BG2 Flashcards
Aristotle homology
distinguished between analagous and those that were ‘the same without qualification’.
- stimualted by enquiry into origin of cephalopod and vertebrate brain
renaissance homology
peirre belon
noticed simlarities between human and avian skeletons
idealistic homology
richard owen in the 19th century, founder of the NHM,
archetypal vertebrate, from which all vertebrate arose - fanciful and failed when tested against embryology
darwins homology
same by virtue of common descent
modern homology defintion
character are homologous when the genetic info required to specify them is similar by descent.
homoplasy in Algae Eaters
Homoplasy:
Galapagos marine iguana, Lake Tanganyika/Malawi cichlids and russian periwinkle all have flexible rasping stalked teeth like structures with multiple cusps.
snail = radula.
convergent evolution for algal feeders, most efficent method of algal scarping.
evidence for homology
- structural similarity
- fossils
- embryology
- genetic programme
Structural evidence for homology of the mammalian middle ear
homologous to reptilian jaw * same blood vessels and nerves involved in both Quadrate -> incus Articular -> malleus Stapes -> stapedial cartilage.
fossil evidence for homology of mammalian middle ear
in the synapsid reptile can see jaw bones have become smaller and been translated further and further into the middle ear
embryological evidence for homology of mammalian middle ear
incus and malleus of mammals and quadrate and articulate of reptiles all develop from first pharyngeal pouch and stapes and stapedial cartilage develop from the second
- early in vertebrate development neural crest cells migrate out of the newly formed dorsal neural tube.
- NC forms a series of pharyngeal pouches.
- — In mammals the incus and malleus develop from the first pharyngeal pouch, in reptiles the quadrate and articulate bones do.
- — in mammals stapes develops from second pharyngeal pouch and in reptiles stapedial cartilage does.
Haekel on homology
ontogeny recapitulate phylogeny
** development of an organism expresses all the intermediate forms of its ancestors throughout evolution.
ontogeny recapitulate phylogeny in mammalian middle ear
mallus can be see to remain attached to the jaw cartilage until late in development.
what are the different eye types
Vertebrates and cephalopods: lens eyes
Arthropod and Annelid: compound eye
Pax6 KO mutations
what is pax6
pax6 = TF found in metazoan eyes`
Aniridia: lack iris in humans, cant regualte size o pupils due to pax6 mutation,
eyeless drosophila: mutations in pax6
Pax6 overexpression drosophila
cause ectopic eye formation
are eyes homologous
all metazoan eyes are descended from a CA structure, a simple photorecepto specified by Pax6. general mechanism is maintained.
but eyes are not homologous
homology should be considered as degree of homology
Gene regulatory network for eye formation
pax6 is one commponent of network of TFs involved in eye specification.
vertebrates and insects both have GRNs of TFs that regulate eye development.
Topology of network differs but similar TFs are involved.
Homology of sine occulus and optx2
argument for homology
sine occulus gene in drosophila have two mammalian homologs six1 and six2, however these dont work in eyes but earlier in sensory placade.
mammalian homologs of another drosophila gene: optix have been found that work in the eye: six3/6
some argue that differences are evidence against eye homology other argue differences due to divergence.
Describe the relevance of thr GRN for limb formation for dung beetle horns
insects have a GRN that regulates limb formation
in dung beetles, this GRn have been clearly co-opted for use in horns.
evidence for cooption of leg genes in dung beetles
clone homologs of Drosophila genes from dung beetles and look for their expression patterns.
found dung beetle horns are made using leg genes: hth, n-exd, dac, DII
what is DII
distalous
controls proximal distal limb growth in insects,
loss causes loss of outgrowth of limb.
what does investigating genetic programmes for homology allow us to do
refine our notion of how structures are built when begin to attend to underlying genetic info.
