Rise of Animals 3 Flashcards
What has largely been responsible for the revolution in our understanding of the relationships among animal phyla?
Sophisticated analysis of very large molecular data sets
What are the changes that have been made to the phylogenetic tree after the use of molecular techniques?
These confirmed that sponges sit at the base of the tree but they may be a paraphyletic rather than monophyletic group
Cnidarians may also by paraphyletic while ctenophoras appear to from a clade
Bilaterians are thought to represent three major clades, the protostomes (split into ecdysozoans and lophotrochozoans) and the deutrostomes
What are the ecdysozoans?
These are moulting animals taking their name from ecdysone which is the hormone which triggers ecdysis
Phyla belonging to this group include arthropods, tardigrades, onycophorans, nematodes and priapulids
What are lophoptrochozoans?
This group includes the annelids, molluscs, Platyhelminthes, brachiopods and bryozoans
It takes its name from the fact it unites the lophophore bearing phyla with those displaying a ciliated trochophore larval stage
What traditional evolutionary idea has been challenged by the new molecular based phylogeny?
Traditionally it was thought that life went from simple to more complex forms through gradual steps with extant forms representing grades of intermediate complexity that resembled ancestral forms
This meant that flatworms which lack a coelom were traditionally thought of as the most primitive of the bilaterians however the new phylogeny places this group as a monophyletic group in the lophotrochozoans suggesting they lost their coelom and became simpler organisms
How has the view of the development of the coelom changed with the advances in molecular phylogeny?
The traditional groupings placed pseudocoelmate animals above the acoelomate flatworms in one group as it was belived this reflecting what was thought to be the next grade in the evolution of the coelom
However the modern phylogeny splits this character with priapulids and nematodes belonging to the ecdysozoan calde and rotifers belonging to the lophotrochozonas
What results from the fact that modern phylogeny causes us to reject the view that phyla can be seen as grades in the evolution of bilaterian animals?
We must consider how they have evolved this is challenging as phyla are usually defined as taxonomic groups but have traditionally been differentiated on the basis of having distince body plans
This is a cladisitcially invalid term as it suggests all phyla are of the same taxonomic rank despite some clearly evolving from others
Why are all phyla old?
Because the body plans used to define them appeared early in animal evolution, the fact that few animal phyla seem to have evoled since the Cambrian is therefore not evidence that for lack of recent evolution in body plans in general
What occurs assuming that phyla have an evolutionary origin and that their characters and body plans must have originated in a particular order?
There is a logical decoupling between the body plan that the extant members of a phylum share and their phylogenetic relationships to each other meaning that early on in the history of the clade before many of the features of the group had yet to emerge members of sister group lineages belonging to different phyla must have been very similar to each other
What is Budds solution to the decoupling of body plan from phyla?
To define body plans as a set of features shared by extant taxa in a monophyletic taxa, this restriction of a concept of a body plan to extant phyla avoids the problem caused basal members of stem groups which may barely differ from the members of other stem groups
What does the new molecular tree tell us about the Cambrian explosion?
This new tree suggests that Cambrian explosion represented the simultaneous diversification of the three great bilaterian clades perhaps indicating that whatever occurred in the Cambrian was triggered by by an external ecological factor rather than a single genetic innovation
How does the molecular tree of life compare with the fossil record?
The oldest fossils of most bilaterian phyla appear fairly close together in the early Cambrian between 515 and 530 MYA (though trace fossils may push this back to 550 MYA)
The molecular clock however provides an estimate of the bilaterian clade divergence in excess of 1000 MYA, or if the fastest estimate is used the protostome and deuterstome split can be placed at 588 MYA but this is still not compatible with the divergence of all animal phyla in the early Cambrian
What explanations have been suggested for the discrepancy between the fossil record and clock divergence estimates?
Early animals may have been tiny and therefore did not leave recognizable fossils for a period of over 500 MY
Early bilaterian lineages evolved as larval forms and the sudden development of complex, large bodies was what we see as the Cambrian explosion
Early animals may have not fossilized because they were soft bodied or very rare so the Cambrian explosion represents the sudden evolution of hard skeletons or the sudden availability of suitable habitats for animals
Assumptions about the rate of the molecular clock divergence dates may be invalid
