Lecture 07 Tuesday March 25 2008

Homology

• common ancestry
• convergent evolution

for example wings

• arthropods and insects are the only species to have developed wings
• arthropods developed frame rodents
• 7 steps to get to the bat - bat is deeply imbedded evolutionary wise to mammals
• it doesn't have close relatives that fly
• when you see an anomalous trait deeply imbedded within the phylogeny
• when considering parsimony it is clear that the simpler solution would for the trait of wings to have occurred within the phylogeny rather than the majority of species loosing wings
• common ancester: the common ancestor did not have wings
• the wing structure of birds and bats are not homlogous

analgous

• convergence of function but independently derived
• still talking about wings
• evidence
• wings are analogous, but the bones are homologous
• using the forearm as a wing was a novelty arrived at independently

Enter Molecular Biology

• wingless gene in fruit flies - studied significantly even before micbio
• wingless was a popular molecule found all over the place
• responsible for not just making wings but segments in general
• a popular evolutionary gene
• but the name stuck

+

• fringe
• Distilliss
• genes are conserved in evolution and used in lots of different places
• embryonic buds or disks that turn in to chick and insect wings were suprisingly similar

• Dorsal ventral sides - back looks different from the front
• distilliss and fringe were very important in dorsal ventral variation
• some genes have the function of interacting with different molecules
• gene actoris has a specific region - it shows homologous to actoris
• you can usually take a limb gene from a mouse and save a fruit fly

fruit fly bones

• anterior posterior - fore arm biceps
• same protein as in fruit flys was found in mammals
• this was initially strange when approaching genetics from the one gene one protein hypothesis
• this was because the protein coded for bones but fruit flies don't have bones
• But off course molecules and proteins are a tool kit that can serve many functions
• this did not prevent early scientists from thing there was a deep homology

eyes

• 7 8 hundred cuticular lenses - made of skin cells
• vertibrates have their eyes developed from nerve or brain tissue
• sephlipodes - squid, octopus - molesques - thye have eyes like human eyes - single lense with retina
• the difference is that the photoreceptor faces backward - light has to go through a lot of tissue
• in octopus the light comes through and immediately hits the photorecptor
• in humans light hits the back of the photo receptor and passes through the pigmints
• all animals with eyes look for food, move quickly, actively search out food - a hunting strategy
• image forming eyes vs. light sensors
• these vision forming eyes were associated with a hunting strategy and arose independently

What genes are associated with sight?

• eyeless mutation
• mutations in mice - no iris forms
• eyeless was homologous to a gene in mice that destroyed the iris
• a mutation in one gene caused both eyeless and irisless - pax 6
• eyes seem to be convergently evolved

this paradox has to be resolved the same genes are able to destroy a trait

Old Genes for New Eyes

• ancient genes - some invented before animals were (pax 6)
• combines molecular basis of development and formational structure with homology and convergent evolution
• all genes are homologous but the structures they make are not homologous
• you can have bone homology without wing homology

April 10th