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Speciation Speciation is the evolution of more than one species (or, at least, a single different species) where previously there was only one. => species are the unit of evolution Species Concepts But what is a species????? • Scientists have been arguing about this for decades and still don't agree on the exact definition of a species. This is a very hotly debated issue. • They have rather come up with whole range (about 15) of species concepts • In this lecture I will talk about 7 of them. E.g. Parts of a seahorse Pectoral fin coronet 1. Morphological Species concept Linnaeus: A species is the smallest group of individuals that are distinct and distinguishable from all others eye Dorsal fin – Morphology refers to the form and struc ture of an organism or any of its parts. Tail Reproductio n Strengths: • Is in fact the way we recognise species differences. • Works well for sexual and asexual organisms • Applies well to past species (fossils can be rela ted to ex tant species) e.g Coelacanth was previously only known from fossils Polymorphism Weaknesses: •Does no t connec t with genetics. Morphological differences can be presen t withou t significant genetic divergence! •Polymorphism, sibling species, sexual dimorphism, mimicry complexes Light-morph jagu ar (typi cal ) Dark-mo rph jagu ar (rare) Sibling species - Differences between sexes Argyroso mus japoni cus (RSA) Ar gyros omus coronus (Angol a) Reprod uctiv ely isolated, yet very similar morphology Mimicry 2. Biological Species Concept (BSC) – Saddl ed Puf fer Canthigaster v alentini 1 dorsal fin Mimic Leather jac ket Ernst Mayr, 1942: • Most dominant concept in ecology studies “ A species is a group of interbreeding natural populations that are reproductively isolated from other such groups." Paralut er es prionurus 2 dorsal fins! 2 pairs of spines on the caudal peduncle This concept assumes that Genetic divergence is key to speciation Genetic divergence is when local units of a popula tion become repro duc tively isolated fro m o ther units. The driving forces of evolution (genetic drift, na tural selec tion and mu ta tion) may opera te in each isolated population and thus lead to changes in gene frequencies. Biological Speciation is the process by which species form when two genetically diverging species remain isolated from each o ther. Because genetic divergence is a gradual process, it is impossible to say precisely when species form. As long as gene flow continues individuals will remain members of a species even though t hey may be geographically distant. Reproductive isolation is key to the BSC: 2. Biological Species Concept Strengths -Criterion of reproduc tive isolation can be tested, observed, and / or inferred -Includes genetic information Weaknesses - Ignores hybridization - Does no t work well for asexual organisms - Depen den t on g eographic isolation to achieve sp. sta tus - Reproducti ve iso lation evo lve s g radually - Can no t be used for paleo species (ex tinct or fossils)! 3. Evolutionary Species Concept Or Genealogical species concept An evolutionary species is a lineage (an ancestral-descendant sequence of populations) which maintains its identity evolving separately from others and with its own unitary evolutionary role and tendencies” –Iden tity ; biologically distinct Includes concep ts associated with BSC e.g. Species Recognition Mechanisms –Tendencies and historical fate; Species have an origin Undergo evolu tion May disappear by ex tinction. 3. Evolutionary Species Concept: Concept : • Recognizes more than just genetic and morphological differences 4. Phylogenetic Species Concept: Concept: A species is a cluster of organisms tha t is diagnostically distinguishable from o ther such clusters, and within which there is a paren tal pattern of ancestry and descen t." •However; –Change within lineages does no t c rea te a new sp. –difficult to determine “evolu tionary fate” – how much diversity is allowed within a common evolutionary fate? –The stu dy of speciation is key, no t the study of microevolution – It rates trai ts as ances tral or deri ved and then looks for gr oupi ngs bas ed on si mi lari ti es. Avoids all reference to repro duc tive isolation and focuses instea d on phylogenetic histories of populations. Lineage; ancestor descendant members of a species have a common ancestor cartilaginous Sharks, rays and chimaeras Coelacanth 4. Phylogenetic Species Concept: Concept: a s pecies is a “tip” on a phy logeny , th at is, th e s mall est s et of organ isms th at sh ar e an a nc est or and c an b e d istin guished fr om other such s ets. Und er this d ef in ition, a r in g spec ies is a single s pecies that en compass es a l ot of ph enot ypic var iat ion. Lobes Lungfish bony Rays One speci es 5. Recognition species concept: concept: Problems (1) confuses histories of traits with histories of organisms, may divide species into groups based on characteristics that do not have any clear biological relevance (2) classifications change with more data (3) creates taxonomic inflation (new species names without the discovery of new species) 6. Cladistic species concept A speci es i s a set of organi sms (an evoluti onar y li neage) betw een tw o branch poi nts or betw een one branch poi nt and an exti ncti on ev ent or a moder n populati on (Ri dley 1993). a) Problem: -Teleosts - Adaptive radiation - Phylogeneti c s tatus of s peci es changes over ti me b) c) 3 species using this definition, but 1 and 2 phenotipically identical 3 species, and all are phenotypically different 1 species, but with significant phenotypic change over time ”A species is the most inclusive population of bipar ental organisms t hat share a common fertilization system" (Paterson, 1985). Comment: Si mi lar to BSC i n vi ewi ng cons peci fi c populati ons as a fi el d for r ecom bi nati on, but focuses on repr oducti on faci li tati ng mechani sms wi thi n speci es rather than on r epr oducti ve barri ers. di sadv antage Li ke BSC can only be appli ed to contem porari es, sex ual, and geographi cally conti guous populati ons, hence to a mi nori ty of natural populati ons Other concepts… concepts … 7. Cohesive Species concept A species is the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability. (Templeton, 1989) Atte mp ts to incorpora te strengths o f BSC, E SC, and R SC. Cohe sion mechanisms a re genetic e xchangeabili ty (factors that de fine gene flow) and de mograp hic exchangea bility (facto rs tha t de fine and sprea d new gene tic variants through gene tic drift and natu ral selection). Difficult to use based on the fact tha t di fferen t scienti sts may choo se to e mpha si ze di fferen t cohe sive mechanisms Problems in defining species 2. Geographic variation 1. Polymorphisms Refers to variation over geography variation within a population. Different individuals in the population have distinctly differ ent structures, colors, biochemistry, etc. but clearly belong to the same population since they reproduce with one another. populations in different areas look different from each ot her. One common pattern of geographic variation is clinal variation. Clinal variation refers to a gradual change in some featur e across geography. e.g. Hima ntur a ua rnak 3. Hybrid zones Geographic areas wher e two distinctly different for ms of organism contact each other and interbreed. Outside this zone, the two for ms retain distinct differ ences from one anot her but within the zone inter mediate for ms between the two occur as a result of interbreeding. e.g O. niloticus in southern Africa As a cons equ ence o f the 2 million year isolation, there is a high level o f geneti c divergen ce (4.5% COI gen e) bet ween the A. aequid ens populations. ANGOLA (n=5 2) So uth Africa (n=33 ) Mo de Min Ma x Mo de Min Ma x 1 st Do rsa l Fin Spine Co unt 10 10 10 10 10 2 nd Do rsa l Fin Spine Co unt 1 1 1 1 1 10 1 2 nd Do rsa l Fin Ra y Co unt 27 24 28 27 26 29 Ana l Fin Spine Co unt 1 1 1 1 1 1 Ana l Fin Ra y Co unt 9 9 9 9 9 9 From Hendri ques , 2008 No s hare d hapl otype s Pecto ra l Fin Ra y Co unt 18 16 19 17 16 18 La tera l L ine Sca l e Co unt 72 70 77 75 71 79 Krug er 200 9 Max. Size (Fork Length) Max. Weight ? Length at Maturity (Fork Leng th) Angola (1) South Africa (2) 960 mm 1300 mm 6.8 Kg 25 Kg 430 mm 900 mm • • • • • • • Morphological species concept X Biological species concept Evolutionary species concept X Phylogenetic species concept Recognition species concept can’t test 3 speci es Cladistic species concept Cohesive species concept X Angola Species Concepts • RSA • Value of each concept depends on its use. Various authors have argued that a combination is the best met hod. – – mor phologica l, ph ysiol ogica l, b ehav ior al geogra phic – – – – – lif e h ist ory & d ev el opment habitat & feed in g ec ol ogy phyl ogen etic s evolut ionar y fat e And Genet ics etc . b arc oding Note: 1. There i s uncer tai nty of the s peci es rank 2. The term “s peci es” can mean di ffer ent thi ngs i n di ffer ent tax onomi c groups