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Species and Speciation
I. Species Concepts
II. Recognizing Species
III. Making Species - Reproductive Isolation
A. Pre-Zygotic Barriers
1. Geographic Isolation (large scale or habitat)
2. Temporal Isolation
3. Behavior Isolation - don't recognize one another as mates
4. Mechanical isolation - genitalia don't fit
5. Gametic Isolation - gametes transfered but sperm can't fertilize egg
B. Post-Zygotic Isolation
1. Patterns:
2. Mechanisms:
- epistatic incompatibilities
AAbb
x
aaBB
AaBb – A and B don’t work together
QTL mapping
Locus 1
Hms1
1g1g
Locus 2
Hms2
M. guttatus
2g2g
X
1n1n
1g1n
M. nasutus
F1
2n2n
2g2n
F2:
All ok except if:
1g,1g, 2n2n
Male sterility
Epistatic incompatibility
Species and Speciation
I. Species Concepts
II. Recognizing Species
III. Making Species - Reproductive Isolation
A. Pre-Zygotic Barriers
1. Geographic Isolation (large scale or habitat)
2. Temporal Isolation
3. Behavior Isolation - don't recognize one another as mates
4. Mechanical isolation - genitalia don't fit
5. Gametic Isolation - gametes transfered but sperm can't fertilize egg
B. Post-Zygotic Isolation
1. Patterns:
2. Mechanisms:
- epistatic incompatibilities
- inversion incompatibilities
Inversion (changes the order of genes on a chromosome)
The only functional
gametes are those that
DID NOT cross over –
and preserve the
parental combination of
alleles
Inversions in different populations of D. pseudoobscura
(Dobzhansky& Sturtevant 1938)
Visualized inversion complexes in giant
chromosomes of salivary glands of hybrids, where
homologous chromosomes replicate 100’s of times
and pair up
Relative frequencies (percentages)
of five chromosomal inversions in D.
pseudoobscura in different
geographic regions.
1. Distribution suggests
these inversions may be
preserving co-adapted
gene complexes; ST high
at warm temperature.
2. Experiment confirmed
that the ST arrangement
was selected for at
warmer temperatures.
Species and Speciation
I. Species Concepts
II. Recognizing Species
A. Morphology
B. Genetic Analysis
C. Hybrid Analyses
- Create hybrids and examine their fertility. Infertility may be due to:
- Epistatic interactions between loci derived from different parents. Maybe
species one has A1A1B1B1 and species 2 has A2A2B2B2, and maybe A1 and B1
don't work together. If one is a sex linked gene, then sterility might be sex-specific.
- Hybrids that receive different inversion chromosomes may have lower
fitness because crossing over produces aneuploid gametes - with chromosomes that
lack centromeres and are lost from the cell line.
- Hybrids receiving chromosomes from parents with different reciprocal
translocations may not have neat homologous sets.
Species and Speciation
I. Species Concepts
II. Recognizing Species
III. Making Species - Reproductive Isolation
IV. Speciation
Speciation
Speciation is not a goal, or an adaptive product of selection. It is simply
a potential consequence of genetic changes that occurred for other
reasons (selection, drift, mutation, etc.).
Speciation
I. Modes:
Speciation
I. Modes:
A. Allopatric: Divergence in geographically separate populations
- Vicariance - range divided by new geographic feature
A
B
C
Almost all most
recent divergence
events date to 3my,
and separate
species on either
side of the isthmus
Speciation
I. Modes:
A. Allopatric: Divergence in geographically separate populations
- Vicariance - range divided by new geographic feature
- Peripatric - divergence of a small migrant population
A
B
Abert’s Squirrel
Kaibab Squirrel
Crossed grand canyon to
the north during Ice Age
and isolated.
Speciation
I. Modes:
A. Allopatric: Divergence in geographically separate populations
- Vicariance - range divided by new geographic feature
- Peripatric - divergence of a small migrant population
B. Parapatric - neighboring populations diverge, even with gene flow
Speciation
I. Modes:
A. Allopatric: Divergence in geographically separate populations
- Vicariance - range divided by new geographic feature
- Peripatric - divergence of a small migrant population
B. Parapatric - neighboring populations diverge, even with gene flow
B. Parapatric - neighboring populations diverge, even with gene flow
Hybrid Backcross??
Hybrid
Speciation
I. Modes:
A. Allopatric: Divergence in geographically separate populations
- Vicariance - range divided by new geographic feature
- Peripatric - divergence of a small migrant population
B. Parapatric - neighboring populations diverge, even with gene flow
C. Sympatric: Divergence within a single population
C. Sympatric: Divergence within a single population
Maynard Smith (1966) - hypothesized this was possible if there
was disruptive selection within a population - perhaps as a specialist
herbivore/parasite colonized and adapted to a new host.
C. Sympatric: Divergence within a single population
Maynard Smith (1966) - hypothesized this was possible if there
was disruptive selection within a population - perhaps as a specialist
herbivore/parasite colonized and adapted to a new host.
Example: Hawthorn/Apple Maggot Fly (Rhagoletis pomonella)
Hawthorn maggot fly is a
native species that breeds on
Hawthorn (Crataegus sp.)
C. Sympatric: Divergence within a single population
Maynard Smith (1966) - hypothesized this was possible if there
was disruptive selection within a population - perhaps as a specialist
herbivore/parasite colonized and adapted to a new host.
Example: Hawthorn/Apple Maggot Fly (Rhagoletis pomonella)
Europeans brought apples to
North America. They are in
the same plant family
(Rosaceae) as Hawthorn.
C. Sympatric: Divergence within a single population
Maynard Smith (1966) - hypothesized this was possible if there
was disruptive selection within a population - perhaps as a specialist
herbivore/parasite colonized and adapted to a new host.
Example: Hawthorn/Apple Maggot Fly (Rhagoletis pomonella)
Europeans brought apples to
North America. They are in
the same plant family
(Rosaceae) as Hawthorn.
In 1864, apple growers
noticed infestation by Apple
Maggot flies...which were
actually just "hawthorn
flies"...
C. Sympatric: Divergence within a single population
Maynard Smith (1966) - hypothesized this was possible if there
was disruptive selection within a population - perhaps as a specialist
herbivore/parasite colonized and adapted to a new host.
Example: Hawthorn/Apple Maggot Fly (Rhagoletis pomonella)
races breed on
their own host
plant, and have
adapted to the
different seasons
of fruit ripening.
Only a 4-6%
hybridization rate.
Temporal, not
geographic,
isolation.
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977a and 1977b. Science.
Two species of green lacewings - generalist insect predators
Chrysopa downesi has one generation in early spring
C. carnea breeds and has three generations in summer
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977a and 1977b. Science.
Two species of green lacewings - generalist insect predators
Chrysopa downesi has one generation in early spring, then
diapause
C. carnea breeds has three generations in summer, no diapause
The differences are due to responses to photoperiod
C. downesi stops reproducing and goes into diapause under long
day length (summer), whereas C. carnea reproduces under long day
length.
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977a. Science 197:592.
The species are completely interfertile in the lab:
Did reciprocal matings: C. downesi x C. carea
Reared F1 offspring under long day length (16L:8D). Found all F1 did not
enter diapause (C. carnea photoperiod response is dominant).
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977a. Science 197:592.
Did F1 x F1 cross: Found 7% (~1/16) of F2 exhibited diapause at 16L:8D.
This is consistent with a model of 2 independently assorting autosomal
genes with complete dominance at each and an additive effect.
AABB
x
aabb
F1
all A-B- phenotype
F2
A-B- = 9/16
A-bb = 3/16
C. carnea photoperiod
aaB- = 3/16
aabb = 1/16.... ~ 7% C. downesi photoperiod
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977a. Science 197:592.
F1 x C. downesi backcross had 3:1 ratio, as expected of model.
AaBb x aabb
AaBb = .25
Aabb = .25
C. carnea photoperiod
aaBb = .25
aabb = .25
C. downesi photoperiod
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977b. Science 197:1298.
How did this temporal separation get established?
C. downesi is dark green and prefers hemlock forests
C. carnea is light green and prefers fields and meadows
Difference governed by a single locus where dark is incompletely
dominant.
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977b. Science 197:1298.
How did this temporal separation get established?
C. downesi is dark green and prefers hemlock forests
C. carnea is light green and prefers fields and meadows
Difference governed by a single locus where dark is incompletely
dominant.
Hypothesize that selection for different morphs in different habitats
created the stable dimorphism, reinforced by inbreeding within the
habitats.
intermediate
heterozygote
C. Sympatric: Divergence within a single population
But can a generalist speciate sympatrically?
Tauber and Tauber. 1977b. Science 197:1298.
How did this temporal separation get established?
C. downesi is dark green and prefers hemlock forests
C. carnea is light green and prefers fields and meadows
Difference governed by a single locus where dark is incompletely
dominant.
Hypothesize that selection for different morphs in different habitats
created the stable dimorphism, reinforced by inbreeding within the
habitats.
Selection then favored early breeding in C. downesi, as that is when
insects feeding on conifers are most abundant.
Speciation
I. Modes
II. Mechanisms
Speciation
I. Modes
II. Mechanisms
A. Progressive Genomic Incompatibility
Tilley et al. 1990. PNAS. Desmognathus ochrophaeus in western NC
1. correlation between geographic
distance and genetic distance
Tilley et al. 1990. PNAS. Desmognathus ochrophaeus in western NC
2. Placed sympatric and allopatric males
and females (reciprocal mating design)
together for an evening and examined the
cloaca of female in the morning for
presence of sperm packet.
Calculated "Coefficient of Isolation":
(sum of % of sympatric matings) - (sum of
% of allopatric matings)
2 = total isolation by sexual selection
0 = no differentiation by sexual selection
Speciation
I. Modes
II. Mechanisms
A. Progressive Genomic Incompatibility
B. Hybrid Incompatibility
- Dobzhansky and Müller (1930's)
Pairs of genes that work together diverge in
different populations; epistatic
combinations broken down in hybrids.
Speciation
I. Modes
II. Mechanisms
A. Progressive Genomic Incompatibility
B. Hybrid Incompatibility
- Dobzhansky and Müller (1930's)
Pairs of genes that work together diverge in
different populations
A1A1B2B2 works
A1A1B1B1 lethal
A1
A2A2B2B2 works
B1
A2A2B1B1 works
Speciation
I. Modes
II. Mechanisms
A. Progressive Genomic Incompatibility
B. Hybrid Incompatibility
C. Differential Selection
C. Differential Selection
- Assumed to be primary, but few studies documenting that
reproductive isolation of phenotypes correlates with fitness differential
in different environments.
Rundle et al. (2000). Science 287:306.
C. Differential Selection
- Assumed to be primary, but few studies documenting that
reproductive isolation of phenotypes correlates with fitness differential
in different environments.
Rundle et al. (2000). Science 287:306.
Sticklebacks colonizing lakes...PHYLOGENY:
limnetic
benthic
limnetic
benthic
limnetic
benthic
C. Differential Selection
- Assumed to be primary, but few studies documenting that
reproductive isolation of phenotypes correlates with fitness differential
in different environments.
Rundle et al. (2000). Science 287:306.
Mate selection
correlates with
ecotype, not with
genetic relatedness....
example of parallel
evolution, too.
Speciation
I. Modes
II. Mechanisms
A. Progressive Genomic Incompatibility
B. Hybrid Incompatibility
C. Differential Selection
D. Hybridization
D. Hybridization
- When hybridization occurs, it show increase gene flow between
populations. How are hybrids stabilized as a reproductively isolated
group?
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
Two species of small western butterflies
have overlapping ranges.
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
Two cluster
Three cluster
Probabilities of assigning individuals from these
populations to a particular dendrogram "cluster"
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
Two cluster
Three cluster
Probabilities of assigning individuals from these
populations to a particular dendrogram "cluster"
Are the alpine populations
simply in hybrid zone, or
are they ‘reproductively
isolated’?
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
Two cluster
Three cluster
Probabilities of assigning individuals from these
populations to a particular dendrogram "cluster"
Are the alpine populations
simply in hybrid zone, or
are they reproductively
isolated?
They are fixed for several
alleles, suggesting no
gene flow.
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
Two cluster
Three cluster
Probabilities of assigning individuals from these
populations to a particular dendrogram "cluster"
Are the alpine populations
simply in hybrid zone, or
are they reproductively
isolated?
They are fixed for several
alleles, suggesting no
gene flow.
- Also used coalescence
to estimate time since a
common ancestor within
each 'species". The alpine
populations had a more
recent history (400,000
yrs) than either of the
others (1.2-1.9 my)
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
- What maintains this genetic uniqueness?
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
- What maintains this genetic uniqueness? Fidelity to Host Plant
- adaptation to an extreme habitat
Gompert et al. 2006. Science 314: 1923.
- What maintains this genetic uniqueness?
Fidelity to Host Plant
Also, their eggs don't stick to the leaf; they drop off into litter.
This may be adaptive, as winds blow leaves a long way from
original plant at high elevations. The host plant is a perennial, so
dropping into the leaf litter keeps it close to host plant.
Other species, even if they used the plant, would have eggs dispersed
from the host plant. That's bad for butterflies, 'cuz caterpillars don't
disperse too far...
D. Hybridization
- When hybridization occurs, it show increase gene flow between
populations. How are hybrids stabilized as a reproductively isolated
group?
- adaptation to extreme habitat
- sexual selection
- sexual selection
Mavarez et al. 2006. Nature 441:868
X
BACKCROSS
BACKCROSS
Backcrossing (and
recombination)
creates genotypes in
which a few different
genes from one
species are placed in
the genetic
background of the
other = introgression.
Selection or drift can
work on these
variants.
Mating probabilities in no-choice experiments:
strong Positive Assortative Mating
female
Male
H. mel
H. heur
H. cyn
H. mel
1.00
0.07
0.18
H. heur
0.10
1.00
0.44
H. cyn
0.12
0.02
1.00
Mate Pairing in Tetrads:
strong Positive Assortative Mating
Implications of Hybridization and Introgression
- Hybridization combines whole genomes in the F1.
- Backcrossing creates genotypes with varying %’s of genetic contributions
from both species – at the gene, gene region, chromosome scales.
- Adaptive combinations remain.
- Neutral combinations remain.
H. Anomalus is a hybrid
species. Some regions of
the genome show a mixing
of genes (introgression) –
these are areas of homology
with same gene order (red).
Other areas, like region T,
show 0% introgression,
because gene order is
different and hybrids getting
recombinant types in this
region were inviable.
16 of 26 areas where there
is no introgression are
associated with pollen
sterility (QTL).
(C) The distribution across markers of the proportion of H. petiolaris alleles seen in experimental hybrids. There were three
generations of crossing within the hybrid population, followed by two generations of backcrossing to H. annuus. Therefore, in
the absence of selection, one expects 1/8 of the genes to derive from H. petiolaris, with a distribution concentrated in the 1–
25% class. In regions of genome with the same gene order in H. petiolaris and H. annuus (red ), most markers fail to introgress,
but some introgress more than expected. In regions of genome that differ in gene order as a result of chromosome
rearrangements, there is almost no introgression (blue). (D) Patterns of introgression along the genomes are similar between
experimental hybrids and the natural hybrid species, H. anomalus. Three of the 17 H. anomalus chromosomes are shown. The
letters to the left (R, S, T, Q) indicate homology of these chromosomes to regions of the parental genomes. (The leftmost
chromosome is rearranged, and combines linkage blocks R and S.) Arrows to the right indicate the genetic markers. The
shading indicates the likelihood that the regions derived from H. annuus (blue) or H. petiolaris (yellow). (A, Courtesy USDA; B,
redrawn from Rogers et al. 1982; C, data from Table 1 in Rieseberg et al. 1995a; D, redrawn from Fig. 3 in Rieseberg and
Noyes 1998.)
Implications of Hybridization and Introgression
- Hybridization combines whole genomes in the F1
- Backcrossing creates genotypes with varying %’s of genetic contributions
from both species – at the gene, gene region, chromosome scales
- Adaptive combinations remain.
- Neutral combinations remain.
- Selection eliminates combinations that don’t work. So, genes or regions
that remain different after introgression remain ‘species specific’.
- Indeed, maybe it is only these gene/regions that give the species a unique
genetic identity. These are regions that probably influence reproductive
isolating mechanisms, too.
Harrison and Larsen 2014
Speciation
I. Modes
II. Mechanisms
A. Progressive Genomic Incompatibility
B. Hybrid Incompatibility
C. Differential Selection
D. Hybridization
E. Polyploidy
E. Polyploidy
Autopolyploidy = within one species
Mitotic Error and Selfing:
“bud”
“all flower cells”
“gametes”
“zygote”
E. Polyploidy
Autopolyploidy
May be able to
reproduce (clonally or
parthenogenically)
3n
Meiotic Error, Parthenogenesis:
normal
2n
1n
3n
2n
parents
error
2n
“gametes”
“zygote”
“organism”
E. Polyploidy
Autopolyploidy
May be able to
reproduce (clonally or
parthenogenically)
3n
Meiotic Error, Backcross:
normal
2n
1n
4n
3n
2n
parents
error
2n
“gametes”
“zygote”
“organism”
E. Polyploidy
Autopolyploidy
May be able to
reproduce (clonally or
parthenogenically)
3n
Meiotic Error, Mating, Genome Doubling:
normal
2n
1n
4n
3n
2n
error
2n
“doubling”
4n
parents
“gametes”
“zygote”
“organism”
E. Polyploidy
Allopolyploidy = different species
Spartina
Spartina alternifolia, native to US, was found in southern England
in late1800's. There is a European species Spartina maritima.
Early in the 20th century a sterile hybrid was found and was called
Spartina townsendii This went through a process of diploidization
(increased ploidy) and became a new sexually reproducing
species known as Spartina anglica
S. maritima
sterile hybrid
S. anglica
S. alterniflora
Spartina alterniflora from
NA colonized Europe
X
Spartina maritima native
to Europe
Sterile hybrid – Spartina x townsendii
Allopolyploidy – 1890’s
Spartina anglica – an
allopolyploid and a
worldwide invasive
outcompeting native
species
E. Polyploidy
Allopolyploidy
“Genome Doubling”,
with subsequent gene
silencing, deletions,
and “diploidization”
2n
4n
2n
Speciation
I. Modes
II. Mechanisms
III. Rates
III. Rates
Mark Pagel,* Chris Venditti, Andrew Meade .2006. Large Punctuational
Contribution of Speciation to Evolutionary Divergence at the
Molecular Level . Science 314:119.
A long-standing debate in evolutionary biology concerns whether
species diverge gradually through time or by punctuational episodes
at the time of speciation. We found that approximately 22% of
substitutional changes at the DNA level can be attributed to
punctuational evolution, and the remainder accumulates from
background gradual divergence. Punctuational effects occur at more
than twice the rate in plants and fungi than in animals, but the
proportion of total divergence attributable to punctuational change
does not vary among these groups. Punctuational changes cause
departures from a clock-like tempo of evolution, suggesting that they
should be accounted for in deriving dates from phylogenies.
Punctuational episodes of evolution may play a larger role in
promoting evolutionary divergence than has previously been
appreciated.