Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 32.4 Views of Animal Phylogeny Continue to Be Shaped by New Molecular and Morphological Data
As molecular and morphological data accumulate, our understanding of animal evolution and relationships continues to evolve. The phylogenetic tree of animals reveals major lineages and ongoing debates about the placement of early-diverging groups like sponges and ctenophores, while confirming several foundational clades.
Reconstructing Animal Evolution
- Uses genomic data (rRNA, Hox genes, protein-coding genes) and morphological traits.
- Most animal phyla appeared ~500 million years ago during or before the Cambrian explosion.
- Extinct lineages left no descendants, but relationships among living groups are becoming clearer.
Key Points from Animal Phylogeny
- All animals share a common ancestor: Metazoa is a monophyletic clade.
- Sponges (Porifera) are the sister group to all other animals:
- Supported by morphology, fossil biomarkers, and genomics.
- Lack true tissues, consistent with basal position.
- Eumetazoa: Clade of animals with true tissues.
- Ctenophores and cnidarians are basal eumetazoans, often diploblastic and radially symmetrical.
- Bilateria: Most animals, with bilateral symmetry and three germ layers.
- Origin traced to the Cambrian explosion.
- Three major clades of Bilateria:
- Deuterostomia: echinoderms, chordates, hemichordates.
- Lophotrochozoa: molluscs, annelids, flatworms, etc.
- Named for lophophore (feeding structure) and trochophore larvae.
- Ecdysozoa: nematodes and arthropods.
- Defined by ecdysis, the molting of a cuticle.
- Group supported mainly by molecular data.
Ongoing Debates in Animal Phylogeny
- Sponges vs. Ctenophores as the basal group:
- Traditional view favors sponges.
- Some data suggest ctenophores diverged first—still debated.
- Placement of Acoela (flatworms):
- Some studies place them as basal bilaterians, others within Deuterostomia.
- If basal, the bilaterian ancestor may have had:
- A simple nervous system
- Single-opening gut
- No excretory system
Importance of Molecular Data
- Genomics has revised previous classifications:
- Built phylogenies using molecular rather than morphological data.
- Clarified relationships among morphologically ambiguous invertebrate phyla.
- Sequencing of lesser-known organisms continues to refine animal phylogeny.
In a Nutshell
Animal phylogeny has undergone major revisions with the rise of genomic and molecular data. While core clades like Metazoa, Eumetazoa, and Bilateria are well supported, questions remain about early-diverging lineages. The placement of sponges, ctenophores, and Acoela highlights how phylogeny is a dynamic field that continues to evolve.