Genesis 1: 24, 25
24 And God said, Let the earth bring
forth the living creature after his kind,
cattle, and creeping thing, and beast of
the earth after his kind: and it was so.
25 And God made the beast of the earth
after his kind, and cattle after their
kind, and every thing that creepeth
upon the earth after his kind: and God
saw that it was good.
©2000 Timothy G. Standish
Evolution and
Genetics
Timothy G. Standish, Ph. D.
©2000 Timothy G. Standish
Historical Science
“…Darwin introduced historicity into science.
Evolutionary biology, in contrast with physics and
chemistry, is a historical science -- the
evolutionist attempts to explain events and
processes that have already taken place. Laws and
experiments are inappropriate techniques for the
explication of such events and processes. Instead
one constructs a historical narrative, consisting of
a tentative reconstruction of the particular
scenario that led to the events one is trying to
explain."
Ernst Mayr 2000 July issue of Scientific American.
©2000 Timothy G. Standish
Family Trees
From the late Renaissance, noble families in
Europe had their pedigrees drawn in the
form of trees - geneaological trees. By a
giant leap of scientific imagination and
analogy, these family trees became the
prototype for a new theory of humanity's
place in the Universe: "I believe this simile
largely speaks the truth," wrote Charles
Darwin in The Origin of Species (1859),
imagining evolution as a tree branching
through time.
Hestmark, Geir. 2000. Temptations of the tree. Nature 408:911.
©2000 Timothy G. Standish
Imposing Nature on a Tree
In 1776, the German-Russian naturalist Peter Simon
Pallas proposed in his Elenchus Zoophytorum a
systematic arrangement of all organisms in the
image of a tree; and in 1861 Heinrich Bronn
presented such a tree based on fossil evidence. But
it was Ernst Haeckel, in his monumental Generelle
Morphologie (1866), who drew the first tree of the
common descent of all life on Earth. Haeckel also
coined the term 'phylogeny' from the Greek words
for tribe or race and origin, and presented
phylogenetic trees for all major groups of
organisms.
Hestmark, Geir. 2000. Temptations of the tree. Nature 408:911.
©2000 Timothy G. Standish
They Look So Real
Phylogenetic trees are common in today's scientific journals,
but there it is seldom realized how speculative they are
because they look so real. This rhetorical power was
significant in the popularization and triumph of
evolutionary theory. Yet phylogenies are only sketches of
historical hypotheses, constructed from imperfect
historical evidence: fossils; morphological and anatomical
similarity; biogeographic patterns; and, recently,
comparison of different molecular sequences. Some
phylogenetic trees are certainly more probable than others,
but the inherently imperfect nature of the evidence seems
to guarantee that we will never be able to reconstruct,
except perhaps by accident, the true phylogeny of life on
Earth.
Hestmark, Geir. 2000. Temptations of the tree. Nature 408:911.
©2000 Timothy G. Standish
What is a Species?
Six major concepts:
1 A - Morphospecies - If it looks different, it is a different
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4
species
B - Cohesion - Defined by an integrated complex of genes
and set of adaptations
A - Biological - Reproductively isolated groups of
organisms
B - Recognition - If two organisms don’t recognize one
another as potential mates, they are different species
Ecological - If they do not occupy the same niche, they are
not the same species
Evolutionary - If they share the same common ancestor
and niche, they are related and may be the same species
©2000 Timothy G. Standish
Evolution
Microevolution - Changes in allele
frequency over time (Population genetics)
Macroevolution - Accumulation of
changes in a population until it becomes a
new species
Macroevolution requires the production
of new “information”; microevolution
deals with how that information in the
form of alleles behaves in populations
©2000 Timothy G. Standish
How Species Evolve
Anagenesis - (an = without, genesis =
beginning) Over time the environment in
which a species lives changes and the species
continually adapts to the new environment.
Thus the species changes over time and
eventually becomes a new species
Cladogenesis - (clad = branch, genesis =
beginning) As new niches become available,
members of existing species move into and
exploit them. As these individuals adapt to
their new environment, they become
distinctly new species
©2000 Timothy G. Standish
How Species Evolve
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D Morphology
Anagenesis
D Morphology
Cladogenesis
©2000 Timothy G. Standish
Where Speciation Occurs
Allopatric Speciation - Speciation that
does not occur in the same place. First
two populations are separated, then
they change and become different
species.
Sympatric Speciation - Speciation in
the same place. Species arise within the
same population due to something
other than a physical reproductive
barrier.
©2000 Timothy G. Standish
Reproductive Barriers
If a species is to be produced, some sort of
reproductive barrier needs to come into play
between two populations of the same species
Reproductive barriers fall into two classes:
Prezygotic - Those that occur before a
zygote is produced
Post zygotic - Those that prevent the
offspring of two species (mule) from
reproducing
©2000 Timothy G. Standish
Physical Reproductive Barriers
If a population is separated into two populations by
a physical barrier the Hardy-Weinburg assumption
of random mating will be violated
If different selective pressures are brought to bear
on the separate populations, they will develop
different allelic frequencies
Evolutionary theory extrapolates from here to say
that they will form new species and, if they drift
enough, new genera and so on
This requires the production of new alleles that
improve the fitness of individuals in the
population, as mere changes in allele frequencies
do not make new species
©2000 Timothy G. Standish
Phylogeny Based on
Homologous Structures/Fossils
Bacteria
Archaea
Fungi
Plantae
Animalia
Hypothetical
common ancestors
Last Universal Common
Ancestor (LUCA)
Spontaneous Generation of Life
©2000 Timothy G. Standish
Phylogeny Based on
Homologous Genes/Proteins
Archaea
Bacteria
Fungi
Plantae
Animalia
Hypothetical
common ancestors
Last Universal Common
Ancestor (LUCA) a
eukaryote?
Spontaneous Generation of Life
©2000 Timothy G. Standish
Problems With Homology
Before Darwin, Richard Owen (1840s) defined
homology as structures derived from a common
“archetype”
Darwin redefined homologous structures as those
derived from a common ancestor
He then turned around and said that homology is
evidence of common ancestry
But this is circular reasoning Common
ancestry
Homology
©2000 Timothy G. Standish
Problems Using Molecular Data
Molecular data run into the same problems as any
other data that assumes homology
Molecular information frequently exhibits
dramatically different expression in different
organisms. For example genes with similar
sequences control production of limbs in:
–
–
–
–
Echinoderms- Tube feet
Polychaete worms- Setae
Vertebrates- legs and feet
Insects- jointed legs
Homologous structures can be produced by
different genes
©2000 Timothy G. Standish
Prezygotic Barriers
Habitat isolation - If they live in different places,
they can’t mate
Behavioral isolation - If species recognition is
behavior based, organisms with different behaviors
will not mate (i.e., eastern and western
meadowlarks are identical in almost all things
except song)
Temporal isolation - If they breed at different
times, they will not breed with each other
Mechanical isolation - Need any more be said?
Gametic isolation - Gametes have complex
recognition mechanisms so that gametes from one
species will rarely fuse with those of another
©2000 Timothy G. Standish
Postzygotic Barriers
Inviable Hybrids - Hybrids may develop from a
zygote formed from the sperm of one species and
the egg of another, but they are weak inferior
creatures and may not even survive until birth
Infertile Hybrids - Hybrids may be hardy
creatures, but they are incapable of reproduction,
frequenctly due to difficulties in producing
gametes due to strange chromosome
combinations resulting from meiosis
Hybrid Breakdown - At first hybrids are fairly
successful, but over the course of several
generations problems develop
©2000 Timothy G. Standish
Barriers To Hybrid Formation
Habitat isolation
Behavioral isolation
Temporal isolation
Prezygotic
Barriers
Mechanical isolation
Gametic isolation
Postzygotic
Barriers
Inviable hybrids
+ +
Infertile hybrids
Hybrid breakdown
Happy Hybrid
©2000 Timothy G. Standish
Sympatric Speciation:
Autopolyploidy
Diploid plant
2n = 4
Somatic
nondisjunction
Tetraploid flowers
make diploid gametes
Tetraploid cells
develop into flowers
Self fertilization
results in tetraploid
offspring which
cannot interbreed
with the original
diploid species
©2000 Timothy G. Standish
Sympatric Speciation:
Allopolyploidy - Scenario 1
Plant species A
2n = 4
1n=2 gamete
Gametes
combine to make
a hybrid
1n=3 gamete
Mitotic nondisjunction
produces diploid cells
capable of producing
fertile gametes
1n=5 hybrid
(infertile)
2n=10 hybrid
(fertile)
Plant species B
2n = 6
©2000 Timothy G. Standish
Sympatric Speciation:
Allopolyploidy - Scenario 2
Plant species A
2n = 4
Unreduced
2n gamete
Gametes
combine to make
a hybrid
1n=3 gamete
Meiotic nondisjunction
produces unreduced
gamete
Unreduced
gamete
1n=7 hybrid
(infertile)
Plant species B
2n = 6
Normal
gamete
2n=10 hybrid
(fertile)
©2000 Timothy G. Standish
Tempo Of Evolution
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D Morphology
Gradualism
D Morphology
Punctuated
Equilibrium
©2000 Timothy G. Standish
Eldredge on Punctuated Equilibria
"At the core of punctuated equilibria lies an empirical observation: once
evolved, species tend to remain remarkably stable, recognizable
entities for millions of years. The observation is by no means new,
nearly every paleontologist who reviewed Darwin's Origin of Species
pointed to his evasion of this salient feature of the fossil record. But
stasis was conveniently dropped as a feature of life's history to be
reckoned with in evolutionary biology. And stasis had continued to be
ignored until Gould and I showed that such stability is a real aspect of
life's history which must be confronted - and that, in fact, it posed no
fundamental threat to the basic notion of evolution itself. For that was
Darwin's problem: to establish the plausibility of the very idea of
evolution, Darwin felt that he had to undermine the older (and
ultimately biblically based) doctrine of species fixity. Stasis, to
Darwin, was an ugly inconvenience."
Eldredge N., "Time Frames: The Rethinking of Darwinian Evolution and the Theory of
Punctuated Equilibria", Simon & Schuster: New York NY, 1985, p 188-189
©2000 Timothy G. Standish
The Rate of Evolution
Sometimes evolution has occurred at an amazingly rapid
rate:
Drosophila pseudo-obscura, a native species, has declined
since 1978 when the European species Drosophila
subobscura was introduced into Chile
In Europe D. subobscura exhibits an increase in wing size
as one goes from south to north
A south to north wing size gradient went unobserved when
D. subobscura was studied around 1989, but a decade later
a difference in wing size distribution mimicking that seen in
European flies was evident.
Thus this wing size difference must have evolved in a
decade or less, but no new species and no new alleles have
been shown to arise in the American population
©2000 Timothy G. Standish
The Rate of Evolution
Galapagos finches are also known to have evolved
very rapidly in nature
After a drought in 1978, a dramatic shift in beak
size was observed in a local population of finches
However, it is worth noting that when conditions
returned back to normal, the finches’ beaks also
reverted back
Thus, selection did produce a morphological
change in the finches which appeared to be
heritable, but no new species of finch were
produced and at the end of it all, the same kinds of
finches that were previously there were still
present
©2000 Timothy G. Standish
When the Data Speaks
“For example, researchers have calculated that
‘mitochondrial Eve’--the woman whose
mtDNA was ancestral to that in all living
people--lived 100,000 to 200,000 years ago in
Africa. Using the new clock, she would be a
mere 6,000 years old.
No one thinks that's the case, but at what point
should models switch from one mtDNA time
zone to the other?”
Gibbons, A. 1998. Calibrating the mitochondrial
clock. Science 279:28-29
©2000 Timothy G. Standish
©2000 Timothy G. Standish
Rooted In Eubacteria?
“...this rooting [in the eubacteria] was rapidly
accepted and advertised in the community of
evolutionary biologists and beyond, being now
systematically used to draw universal trees in
review papers, and even textbooks.” (p 511)
“We have shown in this paper that the rooting of
the tree of life in the eubacterial branch has been
based on unreliable phylogenies.” (p 521)
Philippe Herve' and Patrick Forterre. 1999. "The Rooting
of the Universal Tree of Life Is Not Reliable," Journal
of Molecular Evolution 49:509-523.
©2000 Timothy G. Standish