Transcript Lecture PPT - Carol Lee Lab

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
Last Time: Genetic Models:
The roles of: Mutations
Natural Selection
Genetic Drift
This Time: Geographic Models:
Allopatric Model (difference place)
Sympatric Model (same place)
Parapatric Model (adjoining)
(1) Mechanisms of Speciation
Last Time: Genetic Models:
How do Genetic Drift, Natural Selection,
Mutations, etc. create new species?
Are there “speciation” genes?
This Time: Geographic Models:
How does speciation occur in Nature?
Is geographic isolation required?
Mechanisms 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)
Parapatric Model (adjoining)
Geographic Models of
speciation
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Allopatric speciation: geographic isolation
Sympatric speciation: no geographic
isolation
Parapatric speciation: geographic
separation (or gradient), but not isolation
Allopatric Models
Involves Geographic Isolation
Dispersal
Vicariance
Allopatric Models
• Following geographic separation
between populations,
Dispersal
Vicariance
• This geographic separation provides the setting
that allows speciation at the molecular level to
occur (last lecture)
Allopatric Models
Dispersal
Vicariance
• Random Mutations would arise in the separated populations,
and then selection or genetic drift would lead to fixation of
those mutations
• If different mutations are fixed in the different populations,
reproductive isolation could arise through Dobzhansky-Müller
incompatibilities (last lecture)
Allopatric Speciation
Examples (see book)
 Dispersal:
 Colonization
of islands
 Colonization of lakes

Vicariance:
 Highway
going through a forest
 Fragmentation of habitats
 Formation of Panama splitting the Caribbean &Pacific Oceans
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
Sympatric Model
(1) Formation of Polyploids
Polyploidy (extra chromosomes)
Important mechanism for plants
Also occurred possibly in vertebrates,
some crustaceans
(covered in previous lecture)
Sympatric Model
(2) Selection in the face of gene flow:
• Niche Partitioning
• Strong assortative mating and
sexual selection (disruptive selection)
Example of Niche
Partitioning:
•Soapberry bugs
have adapted to
two different host
plants
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
 Reduces
gene flow
 Isolation
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Disruptive Natural Selection
Adaptation to alternative hosts leads to
reproductive isolation (through the genetic mechanisms
discussed earlier, such as Dobzhansky-Müller model)
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But, sometimes hybrid zones do form between
populations that are in the process of speciating
Sometimes hybridization between different species
results in vigorous new species or populations,
especially in plants (hybrid vigor, or heterozygote
advantage)
The effects vary depending on how distant the two
species or populations are… and whether the
different alleles at different loci are able to work
together (coadapted gene complexes)
Hybrids between different populations within a species do
tend to have an advantage (Heterozygote advantage).
However, mating between very distant populations (different
species) can lead to hybrid breakdown.
Increasing genetic distance
Mating between
different species
(Lions x tiger,
Horse x donkey)
Fitness
Mating between
relatives
Inbreeding
Depression
Populations
within a species
Hybrid Vigor
(due to Heterozygote advantage)
Will not mate or
Produce inviable or
sterile hybrids
Outbreeding
Depression =
Hybrid
Breakdown
Reinforcement
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So, when hybrids are formed
between different species, they are
often costly and maladaptive
because of hybrid breakdown
Reinforcement
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So, when hybrids are formed between
different species, they are often costly
and maladaptive because of hybrid
breakdown
In such cases, you would predict that
mechanisms to avoid mating would
evolve to avoid the production of
maladaptive hybrids (= Reinforcement)
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Reproductive isolation could occur at many
different levels
Prezygotic (before the egg is fertilized)
drift and divergence in bird song-won’t mate
 Selection on coat color-don’t recognize each other
 Genetic

Postzygotic (after the egg is fertilized)
 DM
incompatibilities cause embryo to not develop
(enzymes don’t work together)
Postzygotic barriers
Prezygotic barriers
Gametic Isolation
Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown
Fertilization
Viable,
fertile
offspring
Reinforcement
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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
(2) How are Species Defined?
How are species defined?
So, what criterion should be used?
Historically, the most common
criteria had been using
morphological characters (how an
organism looks)
Speciation is a messy process
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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
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Problem:
Populations are in the process of
speciating from one another, and species
boundaries are often difficult to define until
the populations are sufficiently divergent by
all measures
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So then, how do you define species???
Darwin’s view:
Species are arbitrary constructs of the
human mind imposed on a continuum of
variation
Species are dynamic rather
than static entities, with
boundaries changing
constantly
Many groups are in the
process of speciation
Three Main Species Concepts
1. Biological Species Concept
2. Phylogenetic Species Concept
3. Morphological (Phenetic) Species
Concept
1. Biological Species Concept
(Ernst Mayr, 1942)
A group of interbreeding populations that are
evolutionary independent of other populations
1. Biological Species Concept
(Ernst Mayr, 1942)
Example: all human populations belong
to the same biological species
Biological Species Concept
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
Biological Species Concept
PROBLEMS:
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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”)
Ring Species
2. Phylogenetic Species Concept
The smallest group that is
monophyletic is called a
species
2. Phylogenetic Species Concept
There are several
monophyletic
groups here
Monophyletic group:
A group with a shared derived
(descendant) character
A group that contains a
common ancestor and all its
descendents
Phylogenetic Species Concept
Typically, a phylogeny is constructed using DNA or
other types of data (proteins, morphological traits)
The phylogeny reveals hierarchical relationships
among groups
The smallest group that has a shared derived
character and is monophyletic is called a
species
Phylogenetic Species Concept
There is a derived character that is shared by the 4 populations
Monophyly
The smallest monophyletic
group 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
D
E
E
F
F
F
G
G
G
B
Group I
(a) Monophyletic group (clade)
Group II
(b) Paraphyletic group
E
Group III
(c) Polyphyletic group
42
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:
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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)
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A trivial trait (single mutation or trait) can
make a group monophyletic, and may not
warrant calling a group a new species
Examples of
Paraphyletic Groups
Paraphyly: a group which either
does not include all its
descendants or the ancestor.
Phylogenetic Species Concept
Problems:
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A trivial trait (single mutation or trait) can
make a group monophyletic, and may not
warrant calling a group a new species
The cut off for a “species” is often arbitrary.
For example, 3% sequence divergence is
often used for bacteria
Phylogenetic Species Concept
Monophyly
Sometimes a trivial trait,
like a single point mutation
could make a group
monophyletic, and a
“species” according to the
phylogenetic species
concept
The smallest monophyletic
group is a species
3. Morphological (Phenetic) Species Concept
Identifying species using overall similarity
(but not in a phylogenetic context… no
hierarchy – no branching pattern, no
ancestral-derived relationships)
Most often morphological traits are used,
but any phenotype could be used
Morphological (Phenetic) Species
Concept
Strengths
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Most intuitive; the way we recognize species
Easiest. Easier than constructing phylogeny
or intermating
Morphological (Phenetic) Species
Concept
Problems:
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Different species can look similar due to
convergent evolution
Populations that look distinct sometimes
belong to the same species
Speciation can occur without changes in
morphology or other traits (cryptic species)
Which species concept to use?
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When we discuss animals we often use the biological
species concept as the gold standard... complemented
with the phylogenetic and phenetic species concepts
Plants: it depends, since very distant plants can
hybridize… phylogenetic species concept is often used.
Bacteria: poses difficult problems for classification.
 Bacteria do
not interbreed (≠ Biological Species concept).
In some cases massive exchange of genetic material
(horizontal gene transfer) leads to phylogenetic
confusion.
 Often a combination of the Phylogenetic and Phenetic
Species Concepts (biochemical and morphological [like
cell wall] traits) are used.
Darwin’s view:
Species are arbitrary constructs of the
human mind imposed on a continuum of
variation
Species are dynamic rather
than static entities, with
boundaries changing
constantly
Many groups are in the
process of speciation
However, concept of species is
still useful:
Species are considered the
largest group with a common
evolutionary fate
Concepts
Geographic Models
Allopatric
Sympatric
Reinforcement
Problems with the
concept of
“Species”
Species
Biological
Phylogenetic
Phenetic
(Morphological)
Monophyly
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.
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
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
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
5. Which of the following scenarios is likely to lead to the
most rapid formation of new species?
(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
answers
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1D
2E
3B
4C
5D