Transcript How Humans Evolved, 5e

How Humans
Evolved, 5e
Chapter 4: Speciation and Phylogeny
W. W. Norton & Company © 2009
What Are Species?
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Organisms that belong to a species are
generally distinct from the members of other
species.
Species are real biological categories, but
some dissent exists in the true definition.
What are Species?
Biological species
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The biological species concept defines a
species as a group of organisms that
interbreed.
Reproductive isolation states that members of
one group cannot successfully breed with
members outside the group.
The ability to breed leads to gene flow in the
population.
What are Species?
Ecological species
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The ecological species concept emphasizes
natural selection as a key force in creating and
maintaining the species.
What are Species?
Ecological species
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Asexual species, in which gene flow does not
occur, cannot be thought of as reproductively
isolated.
Selection would favor organisms of the same
morphology, which would maintain their
similarity even without gene flow.
The Origin of Species
Allopatric speciation
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When a population is divided by a barrier,
different parts of the population adapt to
different environments.
The Origin of Species
Allopatric speciation
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The longer the groups remain isolated, the
greater the genetic difference is between the
groups.
When the separation results in complete
reproductive isolation, a new species is
formed.
The Origin of Species
Allopatric speciation
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Character displacement refers to an
occurrence after the two separated groups
come together. If there is some gene flow,
such as through hybrid individuals, the
competition between the groups over food or
mates can still result in a divergence of
characters.
The Origin of Species
Allopatric speciation
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Reinforcement reduces gene flow between
two formerly separated populations by
selecting favorable adaptations that prevent
mating between the members of the two
populations.
The Origin of Species
Allopatric speciation
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Allopatric speciation thus requires a physical
barrier that interrupts gene flow and allows
the two populations to diverge by means of
natural selection.
The Origin of Species
Parapatric and sympatric speciation
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Parapatric speciation states that a new species
can be formed if selection is combined with
partial genetic isolation.
At habitat boundaries, a hybrid zone can be
formed.
The Origin of Species
Parapatric and sympatric speciation
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Sympatric speciation states that speciation can
occur even in the absence of a physical barrier.
Adaptive radiation occurs when a single
species diversifies to fill numerous ecological
niches.
Tree of Life
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Carolus Linnaeus created a standard scientific
taxonomy system in 1735, including the
binomial nomenclature to distinguish all
species.
A phylogeny is a family tree that shows the
common ancestor of two or more species.
Why Reconstruct Phylogenies?
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To classify and identify organisms.
To understand why an organism evolved a
particular trait.
To understand morphology and behavior.
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The technique used in this case is known as the
comparative method.
How to Reconstruct Phylogenies
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Establishing comparisons between features,
behavior, or genetics is the first step in
creating a phylogenetic tree.
How to Reconstruct Phylogenies
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Convergence presents a problem in
reconstructing phylogenies.
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Analogous traits are similar because of
convergence.
Homologous traits are similar because of descent
from a common ancestor.
How to Reconstruct Phylogenies
Ancestral and derived characteristics
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Ancestral traits characterize the last common
ancestor of two or more species. They appear
early in the development of an organism,
appear earlier in the fossil record, and are
seen in out-groups.
How to Reconstruct Phylogenies
Ancestral and derived characteristics
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Derived traits have evolved since the
branching off from the last common ancestor.
Only derived traits can be used in
reconstructing phylogenies.
How to Reconstruct Phylogenies
Genetic distance
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A computation of overall genetic similarity
between pairs of species.
Genetic distances with constant rates of
change are known as molecular clocks.
How to Reconstruct Phylogenies
Genetic distance
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The neutral theory states that most changes in
DNA have little to no effect.
Knowing the distance between two species
allows for estimation of how long ago the
species diverged.
Taxonomy
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Taxonomic classification of humans
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Species: Homo sapiens
Genus: Homo
Family: Hominidae
Superfamily: Hominoidea
Taxonomy
Classification Systems
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Cladistic taxonomy says that only descent
matters.
Evolutionary taxonomy says that both descent
and overall similarity matter.
How Humans Evolved, 5e
This concludes the lecture PowerPoint slideshow for Chapter 4.
W. W. Norton & Company © 2009