Transcript Origin of Species

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.
©2001 Timothy G. Standish
The Origin
Of Species
Timothy G. Standish, Ph. D.
©2001 Timothy G. Standish
Observing Speciation
"The evolutionary divergence
of a single species into two
has never been directly
observed in nature, primarily
because speciation can take a
long time to occur.”
Darren E. Irwin, et al. 2001. Speciation in a
ring, Nature 409:333-337.
©2001 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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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
©2001 Timothy G. Standish
Evolution
Microevolution - Changes in
allele frequency over time
(Population genetics)
Macroevolution - Accumulation
of novel genetic changes in a
population until it becomes a new
species
©2001 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
exploit them. As these individuals adapt to
their new environment, they become
distinctly new species
©2001 Timothy G. Standish
How Species Evolve
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D Morphology
Anagenesis
D Morphology
Cladogenesis
©2001 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.
©2001 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
Postzygotic - Those that prevent the
offspring of two species (mule) from
reproducing
©2001 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
©2001 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
©2001 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,
frequency 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
©2001 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
©2001 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
©2001 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
©2001 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)
©2001 Timothy G. Standish
Tempo Of Evolution
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D Morphology
Gradualism
D Morphology
Punctuated
Equilibrium
©2001 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. 1985. "Time Frames: The Rethinking of Darwinian Evolution and the Theory of
Punctuated Equilibria", Simon & Schuster: NY, p 188-189
©2001 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 in 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.
©2001 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
©2001 Timothy G. Standish
©2001 Timothy G. Standish