Fluctualting Asymmetry Flashcards
What is fluctuating asymmetry?
small random deviations from perfect symmetry in a trait which is normally bilaterally symmetrical.
what is Directional asymmetries
when it is normal for one side to be bigger than the other and the same directional bias is shown by most individuals
what is antisymmetry?
normal to have one side larger than the other but there are similar numbers of individuals of each type
why is FA different to anti or directional symmetry?
caused by stress in development
failure of normal dev programme
optimum is known
how can FA be used?
An assay of environmental stress eg in environmental monitoring
Absolute magnitude of FA is an assay of an individual’s ability to cope with developmental stress
Use in evolutionary biology
what are 5 environmental causes of dev stress?
environmental:
1. temperature - eg Mpho 2002, reared mosquitos at different temps, which destabilised development, causing in wing shape asymmetry.
2. Chemical - Zakharov 1997, found FA in grey seals skeleton to be higher in more polluted years
3. pop density - Zakharov 1991, FA higher in shrews in years with high breeding density.
4. Parasites: Moller 1992.swallows with more mites had more FA in males length of tail feathers.
5. dietary - Swaddle and Wittler 1994. higher FA in food deprived starlings primary feathers
what are 4 genetic causes of stress?
- loss of genetic variation - Northern elephant seal pop crash at the end of 19th C. More FA in skull shape after bottleneck. Hoelzel 2002.
- Hybridisation - Wisley et al 1998. hybrid birch tree strains have more FA in leaves.
- New mutations of major effect - blowflies developed resistance to Diazinon due to single mutation. initially big increase in FA of resistant flies. dev destabilised by new allele. soon after a modifier evolved which reestablished development.
- Intense directional selection - De Coster et al, 2013. De Coster et al. (2013)
4 African rainforest birds underwent recent shifts in habitat quality
Directional selection for different body size.increased size = higher tarsus FA.
is FA heritable?
the level but not direction is heritable
unsure whether due to genes or environment.
What does FA tell us about ‘good genes’ model of sexual selection
Are secondary sexual ornaments particularly sensitive indicators?
Moller 1992 - male swallows tails affected by stress, whereas females not affected.neither sexes wings affected.
FA in secondary sexual characters about 10% whereas only 1% in other characters. Balmford 1993 disagrees.
is symmetry assessed in mate selection?
Moller 1992 - manipulated tail streamer length and symmetry. 2 control groups.
Long symmetrical tailed swallows pair earlier and fledge more.
however, in this experiment, flight probably affected too.
another experiment which tests is FA is assessed in mate selection
Swaddle and Cuthill 1994.
zebra finches find particular coloured leg bands attractive.
Preferences in choice chamber correlate with ‘free’ mating patterns
Manipulate leg band symmetry, whilst holding total colour constant
Females spend more time looking at males with symmetrically arranged leg rings.
human asymmetry in sexual selection?
Swaddle and Cuthill 1995
morphing allows easy manipulation of faces
found symmetrical faces less attractive?!
increasing symmetry of initially unattractive faces made them more attractive.
why would symmetry be assessed?
perhaps asymmetry signals a developmental problem.
however, most faces are fairly symmetrical, so maybe symmetry helps in finding the best mate.
unlikely to be able to judge symmetry when a face is usually moving and not facing you directly.
could symmetry be a byproduct of how categories are learned?
Enqvist and Arak 1994
Neural networks trained to prefer some patterns and avoid others
Patterns come in ‘left-bigger’ and ‘right-bigger’ forms.
Networks prefer symmetrical patterns even if they’ve never seen them before, because the average of all patterns is symmetrical
