Speciation 2 • 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 • - 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
"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. Genetics12, 101-109. Haldane's Rule – postzygotic reproductive isolation 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
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 • 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