Download 27. Introduction to speciation, allopatric speciation

Entomology 601 2015
Lecture 27
Allopatric
Speciation
Species
When we trade in ideas in population biology, species
constitute our currency. Their peculiar significance was long
ago appreciated by Darwin’s mentor, Sir Charles Lyell:
“The ordinary naturalist is not sufficiently aware that when
dogmatizing about what species are, he is grappling with the
whole question of the organic world and its connection with a
time past and with man, that it involves the question of man
and his relation to the brutes, of instinct, intelligence and
reason, of Creation, transmutation, and progressive
improvement or development. Each set of geological
questions and of ethological and zoological and botanical are
parts of the great problem, which is always assuming a new
aspect”
(H.E.H. Paterson)
Speciation
•  Perhaps if we
examine the
processes by
which species
are formed, we
can better
understand the
endpoints
anagenetic change
Study of Speciation
We are interested in
events that happened
in this time frame
anagenetic change
A
B
We must reconstruct
these events by
studying attributes of
the products, A and B
anagenetic change
A
B
But the differences
between A and B include:
1- differentiation that
occurred during speciation
2- differentiation in each
species that occurred after
speciation
anagenetic change
A
B
2- differentiation in
each species that
occurred after
speciation
1- differentiation that
occurred during speciation
Perhaps if we look at enough
cases, we can determine
commonalities
•  Is differentiation in any set of attributes
always associated with speciation?
•  Unfortunately, review of literature does not
provide conclusive answer
•  Differentiation in morphology, chromosomes,
DNA sequences, biology, behavior, ecology
etc. may or may not be found in apparent
sister species
Neodarwinian view of species
•  Species are a reproductive community
•  Composed of number of populations
•  Gene pool of species contains
substantial genetic variation
•  Gene flow within and among
populations is a strong cohesive force
Neodarwinian view of species
•  Speciation involves extensive replacement
and reorganization within gene pool
•  “Genetic revolution”
•  Old groups of co-adapted allele complexes
broken up
•  New co-adapted allele complexes formed in
response to new selective pressures
•  This also results in reproductive isolation and
speciation
How about genetics of
reproductive isolation?
•  Templeton (1981*) reviewed literature on
hybrid incompatibility
•  Perhaps these will reveal critical genetic
changes that occur during speciation
•  In some cases, only few specific loci involved
in hybrid incompatibility
•  Hybrid incompatibility due to a breakdown in
some specific system involved in growth and
life history
* Ann. Rev. Ecol. Syst. 12:23-48
Genetic Basis for Hybrid
Incompatibility
•  Type I = polygenic
–  Many segregating units
–  Each with a small effect
•  Type II = one or a few major segregating units
•  Type III = complementary or duplicate pairs of
loci
•  Apparently, notion that speciation ALWAYS
involves massive genetic change (“genetic
revolution”) is not supported by the evidence
Coyne and Orr (1998)
Phil. Trans. R. Soc. Lond. B 353:287-305
Coyne and Orr (1998)
Phil. Trans. R. Soc. Lond. B 353:287-305
Geographical Distributions
• 
• 
• 
• 
Allopatric
Sympatric
Parapatric
Peripatric
–  Central population surrounded by
peripheral populations
•  Allochronic
Neodarwinian Model of Allopatric
Speciation
Neodarwinian Model of Allopatric
Speciation
•  Gene flow between
pops interrupted
•  Causal agent is
geographical
isolation
•  May occur over long
period of time
•  Two sets of
populations exposed
to different selective
regimes
•  Stochastic genetic
changes may also
occur
Stage One
Stage Two
•  Contact is resumed
between populations
•  Two possible outcomes:
•  1- A single gene pool is
re-established
•  2- two groups continue
to diverge due to
reproductive isolating
mechanisms that
evolved during stage I
Speciation!
“Biological” Species Concept
•  Species are groups of populations that
are actually, or potentially,
reproductively isolated from other such
groups
(Ernst Mayr)
Mechanisms for
Pre-Mating (Pre-Zygotic)
Reproductive Isolation
•  Differences in mate recognition system
–  Courtship
–  Pheromones
–  Coloration
• 
• 
• 
• 
Ecological or habitat differences
Seasonal or temporal isolation
Mechanical factors - genitalia
Sperm competition or incompatibility
Mechanisms for
Post-Mating (Post-Zygotic)
Reproductive Isolation
•  Hybrid inviability
•  Hybrid sterility
•  Hybrid breakdown
“Biological” Species Concept
•  Species are groups of populations that
are actually, or potentially,
reproductively isolated from other such
groups
•  Why potentially?
(Ernst Mayr)
Stage Two is seen as a necessary
part of the process
•  Species are
“tested”
•  Reinforcement of
post-mating
reproductive
isolation by
selection for
premating RI
•  Allopatric
distributions are
always a problem
A Common Assumption
•  Post-mating isolating mechanisms may
often be present at end of Stage One
•  Pre-mating isolating mechanisms may
or may not be present at end of Stage
One
•  But if pre-mating R.I. is not present,
hybrid matings result in genetic death
•  This REINFORCES pre-mating R.I.
An important distinction
•  REINFORCEMENT = evolution of
increased reproductive isolation
resulting from selection against hybrids
•  CHARACTER DISPLACMENT =
interactions between sympatric species
causing divergence in some characters
–  REPRODUCTIVE CHARACTER
DISPLACMENT = character displacement
involving mate recognition, etc.
Demonstration of reinforcement
has been difficult
•  Difficult to
separate from
reproductive
character
displacment
•  Difficult to model,
genetically
•  Excellent recent
review in Coyne
and Orr’s book
Reinforcement
A
A+B
B
Selection occurs only here
If hybrids inviable, generally A or B will win out
Other form goes extinct
Reinforcement
A
A+B
B
Selection occurs only here
If hybrids viable, A/B backcrosses lead to
complex genetic backgrounds
Outcome is a cline or spectrum of genotypes
Models of
Reinforcment
•  Requires
substantial
postzygotic
reproductive
isolation and
•  low errors in
mate choice
Models of
Reinforcment
•  Extinction of one or
the other also a
possibility
Coyne and Orr (1989)
Evolution 43(2):362-381
•  119 pairs of Drosophila species
•  Known geographic ranges (sympatric or
allopatric)
•  Known genetic distances
•  Known mating discrimination
–  pre-zygotic reproductive isolation
•  Known sterility and inviability
–  post-zygotic reproductive isolation
Total RI vs. genetic distance
Genetic
distance and
reproductive
Isolation
•  Substantial
prezygotic RI can
occur with very
low genetic
distance
Sympatry vs. Allopatry
•  Much stronger
prezygotic RI in
sympatry!
•  Is this reinforcement?
•  Problem: appears that
generally there is not
much postzygotic RI to
reinforce
Sexual Selection
•  Competition for mates
•  One sex (often the female) preferentially
chooses among many possible mates
•  Choice based on male phenotype
•  Strong selection for genetics underlying the
traits in successful males
•  May or may not have anything to do with
fitness of progeny
Sexual
Selection
The Tufted Coquette
Lophornis ornatus
(male on right)
Sexual Selection
Sexual Selection
Sexual Selection
•  May involve
traits of mate
recognition
systems
•  Evolution in
such systems
may be very
rapid