Taxonomy and Phylogeny

Question 1

Portugese man o war Physalia Physalis
Is it a jellyfish? If not why not?

Answer 1

Most jellyfish are in the classes scyphozoa and cubozoa, man o war is in class hydrozoa but all are in the phylum cnidaria.

Question 2

Linnaen system for Hierarchial classification

Answer 2

• Genera contained within famlies etc
• Taxon is the taxonomic unit at any level of the hierarchy
• Genera and species names are always in italics
• Higher taxa have capitals
• Latin binomials are comprised of Genus and species name
• In scientific papers the person who described the species is sometimes listed at first mention
  Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species names

Question 3

Phylogenetics

Answer 3

The aim of phylogenetics is to make hypotheses about evolutionary relationship
Phylogenetics often informs taxonomy but taxonomy is slow to change and can retain groups in defiance of phylogeny

Question 4

Cladistics

Answer 4

Phylogenetics based on traits (predates DNA sequencing).
* Primary criterion is to identify groups that share a unique common ancestor.

Question 5

Clades

Answer 5

Ancestral species and all its descendants
* Clades can be nested within larger clades
* Taxon (Linnean system) is only equivalent to a Clade if it is monophyletic.

Question 6

Phylogenies

Answer 6

• Made up of Branches of Nodes
• The Phylum Cnidaria is a monophyletic group comprising an ancestor and all its descendants
• This is a hypothesis about evolutionary relationships

Question 7

A polytomy and a sister taxa

Answer 7

A polytomy :more than 2 lineages diverge from a single node
Sister taxa - share an immediate common ancestor

Question 8

Monyphyletic grp

Answer 8

ancesteral species and all its descendants

Question 9

Paraphyletic grp

Answer 9

ancesteral species and some of its descendants (eg invertebrates)

Question 10

Polyphyletic grp

Answer 10

taxa with different ancestors

Question 11

Creating Phylogenies

Answer 11

Important to use traits that result from common ancestry
Similar features due to common ancestry are Homologous structures or “Synapomorphies”
An example is the nervous system- present in all Eumetazoa, including Cnidaria- but not in Sponges

Question 12

Convergent evolution

Answer 12

• Gives rise to Analogous similarities
• Similar environmental pressures & natural selection produce similar adaptations in organisms from different evolutionary lineages
• Appear similar but internal anatomy, physiology & reproductive systems are different
  Distinguishing between homologous & analogous traits is critical in constructing Phylogenies

Question 13

Origin of Sponges and all animals

Answer 13

Choanoflagellates:
Common ancestor animals & choanoflagellates lived 875-675mya
Stationary suspension feeders

Question 14

Evidence for Choanoflagellate ancestor

Answer 14

1. Choanoflagellates & the collar cells (Choanocytes) of sponges are virtually identical
2. Collar cells only found in animals
3. DNA sequence data suggests Choanoflagellates are a sister group to animals
4. Signalling & adhesion genes known only from animals have been found in Choanoflagellates

Question 15

What Phylogenetic Tree doesn’t tell us

Answer 15

• Sequence of branching does NOT indicate age
• If trees are based on morphology /anatomy the branch lengths do not indicate time (but see exercise on molecular clock later).
• Cannot assume a taxon evolved from the taxon next to it; eg humans did not evolve from chimps

Question 16

Phyogenetic trees as hypotheses

Answer 16

Make and test predictions based on your tree (predict features shared by closely related groups would be present in their common ancestor and all of its descendants
Used extensivley regarding dinosuars
Fossil evidence supports the idea that some dinosaurs built nests and brooded eggs

Question 18

Molecular clock hypothesis

Answer 18

DNA and protein sequences evolve at a rate that is relatively constant over time and among different organisms

Question 19

Problems with molecular clocks

Answer 19

Regularity of change over time thought to be due to genetic drift. This is random accumulation of mutations with little effect on fitness
Molecular clock only ticks regularly for mutations that are ‘neutral’
DNA regions under strong natural selection are not neutral and not good for phylogenies
Clock rate can still vary between taxa and different regions of trees
Abundant fossil record only goes back -550my