Ch 24: origin of species
• What is a species?
• Species Concepts
– Morphological
– Biological
• Isolating mechanisms and hybridization
– Other Species Concepts
• How do new species arise?
– Allopatric vs. sympatric speciation
• What is the tempo of speciation?
– Punctuated equilibrium model and fossil record
• How can new, complex structures evolve?
What is a species?
Species
• A Latin word meaning “kind” or “type”
• There are many kinds of living thing!
• Biologists mean something special by
the word species
One species or two?
Species Concepts
• Our ideas about how best to define a
species have changed over time.
• If speciation is occurring, no species
concept will “work” all the time - you
should always be able to find arguable
forms.
Morphological Species
Concept
• morphology = shape or appearance
– Collect a type specimen
– Others that are similar to the type are the
same species
– Very widely used (e.g., insects)
– Problem - doesn’t take intraspecific variation
into account - how similar is similar enough?
Are the similarities we notice the important
ones?
Biological Species Concept
• Formulated by Ernst Mayr: Ornithologist;
one of the contributors to the Modern
Evolutionary Synthesis
• Species are populations or groups of populations whose
members have the potential to interbreed in the wild
and produce viable, fertile offspring. (1942)
• Rationale: hybridization implies mixing species are distinct because they don’t mix
• Advantage: the organisms decide themselves
how different is different enough
Reproductive isolation
• What keeps different species from
interbreeding? (not counting geography)
– Prezygotic barriers
• Before mating or fertilization
– Postzygotic barriers
• After mating or fertilization
• What’s a zygote?
Prezygotic barriers (1)
• Habitat isolation: species occupy different
habitats in the same geographic area
Three-spined stickleback species
differ by lake habitat: benthic vs.
pelagic zones
Prezygotic barriers (2)
• Breeding behavior ->
Prezygotic barriers (3)
• Temporal isolation: species reproduce at
different times in the same geographic area
Prezygotic barriers (4)
• Mechanical isolation: species have
incompatible anatomy
Prezygotic barriers (5)
• Gametic isolation: species have gametes
that recognize only their own species
Postzygotic barriers
• Reduced viablility: embryos die or fail
to develop
• Hybrid offspring born, but have
reduced viability or fertility
• Hybrid breakdown: offspring of
hybrids have reduced survival or
fertility
Horse + donkey = sterile mule
mom
dad
Reproductive
isolation
• Prezygotic barriers
• Postzygotic
barriers
Problems with Biological
Species Concept
• The Biological Species Concept works very well with
some taxa (birds, for instance) but..
• Information on hybridization lacking for most
species
?
X
Problems with Biological
Species Concept (2)
• In some groups (oak trees, for instance, and many
other plants) hybridization between what seem to
be quite different species is common.
Problems with Biological
Species Concept (3)
• In other groups (some birds), hybridization at least
happens occasionally - a little mixing doesn’t always
break down differences between species
Alternative species concepts
• Phylogenetic species concept - minimum
diagnosible unit
• Ecological Species Concept - a set of
organisms adapted to a particular set of
resources, called a niche, in the
environment
• All these species concepts agree most of
the time - they differ mostly when applied
to borderline or poorly understood cases.
Speciation
• Speciation - when one species splits
into two (aka cladogenesis)
• How does it happen?
• Allopatric vs. sympatric speciation
Modes of speciation
Fig. 24.6
Allopatric Speciation
• Allopatry - “different fatherlands”
– 1. One species is divided into two isolated
populations in two geographic regions
– 2. The two populations evolve in different
directions while apart
– 3. Isolating mechanisms evolve in the two
groups
– 4. The two populations are unable to breed
with each other when they reconnect - they are
now two separate species
Allopatric speciation in
ground squirrels
Sympatric Speciation
• Sympatry = living in the same place
• How could individuals evolve reproductive
barriers to others in a local, interbreeding
population?
• Theoretically possible, but evidence
suggests it’s rare at best
• Organisms can undergo sympatric
speciation by producing polyploid offspring
Speciation by production of
tetraploids
Sympatric Speciation (1)
Tree Frogs
Hyla chrysocelis
Hyla versicolor
Sympatric speciation (2)
• Mate choice by cichlids
Pundamilia pundamilia
Pundamilia nyererei
Fig. 24.16
Normal light
Monochromatic light
Adaptive radiation
• Example?
Fig. 24.11
Is speciation gradual or
sudden?
• Paleontologists have a hard time
seeing gradual change in the fossil
record. More often, a form appears
suddenly, then, after persisting for
some eons, disappears from the fossil
record just as suddenly.
• Why is this?
• Niles Eldredge and Stephen Gould
proposed that formation of new
species happened rather quickly, and
that species, once formed, changed
little until they went extinct.
• This model was called punctuated
equilibrium by Eldredge and Gould.
later
Earlier
So are Gould and Eldredge
right?
• The evidence for equilibria comes from the fossil
record only - no living species has shown any
resistance to change under artificial selection.
• The nature of the fossil record: small slices of
time are preserved. Huge slices are lost.
• Nobody believes that the tempo of evolution is
absolutely constant.
later
earlier
later
Earlier
Evolutionary Novelties
• If natural selection is essentially a “weeding”
process, eliminating the less fit, how does a
complex new structure like an eye develop?
• This is one of the arguments of anti-evolution
religious fundamentalists:
– it’s impossible that natural selection acting on chance
mutations could gradually produce something so complex
and perfect as an eye
• Is it really impossible, though? What would have
to happen?
Question: How do complex
structures evolve?
• Answer:
gradually
• Important principles:
– Natural selection is not goal-directed, and cannot
anticipate future needs
– Evolutionary novelties arise by modification of preexisting structures
– All intermediate steps must be beneficial to the organism
- like working on an engine while it is running.
• So for novel complex structures to
arise by natural selection
– 1. Each step must be a plausible small
minor modification of the one before it
– 2. Each step must be an improvement on
the one before it
A simple patch of pigmented
cells detects light
If a depression forms with the
pigment cells in it, can detect
direction of light source
A depression with a small opening
allows formation of images on the
light-sensing cells
A lens within the eye allows
better focussing and allows
more light to enter
A lens that can change shape
allows near and far focussing
Is it plausible?
• Could these different
stages have existed?
• Would they each give their
owners an advantage?