User:NC1328656/sandbox

Factors causing fluctuating asymmetry
Fluctuating asymmetry (FA) is often considered to be the product of developmental stress and instability, caused by factors including infections, mutations, and toxins. There are many possible variations which can contribute towards FA.

Genetic factors
Fluctuating asymmetry (FA) may result from a lack of immunity to diseases, as a positive correlation is seen between FA and the number of respiratory infections.

Asymmetry may also result from increased levels of mutations and perturbations, consequently having a negative impact on genetic fitness.

There is some speculation that inbreeding contributes towards FA, but some evidence shows this is not the case. For example, ant colonies created by an inbreeding queen do not show significantly higher FA than those produced by a non-inbreeding queen.

Environmental factors
Toxins and poisons are considered to increase fluctuating asymmetry (FA). Pregnancy sickness is argued as an adaptation for avoiding toxins during foetal development. Research has reported that when a mother has no sickness or sickness that extends beyond week 12 of gestation, the daughter shows higher FA (as demonstrated by measuring thigh circumferences).

During development, infectious diseases can also lead to FA, as an association between these factors is frequently reported.

Fluctuating asymmetry in human males is also seen to positively correlate with levels of oxidative stress. This process occurs when an organism creates excess reactive oxygen species (ROS) compared to ROS-neutralising antioxidants. Oxidative stress may mediate the association seen between high FA and amount of infection during development.

Developmental instability
Links to fitness and reproductive success.

Developmental instability of an organism can be reflected by their fluctuating asymmetry (FA).

Mate preferences and fluctuating asymmetry
Male physical attractiveness is rated lower when they show higher levels of oxidative stress, which is seen to increase as levels of fluctuating asymmetry increase.

Causes of fluctuating asymmetry
Fluctuating asymmetry (FA) is often considered to be the product of developmental stress and instability, caused by both genetic and environmental stressors. Various specific factors causing FA include infections, mutations, and toxins. According to Waddington's notion of canalisation, FA is a measure of the genome's ability to successfully buffer development to achieve a normal phenotype under imperfect environmental conditions. This suggests that FA is a result of genetic and environmental factors.

Genetic factors
Increased levels of mutations and perturbations can lead to greater asymmetry. In turn, this negatively impacts an individuals genetic fitness. FA may also result from a lack of genetic immunity to diseases, as those with higher FA show less effective immune responses. Supporting this, there is an association between FA and the number of respiratory infections experienced by an individual, such that those with higher levels of FA have more infections. Increased prevalence of parasites and disease in an organism is also seen more in individuals with greater levels of FA. However, the research in this field is predominantly correlational, so caution must be taken when inferring causation. For example, greater levels of FA may weaken immune responses of an organism, rather than a lack of immunity causing FA.

There is some speculation that inbreeding contributes towards FA, but some evidence shows this is not the case. For example, ant colonies created by an inbreeding queen do not show significantly higher FA than those produced by a non-inbreeding queen.

Environmental factors
During development, infectious diseases can lead to FA, as an association between these factors is frequently reported, such that those with higher FA report more infections. Toxins and poisons are also considered to increase FA. Pregnancy sickness is argued to be an adaptation for avoiding toxins during foetal development. Research has reported that when a mother has no sickness or a sickness that extends beyond week 12 of gestation, the daughter shows higher FA (as demonstrated by measuring thigh circumferences). This could suggest that when a mother fails to expel environmental toxins when pregnant, they affect the foetus and lead to FA.

Fluctuating asymmetry in human males is also seen to positively correlate with levels of oxidative stress. This process occurs when an organism creates excess reactive oxygen species (ROS) compared to ROS-neutralising antioxidants. Oxidative stress may mediate the association seen between high FA and amount of infection during development.

Antisocial behaviours
Some studies suggest a link between FA and aggression, but the evidence is mixed. In humans, criminal offenders show greater FA than nonoffenders. However, other studies report that human males with higher FA show less physical aggression and less anger. Females show no association between FA and physical aggression, but research has suggested that older female adolescents with higher facial FA are less hostile.

The evidence is also mixed in other animals. In Japanese scorpionflies (Panorpa nipponensis and Panorpa ochraceopennis), FA differences between members of the same sex competing for food determined outcomes of interspecific contests and aggression better than body size or ownership of food. Furthermore, cannibalistic laying hens (Gallus gallus domesticus) demonstrate more asymmetry than normal hens. However, this link between FA and aggression is questionable, as victimised hens also showed greater asymmetry. Other research corresponds with this. Hen eggs injected prenatally with excess serotonin (5-HT) exhibited more FA at 18 weeks of age, but demonstrated less aggressive behaviours. It is suggested that the stress introduced during early embryonic stages via excess serotonin may create developmental instability, causing phenotypic and behavioural variations.