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Northwestern Garter Snake
(Thamnophis ordinoides)
Speciation
I. What is a species?
II. Isolating Mechanisms
III. Models of Speciation
Aristotle coined the term species.
Genus
-a group that shares generic
properties
Species
-a group that shares specific
properties
Aristotle
(384-322 BC)
Linnaean System
of Classification
Carl von LinnJ
a.k.a. Linnaeus
(1707-1778)
Definitions of Species
• Morphological species
• Biological species
• Evolutionary species
Morphological Species
A species is a group of individuals that are
structurally similar.
Two species
or one?
Morphological Species
A species is a group of individuals that are
structurally similar.
Two species
or one?
♂
♀
Cryptic species of parasitic flies
(Apocephalus paraponerae)
Some flies feed and lay eggs in an injured
worker ant. Larvae develop inside the ant
and consume their host.
Species of flies are separated by the
ability to use specific species of ants.
Distantly related flies have converged on
the same species of ants. The two
species of flies are genetically distinct, but
phenotypically indistinguishable.
Biological Species
A species is a group of potentially
interbreeding individuals.
Note: Not all individuals have to
interbreed, but they must have the
potential to do so.
Ernst Mayr
1927
[Source: Mayr 1942, Systematics and the Origin of Species, Columbia University Press]
The Ring
Biological Species Concept
A species is a group of potentially interbreeding
individuals.
But what about
asexual organisms?
Oat aphid
(Rhopalosiphum padi)
Evolutionary Species Concept
A species is a lineage that maintains its integrity
through space and time.
A
B
Ancestor
[Source: Wiens 2004, The American Naturalist 163: 914-923]
Speciation depends on ecological distributions.
?
Allopatry
-two groups don’t occur in the same place
Sympatry
-two groups do occur in the same place
Allopatric speciation
Sympatric speciation
geographic
isolation
Evolutionary species
evolution of
reproductive
isolation
Biological species
Evolutionary and Biological
species concepts
evolution of
morphological
differences
Morphological species
Allopatric speciation
Sympatric speciation
geographic
isolation
Evolutionary species
evolution of
reproductive
isolation
Biological species
evolution of
reproductive
isolation
Evolutionary & Biological species
evolution of
morphological
differences
Morphological species
Isolating Mechanisms
•
•
•
•
•
Spatial
Temporal
Behavioral
Mechanical
Gametic
Spatial & Temporal Isolating Mechanisms
Tradescantia canaliculata
-full sun on edge of cliffs
-flowers in spring
Tradescantia subaspersa
-full shade of deep forest
-flowers in summer
Spider wort
(Tradescantia spp.)
Both spatial and
temporal isolation
prevent interbreeding.
Behavioral Isolating Mechanisms
In Drosophila, a female
attracts a male with
chemicals on the surface of
her body.
Fruit flies courting
The chemicals on females
of Drosophila melanogaster
are unattractive to males of
Drosophila simulans (and
vice versa).
Mechanical Isolating Mechanisms
Pollination of Sage
A
A) Only large
pollinators can pick
up pollen from the
anthers of S. apiana.
B
Salvia mellifera
Salvia apiana
B) Only small
pollinators can land
on the flower of S.
mellifera.
Gametic Isolating Mechanisms
Sperm or egg is destroyed,
or sperm cannot fertilize
eggs.
Fruit flies mating
For example, when a male
fly inseminates a female of
another species, her vagina
swells and prevents
fertilization.
Isolating Mechanisms
Pre-zygotic
•
•
•
•
•
Spatial
Temporal
Behavioral
Mechanical
Gametic
Hybridization
Isolating Mechanisms
Pre-zygotic
•
•
•
•
•
Spatial
Temporal
Behavioral
Mechanical
Gametic
Post-zygotic
• Hybrid Inviability
• Hybrid Sterility
Hybrid Inviability
Many hybrids die either in
the embryonic or juvenile
stage.
Green frog
For example, frogs of the
genus Rana hybridize but
the resulting offspring die
as embryos.
Hybrid Sterility
Some hybrids survive just
fine, but cannot reproduce.
For example, mules are
the sterile hybrids of
horses and donkeys. Yet,
they are quite viable as
hybrids (see left).
A mule killing a mountain lion
Pre- & Post-zygotic Isolating Mechanisms
Drosophila pseudoobscura
-low elevation (warm)
-mates in evening
Drosophila persimilis
-high elevation (cool)
-mates in morning
Fruit flies
Even if interbreeding
occurs, hybrids have very
low fertility.
How do new biological species form?
OR
How do isolating mechanisms evolve?
Mayr’s Model of Allopatric Speciation
A single species over a broad range.
A geographic barrier arises that
prevents gene flow. Populations
evolve independently.
The barrier disappears but the
populations are no longer capable
of interbreeding.
Seven Pairs of Species
(P =Pacific; C= Caribbean)
snapping shrimp
(Alpheus spp.)
[Source: Knowlton et al. 1993, Science 260:1629-1632]
Reinforcement
When contact reoccurs between populations,
selection can produce isolating mechanisms
that finalize speciation.
Usually, pre-zygotic isolating mechanisms
evolve to reinforce post-zygotic isolating
mechanisms.
Why do you think reinforcement occurs?
Sympatric Speciation
• Speciation between populations living in the
same area
• Proposed in the 1930’s but not appreciated
until the mid 1980’s
• Usually requires strong disruptive selection
[Source: Via 2001, Trends in Ecology & Evolution 16:381-390]
Disruptive Selection & Assortative Mating
more fit
Assortative
mating
least fit
more fit
Assortative
mating
On Isla Santa Cruz, beaks sizes of
Darwin’s finches vary greatly.
two birds caught in the same net
Source: Hendry et al. 2006, Proc. R. Soc. Lond.
Beak size constrains
the song of a bird.
Frequency (kHz)
Frequency (kHz)
Large-beaked male
Small-beaked male
Time (s)
Factors Promoting Sympatric Speciation
• Habitat choice and mate choice are each
determined by one gene
• Genes for habitat choice and mating
choice are physically linked (i.e., on the
same chromosome).
• Even better, one gene could affect both
habitat choice and mate choice.
[Source: Via 2001, Trends in Ecology & Evolution 16:381-390]
Sympatric Speciation by Polyploidy
(duplication of chromosomes within a species)
Diploid
cell
Most sexually reproducing
organisms are diploid (two
copies of each gene = 2N).
These copies segregate
during meisosis.
Tetraploids (4 N) are
formed when chromosomes
do not segregate during
meiosis.
Tetraploid
cell
Hybrids
are sterile
Allopatric speciation
Sympatric speciation
geographic
isolation
Evolutionary species
evolution of
reproductive
isolation
Biological species
evolution of
reproductive
isolation
Evolutionary & Biological species
evolution of
morphological
differences
Morphological species