- The Phanerozoic: what happened;
- The Cambrian Explosion;
- The New Phylogeny;
- Extinction and punctuated equilibrium;
- Animal Origins: The evolution of multicellularity;
- The axis-formers: The origin and evolution of bilaterial symmetry;
- Major radiations: invertebrates;
- Major radia...
Determining animal relationships
Two assumptions have influenced thinking about the relationships between animal groups:
1. Animals that look (or develop) more like one another are more closely related
2. Animal complexity has increased over evolutionary time within lineages
Question: Are these assumptions always justified?
Often, but not always
Animals are often similar due to convergence
Increases in ‘complexity’ within animal lineages are not, in fact, inevitable (some examples are
given in the following lectures).
Assumption/principle/concept that continues to inform our understanding of animal
relationships:
Parsimony
- Hypotheses (i.e. trees) that explain the evolution of the ‘observed’ animal diversity in the fewest
evolutionary steps are preferred
- We work on the assumption that if a tree can explain the animal diversity out there in a fewer
number of evolutionary changes then its more likely to be true than one that requires a large
number of evolutionary changes
- That in itself is an assumption - may not always be true
Evolutionary change- doesnt give the amount of change or the direction of the change, it can be a
gain or a loss of a trait can be increase or loss of complexity
Morphological phylogenies
2 misconceptions:
Morphological phylogeny - trees of animal relationships based on morphological characteristics
Molecular phylogeny - trees of animal relationships based on molecular characteristics
You can now have trees that are based on both characters together
- Morphological trees are based on extinct animals (fossils) and extant (living) animals
- Morphological characters are identified and scored (usually as present/absent).
- These data are then used to construct trees of animal relationship following defined principles
(one example being parsimony).
- Compare them by the number of characters the animals share with each other
- Subjective- characters you choose and processes you use to construct trees
Characters that unite a particular group of animals (i.e. that are not present in more ancestral
animals) are assumed to have arisen once shortly prior to the radiation of that group.
Synapomorphies- shared, derived characters
Animals that share more- the assumption is that they’re more closely related
The character evolved in the ancestor of the group and has been retained by all members of that
group
- Apomorphy- a newly evolved character
- Synapomorphy- apomorphy that is shared by multiple different groups of animals
,Most animals look different to each other
Embryological characters are often used since the major morphological differences between
animals arise during their embryonic development
An example of a ‘traditional’ morphological phylogeny
Potential
1. Synapomorphy:
a. Protostomy
b. Deuterostomy
2. Coelom (unites Coelomates)
3. Segmentation (unites Arthropod + Annelids into the Articulata)
• Closely related groups of animals
• Puts animals together based on their segmented body plan
• Earthworms and marine polyps have segmented body plan
• Arthropods, insects, millipedes, spiders- closely related - segmented body plan down anterior
posterior axis
Early animal embryogenesis
- Gastrulation leads to the formation of two (diploblast animals) or three (triploblast animals)
embryonic cell layers.
- Inner endoderm and outer ectoderm in diploblasts.
- In triploblasts a third cell layer forms between the ectoderm and endoderm - the mesoderm.
- Mesoderm can give rise to skeleton and muscle.
• Egg
• Complete cell division- 8 cell stage
• More divisions which gives rise to a ball of cells - blastula
• Cells cover the outside of the cell
• The inside of the ball is hollow - blastocoel
• Contains yolk which is feeding the cells
• Gastrulation happens
• Cells come into the interior of the cell and form a hole- blastopore
• Cells inside- endoderm - mid-gut
• Cells outside- ectoderm- epidermis and the lining of foregut and hindgut
1. Protostomy vs. Deuterostomy
Protostome
- ‘First mouth’
- The blastopore becomes the mouth
Deuterostome
- ‘Second mouth’
- The blastopore becomes the anus, and the mouth forms secondarily by fusion of ectoderm/
endoderm
,2. Acoelomates vs. Coelomates
Body cavity vs no body cavity
Diploblasts (Acoelomates):
2 layers
Blastocoel (B) largely disappears
Endoderm comes up against the ectoderm
Archenteron (A) becomes the ‘gut’(diploblastic
Small gap
In jellyfish- there are some cells that function and
some nerves that function within this gap- newly
understood
E.g. Cnidaria, such as anemones, jellyfish).
Triploblastic (Acoelomates):
3 layers
A solid mesoderm (M) replaces the blastocoel
E.g. platyhelminth, flatworms)
Triploblastic, (Pseudocoelomate):
3 layers
A mesoderm partially replaces the blastocoel
Cavity is said to be ‘disorganised’ and lacks a
membrane delimiting it – a peritoneum
E.g. nematodes
Triploblastic, (Coelomate):
3 layers
A mesoderm forms, itself invaded by a body cavity
(coelom)
E.g. polychaete annelid
Muscle fibres forming within there that work against the
body wall and allow these within the body cavity to work
against the limbs
Allows the animal to put pressure against the outside world
Animals historically would be quite passive and responsive
to water currents and movement on the sediment
Whereas with the evolution of a body cavity the animal is
much more likely to be able to interact with its environment
Grouping of annelids and arthropods on the basis of their body segmentation
Early embryogenesis -spiral cleavage- third cell division which gives rise to 8 cells –cells
twist – big macro cells on outside and micro cells
Assumption of putting these animal together based on their segmentation:
• Arthropods lost their spiral cleavage
OR
• Molluscs and annelids gained this independently which is less parsimonious- assuming 2
changes rather than 1
Molecular Phylogenies
Observed and compared molecules as well as morphology
Gene (nucleotide) and protein (amino acid) sequences have been used to determine
animal relationships
Phylogenomics- whole genome sequences have been sequenced and compared to
determine animal relationships with much more confidence (i.e. more data)
- Molecular characters are identified by making alignments of the sequence of conserved
molecules (genes or proteins) and scoring similarities and differences between them at
each conserved position
- Where divergence has occurred in the DNA or protein sequence of the animals
- You can count differences and calculate the level of similarity in the molecular sequence
across animals
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