Palaeo - Higher classification

Question 1

Portraying evolutionary history

Answer 1

• A phylogeny is the evolutionary history of a lineage, most clearly represented as a phylogenetic tree.
• A tree implies a number of branching points (nodes), corresponding to cladogenesis: one lineage becoming two.
• Between each pair of branching points is a branch, corresponding to some period of evolutionary time, in which a lineage may have accumulated morphological change.
• Morphological changes that arise once (i.e. on a single branch) and are retained in all descendants are the backbone of phylogenetic reconstruction.

Question 2

Groups on an evolutionary tree

Answer 2

• Such characters define a monophyletic group (from greek mono-, single, - phylum, tribe; also referred to as a clade), which comprises a single common ancestor and all its descendants.
o Single group with single common ancestor
• A paraphyletic group contains a common ancestor and some, but not all, of its descendants (e.g. fish)
o Group includes common ancestor and some but not all descendants
• Whereas a polyphyletic group (many-tribes) comprises lots of different groups, but not their common ancestor (e.g. vermin).
o Multiple tribes, no connected common ancestor

Question 3

Classifying extinct organisms

Answer 3

• Fossils, particularly old ones, are often harder than living taxa to classify. This is because, on account of their ‘primitive’ condition, they often lack some or all of the key characters that identify a modern group. (Cladogenesis is not necessarily accompanied by immediate morphological change.)
• Rigid Linnean terms – the hierarchy of kingdom, phylum, class, order, family, genus and species – lack the flexibility to accommodate early offshoots of the evolutionary lineages leading to extant clades.
The stem group concept is what allows ‘transitional forms’ at the ‘fuzzy edges’ of categories that are defined based on living taxa to be related to a rigid hierarchical system.

Question 4

History of defining taxa?

Answer 4

•	Linnaeus (1735) - six ‘body plans. 
•	Cuvier (1817) emphasized the disparity of Linnaeus’s “Vermes”, instead using the pre- Darwinian concept of a “chain of being” to classify organisms by complexity, and thus sophistication – with man just one step lower than the angels. 
o	Body plans
	Spinal cord
	Ventral nerve
	Diffuse nervous system
	Linked nerves

• Until the 1990s, groups were defined based on a shared evolutionary history, inferred from shared homologous features – for example, body segmentation.
• But morphological characteristics are prone to convergent evolution: similar features – indeed similar overall morphologies – may arise in distantly related organisms in response to similar physical, environmental or ecological pressures.

Question 5

Modern ways of defining taxa

Answer 5

• In the new age of cheap molecular sequencing, some long-standing relationships have been overturned on the basis of shared genetic sequences that can only be so similar (we assume) if they were inherited from a common ancestor.
• How different do things need to look before they are different phyla / classes / orders?
• There’s no hard and fast rules, but higher taxa seem nevertheless to be useful units of study.

Question 6

Morphological classification

Answer 6

• Phenetics aims to group taxa according to their similarity, rather than their evolutionary relationships.
• Taxa that look similar are grouped together, whether or not their similarity is inherited from a common ancestor. (For this reason, phenetic groups are often polyphyletic.)
• Phenetics can sometimes get a bad press, and there is no consistent framework for ‘phenetic classification’, but it offers a neat way of exploring how the nature of organisms has changed through time.
• Organisms might be grouped by morphological similarity, or by ecological similarity.
o A fundamental ecological distinction might be made between autotrophs (self-feeding – often photosynthetic) and heterotrophs (other-feeding – grazers or predators, perhaps)

Question 7

3D classification cube

Answer 7

• uses three primary dimensions of tiering, feeding and motility, which can be exploded into a 6×6×6 cube; the occupancy of this cube through time offers an interesting window on how ecological possibility has been more thoroughly explored through the Phanerozoic.
• Communities that look very different – perhaps separated by space or time – may have a similar underlying ecological structure; equivalent niches may have different occupants, yet the overall structure of the ecosystem may be the same.
• Bambach et al, 2007