1 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, 18592 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.3 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.4 The process of how species evolve: Speciation As discussed above, the concept of a species is