Origination and diversification of life -- the grand
scale.
As a special case later in this lecture, we will consider the origination and diversification of arthropods.
In two weeks, we will take up this problem when we talk
about development and evolution.
A Brief History of Time:
Before Metazoa (time is measured in billions of years
ago = bya).
1. First fossil life -- prokaryote
The earth is 4.5 bya.
Oldest sedimentary rocks are 3.7 bya. Oldest "fossil-like
objects" are 3.4 bya.
The fossils are largely single cells or mats of connected
single-cells.
Evidence for oxygen increase in atmosphere 2 bya.
Diversification of aerobes and presumably extinction of
some anaerobes.
2. When did the eukaryotes evolve (unicellular algae)?
What of the trends for increase in size? Compare cell size
of extant prokaryotes (and eukaryotes) and compare this with the size distribution
in the fossil record. From this it is clear that eukaryotes first appeared
~1.45 bya.
What about the molecular phylogeny of prokaryotes and
eukaryotes?
The molecular phylogeny has a split in prokaryotes into
two groups archaebacteria and eubacteria that occurred prior to the eukaryotes
orgination.
Subsequent to or coincident with the evolution of eukaryotes,
was the sybiotic origin of the cell organelles: mitochondria and chloroplasts
(derived from members of the eubacteria).
3. When did the metazoa (multicellular eukaryotes) evolve?
First large algae and invertebrates found 0.7 bya
= 700 mya.
First hard parts in algae and metazoa 0.6 bya = 600 mya.
prior to this there was a preservation bias -- hard parts are much more likely to preserve.
Origin of the metazoa (with emphasis on Arthropods)
Change time scale. For Metazoa we deal in millions of years
ago = mya.
Question: Can we use cladistic analysis to refine the
resolution of the phylogenetic relationships among members of a fossil assemblage?
Ediacarian Assemblage -- soft bodied, Precambrian fauna
Best known Precambrian Fossil assemblage is Ediacarian
Period 670-550 mya
What was present (overhead). genera species
Cnidaria (Jelly fish, polyps, etc.)(67 percent) 14 19
Annelida (marine worms) (25 percent) 3 7
Arthropods (segmented) (5 percent) 2 2
Phylum uncertain (3 percent) 1 1
+ Trace fossils 6 7
Burgess Shale -- hard parts, Middle Cambrian fauna
Burgess shale 44 genera of arthropods - 14 genera
of trilobites = 30
What our basic aim with this group of fossils is to construct
a phylogeny based on a couple of different character sets
1) based on the Bauplane (Not all Bauplane are seen in real life).
a) presence of carapace or cephalic shield
b) the trunk tergites (components of the biramous limb)
Failure of assumptions:
Presence or absence of carapace is adaptively plastic (not slow evolutionry
change we would like in parsimony reconstruction). Evidence in later forms
-- can be gained or lost.
2) based on cephalic appendages per se -- CEPHALIZATION
preoral appendages + post oral appendages = appendage
formula
Evolutionary assumptions used
in constructing this tree:
1) primitive form is 1+1 or ~0+1 -> Spriggina
from the ediacarian
2) concept of cephalization (specialization of limb function)
a sequential process, commencing anteriorly, incorporating appendages posteriorly
For example here is an evolutionary progression (nested set of clades).
Trilobites -> Chelicerates -> Crustaceans
1+3 -> 1+5 -> 2+3
What about subsequent diversification of Arthropods?
Within an adaptive zone (marine arthropods):
1) Do we see a similar pattern of diversification from
specialization?
2) Is specialization within a lineage related to diversification
of a lineage? Is there a tendency to make finer and finer partitioning of
the available ecological space into finer scale "niches"?
3) Are there limits to specialization and diversification
(competition)?
Expand on the concept of CEPHALIZATION Briggs 1984
TAGMATIZATION Cisne 1974 Evo. 28.
Fundamental assumptions:
1) Tagmatization -- The degree of arthropod specialization
is proportional to morphological specialization of limb pairs
2) Calculate a diversity index for specialization down
the length of an arthropod. This captures the specialization of function.
Tagmosis index - a measure of diversity along the length of a species.
3) Specialization along the length of the arthropod
is in some way related to niche specialization of the arthropod in an ecological
sense.
An example of a Key Innovation: Malacostracan evolution (crabs, lobsters)
1) primitively 8 sets of filtering appendages in brine shrimp
2) Malacostracans evolved a filtering maxilla - 1 limb specialized for filtering
3) This frees up the remaining 8 limbs to specialize for
other tasks.
Questions (see Price for these points, Chapter 16):
1) How does tagmosis change over time from ancestor to descendant groups? Increases.
2) How does tagmosis change within a lineage?
Increases and then plateaus.
3) What is the correlation between between tagmosis (specialization)
and diversification of a lineage? Very strongly positive.
Final point: Are analyses of patterns in the fossil record confounded by preservational biases?
think about it a little...