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10/12/11 SF Evolution Module Lecture 8 What is a species and how do they form? Presenta(on on: Tuesday 18th October at 1pm LTEE2, Hamilton Building Trinity College Dublin Volunteering with Operation Wallacea! • Gain valuable field experience • Work with a world renowned conservation network and academics from around the world • Study spectacular wildlife • Experience living in some of the few truly wild places left on earth • Make a real contribution to conservation in some of the most vulnerable and important ecosystems in the world! 1 10/12/11 Timetable changes Original Order: Fossils and diversity Endosymbiosis Evolution of development New Order: What is a species Speciation in plants Bacteria Speciation What is a species Speciation in plants Bacteria Speciation Endosymbiosis Fossils and diversity Evolution of development Practical on Phylogenies In your own time between now and Fri 21st Oct (2 weeks) In the Zoology Museum (1st floor of Zoology building) Follow instructions in your handbook If you can’t find a demonstrator to sign you off, no problem, hand it in anyway. Practical on Altruism – also in your own time, until end of reading week. SF Evolution Module Lecture 8 What is a species and how do they form? What is a species? When does it stop being a subspecies, or a distinct population? “There is probably no other concept in biology that has remained so consistently controversial as the species concept” (Mayr, 1982) “What are species? Perhaps no other issue in comparative or evolutionary biology has produced quite so much disparate opinion as this simple question” (Eldredge 1995) 2 10/12/11 We consider blue tits from here and France to be the same species. There are 5 main ways scientists have considered defining the species: We consider humans and chimps to be clearly different species. 1) Biological species concept Yet the blue tits are genetically more different than the man and the chimp. 2) Evolutionary species concept In Indonesia we found a black-backed version of the red-backed thrush. 3) Recognition species concept 4) Morphological species concept Is it a new species, a sub-species or just a local colour variant? 5) Genetic difference JMS: “if it will save a rainforest by being a species, then it’s a species” Each has strengths and weaknesses, and different ones are useful for different groups. How DO we decide? We will look at the uses and drawbacks of each. 1) Biological Species Concept (or Isolation Species Concept) “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” Mayr 1970 If populations do not breed together, they have separate gene pools, so remain separate, and will become more separated with time. Definition should specify the young need to be fertile. Zebroid is offspring of zebra and horse, but cannot breed with either parent species. Liger is lion and tiger and the males are infertile. A Mule finch is an infertile cross between a canary and another finch species, bred for their song. Other problems with the Biological species concept. 1) Inappropriate for asexual organisms. Do we count all amoeba as the same species? 2) Too difficult to apply to fossils, as can’t know who they would have bred with. 3) Species isolated on islands are problematic as they never get the chance to show whether they would breed. Trying in the lab is unsatisfactory as some populations who breed in the wild don’t in the lab and v.v. 3 10/12/11 4) Organisms which just release gametes are problematic. 2) Evolutionary species concept If mating is never observed, hard to tell who is mating with whom or how often. Rare enough to see it in large animals, but impossible in wind pollinated plants, or plankton. A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space 5) Some species which do occasionally interbreed, are clearly good species. May or may not recognise sub-species and recognises sister-species only when they have become distinct enough to be clearly different. E.g. wolf and coyote, have remained separate for 500,000 years, despite cross-breeding occasionally and producing perfectly fertile offspring. 3) Recognition species concept HEH Paterson 1980s Species are separated by differences in their fertilization system (complex of behaviours and morphology, physiology and biochemical features). They are different species if they would not recognise each other as mates, or could not physically mate. Supposedly covers fossils, and species on different islands But Does not cater for asexual organisms How different is too different to allow mating? (Surely that’s the debate?!) Depends on the evolutionary history being known, and the phylogenetic tree being correct, which isn’t universal. 4) Morphological Species Concept In practice this is most often used in difficult cases - based on the visible differences between species. Taxonomists look for discontinuities in the morphological variation between the populations in question. If variation is continuous then it’s not a species e.g. different galapagos finches are sufficiently different on each island that they are clearly distinct species, with no gene flow between them. The red backed thrush, on the other hand, was defined as a sub-species. 4 10/12/11 But what do these genetic differences actually mean? 5) Use of genetic markers Genetic analysis can also show whether populations are interbreeding. Wakatobi (Tomia) We share 98% of the same genes with a chimp, but are those genes the same? (No.) They have different genetic sequences, even if they both have similar phenotypic effects. Problem: the degree of morphological differences and genetic ones don’t agree. Runduma To determine genetic differences between subspecies and sister species, you need to look at differences in base pairs within homologous sequences of DNA. Frogs are all morphologically very similar but have huge genetic diversity. On the other hand humans and chimps share 98% of genes but are morphologically clearly good species. Mainland Sulawesi We have been looking at genetic markers in White eyes of South East Sulawesi We share 95% with mice! These allow us to investigate the degree of separation between the different populations of closely related birds, living on different islands. Arbitrary how much difference needed to count as a different species. WWWE Borneo Runduma Bali Australia Wakatobi islands 5 10/12/11 Second (new) white-eye species Presumed source population Different LBWE colonisation event Closely related group of populations of LBWE So there is a range of methods for defining a species, and people use whatever method works best for their group. Bacteria, which exchange plasmids of DNA at will, don’t have genetic isolation, so the idea of species is merged with the term “strain” (see Mario’s lecture on this). Virologists aren’t even sure their study organism is alive, and call them “phage types”, again avoiding the term species. So the term “species” is neither well defined, nor universally agreed upon, and JMS is not wrong when he chose a practical definition. “A species is that which is defined as a species by a competent taxonomist!” Routes to Speciation However speciation occurs, the two populations need to stop breeding for long enough for them to become too different to breed any more. There are 3 possible ways: 1) Allopatric speciation Some form of barrier splits the population, or part of the population gets stranded on an island. The different conditions in the two populations cause slightly different selection to occur, so they diverge. When the two populations meet once more they can no longer interbreed. 6 10/12/11 But why would the two populations become different when one is isolated on an island? a) Founder effect b) Genetic drift c) Less competition for resources on the island as population smaller d) Different selective pressures on the mainland from those on the island e) Co-adaptation of genes e.g. imagine greenness and hiding only beneficial when present together. If no predators on new island, greenness dies out and redness takes over. When new species spreads to mainland red individuals are not recognised as same species by the green ones Speciation has occurred using colour as Specific Mate Recognition System (SMRS). Hybrid Zone narrows if hybrid offspring are less fit than the pure breeds of each type, Routes to Speciation 2: Parapatric speciation = speciation at a border which does not prevent gene flow entirely. e.g. 2 species of crickets in the Pyrenees interbreed only at the tops of certain mountains, and are clearly different species further down the mountains Tends to form a hybrid zone at the border as hybrids between the two species may occur here. (Hybrid zones may also arise where two originally allopatric species come together again.) In hybrid zones you often get Character Displacement = exaggeration of signals, especially those related to breeding, to make the signal stronger and prevent hybridisation. Hybrid Zone Call frequency It widens if they are equally fit with pure bred offspring. Normal frequency for species A Character displacement Normal frequency for species B It may even widen so far that it wipes out one or both of the original species. 7 10/12/11 Another example of a hybrid zone is between hooded and carrion crows, which interbreed only at the border between the two species. During the last Ice Age, crows were forced into “refugia” in Spain and the Balkans Hooded Crow In the British Isles this is the distribution of the Carrion Crow… …and this is the hooded crow’s distribution. Carrion Crow By the time they spread back into Europe, the two populations had drifted apart, and now there were two forms – but are they now different species? The crows were classed as one species until work on the hybrids made the experts decide that they are actually two separate species. The hybrids were found to be less fit than either pure bred carrion or hooded crows. This will make a block to gene flow, and over time they should hybridise less and less. Thus they are increasingly separate species. (Interestingly, the hybrid zone in Scotland appears to be moving north with global warming.) In Scotland, where the two forms meet, they form a hybrid zone. Hybrids of different species are often less fit than pure breeds. Routes to speciation: 3) Sympatric Speciation More controversial than the other routes. Requires isolation of a subset of the population within the original population. Interbreeding must stop if two separate species are to form though. If hybrids are less fit, then assortative mating may be favoured i.e. preferentially mating with your own morph. The two forms may become increasingly separate populations until they become species. On the way, you may see stable polymorphisms i.e. more than one morph of the same species e.g. 2spot ladybirds 8 10/12/11 How could it happen? A thought experiment: Hybrid forms if lettuce eater mates with cabbage eater X But hybrids do badly on both food plants, so fitness low for parents. Mutant with SMRS (blue colour) differentiating new species is at advantage as can avoid hybridising with lettuce eater. Perhaps through change in food types: What’s he?? Caterpillars all eat lettuce, and avoid cabbage. One evolves to cope with cabbage, but now can’t eat lettuce. Sympatric competitive speciation The End (of interbreeding) Speciation Summary e.g. Ichneumon flies Lay eggs in beetle larvae buried in wood of tree trunks. If larva already parasitised she doesn’t lay another egg in it, as other larva would eat 2nd egg. Big Mums have long ovipositors so lay in deeply buried larvae, small Mums in shallower larvae. Resulted in disruptive selection for long and short but not middle sized ovipositors, with middle sized least favoured as suffers competition from both. But to get fixed it needed assortative mating. Ugly cow! Allopatric Got stable polymorphism for two extremes of size…. Leading to 2 species. Parapatric Sympatric Original population First step Barrier formed New niche entered Polymorphism occurs Reproductive isolation evolves: In isolation Big Dads could reach further into the wood to mate with emerging females from deep in wood, so big Dads ended up mating more with offspring of big Mums, small Dads with small Mum’s offspring = assortative mating. Character displacement accentuates this difference, and further SMRSs arise (antenna shape and colour) in both species. Now don’t recognise each other at all, and never hybridise In new niche Within the population New genetically distinct species spreads and competes with original species 9 10/12/11 Keywords to revise: Real life examples of sympatric speciation: Founder Effect - a change in allele frequency due to the foundation of a subpopulation by an unrepresentative sample of the parent population. Genetic Drift - A change in allele frequency due to chance events Selectively neutral - A gene which has no effect on fitness. Fixation - when only one allele survives at a locus Effective population size - Those individuals of a population which breed Population bottlenecks - When a population is reduced to a very few individuals. Polymorphic - having more than one form. Monomorphic - having only one form. Inbreeding depression - reductions in fitness due to breeding with close relatives due to increased expression of rare deleterious recessives. Heterozygote advantage - extra vigour due to the genotype being heterozygotic. Assortative mating - mate selection on the grounds of sharing the same allele Disassortative mating - mate selection on the grounds of having a different allele Stable polymorphism – A species with more than one form in the population. 1) In Madeiran petrels Ibis Volume 149 Issue 2, Pages 255 - 263 Published Online: 21 Dec 2006 Review: http://www.sciencedaily.com/releases/2007/11/071113160351.htm 2) In bats Abstract: http://www.nature.com/nature/journal/v429/n6992/full/nature02487.html 3) In Darwin’s finches http://www.journals.uchicago.edu/doi/abs/10.1086/339987 Background Reading: Peter Skelton “Evolution: A Palaeontological Approach” (1998) Addison Wesley Chap. 3.5 “Measuring Variation” Chap. 3.6 “Variation and Population structure” Chap. 9 “Species, Speciation and Extinction” Other interesting books relating to speciation: Song of the Dodo by David Quammen The Third Chimpanzee by Jared Diamond 10