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Species Concepts and Speciation

Non-Evolutionary Approaches I. Typological Species Concept: species are a 'type' of organism

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Species Concepts and Speciation

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    1. Species Concepts and Speciation

    2. Non-Evolutionary Approaches I Typological Species Concept: species are a 'type' of organism…Plato Existence of a limited number of universals No regard to variation in a species "Species are as many as were created in the beginning by the Infinite." (Linnaeus, 1758)

    3. Non-Evolutionary Approaches II Essentialism Similar to typological thinking Species consist of individuals sharing the same essence Species are separated by sharp discontinuity Each species is constant through time Limits to variation within a species

    4. Non-Evolutionary Approaches III Nominalistic Only Individuals exist There are no “real” universals Popular in France in late 1800s Nature produces individuals and nothing more…no species. Species are only mental constructs

    5. Species Concepts Based in Evolution The lineage species concept. A single species forms an evolutionary lineage separated from other species. They are not in genetic contact with other species Therefore, the concepts you may have learned are actually criteria for determining the split between species.

    6. Biological Species Criterion (Concept) Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups (Mayr, 1940).

    7. Problems with the BSC What do you do with asexual organisms, and what do you do with organisms that occasionally form hybrids with one another? Other difficulties include: What is meant by “potentially interbreeding Fossils---for example, it is not really possible (or very meaningful!) to figure out whether a trilobite living 300 million years ago would have interbred with its ancestor living 310 million years ago.

    8. Evolutionary species criterion (concept):: Evolutionary species concept: A species is a single lineage of ancestor-descendant populations which maintain its identity from other such lineages and which has it own evolutionary tendencies and historical fate (Simpson, 1961; Wiley, 1981). Difficult to apply, so we use the following:

    9. Phylogenetic species criterion (concept) A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent (Cracraft 1983).

    10. Other criteria (concepts) Cohesion species concept: A species is the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability. (Templeton, 1989) Ecological species concept: A species is a set of organisms exploiting (or adapted to) a single niche (Ridley 1993). Recognition species concept: A species is the most inclusive population of individual biparental organisms which share a common fertilization system. (as defined by Paterson, 1985; in Templeton, 1989). Isolation species concept: Species are systems of populations: the gene exchange between these systems is limited or prevented by a reproductive isolating mechanism or perhaps by a combination of several such mechanisms. (as defined by Dobzhansky 1970; in Templeton, 1989)

    11. Now that we know some concepts, how does speciation occur? Geographic Isolation A physical barrier (mountain, river etc.) reduces gene flow Poor habitat separating organisms From: http://evolution.berkeley.edu/evosite/evo101/VC1eSympatric.shtml

    12. Reduction of Gene Flow Physical barrier Or no contact at the edges of a large range

    13. On a phylogeny a node indicates a speciation even

    14. Modes of speciation I: Allopatric In this mode of speciation, something extrinsic to the organisms prevents two or more groups from mating with each other regularly, eventually causing that lineage to speciate. Isolation might occur because of great distance or a physical barrier, such as a desert or river, as shown below.

    15. Modes of speciation II: Peripatric An isolated group is severed from the original population Rare genes move to fixation If these genes are associated with reproduction, sexual selection takes over, then speciation has occurred as they may not mate with the original population

    16. Modes of speciation III: Parapatric In parapatric speciation there is no specific extrinsic barrier to gene flow. The population is continuous, but nonetheless, the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population’s range. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range.

    17. We may be observing the first steps of parapatric speciation in the grass species Anthoxanthum odoratum

    18. Modes of speciation IV: Sympatric Unlike the previous modes, sympatric speciation does not require large-scale geographic distance to reduce gene flow between parts of a population. How could a randomly mating population reduce gene flow and speciate? Merely exploiting a new niche may automatically reduce gene flow with individuals exploiting the other niche. This may occasionally happen when, for example, herbivorous insects try out a new host plant.

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