Download Speciation - Carol Eunmi LEE

Carol Eunmi Lee
11/17/16
Speciation
Today’s OUTLINE:
(1) Geographic Mechanisms of
Speciation
(What circumstances lead to the formation
of new species?)
(2) Species Concepts
(How are Species Defined?)
Mechanisms of Speciation
(1) Mechanisms of Speciation
Last Time: Genetic Models:
Last Time: Genetic Models:
How do Genetic Drift, Natural Selection,
Mutations, etc. create new species?
Are there “speciation” genes?
The roles of: Mutations
Natural Selection
Genetic Drift
This Time: Geographic Models:
This Time: Geographic Models:
How does speciation occur in Nature?
Is geographic isolation required?
Allopatric Model (difference place)
Sympatric Model (same place)
Parapatric Model (adjoining)
Mechanisms of Speciation
Geographic Models of
speciation
Geographic (Ecological) Models:
Allopatric Model (different place)
Disperse to Another Location
Vicariance: a barrier is formed
This geographic split could lead to Dobzhansky-Müller
incompatibilities
Sympatric Model (same place)
Polyploid speciation
Mate Choice
Niche Partitioning (e.g. different food source, Host Plant)
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Allopatric speciation: geographic isolation
Sympatric speciation: no geographic
isolation
Parapatric speciation: geographic
separation (or gradient), but not isolation
Parapatric Model (adjoining)
Title goes here
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Carol Eunmi Lee
Allopatric Models
Involves Geographic Isolation
11/17/16
Allopatric Models
• Following geographic separation
between populations,
Dispersal
Dispersal
Vicariance
Vicariance
• This geographic separation provides the setting
that allows speciation at the molecular level to
occur (last lecture)
Allopatric Models
Dispersal
Allopatric Speciation
Examples (see book)
■ Dispersal:
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Vicariance
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Vicariance:
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• Random Mutations would arise in the separated populations,
and then selection or genetic drift would lead to fixation of
those mutations
Colonization of islands
Colonization of lakes
◆
◆
Highway going through a forest
Fragmentation of habitats
Formation of Panama splitting the Caribbean &Pacific Oceans
• If different mutations are fixed in the different populations,
reproductive isolation could arise through Dobzhansky-Müller
incompatibilities (last lecture)
Sympatric models
Speciation with no geographic separation
Speciation despite gene flow
(1) Formation of polyploids
(discussed in previous lecture)
(2) Natural Selection due to
Niche Partitioning
Sexual Selection
Title goes here
Sympatric Model
(1) Formation of Polyploids
Polyploidy (extra chromosomes)
Important mechanism for plants
Also occurred possibly in vertebrates,
some crustaceans
(covered in previous lecture)
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Carol Eunmi Lee
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Sympatric Model
Example of Niche
Partitioning:
(2) Selection in the face of gene flow:
•Soapberry bugs
have adapted to two
different host plants
• Niche Partitioning
• Strong assortative mating and
sexual selection (disruptive selection)
Selection drives beak length apart
Evolutionary change in beak length on
the new small fruit
trend toward
smaller beaks on
smaller fruit
Niche Partitioning
Soapberry bugs mate on
different host plants
■
The populations are unlikely to encounter each
other
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◆
■
■
Reduces gene flow
Isolation
■
■
■
Disruptive Natural Selection
Adaptation to alternative hosts leads to
reproductive isolation (through the genetic mechanisms
discussed earlier, such as Dobzhansky-Müller model)
Title goes here
■
But,sometimeshybridzonesdoformbetween
populationsthatareintheprocessofspeciating
Sometimeshybridizationbetweendifferentspecies
resultsinvigorousnewspeciesorpopulations,
especiallyinplants(hybridvigor,orheterozygote
advantage)
Theeffectsvarydependingonhowdistantthetwo
speciesorpopulationsare…andwhetherthe
differentallelesatdifferentlociareabletowork
together(coadaptedgenecomplexes)
Hybridsbetweendifferentpopulationswithinaspeciesdo
tendtohaveanadvantage(Heterozygoteadvantage).
However,matingbetweenverydistantpopulations(different
species)canleadtohybridbreakdown.
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Carol Eunmi Lee
11/17/16
Increasing genetic distance
Mating between
different species
(Lions x tiger,
Horse x donkey)
Fitness
Mating between
relatives
Inbreeding
Depression
Populations
within a species
Will not mate or
Produce inviable or
sterile hybrids
Hybrid Vigor
(due to Heterozygote advantage)
Outbreeding
Depression =
Hybrid
Breakdown
Reinforcement
Reinforcement
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■
So, when hybrids are formed
between different species, they are
often costly and maladaptive
because of hybrid breakdown
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Reproductive isolation could occur at many
different levels
In such cases, you would predict that
mechanisms to avoid mating would
evolve to avoid the production of
maladaptive hybrids (= Reinforcement)
Reinforcement
Prezygotic (before the egg is fertilized)
◆
◆
■
So, when hybrids are formed between
different species, they are often costly
and maladaptive because of hybrid
breakdown
Genetic drift and divergence in bird song-won’t mate
Selection on coat color-don’t recognize each other
■
Postzygotic (after the egg is fertilized)
◆
DM incompatibilities cause embryo to not develop
(enzymes don’t work together)
■
Prezygotic barriers
Postzygotic barriers
Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown
Fertilization
Title goes here
So, the prediction is that in sympatry (when
two different species are in the same place),
mechanisms to avoid mating (prezyotic
isolation) would be strong
Whereas in allopatry, prezygotic isolation
would not be needed because the different
species would not come into contact
Viable,
fertile
offspring
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Carol Eunmi Lee
11/17/16
(2) How are Species Defined?
How are species defined?
Speciation is a messy process
■
So, what criterion should be used?
Historically, the most common
criteria had been using
morphological characters (how an
organism looks)
■
Problem :Populationsareintheprocessof
speciatingfromoneanother,andspecies
boundariesareoftendifficulttodefineuntil
thepopulationsaresufficientlydivergentby
allmeasures
Title goes here
■
■
Rates of molecular, phenotypic
(morphological) evolution and
reproductive isolation are not
necessarily concordant, but often
discordant
Speciation is a jagged messy
idiosyncratic process, where species
boundaries are often difficult to define
Sothen,howdoyoudefinespecies???
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Carol Eunmi Lee
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Darwin’s view:
Species are arbitrary constructs of the
human mind imposed on a continuum of
variation
ThreeMainSpeciesConcepts
1. BiologicalSpeciesConcept
Speciesaredynamicrather
thanstaticentities,with
boundarieschanging
constantly
2. PhylogeneticSpeciesConcept
Manygroupsareinthe
processofspeciation
3. Morphological(Phenetic) Species
Concept
1. Biological Species Concept
1. Biological Species Concept
(Ernst Mayr, 1942)
(Ernst Mayr, 1942)
Agroupofinterbreedingpopulationsthatare
evolutionaryindependentofotherpopulations
Example: all human populations belong to
the same biological species
Biological Species Concept
Biological Species Concept
PROBLEMS :
Strengths
An unambiguous empirical criteria which is
clearly linked to speciation (if populations can’t
intermate they can’t belong to the same species)
Using reproductive isolation as the criterion is
meaningful as it confirms the lack of gene
flow between groups
Title goes here
■
■
■
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Many ‘species’ are asexual and do not intermate
(viruses, bacteria, protists)
Many highly divergent species can hybridize
(plants)
Only applicable to present (not fossil taxa)
Ability to intermate sometimes drops off gradually
(“ring species”)
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Carol Eunmi Lee
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Ring Species
2. Phylogenetic Species Concept
The smallest group that is
monophyletic is called a
species
2. Phylogenetic Species Concept
Phylogenetic Species Concept
There are several
monophyletic
groups here
Typically, a phylogeny is constructed using DNA or
other types of data (proteins, morphological traits)
Monophyletic group:
A group with a shared derived
(descendant) character
A group that contains a
common ancestor and all its
descendents
Phylogenetic Species Concept
There is a derived character that is shared by the 4 populations
The phylogeny reveals hierarchical relationships
among groups
The smallest group that has a shared derived
character and is monophyletic is called a
species
A monophyletic clade consists of an
ancestral taxa and all its descendants
A
A
A
B
B
C
C
C
D
D
E
E
F
F
B
Monophyly
Group I
G
(a) Monophyletic group (clade)
D
Group II
G
(b) Paraphyletic group
E
Group III
F
G
(c) Polyphyletic group
The smallest monophyletic
group is called a species
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Title goes here
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Carol Eunmi Lee
11/17/16
Phylogenetic Species Concept
Strengths
Easy to see evolutionary relationships on
large and small taxonomic scales
It can be used on any species (sexual, asexual) for
which there is phylogenetic information (molecular,
morphological, biochemical data) on extant or fossil
species
Phylogenetic Species Concept
Problems:
■
■
■
Need a good phylogeny – time consuming
and can be expensive
Not recognize paraphyletic groups (a
monophyletic group that does not include all the
descendents; reptiles are paraphyletic, as they do not
include birds, because birds emerged from within
reptiles)
A trivial trait (single mutation or trait) can
make a group monophyletic, and may not
warrant calling a group a new species
Phylogenetic Species Concept
Examples of
Paraphyletic Groups
Paraphyly: a group which either
does not include all its
descendants or the ancestor.
Phylogenetic Species Concept
Monophyly
Problems:
■
■
A trivial trait (single mutation or trait) can
make a group monophyletic, and may not
warrant calling a group a new species
Sometimesatrivialtrait,
likeasinglepointmutation
couldmakeagroup
monophyletic,anda
“species”accordingtothe
phylogeneticspecies
concept
The cut off for a “species” is often arbitrary.
For example, 3% sequence divergence is
often used for bacteria
The smallest monophyletic
group is a species
Title goes here
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Carol Eunmi Lee
11/17/16
3. Morphological (Phenetic) Species Concept
Identifying species using overall similarity
Morphological (Phenetic) Species
Concept
(but not in a phylogenetic context… no
hierarchy – no branching pattern, no
ancestral-derived relationships)
Strengths
■
Most often morphological traits are used,
but any phenotype could be used
Morphological (Phenetic) Species
Concept
■
■
Different species can look similar due to
convergent evolution
Populations that look distinct sometimes
belong to the same species
Easiest. Easier than constructing phylogeny
or intermating
Which species concept to use?
■
Problems:
■
Most intuitive; the way we recognize species
■
■
Whenwediscussanimalsweoftenusethebiological
speciesconceptasthegoldstandard...complemented
withthephylogeneticandpheneticspeciesconcepts
Plants:itdepends,sinceverydistantplantscan
hybridize…phylogeneticspeciesconceptisoftenused.
Bacteria:posesdifficultproblemsforclassification.
◆
■
Speciation can occur without changes in
morphology or other traits (cryptic species)
◆
Bacteriadonotinterbreed(≠BiologicalSpeciesconcept).
Insomecasesmassiveexchangeofgeneticmaterial
(horizontalgenetransfer)leadstophylogenetic
confusion.
OftenacombinationofthePhylogeneticandPhenetic
SpeciesConcepts(biochemicalandmorphological[like
cellwall]traits)areused.
Darwin’s view:
Species are arbitrary constructs of the
human mind imposed on a continuum of
variation
Speciesaredynamicrather
thanstaticentities,with
boundarieschanging
constantly
Manygroupsareinthe
processofspeciation
Title goes here
However, concept of species is
still useful:
Species are considered the
largest group with a common
evolutionary fate
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Carol Eunmi Lee
Concepts
11/17/16
Geographic Models
Allopatric
Sympatric
Reinforcement
Problems with the
concept of
“Species”
Species
Biological
Phylogenetic
Phenetic
(Morphological)
Monophyly
2. Which of the following is NOT a reason that Species are difficult to
define?
(A) Many plants that are genetically divergent are able to mate
(B) Many organisms that are morphologically similar are genetically
distinct
(C) Many organisms are asexual
(D) Sometimes groups split off from within a monophyletic group (such
as birds splitting off from the reptiles)
(E) Sometimes sexual populations that are unable to interbreed could
still be the same biological species
1. Which of the following is a species according to the biological
species concept?
(A) All hominin species (most are fossil species).
(B) A population of bacteria for which 80% of their DNA
sequences are identical.
(C) All allopolyploid plants.
(D) A group of beetles that can intermate and produce offspring
for multiple generations.
3. Which of the following is most likely to be a "species"
according to the Phylogenetic Species Concept?
(a) A population of bacteria that has a gene that allows
glucose metabolism
(b) Bird populations, which share a unique heritable feather
structure
(c) Spider populations that can interbreed and produce fertile
offspring
(d) Crustacean populations that form a clade (geneticallyrelated group), except for one population within the clade
that colonized land and became insects
(e) Populations of deer that share similar antler shape
5. Which of the following scenarios is likely to lead to the
most rapid formation of new species?
4. Under which of the following scenarios is reinforcement
most likely to evolve?
(a) Different fish species, with each living in a separate
pond
(b) Two snail species, where each lives on opposite sides
of a freeway
(c) Different species of crickets living together in a park
(d) Different insect species, each living on a different
species of fruit in a forest
(e) Different species of allopolyploid plants living in a field
Title goes here
(a) Two populations become geographically separated, and
there is continued migration between the populations
(b) Two populations become geographically separated, and
then new mutations arise in each population that become
fixed due to genetic drift
(c) Two populations that are in the same location diverge
due to sexual selection for different traits in the two
populations
(d) Two populations become geographically separated, and
then new mutations arise in each population that become
fixed due to selection favoring different egg coat proteins
in the different habitats
(e) All of the above would on average lead to equivalent
rates of speciation
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Carol Eunmi Lee
11/17/16
answers
■
■
■
■
■
Title goes here
1D
2E
3B
4C
5D
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