Speciation and Phylogeny

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• Speciation and Phylogeny
• Goals
• Understand how evolution produces new species.
• Understand how to recognize relationships among
  species.

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Speciation Where do new species come
from? First, what is a species? Defined by
reproductive isolation. Members of different
species dont share genes they cant interbreed.
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But, all organisms are connected in an unbroken
ancestor-descendant chain, back to the first
gene-based life forms! So how is that the various
descendants become reproductively isolated? All
living chimps and all living humans share an
ancestor so why cant we breed with chimps?
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This is the same as the question, Where do new
species come from? It is a question about the
origins of reproductive isolation. The simplest
explanation (though not the only one) is called
the allopatric model.
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Allopatric model
Geographic range of species A
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Allopatric model
Range divided by a physical barrier.
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Allopatric model
Individuals in the west cant breed with those in
the east.
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Allopatric model
No passage of genes between two sub-populations.
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Allopatric model
If environment is different in the east and the
west
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Allopatric model
Selection might favor different traits in the two
areas
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Allopatric model
If this went on long enough
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Allopatric model
Then when the barrier disappeared
The two types might be too different to
interbreed.
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Any factor that discourages gene flow can serve
in place of the physical barrier. e.g.,
ecological separation within the same geographic
range.
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• Two lines of evidence suggest that the
  accumulation of small change can add up to a
  species-level difference
• Domestic breeds
• Ring species

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Classification and Phylogeny Classification
clustering organisms in natural
groups. Phylogeny deducing ancestor-descendant
relationships. Principle Classification should
reflect phylogeny.
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Classification is hierarchical
vertebrates
mammals
birds
primates
swifts
bats
ducks
cetaceans
penguins
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• But what are the natural groupings?
• They could be inferred from overall similarity?
• Any problems with that?
• What causes similarity between organisms?
• Shared ancestry
• Convergence!

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But what are the natural groupings? The ones
created by evolution, of course. Those that share
a common ancestor. But all life shares a common
ancestor Those with a more recent common
ancestor are more closely related!
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Cladistic classification is based on order of
speciation.
a b c d
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a b c d
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mammals

primates
apes
a b c d
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Primitive versus derived similarities.
Primitive trait present in the common ancestor
of the group. Derived trait evolved since
divergence from the common ancestor.
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Primitive versus derived similarities.
Pentadactyly (having 5 digits) is primitive for
mammals. It should not be used to find natural
groups of mammals. But having one toe is derived
for mammals. So those that have one toe may
represent a natural group.
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• How to decide what is primitive and what is
  derived?
• Two methods
• fossil record
• out-group comparison

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• 2 3 4 5 6
  species
• b a b b a b
  trait

Species 6 is the outgroup. Cladistic principle
Trait present in the outgroup is primitive.
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• 5 1 3 4 6
  Species
• a a b b b b
  Traits

Species 6 is the outgroup. Cladistic principle
Trait present in the outgroup is
primitive. Derived traits show recent ancestry.
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This process can be repeated for other
(phenotypic or genetic) traits. Not all traits
will give the same answer but each trait can
count as a vote for one grouping arrangement or
another. The arrangement that gets the most votes
wins Maximum parsimony.