Species and Speciation I. Species Concepts II. Recognizing Species III. Making Species - Reproductive Isolation

Species and SpeciationI. Species ConceptsII. Recognizing SpeciesIII. Making Species - Reproductive Isolation

Species and SpeciationI. Species ConceptsII. Recognizing SpeciesIII. Making Species - Reproductive Isolation A. Pre-Zygotic Barriers


1. Geographic Isolation (large scale or habitat)

Drosophila speciation on the Hawaiian Islands.

Obbard D J et al. Mol Biol Evol 2012;29:3459-3473

© The Author 2012. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.


1. Geographic Isolation (large scale or habitat) 2. Temporal Isolation


1. Geographic Isolation (large scale or habitat) 2. Temporal Isolation 3. Behavior Isolation - don't recognize one another as mates

Western Meadowlark Eastern Meadowlark

http://www.youtube.com/watch?v=lvAUgFb1cLY

http://www.youtube.com/watch?v=Th4T4YrtR2o&feature=related


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; limit pollinators


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; this

is a common isolation mechanism in species that spawn at the same time


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



B. Post-Zygotic Isolation1. Genomic Incompatibility - zygote dies



B. Post-Zygotic Isolation1. Genomic Incompatibility - zygote dies 2. Hybrid Inviability - F1 has lower survival

Crazy hybrids

http://www.ligerliger.com/articles.php?a=0



B. Post-Zygotic Isolation1. Genomic Incompatibility - zygote dies 2. Hybrid Inviability - F1 has lower survival 3. Hybrid Sterility - F1 has reduced reproductive success

Horse: 64 chromosomesDonkey: 62 chromosomesMule: 63 chromosomes



B. Post-Zygotic Isolation1. Genomic Incompatibility - zygote dies 2. Hybrid Inviability - F1 has lower survival 3. Hybrid Sterility - F1 has reduced reproductive success 4. F2 breakdown - F1's survive but F2's have incompatible combo's of genes

AABB x aabb

F1: AaBb = ok

F2:A-B- = okA-bb = noaaB- = noaabb = ok

Species and SpeciationI. Species ConceptsII. Recognizing SpeciesIII. Making Species - Reproductive IsolationIV. Speciation

Speciation

Speciation is not a goal, or a selective product of adaptation. It is simply a 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 3 my, and separate species on either side of the isthmus

Speciation

I. Modes:



- 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:




B. Parapatric - neighboring populations diverge, even with gene flow

Speciation

I. Modes:






Hybrid

Hybrid Backcross??

Speciation

I. Modes:





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.)




Europeans brought apples to North America. They are in the same plant family (Rosaceae) as Hawthorn.




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"...




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.


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



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.



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).




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

aaB- = 3/16

aabb = 1/16.... ~ 7%

C. carnea photoperiod

C. downesi photoperiod




F1 x C. downesi backcross had 3:1 ratio, as expected of model.

AaBb x aabb

AaBb = .25

Aabb = .25

aaBb = .25

aabb = .25

C. carnea photoperiod

C. downesi photoperiod



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








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


B. Hybrid Incompatibility

- Dobzhansky and Müller (1930's)

Pairs of genes that work together diverge in different populations

Speciation

I. Modes

II. Mechanisms



- Dobzhansky and Müller (1930's)

Pairs of genes that work together diverge in different populations

A1

B1

A1A1B2B2 works

A2A2B1B1 worksA2A2B2B2 works

A1A1B1B1 lethal


D. melanogaster and D. simulans

http://www.sciencemag.org/cgi/content/full/314/5803/1238/F2



Cross female D. mel. x male D. sim - no sons





- Watanabe - 1970 - isolated a mutant strain of D. simulans (w) that could make males with D. melanogaster....






- Hypothesized that this strain had a mutant gene partner that reestablished function with the D. melanogaster partner gene... called it "lethal hybrid rescue" (lhr).






- Hypothesized that this strain had a mutant gene partner that reestablished function with the D. melanogaster partner gene... called it "lethal hybrid rescue" (lhr).

- Ashburner - 1980 - isolated a mutant strain of D. melanogaster (a) females that could breed with D. simulans males and produce sons ...called it "hybrid male rescue" - hmr - X-linked




SYSTEM: (s-lhr dominant)

Ancestor: lhr, mhr

Male D. simulans: s-lhr, mhr Female D. melanogaster: lhr, m-mhr(X)

s-lhr/lhr, m-mhr(X) = INVIABLE SONS




D. sim = s-lhr, hmr (X) x D. mel = lhr, m-hmr (X) SONS GET : s-lhr/lhr, m-hmr/Y (only X) .... INVIABLE



SYSTEM:

D. sim = s-lhr, hmr (X) x D. mel = lhr, m-hmr (X) SONS GET : s-lhr/lhr, m-hmr (only X) .... INVIABLE

(w)D. sim = lhr/s-lhr, hmr (X) x D. mel = lhr, m-hmr (X) 1/2 SONS GET lhr/lhr, m-hmr (ONLY X) = VIABLE



SYSTEM:

D. sim = s-lhr, hmr (X) x D. mel = lhr, m-hmr (X) SONS GET : s-lhr/lhr, m-hmr (only X) .... INVIABLE

(w)D. sim = lhr/s-lhr, hmr (X) x D. mel = lhr, m-hmr (X) 1/2 SONS GET lhr/lhr, m-hmr (ONLY X) = VIABLE

D. sim = s-lhr, hmr (X) x (a) D. mel = lhr, m-hmr(X)/hmr (X) 1/2 SONS GET: s-lhr/lhr, hmr (only X) = VIABLE




Ancestor: lhr, mhr

D. simulans: s-lhr, mhr D. melanogaster: lhr, m-mhr

s-lhr, m-mhr = INVIABLE



Brideau et al. 2006. Science 314: 1292-1295

- isolated location of lhr gene. - put NORMAL D. simulans gene into D. melanogaster. - mated these D. melanogaster with Watanabe's mutant strain of D. simulans.

- IF these two genes are partners, then 3/4 hybrids should die.

(w) D. sim = lhr/s-lhr, hmr (X) x (b)D. mel = s-lhr/lhr, m-hmr (X) (doesn't die....)

1/4 SONS GET : lhr/lhr, m-hmr (only X) .... VIABLE

3/4 get some other combination including s-lhr and m-hmr.. INVIABLE

Speciation

I. Modes

II. Mechanisms



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.



Sticklebacks colonizing lakes...PHYLOGENY:

benthic

benthic

benthic

limnetic

limnetic

limnetic



Mate selection correlates with ecotype, not with genetic relatedness....

example of parallel evolution, too.

http://www.sciencemag.org/content/vol287/issue5451/images/large/se5298174001.jpeg

Speciation

I. Modes

II. Mechanisms




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 habitatGompert et al. 2006. Science 314: 1923.


Two species of small western butterflies have overlapping ranges.


Two cluster

Three cluster

Probabilities of assigning individuals from these populations to a particular dendrogram "cluster"


Two cluster

Three cluster


Are the alpine populations simply in hybrid zone, or are they reproductively isolated?


Two cluster

Three cluster



They are fixed for several alleles, suggesting no gene flow.


Two cluster

Three cluster



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)


- What maintains this genetic uniqueness?


- What maintains this genetic uniqueness? Fidelity to Host Plant


- 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

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

Mating probabilities in no-choice experiments:

strong Positive Assortative Mating

Male

female

Mate Pairing in Tetrads:

strong Positive Assortative Mating

Speciation

I. Modes

II. Mechanisms




D. Hybridization

Several ways that new gene combinations can form and become stabilized.

(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.)

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Species and Speciation I. Species Concepts II. Recognizing Species III. Making Species - Reproductive Isolation