Bilateria and Bilaterian Phylogeny Flashcards
What is a phylogeny
An evolutionary tree showing relationships between organisms and who is closely related to whom
What is taxonomy
Classification system — naming and grouping organisms based on shared characteristics (not necessarily evolutionary)
How should a phylogenetic tree be read
From bottom up - the bottom represents ancestral forms
Can nodes rotate in a phylogenetic tree
Yes, rotating nodes doesn’t change relationships
Does the horizontal branch length in a sideways tree represent time
No - sideways branches don’t represent time or distance
What is a monophyletic group (clade)
A group that includes a common ancestor and all its descendants
What is a paraphyletic group
A group with a common ancestor and some, but not all, descendants
What is a polyphyletic group
A grouping of unrelated organisms from different ancestors
How are phylogeny and taxonomy connected
Phylogeny informs taxonomy by providing evolutionary relationships; taxonomy names/classifies those groups
What morphological features were traditionally used to create phylogenies
- Symmetry (bilateral vs radial)
- Germ layers (bilaterians have 3)
- Body cavities
- Segmentation
- Cleavage patterns in embryos
What are three major problems with using morphology for phylogeny
- Loss of traits in some lineages
- Convergent evolution (similar traits evolved separately)
- Too few characters to resolve deep relationships
Why is DNA more reliable for phylogenies
Because mutations accumulate gradually, and closely related species have more similar sequences
What features make a gene useful for phylogeny
Evolves slowly
Same function in all species
Conservative, universal, and informative
What gene was first used widely for phylogeny
18S rRNA (ribosomal RNA)
What are the benefits of 18S rRNA
Same function in all species
Short (~2000 bases)
Well-studied, with a large database
What is the advantage of using 1000+ genes now
More data reduces errors and gives a more robust tree, thanks to cheaper sequencing
What problems can still occur in molecular philogynies
- Loss/reversal of genes
- Convergent mutations at single bases
- LBA (Long Branch Attraction) – fast-evolving species incorrectly grouped
What symmetry defines Bilateria
Bilateral symmetry (left-right body plan)
What key body features define bilaterians
Head-to-tail axis
Centralised brain & sensory organs at the front (anterior)
Nerve cord
Muscle blocks
Through-gut (separate mouth and anus)
What are the three major groups of Bilateria
Lophotrochozoa
Ecdysozoa
Deuterostomia
Name organisms in Lophotrochozoa
Annelids
Molluscs
Platyhelminths (flatworms)
Bryozoans
Nemerteans
Name organisms in Ecdysozoa
Arthropods
Nematodes
Name organisms in Deuterostomia
Chordates (includes vertebrates)
Echinoderms (e.g. starfish)
What environmental change did Bilateria bring
They began burrowing and moving through sediment - transforming ecosystems from 2D surfaces into 3D environments (bioturbation)
