Lecture 2_Phylogenies and the History of Life (Chap 27) Spring 2010

8/7/2019 Lecture 2_Phylogenies and the History of Life (Chap 27) Spring 2010

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Chapter 27

PHYLOGENIES ANDTHE HISTORY OF

LIFE


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Tools for Studying the History of Life

- Phylogenies: evolutionary histories of

groups (taxa) of organisms usually

depicted as trees.

- note, phylogenies are hypotheses.

- Fossils collection continues to grow,

transitional forms are found and

relationships are refined.


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Phylogenies as Trees

Estimates or hypotheses of the relationships amongorganisms.

Reading the tree:

Root

Branches

Nodes (forks)

Branch Tips

Outgroup


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Types of phylogenies

Monophyletic group or Clade -

Paraphyletic group

Polyphyletic group


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How are they constructed?

What would you look for or compare?

Comparisons:

Phenetic based on computing a statisticshowing overall similarity among populations

Cladistic relationships among organisms based

on shared derived characteristics orsynaptomorphies

- characteristics derived from a common ancestor



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Fig 27.1


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How are they constructed?

Problems:

- Characteristics that arose independently in 2 or

more distantly related groups (analogous traits or

homoplasy)

- Loss of a derived characteristic in 1 or more

closely related groups

- Lack of transitional forms

Solution: Parsimony, the most logical, simplest

explanation, involves the least amount of change.



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Fossils Traces of Organisms from

the Past How are fossils formed?

- Rapid burial

- Lack of or very slow decay


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Fig 27.5


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Types ofFossils fig 27.6


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Types ofFossils Fig 27.6


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Fossils

Limitations: the formation of fossils requires

that conditions be just right. Fossils may be

formed or found preferentially according to:

Habitat sediments form routinely Composition of organism hard substances

fossilize better.

Time line ancient fossils disappear into the

earth.

Abundance more abundant, more likely to

find fossils.


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How do we determine the age of fossils? - Radiometric dating: using radioactive isotopes

to predict the age of rocks and other formations

The ages of the Earth and its organismshave been used to create Lifes Timeline,fig 26.9. - Divided into 4 major segments or eras, with

further divisions demonstrating majoradvancements, etc.

Fossils


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Fig 27.8


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Fig 27.8


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Fig 27.8


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Fig 27.8


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Cambrian Period (Paleozoic era) Significance: rapid diversification of the species.

- Multicellular organisms

- Rapid increase in size and complexity of organisms.

Fossils from China (Doushantuo formation),

Australia, Ukraine, Siberia, etc. (Ediacaran

period), and Canada (Burgess Shale fossils)

deposits are unique soft bodied animals.


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Cambrian Period

Fossil record How did this explosion of

diversity happen?

Diversity of Habitats Genetic changes


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Evolution the Process

Natural Selection

Natural selection and evolution requires genetic

change. Genetic changes:

Gene duplication multiple copies of genes

Gene expression timing and duration of

expression during development.


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Genetic Mechanisms

Newest Approach: evo-devo, evolution

and developmental studies combined.

- Correlates evolutionary history, morphologyand genetic make-up of organisms.

- The more characteristics that suggest a

relationship among organisms, the more

plausible the phylogenetic hypothesis.


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Genetic Mechanisms

Homeotic genes: direct the pattern of

development of organisms, universal.

Hypothesis: new genes = new bodies Gene duplication studies, fig 27.10

Correlate well with the hypothesis. Early

branches show fewer homeotic gene clusters

and simpler bodies. Later branches show moregene clusters and more complex bodies.


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Fig 27.10


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Genetic Mechanisms

Gene duplication discrepancies

- The number of clusters and the complexity of

the organism do not always correlate, ex. zebrafish and the mouse.

- Loss of gene clusters (?)


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Genetic Mechanisms

Gene Expression

- The time during embryogenesis when homeotic

genes are expressed and the duration of theirexpression affects development.


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Adaptive Radiations or Star

Phylogenies What are they? Fig 27.11.


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Fig 27.11


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Adapti

ve Radiations

What causes radiations?

Ecological Opportunity:

Morphological Innovation:


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Extinctions

Mass extinctions - disappearances of large

numbers of species by an extraordinary,

sudden, temporary change in theenvironment.

Background Extinctions the average rate

of disappearances of organisms caused by

normal variations in the environment or

competition.


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Fig 27.14

Lecture 2_Phylogenies and the History of Life (Chap 27) Spring 2010

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