Genetics and Evolution IB 201 What Are Species and How Do They Evolve All cultures early and modern group organisms based on similarities and differences in 1 morphology 2 behavior 3 ecology 4 reproductive compatibility Species are the units of biodiversity they are the basis of our classification systems Thus we are interested in grouping organisms based on evolutionary history The tree of life is the result of millions of speciation events over time What is a species Exactly what is a species and how it comes into existence has been controversial in biology since the 19th century Numerous definitions have been proposed to distinguish a species depending upon the focus and interests of the biologist A biologist studying variation among populations has one perspective a phylogeneticist studying variation among higher taxa may have another plants may behave differently to animals e g undergoing frequent hybridization or polyploidization microbes do not behave like tigers This does not invalidate the notion of a species but leads to a multivariate view within biology Most usable definitions converge on two major principles species are real entities of nature forming natural groups species are evolutionarily independent units they form a boundary for the spread of alleles different species have independent evolutionary trajectories Differences among species concepts concern the criteria for identifying evolutionary independence The differences arise because different organisms lead to different ideas about species and how to define independence different disciplines lead to different concepts of species General criteria of a species concept encompass all taxa sexually reproductive parthenogenetic or asexual should define the practical aspects of species recognition Species concepts I Descriptive taxonomic phylogenetic 1 Darwinian species concept Wallace 1865 Darwin 1859 1 In Darwin s view species had evolutionary integrity descent from a common ancestor In other words they were real entities although plastic and mutable Problems No strict criteria 2 Morphological species Cronquist 1978 Most practiced by plant taxonomists especially museum taxonomists morphospecies still most useful concept for taxonomists working with groups of many undescribed species effective and efficient in most cases Criterion if morphologically distinct from other groups it is a species Problems species can be arbitrary depending what an expert thinks is morphologically distinct cryptomorphic sibling species often closely related DNA can be different but morphology same DNA similar but large differences in morphology 3 Phylogenetic species Cracraft 1989 Based on the concept of the monophyletic group a group of taxa that contains all of the known descendents of a single common ancestor The tips of a phylogenetic tree of populations form species the irreducible units are differentiated via process of speciation not arbitrary each entity can be distinguished by one or more novelties morphological behavioral ecological or genetic each entity is reproductively cohesive includes sexual and asexual organisms Criterion If totally fixed differences between populations they are considered different species Problems multiplication of species names may find populations with almost but not completely fixed differences II Evolutionary process based concepts Biological 4 Biological Species concept Mayr 1963 also known as the Isolation concept Criterion reproductive isolation lack of interbreeding in sympatry measured by observation or genetic differences Problems what about asexual species Fossils hybridization especially among plants is a common phenomenon rarely does the biologist have information on the reproductive isolation of the populations they work with it is assumed 2 5 Mate Recognition Paterson 1985 This was proposed as a reaction to the biological species concept emphasizing the importance of mate finding and mate recognition Criterion demonstrate that the mate recognition system is different enough to prevent mating from close relatives species share a fertilization system Problems what about asexual species Fossils hybridization especially among plants is a common phenomenon exclusive use of fertilization mechanisms to define a species What about the rest of the life cycle References on species concepts Freeman S and Herron J C Evolutionary Analysis 3rd Ed Prentice Hall New Jersey pp 583 608 includes speciation mechanisms below discussion Futuyma D J 1998 Evolutionary Biology 3rd ed Sinauer Sunderland Mass Cracraft J 1989 Speciation and its ontology the empirical consequences of alternative species concepts for understanding patterns and processes of differentiation pp 28 37 55 59 In Otte D and J A Endler Speciation and Its Consequences Sinauer Sunderland MA Cronquist A 1978 Once again what is a species Belstville Symp Agricult Res 2 3 20 Hey J 2001 The mind of the species problem Trends in Ecology and Evolution 16 326 329 Harrison R G 1998 Linking evolutionary pattern and process the relevance of species concepts for the study of speciation pp 19 31 In Endless Forms Species and Speciation Eds Howard D J and S H Berlocher Oxford University Press NY and Oxford Howard D J and S H Berlocher 1998 Endless Forms Species and Speciation Oxford University Press NY and Oxford Mayr E 1963 Animal Species and Evolution Harvard University Press Cambridge MA Mayr E and P D Ashlock 1991 Principles of Systematic Zoology 2nd Edition McGraw Hill NY Chap 5 pp 86 109 Speciation and taxonomic decisions Paterson H E H 1985 The recognition concept of species In Vrba E S ed Species and Speciation pp 21 29 Transvaal Museum Monograph No 4 Transvaal Museum Pretoria Templeton A R 1989 The meaning of species and speciation a genetic perspective In Otte D and J A Endler Speciation and Its Consequences pp 3 27 Sinauer Sunderland MA Wiley E O 1981 Phylogenetics The Theory and Practice of Phylogenetic Systematics Wiley NY 3 The process of how species evolve Speciation As discussed above the concept of a species is variable and depends largely on the kinds of organisms under examination and the particular subdiscipline of biology in which one practices Species taxa Nonetheless species are real entities living in interbreeding populations and communities in time and space they are real taxa which often exhibit greater or lesser amounts of variation These subdivisions of a species into different populations assumed to be actually or potentially