Download Speciation

Speciation
Chapter 16
The big problem in Speciation
Define Species!!!!!!!!!!!!!!
Biological species concept
Phylogenetic species concept
Morphological species concept
Cohesion species concept
Evolutionary species concept
Lots of other species concepts.
Why is this a problem
Without a clear species concept we cannot be clear
about what we are talking about!
How can we talk about mechanisms of speciation if we
don’t know what a species is?
More fundamentally:
Mayr thought that species were a “natural” unit.
Is this true?
The Biological Species
Concept
BSC: A species is a group of actually or potentially
interbreeding individuals that are reproductively isolated
by other such groups. (Ernst Mayr)
Species is a group of organisms sharing a gene pool
Species boundaries are defined by the limits of gene flow.
Ultimately this definition is about reproductive isolation
It is the most widely accepted and conceptually clearest
definition, and underlies our common ideas of “species”.
Ernst Mayr
1904 – 2005
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Mayr circa 2000
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Mayr 1928
The importance of
reproductive isolation
Phylogenetic Species concept
A monophyletic group of organisms that is
phylogenetically distinct from other such groups.
This is consistent with the BSC.
How “far down” the tree you need should go is an issue.
Phylogenetic Species concept
A monophyletic group of organisms that is
phylogenetically distinct from other such groups.
This is consistent with the BSC.
How “far down” the tree you need should go is an issue.
Phylogenetic Species concept
A monophyletic group of organisms that is
phylogenetically distinct from other such groups.
This is consistent with the BSC.
How “far down” the tree you need should go is an issue.
Morphological Species
concept
A group of organisms that are sufficiently similar in
appearance behavior etc. that they can be logically
grouped.
Don’t kid yourselves. This is the one we use!
Biological Species Definition
Problem: What is potentially interbreeding?
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
“Artists” rendition
It's pretty much my favorite
animal. It's like a lion and a
tiger mixed... bred for its
skills in magic.
Napoleon Dynamite
The liger is the world's largest big cat. An average male liger weighs over 900 pounds and
standing almost 12 feet tall.The reason that they are called a liger is because the father
was a lion and the mother was a tiger. If the situation was reversed and the mother was a
lion and the father was a tiger, he would be called a tigon, and would be a dwarf instead
of a giant. A fully grown tigon is usually less than 350lbs. Ligers are not sterile, and they
can reproduce.
Morphological Species Definition
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Heliconius/Ithomiine
mimicry complex
Isolating Mechanisms
The BSC is really about isolating mechanisms
Two general classes of isolating mechanisms
Premating isolating mechanisms
Postmating isolating mechanisms
Generally Postmating isolating mechanisms are considered
to be artifacts of phylogenetic distance
Premating isolating mechanisms are thought to evolve to
avoid “wasting gametes”
Premating Isolating
Mechanisms
Some examples:
Potential Mates do not meet (seasonal or habitat isolation.)
Lions and Tigers live in different habitats. For
this reason there are no wild Ligers or Tigons
Potential Mates meet but do not mate (behavioral Isolation)
Many species have elaborate mating rituals
that (among other things) keep species
separate
Copulation is attempted, but no transfer of sperm takes
place (mechanical isolation)
Post Mating Isolating
Mechanisms
•Sperm Transfer takes place, but egg is not fertilized
(gametic mortality).
•Egg is fertilized, but zygote dies (zygote mortality)
•Zygote produces an F1 hybrid, but zygote dies (zygote
mortality)
•F1 hybrid is fully viable, but partially or completely
sterile (hybrid sterility)
•F1 hybrid is viable, but ecologically unable to survive
(hybrid inviability)
Thought to be the side product of evolutionary divergence.
Speciation: the “Patrics”
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Speciation
Common elements to all models of speciation:
•Reproductive isolation
•Time for independent evolution to occur
Mayr introduced the allopatric speciation model. It is the
only model accepted as definitely true.
Sympatric models have been controversial, but evidence
is accumulating that sympatric speciation does occur.
Dobzhansky-Muller Model
The Dobzhansky-Muller model argues that at least two
interacting genetic changes must occur for there to be
speciation
mutation
mutation
mutation
mutation
Sympatric Speciation
Sympatric speciation is the most controversial
Postulates NO barrier to gene flow at the
beginning
Speciation arises due to disruptive selection
Theory says that even a small amount of gene flow
should stop this from happening.
Speciation in Nicaraguan
Cichlids
Marta Barluenga et al
Nature 439, 719-723 (9 February 2006
Speciation in Oceanic Palms
Howia forsteriana is shown in black (n = 1,677)
and H. belmoreana in grey (n = 4,542)
Lord Howe Island
Vincent Savolainen et al
Nature 441, 210-213(11 May 2006)
Causes of Divergence
Genetic Drift -- SLOW
Mutation -- SLOW
Divergent Selection -- Probably the most common cause
Different Environments -- see table 16.1
Different genetic complexes
Sexual selection
Genetic drift and selection alone do not cause
reproductive isolation unless (1) there is mutation, or (2)
they diverge into sufficiently different habitats that the
hybrid has low fitness
Accelerating Speciation
Sexual Selection: Fisher’s
Runaway Process
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Founder Events
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Gene Interaction and Speciation:
Dominance by Additive Epistasis
B1B1 B1B2 B2B2
Freq (A1) = 1
Freq (A1) = 0.5
Freq (A1) = 0
In a Metapopulation fixed for the B2
allele the introduction of a B1 allele (by
mutation or migration) can lead to
speciation due to underdominance at the
A locus
1
0
1
0
0
0
—1
0
—1
Reinforcement
After the initial stages of reproductive isolation have
occurred selection may favor further isolation
1) Initial divergence
2) Species come back into contact
3) Hybrids have low fitness
4) Selection favors females that mate with their own
species
Reinforcement favors the evolution of premating isolating
characters. (why?)
The effects of Reinforcment
Pairs of Drosophila Taxa
Galapagos and Cocos Islands
Galapagos Finches
Another tree
Finch Ecology
Speciation in Galapagos
Finches
13 species on Galapagos, 1 on Cocos
Galapagos has more diverse habitats than Cocos
Galapagos has more area than Cocos
Galapagos has more islands than Cocos (1 island)
Which is the cause of speciation on Galapagos?
Hybridization and Speciation
Hybridization can be a source of new species
Loren Rieseberg
Hybrid Zones
Hybrid Zones cont.
Forces on hybrid zone
Fate of the Hybrid?
Hybrid has
low fitness
Hybrid has high fitness
throughout region
Hybrid has high
fitness in hybrid
zone
Speciation in Continuous
Populations
a)
AA 1 .5 1
Aa .5 0 .5
aa 1 .5 1
BB Bb bb
b)
AA 1 .5 .92
Aa .5 0 .5
aa .92 .5 1
BB Bb bb
In theory speciation can take
place in a continuous population.
Consider a population with:
•multiple loci
•Underdominance
•Multiply underdominant
genotypes have zero fitness.
Speciation in Continuous
Populations
t = 50
t = 250
t = 150
b)
c)
d)
e)
f)
g)
=0
t=1
a)
= 0.1
Speciation in Continuous
Populations
• Generations to Speciation.
When mating is spatially localized,
mild underdominance persists for
extended duration (blue and red
lines).
• Decreasing the epistatic coefficient
by an order of magnitude increases
the time to speciation by an order
of magnitude.
• Generations to fixation of a
single genotype. Speciation never
occurs with panmixia.