The Reynolds number is a fluid mechanics principle that describes relevance to protists like Paramecium. There were three hypotheses for how multicellular organisms evolved from protists: symbiotic, colonial, and cellularization. Ernst Haeckel promoted the idea of recapitulation, that ontogeny recapitulates phylogeny. Haeckel's colonial hypothesis reflected this idea by suggesting colonies formed through repeated cell divisions.

1. Explain the fluid mechanics principle known
as a “Reynolds Number” and describe its
relevance to the life of single cell protists
like Paramecium.

2. 3 ways in which
a multicellular
organism could
evolve from a
protist.
Symbiotic Hypothesis
Colonial Hypothesis
Cellularization Hypothesis

3. Ernst Haeckle (1834-1919)
and the Theory of Recapitulation
Haeckel promoted the influential (but no
longer widely held) recapitulation theory
("ontogeny recapitulates phylogeny") claiming
that an individual organism's biological
development, or ontogeny, parallels its
species' evolutionary history, or phylogeny.
In what way does Haeckle’s colonial
theory for the origin of animals reflect
his believe in “recapitulation”?

4. Blastea
Gastrea
(Haeckle’s favorite)
Strengths: models exist (Volvox, choanoflag.)
: parallels animal embryology
Weaknesses: ???
Gastrea

5. Blastea
Gastrea
(Haeckle’s favorite)
Strengths: models exist (Volvox, choanoflag.)
: parallels animal embryology
Weaknesses: ???

6. Similarities of Choanocytes (sponge cells)
and choanoflagellates (protozoan)
Choanocyte Choanoflagellate

7. Choanoflagellates live as solitary and colonial forms

8. b-d. choanoflagellate cells bear a single
Flagellum and an apical collar of actin-
filled microvilli (bracket, c). D. an
overlay of b-tubulin (reen), polymerized
actin (red) and DNA (blue) localization
reveals the position of the flagellum
within the collar of microvilli.
a. Phylogeny based on protein
domain comparisons

9. Monosiga genome:
Encodes a diverse array of cell adhesion and extracellular
protein domains previously thought to be restricted to
metazoans. (cadherin required for cell sorting and adhesion
during embryology (why in protist????)
Lectins….cell recognition, adhesion
Integrin and immunoglobulin…adhesive and immune functions
Collagen domains: Key component of basement membrane in
metazoan tissues
Why present in a species that is unicellular?

10. Conclusions on Metazoan Origin
Evidence best supports the colonial theory
Choanoflagellate group is most likely ancestor;
Evidence includes cell structure and genetic
similarities
Idea that metazoans are monophyletic is supported

11. Systematics, Classification
and Phylogeny
Accompanying textbook
pages 19-31

12. II. Some Important Terms and Concepts
attributes or features of organisms
similarity due to descent from a
common ancestor
similar-appearing structure evolved in
entirely unrelated groups
various conditions of homologous
character ex. fins of fish, arms of
humans, forelegs horses
Direction of change...Ancestral and
derived...is relative
• Character
•Homologous
Character
•Convergent
Character
• Character
states
• Character
Polarity

13. Convergent vs. Homologous
Ancestral vs. Derived
Distinguishing characters by these criteria is key to
learning about evolution and phylogeny.
Why?

14. Key rules of cladistics taxonomy
• Only shared derived homologous characters determine
phylogeny : synapomorphies
• All species in a grouping must share a recent common
ancestor
• All species derived from a common ancestor as well as
that ancestor must be included in the taxon
• All species must be in monophyletic taxa
( true reflection of evolutionary history)

15. Polyphyletic Taxon:
An incorrect taxonomic grouping of species that arose
from two or more recent ancestors
A taxonomic grouping of species that evolved from a single
ancestor and includes all descendants of that ancestor
Monophyletic Taxon?
Paraphyletic Taxon:
An incorrect grouping of species sharing an immediate
ancestor but not including all descendants of that
ancestor.

17. Key rules of cladistics taxonomy
All species in a grouping must share a common ancestor
All species derived from a common ancestor as well as
that ancestor must be included in the taxon
Thus all species must be in monophyletic taxa
all species in this taxon share a
common ancestor and all species
derived from that common
ancestor are included
Reptiliomorpha

18. Key rules of cladistics taxonomy
All species in a grouping must share a common ancestor
All species derived from a common ancestor as well as
that ancestor must be included in the taxon
Thus all species must be in monophyletic taxa
This is polyphyletic as the
Two lineages do not share
A recent common ancestor

19. Key rules of cladistics taxonomy
All species in a grouping must share a common ancestor
All species derived from a common ancestor as well as
that ancestor must be included in the taxon
Thus all species must be in monophyletic taxa
Grouping contains the
immediate common ancestor
but not all descendants of that
ancestor are included

20. Ex. Vertebrate lineage
Orthodox classification of
Vertebrate Animals.
Class Pisces?
Class Reptilia?

21. Three important cladistics conventions in converting cladograms to
taxonomic categories:
1. Use the Linnean categories: PCOFGS
2. Minimum taxonomic decisions will be made
a. Minimize use of empty or redundant categorial ranks
Division Ginglymodi
Family Lepisosteidae
Genus Atractosteus
Genus Lepisosteus
b. Ranks of well known clades will be retained
3. Asymmetric trees containing terminal taxa may be placed at the
same hierarchal rank and listed in order of their branching sequence
(Sequencing convention)

22. Ex. Vertebrate lineage
Orthodox classification
of Vertebrate Animals.
Phylogenetics nested
hierarchy
Phylogenetics
Sequencing convention
Ph Chordata
SubPh Vertebrata
Class Pisces
Class Amphibia
Class Mammalia
Class Anapsida
Class Lepidosaura
Class Crocodilia
Class Aves