How and why do sex ratios vary?
Sex ratio will be interpreted as referring to the number of females in
respect to the number of males in a given population. Sex ratios have been
demonstrated to usually present a consistency within a community, in animals
the sex ratio being 100:100, that meaning for every 100 males born, 100
females are also born (Grant, 1998).
The sex ratio although usually
consistent, is not constant; it may alter over the course of a generation’s life or
from one generation to the next. Variations between populations and species
through space and time do occur (Warren & Beukeboom, 1998). It is how and
why this variation from the norm occurs that shall be considered.
As mentioned, the accepted standard approximate sex ratio is 1:1,
termed the Evolutionary Stable Strategy (ESS) (Hans et al, 1996). Sex
determination factors act to affect what the sex of an individual will be, thus if
these factors are biased towards either gender, the resulting sex ratio within a
population would be skewed (Charnov et al, 1981). In dioecious species,
numerous mechanisms exist to cause sex determination; haplodiploidy,
paternal genome loss, male/female hetrogamety, polygenic sex determination
or even environmental factors (Warren & Beukeboom, 1998). These sex
determination factors can alter within a species or between closely related
species, even changing if destabilized, leading to the evolution of new
mechanisms (Warren & Beukeboom, 1998). During this essay, several
different sex ratios may be referred to, each relating to a differing stage;
primary sex ratio is at fertilisation, secondary sex ratio is at birth, tertiary sex
ratio is upon sexual activity and finally quaternary sex ratio refers to post-
reproductive activity (Hardy, 2002).
It was Fisher (1930), who first suggested the theoretical explanation of
why the two sexes are usually produced in the same number, and thus a
basis of standard from which variation can be viewed (Hamilton, 1967). Fisher