EVOLUTION AND
FIXITY
© 2008 Paul Billiet ODWS
Evolution
 The process of cumulative change in the
heritable characteristics of a population
Fixity
 The creation of life followed by no further change
© 2008 Paul Billiet ODWS
The Greek Philosophers
Aristotle developed an idea of the
organisation of life through a ladder of
life (Scala Natura) and that this
organisation was fixed
 Other Greek philosophers, such as
Diogenes, thought that life evolved

© 2008 Paul Billiet ODWS
The Biblical interpretation


Literal interpretations of the Bible
suggested that the Earth was only
a few thousand years old
This short time span seemed
appropriate for creation of life by a
divinity followed by fixity
Archbishop Ussher
1580-1656
© 2008 Paul Billiet ODWS
Intelligent design
William Paley (1802)
Developed the idea of
intelligent design through
special creation
 Living organisms were too
complex and well adapted
to have evolved by chance
 Their complexity was the
sign of a creator
© 2008 Paul Billiet ODWS
The Dalvey Depot
Time to evolve
The development of ideas in
geology
Starting in the 17th century
studies in geology pushed back
the estimated age of the Earth
from thousands to millions of
years
 Time for evolution to occur was
now available
 Fossils of animals and plants no
longer found on Earth, were
discovered in strata
 They are similar to those alive
today
BSAC Travel Club
Ammonite
© 2008 Paul Billiet ODWS
Catastrophism



Advocates of special
creation explained fossils
in terms of the victims of
natural (biblical)
catastrophes
(e.g. Noah’s Flood)
Those that did not
survived the flood were
the fossils of creatures
that do not exist today
Georges Cuvier 1769 – 1832
Public Domain Image
© 2008 Paul Billiet ODWS
Radioactivity
Estimates on the age of the Earth were
made from experiments in cooling,
observations of sediments and the salinity of
the seas
 Absolute dating for rocks came with the
discovery of radioactivity and radioisotopes
 This revealed fossils to be millions or even
billions of years old

© 2008 Paul Billiet ODWS
Systematics



Jardins des Plantes Paris

© 2008 Paul Billiet ODWS
Collections of animals and
plants in museums
increased from 17th century
The need for systematic
classification became
apparent to organise
organisms
The binomial classification
scheme was developed by
Carl Linneus in 1735
To “put order into God’s
creation”
Comparative Anatomy
Classification led to comparisons of
shape and form that gave rise to
comparative anatomy
 Comparative anatomists noticed that
different species have similar
structures used for different functions
(e.g. the pentadactyle limb of
terrestrial vertebrates).
 These are called homologous
structures

© 2008 Paul Billiet ODWS
The pendadactyle limb
Frog
Lizard © Chereka Keaton
Bat
© 2008 Paul Billiet ODWS
Human
Common ancestors &
missing links




Evolutionists argued that if species
had been created independently by
a creator then there was a great
deal of coincidence in their design
If all organisms evolved from a
common ancestor, this could
explain their common features
Occasionally a fossil is found close
to the origin of a common ancestor
These are called missing links
(e.g. Archaeopteryx a fossil
dinosaur with bird-like features)
© 2008 Paul Billiet ODWS
Archaeopteryx
Phylogenetics
Classification led to phylogeny: the
study of related groups as revealed by
systematic classification
 Closely related organisms are more
similar than distantly related
organisms
 Currently the systematic analysis of
relatedness uses a technique called
cladistics

© 2008 Paul Billiet ODWS
Cladogram for the birds
Archeopteryx
reversed first toe, fewer
than 26 tail vertebrae
Velociraptor
halfmoon-shaped
wrist bone
Modern birds
no teeth,
horny beak
Allosaurus
three fingered hand
Coelophysis
three functional toes
and hollow bones
Titanosurus
Node
Other
dinosaurs
© 2008 Paul Billiet ODWS
Comparative Embryology
Early embryos of animals show surprisingly
similar features
revealing a common ancestry
© 2008 Paul Billiet ODWS
Comparative biochemistry



Protein molecules the bottom line in studying the
phenotypes of organisms
Similarities and differences in the amino acid
sequences of the same molecule
(e.g. haemoglobin) taken from different species
produce a phylogeny
The phylogeny revealed by studying protein
structure reflects the same phylogeny as
comparative anatomy and embryology but with a
much finer resolution
© 2008 Paul Billiet ODWS
Molecular genetics




Comparison of the base sequences of variable
regions of DNA (in particular mitochondrial DNA)
taken from different organisms
The genotype is being analysed
Rates of mutations are assumed to be constant
The analysis of DNA provides a molecular clock
against which the geological clock can be
compared
© 2008 Paul Billiet ODWS
Biogeography





Organisms are not found in every habitat that
they could occupy
Travelling around the world the distribution of
organisms follows a simple pattern
Two similar habitats that are close will contain
species that are closely related
Two similar habitats separated by a great
distance (e.g. an ocean) will contain unrelated
species
This distribution suggests that all organisms were
not created at the same time or they would be
evenly distributed all over the world
© 2008 Paul Billiet ODWS
Disjunct distributions
Living or extinct organisms found in widely
different parts of the world were difficult to
explain
 The problem was resolved with the
development of the theory of plate
tectonics in geology

© 2008 Paul Billiet ODWS
Marsupial fauna






Australia was once attached to India, Africa,
Antarctica and South America (Gondwana)
This large continent broke up before the
placentals evolved
The marsupials of Antarctica died out as it froze
over
Those of Africa suffered from competition when
the placentals evolved
Those of South America survived until the
Panama Isthmus was formed
Only the Possum survived the competition form
North American placentals
© 2008 Paul Billiet ODWS