BILD 3 Lecture Notes - Lecture 17: Symmetry In Biology, Molecular Clock, Polyphyly
13 Jun 2018
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BILD 3 Lecture 17
5/11/2018
• Choanoflagellates
o Sister group of our common ancestor
o There are cell types in multicellular organisms that look like
choanoflagellates→called collar cells
o They are a single-celled protists
o They are the closest living relative to the sponges
o They are the closest living relative to the animals
• Traits that define animals
o All are chemoheterotrophs
o Multicellular eukaryotes that lack cells walls; cells are supported by structural
proteins (like collagen) and skeletons
o Tissues that are organized with subspecialized functions
o Most reproduce sexually
o They are many developmental homologies
• Animal phylogeny is not fully resolved; molecular and morphological data provide
different views of diversification
• Major points of agreement:
o Animals are monophyletic (remember: algae is polyphyletic)
o Sponges are likely basal
o Most animals have true tissues (groups of similar cells that act as a functional
unit are isolated from other tissues by membranous layers)
o Most animals have bilateral symmetry and three germ layers (the bilateria)
(ectoderm, mesoderm, and endoderm that will subdiversify to become other
tissues)
• Worms are polyphyletic
• Eukarya (animals)
o Oldest animal fossils are from 565-550 MYA
o Molecular clock estimates that they are older than that, which would have been
soft-bodied orgaiss that do’t fossilize well
o Aside from the Porifera, Ctenophora, and Cnidaria all animals have three tissue
layers: endotherm (gut), mesoderm (muscle), and ectoderm (skin and
neurons)→they are diploblastic
o Lophotrocozoa: most diverse clade
▪ Have a lophophore (specialized form of its mouth) →is a synapomorphy
that unites the Lophotrocozoa
▪ Includes the Platyhelminthes, Mollusca, and Annelida
o Ecdysis
▪ A sister group of the Lophotrocozoa
▪ To molt
▪ Comprised of the nematoda and arthropoda
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