Macroevolution and Speciation:
Darwin’s great dilemma
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The Biological Species Concept
Reproductive Isolation
Patterns of Speciation
Modes of Speciation
Gradual versus Punctuated Change
A unifying theory for micromicro-, macromacroand megaevolution:
‹ Facilitated Variation
‹ Developing an evolutionary tool
box
What is a species?
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A summary of reproductive barriers between
closely related species
Two patterns of speciation
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Two modes of speciation
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1. Allopatric Speciation when geographic isolation
leads to speciation.
‹ Grand canyon ground
squirrels, CA lizards
‹ On islands leads to adaptive
radiation
 Galapagos Islands
 Hawaiian Islands vs
Florida Keys
The Biological Species Concept - A natural
population capable of interbreeding and producing
fertile offspring.
‹ Genetic or reproductive isolation
‹ Barriers fall into 2 categories:
 Prezygotic barriers - impede mating or
prevent fertilization of the egg.
 Postzygotic barriers - prevent the zygote
from developing into a viable, fertile adult.
‹ The impossible challenge of applying this to the
fossil record, hence a different species
definition is required.
Anagenesis - phyletic evol.
occurs when new traits
accumulate in a pop.
yielding a new species.
Cladogenesis - branching
evol. occurs when new
species arise from a parent
species. This is the basis of
increasing biological
diversity.
Has speciation occurred during
geographic isolation?
Two modes of
speciation, cont’d
Two models for the tempo of
speciation:
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2. Sympatric Speciation where new species arise
within the range of parent
populations.
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In plants occurs as a result
of polyploidy
 Autopolyploidy
 Allopolyploidy
In animals occurs if
variations in offspring cause
them to be dependent on
resources not used by the
parent.
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The fossil record
indicates that, on average,
a successful species only
lasts a few million years,
so accordingly, speciation
is a normal process.
The fossil record tends to
favor the latter view
relying on gradual
accumulation of variation,
then stasis, then
speciation.
The branched evolution of horses
Punctuated Equilibrium
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20 million years of
mollusc evolution in Lake
Turkana, Africa
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Gould and Eldredge 1977:
“Stasis is data”
data”
Punctuated equilibrium
Neo-Darwinism: A swing and a
Megaevolution and the
emergence of novel variations
Horizontal and vertical evolution
‹ Speciation vs. Complexification
„ Ernst Myer and the Modern Synthesis (aka
NeoNeo-Darwinism)
‹ Anything can be accomplished with
enough time
‹ With time and diversity genetic distance
increases
„ EvoEvo-Devo and the conservation of genes
forming an ancestral evolutionary toolbox
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miss…
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"Much that has been learned about gene
physiology makes it evident that the search for
homologous genes is quite futile except in very
close relatives. If there is only one efficient
solution for a certain functional demand, very
different gene complexes will come up with the
same solution, no matter how different the
pathway by which it is achieved. The saying
"Many roads lead to Rome" is as true in evolution
as in daily affairs." Ernst Mayr,
Mayr, Animal Species
and Evolution (Harvard University Press, 1963),
p. 609
Evolution through changes in the
timing of developmental
processes
Experimental attempts at explaining
macroevolution: The eye…
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Discerning the path
toward new and diverse
species is difficult
because we cannot replay
the course of evolution
and are only left to glean
what we can from the
fossil record or make
deductions based on the
diversity of extant
species.
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Heterochrony and allometric growth
Heterochrony and the evolution of salamander feet
among closely related species
(developmental proportioning)
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How evolution doesn’
doesn’t require rere-invention,
simply modification.
Evolution of novelty by
developmental modification of
existing structures
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Heterochrony
‹ Disproportionate
growth
‹ Eg. Horse evolution
Neoteny
‹ Early sexual maturity
 Mud puppies
Reptilian jaw
bones become
modified in
mammals for
sound
transmission
Compartmentation
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Understanding the Cambrian Explosion
The Hox pattern formation genes
determine the headhead-toto-tail animal body
platform.
Allowing complexity to arise out of
regional specializations on a common
platform.
Hox homologies and alterations:
‹ The brains of insects and mammals
‹ The bodies of lobsters and
dragonflies
Hox mutations and the origin of vertebrates
Hox genes and the evolution of tetrapod limbs