error.
The story about organelles is an implementation
detail - not the sole cause.
Gender and anisogamy would still exist if organelles did not.
- The origin and evolution of gamete
dimorphism and the male-female phenomenon. Parker, G. A., R.
R. Baker, V. G. F. Smith. 1972. J. Theoret. Biol. 36
Selection for high gamete encounter rates explains the success of male and female mating types
Selection for high gamete encounter rates explains the evolution of anisogamy using plausible assumptions about size relationships of swimming speed and duration
Inheritance and Intragenomic Conflict by Leda Cosmides And John Tooby
- The Selfish Gene, Richard Dawkins, 2nd ed.
- The evolution of anisogamy: a game-theoretic approach - M. G. Bulmer, G. A. Parker
- The Red Queen, Matt Ridley, Chapter 4
- Mendel's Demon, Mark Ridley, 2000, around page 150