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“They all look alike to me.”
Speciation and Macroevolution
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Species concepts
Reproductive isolating mechanisms
Allopatric speciation
Sympatric speciation
Adaptive radiation
Mass extinctions
Key Concepts
Speciation occurs when populations of the same species become
genetically isolated by lack of gene flow and then diverge from
each other due to natural selection, genetic drift, or mutation.
Populations can be recognized as distinct species if they are
reproductively isolated from each other, if they have distinct
morphological characteristics, or if they form independent
branches on a phylogenetic tree.
© 2011 Pearson Education, Inc.
Key Concepts
Populations can become genetically isolated from each other if
they occupy different geographic areas, if they use different
habitats or resources within the same area, or if one population is
polyploid and cannot breed with the other.
When populations that have diverged come back into contact, they
may fuse, continue to diverge, stay partially differentiated, or have
offspring that form a new species.
© 2011 Pearson Education, Inc.
Speciation
• Origin of new species from existing
species
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What is a species?
Concept recognizes discontinuity
in variation among organisms
Early concepts based on
morphology
Morphological species concept
If it looks like a duck…
Phylogenetic species concept:
Species are the smallest
monophyletic group
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Problems with early species
concepts
Darwin recognized variation within
domesticated species exceeded that
among some similar species
Some species are indistinguishable
morphologically = Cryptic species
Others differ radically within species
Sexual dimorphism
Phenotypic plasticity
Photos:Patrick Coin (left), L.A. Dawson (top right), Robert A. Coggeshall (lower right)
Eclectus roratus (female on rt!)
• Photo: Doug Janson
Biological Species Concept
• Ernst Mayr (1904-2005)
Biological Species Concept
(1940)
• Ernst Mayr (1904-2005)
• Actually or potentially interbreeding
to produce viable, fertile offspring
• Have at least the potential to share
a common gene pool
• Depends on reproductive isolation
from other such groups
Tab. 22.1
Prezygotic
isolation prevents
fertilization, so no
hybrid zygote is
ever formed
Postzygotic
isolation
(bottom panel)
prevents
hybrids from
having much of
an evolutionary
future
Hybrid
breakdown: if
F1 hybrids
viable and
fertile, later
generations
may not be
Behavioral isolation in
lacewings
Mating activity
In frogs, both
temporal and
courtshipbased
isolation
Wood
frog
March 1
Leopard
frog
April 1
Fig. 19.01c
May 1
Mechanical Isolation
• Differences in
flower structure in
black and white
sage require
different pollinators
• Big bees can’t land
on black sage
• Small bees don’t
contact anthers of
white sage
Fig. 19-3
The Phylogenetic Species
Concept
• A species is defined as the smallest
monophyletic group on the tree of life.
• On phylogenetic trees, an ancestral
population plus all of its descendants is
called a monophyletic group or clade.
– Clades are identified by
synapomorphies, or shared derived
characteristics
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Problems with phylogenetic
species conept:
• Still does not tell us which clade to
count (Smallest? Single mom and her
kid?)
• Ultimately, concept should reflect gene
flow within, but not between, species
• Data are lacking for many groups of
organisms.
How are new species formed?
• Question really becomes “how
does reproductive isolation occur?”
• Allopatric speciation requires a
period of geographic isolation of
populations
• Sympatric speciation can occur via
–Polyploidy
–Ecological isolation
Allopatric Speciation
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Ernst Mayr
Allopatric = “different fatherland”
Begin with single population
Isolate one or more subpopulations
Allow populations to diverge genetically
(recall molecular clock?)
• If sufficient time passes without gene
flow, two subpopulations no longer
compatible
Fig. 19-15
Opportunity for isolation
fosters speciation
•Island archipelagos
•Chains of
lakes
•Small
populations
 Rapid drift
Fig. 22.11
Adaptive Radiation
• Rapid diversification of a lineage to fill many
ecological niches
• Often follows mass extinction and/or
associated with some key innovation (e.g.,
flight in bats, birds, insects…)
• Especially common on archipelagos
– Many opportunities for allopatric speciation
– Many ecological opportunities because
island biota generally depauperate
– Hawaiian Drosophila and silverswords,
Galapagos finches, African cichlids
Allopatric Speciation in Action
• Bullock’s and Baltimore Orioles
– Once considered two species, now just
one
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Sympatric Speciation
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Sympatric = “same fatherland”
No geographic isolation required
Can result from polyploidy
Or a behavioral shift
Allopolyploidy
• “Allo” = “different”
• Hybridization sometimes produces viable
polyploid progeny with 2n = sum of the two
parents’ haploid chromosome numbers
• Normally meiosis fails because homologs
cannot pair correctly
• Nondisjunction prior to meiosis can
produce 2n gametes capable of selfing
Allopolyploidy
• 2n = 6 and 2n = 4 parents
• Hybridize to form 2n = 5
progeny (usually sterile)
• Error in mitosis doubles
chromosomes to 2n = 10
• Meiosis can now proceed
normally to produce n = 5
gametes
• Can self to form 2n = 10
progeny capable of
interbreeding
Allopolyploidy is well-documented
• Many plant species experimentally
reconstruced
– E.g., Galeopsis tetrahit (2n=32) appears
to be allopolyploid hybrid of G.
pubescens and G. speciosa (each 2n =
16)
• Spontaneous formation of a new species
occurred in Kew Gardens
– Primula kewensis (2n = 36) is a hybrid of
P. floribunda and P. verticillata (each
2n=18)
Primula
kewensis
Primula
floribunda
Primula
Primula
verticillata
Fig. 19.9
Novel ecological situations may
lead to sympatric speciation
• Rhagoletis pomonella
the haw fly or apple maggot
• Original host haws
• Moved onto apples in 1860s
• Reproductively isolated because timing
of fruit used for mating differs
• Populations have distinctive alleles, but
still considered one species by many
Extinction
• Five mass extinction events
• A significant factor in evolution
• Virtually all species that ever
existed are now extinct
• Mass extinctions in Earth’s
history radically altered the
course of evolution
– Presumably in unpredictable
ways
– Often followed by adaptive
radiation
• Humans responsible for what may be largest
(sixth) mass extinction
Speciation and Macroevolution
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Species concepts
Reproductive isolating mechanisms
Allopatric speciation
Sympatric speciation
Adaptive radiation
Mass extinctions