Taxonomy n Systematics 2
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There are many different concepts of what constitutes a species. These concepts include biological, ecological, evolutionary, and phylogenetic species concepts. There is no universal agreement on how to define species. Determining whether species are real, and delineating species boundaries accurately is challenging given the various concepts and lack of consensus on an approach. Higher taxa concepts are also debated in terms of their philosophical reality.
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Taxonomy n Systematics 2
- 5. Key questions to be answered How to define species? How to decide between species concepts? Do species really exist?
- 7. Similarity Concepts Overall similarity and/or gaps in character distributions (<MorphSC, PhenotSC, TaxSC,...) Evolutionary Concepts Theoretical commitment to evolutionary theory (BioSC, EcolSC, EvolSC, RecogSC, CohSC,...) Phylogenetic Concepts Commitment to phylogenetics (<CladSC,PhyloSC, HennigSC,...) Three main breeds of species concepts
- 8. Some Definitions Biological species A group of interbreeding natural populations that do successfully mate or reproduce with other such groups The smallest group of cohesive individuals that share intrinsic cohesive mechanisms (e.g. interbreeding ability, niche) A lineage which occupies an adaptive zone. Ecological species concept defines a species as a group of organisms that share a distinct ecological niche. A single lineage of ancestor-descendant populations which is distinct from other such lineages and which has its own evolutionary tendencies and historical fate Cohesion species Ecological species Evolutionary species
- 11. Biological species Cohesion species Ecological species Evolutionary species Cohesion is difficult to recognize, prezygotic and postzygotic isolating mechanisms are mostly unknown Species concept Practical application Strengths / weaknesses Difficult Difficult Difficult Difficult Popular, explains why the members of a species resemble one another and differ from other species (shared gene pool + reproductive isolation). Irrelevant to fossils, asexual organisms, complicated by natural hybridization, polyploidy, etc. Adaptive zones/ Niches, difficult to define, assumes two species cannot occupy the same niche for even a short period (but what to do with life stages…) Criteria vague and difficult to observe (see also PSC)
- 12. Morphological species Phylogenetic species Recognition species Morphological criteria may not reflect actual links that hold organisms together into a natural unit; only possibility for paleontologists; but what with cryptic species? Will give rise to recognition of many more species than more traditional concepts; but from what point onwards do we conceive différences to be ‘statistically significant’? Determining if a feature is used to recognize potential mates is difficult or impossible in many populations (note that this concept has been succesfully demonstarted with amphibians, crickets,…) Species concept Practical application Strengths / weaknesses Common Increasing Difficult
- 13. The phylogenetic species concept Speciation and phylogenetic relationships Dispersal + subsequent character change Vicariance Sympatric
- 17. Higher taxon reality Are genera, families, etc., real? At the species level there are a number of competing concepualizations. At higher levels we have the opposite problem. We lack criteria to use in evaluating the question.
- 18. Higher taxon reality If a genus is just a mono- phyletic group larger than a species and smaller than a family, we can determine if such groups exist.
- 19. Cited works: R. Descartes, 1641. Meditationes de prima philosophia. Published by someone a really long time ago. M.T. Ghiselin, 1974. A radical solution to the species problem. Systematic Zoology, 23(4): 536-544. M. Pigliucci, 2003. Species as family resemblance concepts: the (dis-)solution of the species problem? BioEssays, 25(6): 596-602. F. Pleijel & M. Harlin, 2004. Phylogenetic nomenclature is compatible with diverse philosophical perspectives. Zoologica Scripta, 33(4): 587-591. L. Wittgenstein, 1921. Tractatus Logico-Philosophicus, trans. by D. Pears and B. McGuinness. London: Routledge & Kegan Paul.
- 20. “ Damn your principles, stick to your party” Benjam Disraeli (1804-1881)
Editor's Notes
- In Lecture one we had some discussion of phylogenetic relationship of some mammals groups. These groups were species.
- philosophical doctrine of essences: the doctrine that things have an essence or ideal nature that is independent of and prior to their existence - es·sen·tial·ist , noun Microsoft® Encarta® 2007. © 1993-2006 Microsoft Corporation. All rights reserved. ince Aristotle, species have been paradigmatic examples of natural kinds with essences. An essentialist approach to species makes sense in a pre Darwinian context. God created species and an eternal essence for each species. After God's initial creation, each species is a static, non evolving group of organisms. Darwinism offers a different view of species. Species are the result of speciation. No qualitative feature—morphological, genetic, or behavioral—is considered essential for membership in a species. Despite this change in biological thinking, many philosophers still believe that species are natural kinds with essences. Let us start with a brief introduction to kind essentialism and then turn to the biological reasons why species fail to have essences. es·sence [éss'nss] ( plural es·sences ) noun 1. identifying nature: the quality or nature of something that identifies it or makes it what it is You've described the city, but you haven't communicated its essence. 2. most important feature: the most important element or feature of something The essence of leadership is said to be the willingness of other people to follow. Microsoft® Encarta® 2007. © 1993-2006 Microsoft Corporation. All rights reserved.
- SC: Species Concept The saying that someone “can't see the forest for the trees” is a reference to people who get so involved with the details of an issue that they lose sight of the larger issue.
- Ecological species concept defines a species as a group of organisms that share a distinct ecological niche. Shortcoming: This concept, which is based on the niche occupied by a species, is problematic because widespread species generally have local populations that differ in their niche occupation, which would require they be recognized as different species, even though based on all other criteria they would not be. As noted by Mayr, &quot;fatal for the ecological species concept are the tropic species of cichlids,&quot; which differentiate niche within a single set of offspring from the same parents. There are also common cases where two sympatric species seem to occupy the same niche.
- Different Cichlids
- Prezygotic: species isolating mechanisms which prevents fertilization among different species. Postzygotic, which prevent the development of a fertilized egg, resulting from two different species.
- We have described some 1.7 to 1.9 million species At least 8 million species are yet to be discovered and described Most of our existing (and comming) species knowledge comes (will come) from a single point in space (single locality) and time (no fossil evidence to back up) and hence populational variability and concern with the process of speciation remain “luxury concerns” J. Ray (1653): “Species are merely what competent naturalists says they are” ... We need more competent naturalists, and hence taxonomists!
- Suppose we go with the biological species concept , wherein species are the smallest groups of organisms that reproduce with each other. Then the species category is real insofar as organisms are actually divided up into such groups. If we looked into the world and saw only clonal, asexual lineages, we would conclude that the species category (so defined) is not real. If we saw sexual lineages forming groups with clear reproductive boundaries, we’d say the species category is real. What we actually see is some-where between the two.
- I’m suggesting we evaluate the reality of the species category in reference to species concepts; in reference to what we’ve decided it means for a thing to qualify as a species.