Sex ratio variation and evolution
. The present study has two parts. In the first (Chapters
2 an 3) Fisher's theory of sex ratio is extended and in the
second (described in Chapters 4, 5 and 6) the methods of
quantitative genetics were applied to a laboratory population
of Drosophila melanogaster to evaluate the amount of genetic
variance in sex ratio.
In Chapter 2 the relationship between birth control, sex
preference and sex ratio was considered. It has been suggested
that in some human populations the desire for male or female
offspring coupled with the knowledge of methods of birth
control can influence the distribution of family sizes. The
consequences of this situation have been analysed using the
evolutionarily stable strategy concept. .
In Chapter 3, using precise genetic models, I have studied
the evolution of the sex ratio in a population in which interactions
between siblings exist but these interactions are not
symmetrical with respect to the sexes. Some interesting
differences between cooperative and aggressive models have
The second part of the thesis is composed of three
experiments. Experiment I was designed for investigation of
the variability in the sex ratio character and identification.
of the sources of heterogeneity apart from that due to
binomial sampling. The results showed a close agreement with
the expectations from random segregation and indicated that
genetic or environmental factors affecting sex ratio were
It has been reported that environmental factors can
"distort the sex ratio in several species. One such factor is
the age of parents. Experiment II was performed to study this
suggestion in Drosophila melanogaster in the hope of obtaining
some information about the mechanisms involved in this
distortion. The data indicated that this phenomenon, at least
in D, rosophila melanogaster is non-existent.
In Experiment III artificial selection for sex ratio was
performed. Nine generations of selection were unable to
increase the sex ratio of the population. Selection was very
effective in increasing the number of females, but the presence
of sex-linked lethals was shown to be responsible.
It was concluded that all of the empirical evidence
supports a Mendelian interpretation and gives no support" to
any theory invoking" adaptive sex ratio.