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.

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)

Postzygotic reproductive isolation• - 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

• 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 is a controversial topic! Read e.g.

Futuyma: Evolutionary Biology, on this subject

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- 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 biloba, 2n=16, (ancestral

number) wetter habitats, widespread

Clarkia lingulata, 2n=18,

drier habitats, restricted range

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