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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 interbreeding) are known variably as subspecies, races, varieties, etc. Such a subdivided species is known as a polytypic or polymorphic species. The subdivisions are useful only as a means of categorizing the amount of variability within a species, and do not necessarily represent incipient species. The species as groups of interbreeding populations cannot accommodate asexual organisms Species category: A species is also a rank in the Linnaean hierarchy (e.g., phylum, class, order, family, genus, species). The species rank includes all forms of organisms that have been described and given names, including asexual species. Thus, keep in mind that the single word species has several meanings, depending on the context in which it is used. Maintenance of species. A species is maintained via gene flow—exchange of genes between interbreeding neighboring populations. Species are isolated physically and/or biologically Species isolating mechanisms refer to the biological properties of the organism that prevent them from interbreeding. Isolating mechanisms are especially relevant to sympatric species that overlap in the same area and therefore whose populations are exposed to the potential of interbreeding. Because most interbreeding between animal species often leads to inviable (sterile) or inferior offspring, species isolating mechanisms maintain the unity of a species. How do new species form? In spite of the fact that a species has unifying mechanisms, such as gene flow, it nonetheless can give rise to new descendent forms. Speciation is distinct from phyletic evolution (anagenesis) • speciation leads to multiplication of species from a single parental species • phyletic evolution leads to changes over time (generations) of a single species Speciation is ultimately dependent on isolation Spatial Ecological Genetic 4 Behavioral • • Premating mechanisms prevent mating between species Postmating mechanisms limit the survival and success of mating events between species Modes of speciation (emphasis on geographical considerations) Allopatric speciation (allo: different; patric: father, fatherland): one species splits into two species while living in different places. Allopatric: spatially isolated populations achieve reproductive isolation gradually Traditional allopatric: a large distributional area is divided by a physical barrier (geological, geographic, vegetational, etc.), splitting the continuous population into two isolated groups of populations. Not possible to present reliable proof of spp. status if the two subpoplations are differentiated Sympatric speciation (sym: same): one species splits into two species while both living in the same place. Sympatric: the origin of a new, reproductively isolated species population within the dispersal area of the offspring of the parental population. Problem of sibling spp.— difficult to tell apart Problem of intra-specific polymorphism A controversial mode, some have denied the likelihood. Most documented cases involve host-specific plant feeders and host-specific parasites. Parapatric speciation: contiguous allopatric populations are in contact. Mostly developed by John Endler (1977, Geographic Variation, Speciation and Clines) In this mode he postulates that isolating mechanisms build up in a cline, along an ecological escarpment, until the two adjacent pops ultimately acquire reproductive isolation The intergradation between the two subspp, however, could be explained by zones of secondary contact, where two formerly isolated populations come back into contact. You can have all these possibilities: 1. 2. 3. 4. 2 populations intergrade clinally in a wide zone of contact 2 populations interbreed completely in a narrow zone of contact or hybridization 2 populations meet in a zone of contact where occasional hybrids occur 2 populations meet in a zone of contact but do not interbreed at all Populations 1 and 2 treated as subspecies by the Biological Species Concept (BSC) 5 Populations 3 and 4 treated as separate species by most concepts if there Peripatric: a new population is founded outside the continuous species range by a single colonist or small founder group and remains isolated long enough to acquire the genetic basis for reproductive isolation Also called speciation by budding. Stages in Speciation Process Speciation takes place in basically 3 stages: • Isolation phase, when a population becomes separated in time or space • Divergence phase, when various traits change, including mating or habitat preference • Reproductive isolation, when a population is no longer able to undergo mating and exchange genetic material with the parental population Relevant References Freeman, S. and Herron, J. C. Evolutionary Analysis, 3rd Ed. Prentice Hall, New Jersey. (pp. 583-608)—(includes species concepts, above) Howard, D.J and S.H. Berlocher. 1998. Endless Forms: Species and Speciation. Oxford University Press, NY and Oxford. Otte, D. and J.A. Endler. 1989. Speciation and Its Consequences. Sinauer, Sunderland, MA. 6