Chordate characteristics
- notochord
stiffening dorsal rod, pre dates spine
made of cartilage.
History of Phylogeny
Darwin 1850’s: tree of life, first phylogeny.
Haeckel (1870’s): hierachal tree of life, humans at the top.
20th cent: anglo american consensus phylogeny: Deuterosomes and Protosomes
what are deuterosomes and protosomes
Deuterosomes: anus forms first in embyonic development (humans, tunicates.)
Protosomes: mouth forms first in embryonic development (molluscs, anneldis).
why did traditional phylogenetics fail?
- phyla are ancient and distantly related to each other.
- objected methods were only developed in 1960s
- lack of suitable characters.
what were three traditional characters in phylogenetics`
- cleavage
division of cells in early embryo to produce blastomere cells which form the morula (compact mass) - blastopore fate
blastula invaginates to create the gastrula = gastrulation (cavity formation) - coelom foramtion.
principle body cavity: begins in gastrula stage
what are the two main types of cleavage
spiral cleavage (protosome): blastomeres divide they shift relative to each oher, dividing assymetrically producing a spiral arrangement. Radial cleavage: (deuterosome): blasometes roughly equal size are produced in a symmetrical arrangement
what are the two processes of coelom formation
- schizocoely (protosomes): coelom forms by splitting mesodermal embryonic tissue into two layers.
- enterocoely coelom forms from pouches pinched off of the digestive tract.
Who is responsible for the origin of cladistics
1950 - will hennig
what did will hennig argue
that we should
- attend order of which taxa branch out, not just degree of divergence.
- –> use cladograms to represent that order. - use shared derived (apomorphic) but not shared primitive (pleisomorphic) characters.
- —> Taxa with many shared derived characters were likely to be related.
what is the maximum parsiomony method
Method developed by hennig.
* assumes tree which requires fewest steps to explain data.
can be justified with probability.
* generally no longer used in phylogenetic reconstruction, as more explict methods used.
what was the origin of DNA as a character of phylogenetic study
18S rDNA sequencing, bp position.
evolves slowly so used for phylum level phylogenetics
what are the four main groups in the current tree of life
- Basal taxa (protostoma no longer used)
- Deuterostomia
- Ecdysozoa: identified by molecular bio, all shed a cutiicle
- Lophotrochozoa
what is a phylogeny
a hypothesis with statistical uncertainty subject to falcification as new data accumulates.
what is bootstrapping
a form of bayesian posterior probabilities.
calculates support or each node based on the random re-sampling of data and rebuilding of each tree.
what is the rate of constancy assumption
most methods of tree construction assume different lineages accumulates character state changes at a constant rate.
what happens if the rate constancy assumption is violated
algorthms will give inaccurate trees since rapidly evolving lineages will tend to be artifactually placed together.
The long branch problem
give an example of the long branch problem
C. elegans have a high rate of molecular evoltuion so analysis classifed them with lophotrochozoa despite high evidence as ecdysozoa.
what is long branch attraction
systematic error,
distantly related lineages which have a a similar molecular rate of change are incorrectly inferred ton be closely realted when they have more substituions in common with each other as they have more substitutions in total.`
what is the relative rate test
tests whether different lineages are evolving at the same rate
describe the relative rate test.
- simple phylogeny ((A,B)C)
K= no. o subs between species. count relative to outgroup. - if molecular clock is true than KAC = KBC.
- if not true this implies unequal rates of molecular evolution in lineages.
