Speciation: BIOL 4212 Lecture Notes - University of Windsor

Speciation BIOL 4212  Lecture 6 Sympatric Speciation Readings Chapter 4 2  Lecture Outline Sympatric speciation –Theory –Experimental evidence –Evidence from nature 3 vicariant peripatric parapatric sympatric  Sympatric Spec...

Speciation BIOL 4212  Lecture 6 Sympatric Speciation Readings Chapter 4 2  Lecture Outline Sympatric speciation –Theory –Experimental evidence –Evidence from nature 3 vicariant peripatric parapatric sympatric  Sympatric Speciation Sympatric speciation: the origin of an isolating mechanism among the members of an interbreeding population Speciation must occur within the average dispersal distance or “cruising range” of a single individual – Otherwise could be considered microallopatric speciation 5  Sympatric Speciation Most controversial aspect of speciation It was Darwin’s idea that species could form in sympatry – Species would arise in sympatry to fill empty niches – But Darwin used a morphological species concept – Without knowledge of genetics, impossible to know that interbreeding could prevent the 6 formation of discrete taxa  Sympatric Speciation This view prevailed for decades Mayr (1963) reviewed and critiqued sympatric speciation Used arguments from natural history and genetics Had a huge impact on the field – sympatric speciation no longer considered the norm 7  Theory All models of sympatric speciation face two problems: 1. Antagonism between selection and recombination – As selection tries to split apart a population, interbreeding continually breaks up gene complexes that produce reproductive isolation – E.g., could break up genes for habitat fitness and habitat preference 8  Theory 2. Coexistence – Populations must develop sufficient ecological differences to coexist – Implies that selection should in part be driven by selection for ecological divergence – Models based on disruptive sexual selection do not easily solve the problem of coexistence 9  Theory Two main types of models: 1. Disruptive sexual selection 2. Disruptive natural selection 10  Theory 11 Disruptive Sexual Selection Models In many adaptive radiations, species appear to differ more strongly in sexual traits than ecological adaptations – E.g., African lake cichlids Has inspired models where sympatric speciation is driven by sexual selection 12 Disruptive Sexual Selection Models Models show some success, but often under unrealistic or restrictive assumptions Most provide no mechanism for the coexistence of new species in the same habitat Some more recent models have added ecological divergence in addition to sexual selection with better success 13  Disruptive Natural Selection Models Two types of natural selection models: 1. Discrete-habitat models 2. Continuous-resource models 14  Discrete-Habitat Models Sympatric species that seek out different niches, are most fit in own niches, and preferentially mate with members of their own niches – E.g., insects that feed and mate on a specific host plant Models focus on three traits: – Niche preference – Niche adaptation – Assortative mating 15  Discrete-Habitat Models Most early models only combine two of these three traits One model explores all three traits simultaneously (Johnson et al. 1996) – This is the most realistic and successful discrete-habitat model 16  Continuous-Resource Models Discrete-Habitat models do not seem appropriate for many species – E.g., Sympatric species of African cichlids do not show dramatic differences in habitat 17  Continuous-Resource Models Continuous-Resource models are based on co-evolution of ecological traits and assortative mating – Splits population into two sympatric groups, each using a different part of the resource distribution 18  Continuous-Resource Models Most famous model: Dieckmann and Doebeli (1999) – Important resource (e.g. seed size) unimodally distributed – Ecologically-relevant trait (e.g. beak size) unimodally distributed – As population grows, frequency- and density-dependent selection favours individuals with more extreme values (larger and smaller beaks) – Splitting occurs if individuals with extreme traits prefer individuals with similar traits 19  Continous-Resource Models Problems with Dieckmann and Doebeli model Works best when individuals have innate preference for mating with other individuals having similar traits If mate preferences are not very strong, speciation takes so long that initial conditions probably change before speciation occurs –1,000,000 generations 20 Change in mean beak size in the medium ground finch (Geospiza fortis) studied by the Grants. A severe drought in the late 70’s selected for larger beak sizes to handle larger, tougher seeds, but the arrival on the island of a larger competitor with a larger beak selected for smaller beak sizes. Finally, a severe drought in the early 2000’s selected for an even smaller size. Such a short time scale for variation makes sympatric speciation in Dieckman and Doebeli model seem less likely.  Continous-Resource Models Problems with Dieckmann and Doebeli model As the ends of the resource distribution become occupied, less competition for central resources, which should favor intermediate individuals –Could result in a continuum of interbreeding forms rather than discrete groups 22  Model Conclusions Some models show reproductive isolation in the face of gene flow – Sympatric speciation is theoretically possible But is it biologically reasonable? Conditions for sympatric speciation are more stringent than allopatric speciation Allopatric speciation should be regarded as the null hypothesis 23  Model Conclusions Some models may have unrealistic assumptions, and few models have varied their parameters to examine effects on probability of speciation More prudent to judge likelihood of sympatric speciation based on laboratory experiments and evidence from nature 24  Experimental Evidence Some lab studies have demonstrated reproductive isolation between selected lines that interbreed But many are “kill the hybrid” experiments where hybrids are removed in each generation – Leads to successful isolation but not realistic 25  Experimental Evidence House fly geotaxis studies (Hurd and Eisenberg 1975) – Applied strong disruptive selection to flies moving up or down from a central chamber – Reared apart but allowed to interbreed – After many generations, flies diverged in geotaxis and mating tests showed strong sexual isolation 26  Experimental Evidence Bristle number in Drosophila (Thoday and Gibson 1962, 1970) – 12 generations of disruptive selection for bristle number – Flies showed complete assortative mating by bristle number – But results could not be replicated by 19 future studies – Other studies found moderate assortative mating 27  Experimental Evidence Most successful experiment was Rice and Salt’s (1990) “no gene” assortative mating maze –Individuals mate only with others who choose the same habitat 28  Experimental Evidence Flies could choose habitats differing in three environmental factors: – Phototaxis (light or dark) – Geotaxis (up or down) – Chemotaxis (ethatnol vs. acetylaldehyde) 29  Experimental Evidence Newly hatched flies released into central chamber A – Sorted themselves into 1 of 8 habitat vials – To simulate selection for extreme habitat preference, only flies choosing extreme habitats B were allowed to breed A Up/dark/acetylaldelhyde B Down/light/ethanol 30  Experimental Evidence Additional selection on development time A – Early eclosing flies from habitat A – Late eclosing flies from habitat B – Used mutations and chemical markers to tell B from a fly’s eye colour which habitat its parents had chosen 31  Experimental Evidence Complete habitat isolation in 30 generations – All flies who chose each extreme environment were offspring of parents who had chosen that environment 32  Experimental Evidence But no evolution of complete habitat preference Offspring flies from extreme habitats continued to choose intermediate habitats but were considered lethal and discarded Flies did not mate assortatively when in a common environment: assortative mating was a product of habitat isolation 33  Experimental Evidence Rice and Salt (1990) study shows that habitat isolation can evolve if selection is strong and assortative mating is a byproduct of habitat choice Strong disruptive selection appears necessary but not sufficient for sympatric speciation Experiments are helpful, but are they biologically realistic? 34  Evidence From Nature Allopatric speciation should be the null hypothesis Easier than sympatric speciation – Selection or drift acting on geographically isolated populations will eventually produce isolating barriers Strong and pervasive evidence for allopatric speciation 35  Evidence From Nature Allopatric speciation should be the null hypothesis Many opportunities for geographic isolation – In past 2 million years, 20 major glacial advances and more frequent cycles of temperature change 36  Evidence From Nature Sympatric speciation should meet four criteria: 1. Species must be largely or completely sympatric 2. Must have reproductive isolation, preferably of genetic origin 3. Must be sister groups 4. Biogeography and evolutionary history must make allopatry very unlikely 37  Evidence From Nature Evidence from habitat “islands” – Species on oceanic and continental islands – Fishes in postglacial lakes – Cichlid fish in African lakes – Host-specific parasites Host races Host-specific species Allochronic (temporal) isolation Comparative studies 38  Species on islands Small monophyletic group confined to a small isolated habitat would be convincing Several endemic Coleoptera and Orthoptera on St. Helena once thought to have speciated in sympatry – But species are all flightless and could have speciated microallopatrically 39  Species on islands Flightless weevils on island of Rapa – Could have speciated microallopatrically or on small islets that were connected to mainland during Pleistocene Lepidoptera on island of Rapa – But groups are not endemic to Rapa; possibility of multiple colonizations – Negative correlation between endemism and 40 mobility suggests allopatry not sympatry  Species on islands Surveys of endemic birds (Coyne and Price 2000) and Anolis lizards (Losos and Schluter 2000) fail to provide evidence of sympatric speciation on islands – No sister species on isolated islands 41  Fishes in Postglacial Lakes Many lakes in North America and Eurasia were formed after the last glaciation only 15,000 years ago – Endemic fish in these lakes likely to have evolved since that time Some lakes contain closely related fish differing in morphology, behaviour, habitat, or life history Often considered as sympatric speciation 42  Fishes in Postglacial Lakes 43  Fishes in Postglacial Lakes But lakes are connected to rivers and the sea – Could allow multiple invasions from allopatric taxa Gene exchange after secondary contact (hybridization) can yield misleading conclusion that the fish are sister taxa – Hybridization makes them seem more closely related than they actually are – Mitochondrial DNA especially misleading; nuclear DNA provides a clearer picture 44  Fishes in Postglacial Lakes Strong evidence comes from Arctic charr in Lake Galtabol, Iceland – Limnetic and benthic morphs more closely related to each other than taxa in four nearby lakes – Genetic similarity does not reflect current hybridization Share no alleles at one of six nuclear loci and strongly differentiated at other loci 45  Cichlid Fish in African Lakes A dramatic adaptive radiation Over 1500 species Very well- studied Sympatric speciation? 46  Cichlid Fish in African Lakes Net diversification interval Victoria, Malawi, and Tanganyika are African rift lakes Nabugabo is a satellite lake separated from Lake Victoria by a sandbar 4000 years ago Barombi Mbo and Bermin are tiny crater lakes 47 Support for sympatric speciation 1. Hard to envision geographic barriers allowing formation of hundreds of species in only 2 million years 2. Speciation has also occurred in limnetic and deep water forms less likely to encounter geographic barriers 3. Movement between habitats may prevent allopatric speciation 4. Closely related species differ more in colour than morphology and habitat use 5. Some pelagic species show no genetic structuring across Lake Malawi; fish are so mobile that allopatry not possible 6. Monophyletic groups of pelagic species show that speciation occurred in pelagic ancestors 48  Support for allopatric speciation 1.Levels of all three major lakes have risen and fallen repeatedly, creating isolated small lakes 2.Many species have highly localized ranges and shorelines have diverse habitats (rocky, sandy, swamps, bays, rivers) 3.Habitat specificity can reduce gene flow 4.Genetic patterns sometimes mirror fragmentation of lakes 5.Littoral fish have limited migration but occasionally colonize new habitats, promoting allopatric or parapatric speciation 6.Some pelagic species return to littoral zone for spawning which could cause allopatric speciation by habitat segregation 7.Some allopatric populations evolved behavioural isolation during periods when there were barriers to dispersal 49  Cichlid Fish in African Lakes Evidence for sympatric speciation in large lakes is inconclusive Fish in small crater lakes are more convincing – Fish differ more in feeding habits and morphology relating to diet – Little sexual dichromatism – Sister species and no evidence of hybridization – Size and formation of lakes suggests no allopatry 50 – Meet the four criteria for sympatric species  Host-specific parasites Host-specific parasites occupying different host species could be considered “islands” – But this could also be considered a form of allopatric speciation 51  Host-specific parasites Parasites occupying different parts of the same host species may be more convincing – Lice in birds occupying different feathers or even different parts of the same feather Possible case 52  Host-specific parasites Head lice and body lice in humans – Genetic studies show probably not different species 53  Host-specific parasites Fig wasps with different ovipositor length and temporal segregation – Could have speciated in allopatry on Melanesian islands 54  Host Races Host races involve two races of phytophagous insects living on different host plants in same area (not true species) Most famous example is the apple maggot fly Rhagoletis pomonella Use both apples and hawthorns as hosts in US – Started feeding on introduced apples in 1850’s But recent genetic evidence suggests that the apple race might have descended from an allopatric hawthorn race 55  Host Races Other possibilities for sympatric speciation, but allopatry cannot be excluded in most cases 56  Host-Specific Species Species living on different hosts and showing little gene flow – Treehoppers: each of six species inhabits a different species of tree, some sympatrically Reproductively isolated But may have differentiated in allopatry – Lacewings: two species that differ in colour and habitat Genetics show they are not sister species 57  Allochronic (temporal) isolation Perhaps large and rapid changes in life cycle could lead to sympatric speciation Two sympatric species of Gryllus field crickets breeding at different times – Not sister species Periodic cicadas that breed in 13-year and 17-year cycles – Cannot exclude biogeography but a good putative example – Shows influence of rapid changes in life cycle on reproductive isolation 58  Comparative studies Calculate degree of range overlap across groups of closely-related species In allopatric speciation, changes in range over time should lead to slightly increased overlap In sympatric speciation, range overlap should decrease over time (since speciation occurred during complete range overlap) 59  Comparative studies 60  In many animals, range overlap almost always increases over time, consistent with allopatric speciation 61  General allopatric pattern in mammals In pocket gophers, perpetual allopatry – No increase in range overlap with time No consistent biogeographic pattern in kangaroo rats but possible case of sympatric speciation (arrow) 62  The problem with mitochondrial DNA – Fig. A shows two very closely related species of chipmunk with large range overlap based on mtDNA Sympatric speciation pattern – Pattern disappears in fig. B when nuclear DNA is used 63  Key Points to Review What is sympatric speciation? Problems with models of sympatric speciation Be familiar with main types of models and conclusions Be familiar with experimental studies and their findings Four criteria for evidence of sympatric speciation in nature Be familiar with categories of and findings for evidence from nature Be familiar with arguments for/against sympatric speciation in African cichlid fish The problem with mitochondrial DNA 64