As
Fluffy the Cat suggests, there is a strong
selective pressure for a 1:1 ratio for many animal
species, including humans, but that is not necessarily the
absoute rule. The study of sex ratios is central to the
biological disciples of
demographics and
life history. In
many animal species, the basal sex ratio is not 1:1, and is so
for easily explained evolutionary reasons.
For example, look at the elephant seal. This species exhibits
extremely strong polygamy, where one male may have a harem of
up to 20 females, which he jealously protects. Thus,
non-dominant males may go years before being able to reproduce
(read: defend a harem themselves), and during this time have
zero fitness. Furthermore, each year that an adult male cannot
reproduce is another year where he risks mortality for zero
evolutionary reward. In this species, the sex ratio is roughly
4 to 5 females per male, and evolution dictates that it should
be such. If you were an elephant seal, betting your genes on
your offsprings' reproductive success, it makes sense to produce
more females; they will reproduce without much difficulty.
Producing a male is more risky, as the animal may never
reproduce, but can carry more reward, as one successful male may
produce hundreds of offspring. Thus, the best strategy to adopt
is to favour female offspring in order to maximize your
chances of passing your genes to subsequent generations, and
throwing the odd male in there in the hopes of winning the
lottery, as it were.