Note for ECOL 600B at UA
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Date Created: 02/06/15
SPECIES Read Futuyma Chap 15 pp 447 454 Table 151 Ignore the Internodal Species Concept which has disappeared almost entirely from consideration Good general source Coyne amp Orr 2004 Speciation Sinauer Terminology 39 Species concept 2 theoretical model of species 39 Species criterion or definition 2 operational criterion by which an individual is assigned to one or another species How do we know species are real biological units vs arbitrary constructs 39 Non scientists identify same species as taxonomists 39 Statistical identification of clusters This has been enshrined as the phenetic species definition but seems to be purely empirical with no underlying species concept 39 Species are successfully treated as fundamental units of population and evolutionary genet1cs Species concepts in rough historical order Phenotypic clusters phenomenon to be explained probably the first species definition Common descent parent and offspring belong to same species common to all concepts Typological Species Concept Linnaeus to 20 11 century morphological characters possessed by all members of a species Tradition enshrined in type specimens placed in museums Biological Species Concept BSC T H Dobzhansky H J Muller Ernst Mayr species as most inclusive group of individuals that actually or potentially interbreed Have cohesion as result of shared gene pool sex as glue that holds a species together Mayr called BSC to contrast with typological species Population genetics emphasized species as populations of individuals that are similar but not necessarily identical Criterion breeding tests or definitive evidence that organisms couldn t interbreed even if given the chance Dominates thinking and research on speciation Research on speciation is research on the origin of reproductive isolation Recognition SC RSC most inclusive group of individuals that share common fertilization mechanism Flip side of BSC not significantly different Pluses Theoretically applicable to many or most sexual organisms works well in practice for many Informed by population genetics Minuses Irrelevant to asexual organisms problematic with organisms like plants that have clear phenotypic clusters with lots of hybridization Templeton BSC fails with organisms that have too little or too much sex Criterion often hard to apply so use surrogate evidence like phenotypic similarity Evolutionary SC EVSC George Gaylord Simpson Species as lineage that evolves independently of other Lineages Plus Applicable to any organism Implicit or a consequence of many other concepts or criteria Minus Original form vague without clear criteria Ecological SC EcSC Leigh Van Valen Species as lineages that are adapted to different niches Note that this provides evolutionary cohesion Criterion sympatric coexistence of clusters Plus Applicable to any organism Can be grounded in population genetic theory Application of molecular data led to new species concepts inspired by population genetics Cohesion SC CSC Alan Templeton Inclusive groups of individuals that are cohesive due to shared gene pool and or adaptation to different niches Criteria various Gedankenexperiment mix individuals from two populations in equal numbers many replicates if belong to same species over time each population will replace the other in half the populations if different one will replace the other more often Plus Applicable to any organism Can be grounded in population genetic theory Minus Calls species while they are still poly or paraphyletic if they are adapted to different niches I d prefer to call these subspecies Genotypic Cluster SC GCSC Species are phenotypic or genotypic clusters with few or no intermediates Plus Focus on clusters and gaps Applicable to any organism Minus Vague about how to define clusters Really a vague criterion not an SC Ecotype SC EctpSC Fred Cohan Bacteria form clusters as a result of periodic selection for adaptation to a particular ecotype ecological niche A special case of EvSC emphasizing role of periodic selection in producing clusters Plus Grounded in population genetic theory Improvement over arbitrary division by 25 or other amount of rRNA sequence divergence Minus Unnecessary focus on periodic selection Rise of phylogenetic methods and concepts and of cladistics produced several Phylogenetic Species Concepts PSCs Multiple versions Coyne and Orr distinguish two as follows Phylogenetic SC 1 PSC1 Joel Cracraft Basal cluster of organisms diagnosably distinct from other such clusters Plus Minus Basically typological Would allow any clade distinguished by any shared trait from other clades but shared traits don t necessarily reflect history Could subdivide biological species into trivial species that are clustered on the basis of one or a few morphological traits Can create species in which some individuals are more closely related to individuals from other species Maybe not applicable to asexuals Phylogenetic SC 2 PSC2 Kevin de Queiroz and Michael Donoghue Smallest exclusive monophyletic group Plus Applies to all organisms Solves problem with PSCl by requiring synapomorphies shared derived traits to distinguish reciprocally monophyletic clades Minus Lacks ranking criterion so one could have reciprocally monophyletic populations created by drift Genealogical Species Concept GSC also called PSC3 by Coyne amp Orr Brent Mishler et al Group in which in which all genes belong to same monophyletic group Plus Applies to most organisms Minus Can have lineage separated from rest of species that becomes monophyletic it is then a species but the original species is now paraphyletic and hence no longer a species In sexual organisms having all genes be monophyletic is impossible if any locus shows balanced polymorphism Not a problem with asexual organisms Takes very long time for all loci to become reciprocally monophyletic long after populations have become reproductively isolated because genes controlling reproductive isolation have already become reciprocally monophyletic Not a problem with asexual organisms General Lineage SC GLSC Kevin de Quieroz Species are segments in time of different evolutionary lineages Other SCs are special cases of the GLSC Plus Very general applicable to all organisms in principle ncorporates most other SCs as special cases Minus Too general doesn t specify when one lineage becomes two lineages and two species Evolutionary Genetic SC EVSC Birky based on theory developed independently by Birky and by Tim Barraclough amp Austin Burt See below General properties seen in many SCs 39 Common descent all 39 Monophyletic 39 Phenotypic andor genotypic clusters 39 Evolutionary units and lineages 39 Reciprocally monophyletic Systematics is a contentious field especially Where species are concerned Eg debate over DNA barcoding The tone and content of this submission reflects the ad hominem nature of the debate We would be astonished if any credible scientists would accept the diversity of cox haplotypes sampled from an area as a valid or meaningful measure of biodiversity Even among the three of us there is little consensus as to the best species concept or even the importance of species as a taxonomic rank a situation that mirrors the broader biological community Blatantly biased editorial comment barriers to progress in defining species 39 Confusing philosophy and science 39 Birders 39 Extreme cladists 39 Ignorance of or just ignoring population genetics 39 Bias toward sexual organisms My Approach to Species Concepts and Criteria Principles 39 Asexual organisms form clusters similar to those in sexuals so SC must apply to asexual species 39 We need to be able to compare species in asexual and sexual organisms so SC must apply to sexual species or define units comparable to those defined by BSC 39 Species are populations so SC must be grounded in populationevolutionary genetic theory 39 If there were only one cluster there would be no species so SC must explain gaps as well as clusters 39 SC must not allow gaps due to drift 39 SC must define evolutionarily independent lineages species must be independent arenas for mutation selection and drift This requires that speciation events be completed and the species be reciprocally monophyletic Read Barraclough Birky and Burt 2003 Evolution for theory pdf is on web site This theory it could be considered a more explicit version of the EVSC But it goes further so I think it can be considered a neW species concept the Evolutionary Genetic Species Concept or EGSC Disclaimer Tim doesn t want to call it a species concept For asexual organisms the theory is as follows W C 314 A 45 W W B D Asexual gene or organism tree With synchronous reproduction and no extinction A With stochastic reproduction and extinction drift B drift plus truncating selection C and drift plus diversifying selection D In due gaps due to to drift selection I 2quot 2quot Phylogenetic trees With extinction random drift and diversifying selection top observed tree based on extant survivors bottom TESZN eu E m I g nucleotide differencessite II ec ll t Z4N 9 generations 95 CI of time to reciprocal monophny D28N eu nucleotide differencessite l Dnza4 Species criterion involves identifying reciprocally monophyletic populations If samples are reciprocally monophyletic and d 2 47 2 96 of populations are reciprocally monophyletic when sample sizes 2 3 We used this criterion to identify reciprocally monophyletic populations in monogonont rotifers and two sets of data from bacteria We found few or no borderline cases perhaps because periodic selection keeps Ne low For sexual organisms two populations would be independent evolutionary arenas if they were 95 reciprocally monophyletic for the gene or genotype that makes them reproductively isolated This is identical with the BSC except that it requires evidence that the majority are reproductively isolated Tim Barraclough detecting species by analyzing branching rates hood mmlel ill 2 I p ltlt 0001 234 clusters 95quot I 225 244 LE me Since coalesce number of lineages This method has been applied to asexual bdelloid and sexual monogonont rotifers and to tiger beetles In sexual org anisms the species so identi ed correspond well to species identi ed under the BSC Summary It is possible to have aspecies concept theoretical model that is informed by population genetics and captures widely but not universally accepted features ofspecies and is potentially applicable to all organisms
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