Evolution #13, pg. 1 Bio 1B Lecture Outline (please print and bring along) Fall, 2008 B.D. Mishler, Dept. of Integrative Biology 2-6810, [email protected] Evolution lecture #13 -- Speciation -- Dec. 5th, 2008 487-506 (ch. 24) in 8th ed. 472-480 (ch. 24) in 7th ed. • Summary of topics • Describe the biological species concept and understand its limitations; be aware of other species concepts, especially the phylogenetic species concept • Understand different mechanisms of reproductive isolation • Contrast allopatric and sympatric modes of speciation • Describe examples of species clusters in connection to concepts of niche, adaptive radiation, and convergent evolution • Species concepts Why classify organisms? All societies classify local plants and animals because they are important to them. We want to eat some; we want to exploit some; we want to avoid eating others; and we want to avoid being eaten or harmed by still others. In Europe, the age of exploration and especially the discovery of North and South America created the need to classify a huge number of new organisms, some of which were very similar to what was known in Europe and others of which were completely different. Linnaeus was the first to successfully devise a system of classifying all organisms and be able to accommodate new types. Now, one of the main reasons to refine our methods of classification is to understand and preserve natural diversity. Species: A basic "kind" of organism; designated through a concept most appropriate to the group of interest, often limited by criteria available to diagnose membership in and boundaries of this group. This is a very controversial topic -- there are many competing species concepts (more than 20!). For our purposes, these four are sufficient to discuss: • The typological species concept (sometimes known as the morphological species concept) groups organisms into species based on a common morphology, or body plan. This was the concept of the ancient Greeks up through Linnaeus, but still may be used today (e.g., by historical biologists such as paleontologists who have morphology and want to estimate time of presence in the fossil record).Evolution #13, pg. 2 Linnaeus’s definition was that a species is a group of organisms that share important characters, and are dissimilar enough to in these essential characters from other groups be regarded as distinct. The emphasis is on morphology rather than behavior or environment because much of the classification was and still is done with preserved specimens. The morphological definition works in many cases. Traditional societies, which depend on their environment for food, recognize different species. Western scientists usually agree with their distinctions although they usually recognize more species. • The biological species concept, explicated by Mayr (1942), is widely used in biology for sexually reproducing organisms (Fig. 24.4 (8th); Fig. 24.3 (7th)). Here a species consists of members of a group of populations that interbreed, or potentially interbreed, with each other under natural conditions. The biological species concept hinges on reproductive isolation with each species isolated by factors (barriers) that prevent interbreeding, thereby blocking genetic mixing with other “types”. Even though the BSC is widely accepted, it cannot always be used in practice. In one location, it is usually possible to tell which groups are interbreeding, so identifying species is not too difficult, at least to people very familiar with particular groups. Sometimes, close observation will allow splitting of species previous thought to be a single species. In groups in different areas or groups known only from specimens or fossils, it is usually impossible to know whether interbreeding would occur. In that case, the extent of morphological variation between species within a region is used as a guide so say whether groups in different areas are species, subspecies, geographic races. It is ultimately a subjective judgment and, as in all such situations, people defend vigorously their own interpretations. Ensatina is an example. If the intermediates were now known, the ends would be good species. Two species of birds (yellow and red tailed flickers) were thought to be good species. When one of them expanded its range, they hybridized, so they are now subspecies of one species. If you accept that species have evolved, then you should expect a whole spectrum of intermediate cases to exist, making it impossible to say exactly how different two species are. • The phenetic species concept, which views species of clusters of similar organisms whether based on molecular or morphological data, is widely used in biology in cases where researchers think the biological species concept doesn't apply. It is applied for example in bacteria and in many plants that may be either asexual or else engage in widespread hybridization. Researchers who favor this approach often think that ecological factors and stabilizing selection are more important for keeping species separate than barriers to interbreeding.Evolution #13, pg. 3 One problem with a morphological definition is that it is difficult to say how different two groups have to look in order to be called different species, or how similar they have to look to be put in the same species. Another problem with the morphological definition is that males and females might be classified as different species, although they will not be if their reproductive biology is known. In practice, the phenetic species concept tries to take into account such variation. Even though people can look very different, we are all in the same species. New genetic data may help, but the logic is the same as for morphological characters. The difference will be that you do not need specialists in the morphology of each group, once a standard of difference is agreed upon. • The phylogenetic species concept is the most recently developed and looks at species from the standpoint of cladistics. Like cladistic taxa at other ranks, species are defined by the splitting of lineages (cladogenesis) through time. In this approach a species is the smallest monophyletic group recognized in a classification. All types of criteria are used as potential synapomorphies to determine evolutionary relationships (i.e., build a cladogram) of individual organisms.
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