Speciation 2
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Postzygotic reproductive isolation
Barriers in allopatric speciation
Haldane’s rule
Sympatric speciation
Another case of prezygotic reproductive isolation:
Mimulus flowers and and pollinators
• The two closely related flower species differ in multiple
ways so that one species is best adapted to bumblebee
pollination, the other to hummingbird pollination;
differences concern morphology (including placement of
reproductive organs), colour, nectar content. Several of
these may be genetically linked (after Bradshaw &
Schemske)
Postzygotic reproductive isolation
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- for example via hybrid sterility, e.g. in horse and donkey
- mother horse, father donkey: offspring is a mule (sterile)
- mother donkey, father horse: offspring is a hinny (sterile)
- Hybrid inviability eg. sheep-goat embryos
No assortative mating in two neighbouring populations of
Alpine grasshoppers (Profs. Nichols, Hewitt, Barton)
• Podisma pedestris races differ in their sex determination
mechanism: males of one race have the usual XY
combination, males of the other have only a X
• hybrids are less viable than pure strains: eggs tend to die,
or individuals die soon after hatching
• Both populations not adapted to different habitats
• Without selection acting against hybrids the two
populations would have mixed entirely
Possible types of barriers in
allopatric speciation
• Mountain ranges
Possible types of barriers in
allopatric speciation
• The sea (for terrestrial
species), or bodies of
freshwater
Possible types of barriers in
allopatric speciation
• The land (for aquatic,
especially freshwater
species)
• Lakes or ponds are “islands”
for aquatic species!
Possible types of barriers in
allopatric speciation
• Mountain ranges
• The sea (for terrestrial
species), or bodies of
freshwater
• The land (for aquatic,
especially freshwater
species)
• Glacial masses
• Valleys
Can speciation only happen when
new barriers occur/arise within
continuous populations?
• about 800 species of drosophilid flies in
Hawaiian islands – you can generate lots of
species via founder events!
Peripatric speciation
• can be produced by founder events – where
very few individuals (or indeed a single
pregnant female) gets displaced to a
location (e.g. an island, or lake) that was
previously uninhabitated by this species
Haldane's Rule – postzygotic
reproductive isolation
"When in the F1 offspring of two different animal
races one sex is absent, rare, or sterile, that sex is the
heterozygous [heterogametic] sex.
J.B.S. Haldane (1922) Sex-ratio and unisexual sterility in hybrid animals. J. Genetics 12, 101-109.
In most familiar species except birds and butterflies the heterogametic sex is male
It turns out that the X chromosome is often implicated in hybrid sterility.
Haldane’s rule
• Coyne et al. made the following cross:
- females from D. persimilis x D. pseusoobscura F1
- males D. pseudoobscura.
• This produces viable hybrids, with genotypes
which are 50%- 100% pseudoobscura
• Motile sperm are essentially absent from all
combinations with a persimilis X-chromosome –
supporting Haldane’s rule.
Speciation in the laboratory?
• Selection experiment on bristle number on the
abdomen by Thoday & Gibson (1962)
• Removed individuals with intermediate numbers
of bristles, but left the others together.
• After 12 generations, there was premating
isolation between individuals with high and low
bristle numbers -> new species formed?
• has not been replicated
Rice and Salt selection
experiment
Bi-directional selection at work
• .... strong effect on habitat selection. While mating was
assortative, there was no reproductive isolation – hence no
speciation
Allopatric and sympatric speciation
• Allopatric speciation occurs when populations of a
species become geographically isolated
• Sympatric speciation is the evolution of reproductive
isolation within a randomly mating population
• Parapatric speciation is the evolution of reproductive
isolation between populations that are continuously
distributed in space, so that there is movement of
individuals (and hence gene flow) between them (e.g.
ring species in previous lecture)
Sympatric speciation is a controversial topic! Read e.g.
Futuyma: Evolutionary Biology, on this subject
• In the past, many scientists have assumed that the extraordinary diversity of
species could not be explained just by allopatric speciation
• But can new species arise within the same locality?
Sympatric speciation in maggot
flies (Rhagoletis pomonella)
apples
hawthornes
Why is sympatric speciation
thought to be rare in animals?
• The problem is how to avoid intermediate genotypes that
will function as bridges for gene flow, which would
eliminate the difference.
• Needed: polymorphism and assortative mating.
• Problem: if these two are not genetically coupled,
recombination will eliminate the barrier to gene flow.
Sympatric speciation
• “Sympatric speciation is like the measles –
everyone gets it, and we all get over it.”
Theodosius Dobzhansky, 1960s
Sympatric speciation by polyploidy (in plants)
• Sometimes hybrids between two parental flowering plants are
formed.
• This can happen because plants can‘t choose whom to mate with
(i.e. pollinators or wind may move pollen between flowers of
different species
• Hybrids may be sterile if paternal and maternal chromosomes are
incompatible and cannot pair in meiosis.
• But sometimes, chromosome sets “accidentally“ double
(polyploidy)
• This doubling results in compatible partners
• Plants can self-pollinate, produce seeds, and so propagate: a new
species has formed
• Between 30 and 50% of angiosperm plant species may have
formed in this way!
Plants that have had a
hybrid/polyploid episode in their
evolutionary history
• wheat, tobacco, cotton, bananas, potatoes
• The widespread occurrence of hybridisation
in plants might mean that phylogenetic trees
are more appropriately “phylogenetic nets”
in plants
Speciation can also occur through chromosome
rearrangement, or doubling of only single
chromosomes
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typically selection will eliminate such anomalies, but genetic drift and
inbreeding might allow fixation in a population.
pairing difficulties between chromosomes will create infertility barriers with the
ancestral population and hence speciation
Clarkia lingulata, 2n=18,
drier habitats, restricted range
Clarkia biloba, 2n=16, (ancestral
number) wetter habitats, widespread
Sympatric speciation in Mimulus
flowers?
• The two closely related flower species differ in multiple ways so that
one species is best adapted to bumblebee pollination, the other to
hummingbird pollination; differences concern morphology (including
placement of reproductive organs), colour, nectar content. Several of
these may be genetically linked (after Bradshaw & Schemske)
• But – are pollinators really so constant as to produce complete
reproductive isolation?
What you should think about:
• Why are there different species in the first place? Why don‘t many
species freely interbreed with individuals of other species, thereby
creating a continuum of different living systems, rather than
different categories? Think about this both in terms of the
mechanistic difficulties involved, and in terms of the ultimate
consequences (would the results of such random matings be fitter
than “pure“ offspring?)
The hippocampus
“The mermaid” by JW Waterhouse 1849-1917
The Centaur
What you should know
• Why do evolutionary biologists think that most
animal species arose by allopatric, rather than by
sympatric speciation?
• Reproductive isolation can occur at multiple
levels. Which levels are these, and what are the
reasons for reproductive isolation to occur?
• Essential Reading: Chapter 9 of Skelton, also chapter 9 in Kardong
KV – An introduction to biological evolution
• Outside reading:
• Rice & Hostert 1993 Laboratory experiments on speciation - what
have we learned in 40 years? Evolution 47: 1637-1653
•
TRENDS ECOL EVOL Special. Issue on Speciation. Volume 16
Issue 7 Aug 2001
• * Especially read the article by Turelli and Reiseberg
• * Don't forget to read the Introduction by Barton and the Glossary.
Postzygotic reproductive isolation
– mechanism understood!
• Platy fish and swordtail: F2hybrids show higher probability of contracting
cancer.
• Platyfish/swordtail inviability has been localised to an X-linked oncogene and
receptor for tyrosine kinase with an autosomal inhibitor in Platyfish, where the
system generates spots. There is no inhibitor in the swordtail, with consequent
melanoma in the F2 generation
Why is the X chromosome so often
implicated in hybrid infertility?
• it appears that most alleles causing hybrid
sterility and inviability are recessive
• advantageous recessives will be fixed faster
on the X chromosome