rather are under the influence of stabilizing selection. Since behavioural traits are highly variable, mutation accumulation does not negatively affect all traits equivalently. Sex-ratio behaviour The study of sex allocation has provided some of the most convincing tests of adaptive behaviour. Theory predicts that organisms can adjust the allocation of resources to male and female offspring in response to environmental conditions. Sex ratio behaviour is the sex ratio response of a female in various conditions. Mutation accumulation is important because it is one evolutionary cause that increases variation between individuals in sex-ratio behaviour. For example, female wasps can adjust their offspring sex ratios by choosing whether to fertilize an egg because they are haplodiploid. In particular, female Nasonia vitripennis produce less males when laying eggs alone, and more males when laying eggs on a patch with other females. If female parasitoid wasps produce too few male offspring, then some of the female offspring will remain unmated. On the other hand, if too many sons are produced, then resources are wasted that could have been used to produce more daughters. Females of other strains show no similar conditional sex ratio behaviours. Researchers find that these behaviours are indeed subject to genetic variation. However, genetic variation in natural population is low and it has low heritability as for other fitness-related traits. The observation of this type of behavioural mutation has been argued to pose a problem for sex-ratio theory because the mutations are likely to have decreased fitness. Mutations affecting passive/aggressive characteristics Aggression is a survival trait that can be favoured by natural selection in nearly any species. Aggressive individuals can be better able to compete for resources including food, territory and mates, as well as more successful in protecting themselves and their progeny from predators. It can also be energetically costly, and extreme or out of context aggression can be disadvantageous or deleterious, especially in social organisms. Aggression is a complex trait that is regulated by many interacting genes and gene expression is highly variable depending on environment (phenotypic plasticity). Mutations in genes that influence aggressive behaviours can potentially increase aggression or passivity. Neurotransmitters Neurotransmitters, dopamine and serotonin in particular, play an important role in the regulation of aggressive behaviours. Many studies are focused on genes that change the way neurotransmitters interact with receptors within the organism. For example, when individuals suffer from a mutation that causes them to have low