Species Concepts

1. Species Concepts

2. Species in Theory and Practice • Biologists have not been able to agree on exactly what a species is, or how species should be abstractly define - the controversy is theoretical, not practical • Under most circumstances, there are no practical problems when defining species

3. Species in Theory and Practice • Practical problems do arise when species are recognized and identified based on phenotypic characters • If we accept that species have evolved from a common ancestor, then we would expect that there may be some conditions in which organisms are phenotypically intermediate • Variation poses most of the practical problems of species recognition using phenotypic characters • Geographic variation creates difficulties • If a species varies geographically, then a good character for species recognition in one place may become useless in another place

4. Theoretical Considerations • Practical difficulties have, in part, lead to theoretical questions regarding species • Species are in practice mainly recognized by phenotypic characters - but the interest of the subject of evolutionary biology lies elsewhere • What we want to determine is whether there is some deeper theoretical concept beyond individual characters that can be used to recognize individual species

5. Morphospecies Concept • Species traditionally have been described and identified on the basis of morphological criteria, a classification system referred to as the morphological ortypological species concept • Species are groups of individuals that are morphologically similar and clearly distinguishable from individuals of other groups • Species had traditionally been defined by reference to a morphological type • Usually any geographic variation among members of the group was not detected or simply ignored

6. Concerns Regarding the Morphospecies Concept • It became apparent that what appeared to be distinct morphological species at the local level were merely one in a series of morphologically intergrading populations on a broader geographic scale • Geographic variation became commonplace, and species were viewed as multi-populational systems distributed over a broad geographic range • Emphasis shifted from characterizing individuals from local populations to describing populational systems

7. Phenetic Species Concept • A quantitative approach to systematics that seeks to classify organisms on the basis of their overall similarity • Based on numerical taxonomy, which measures and records similarities for large numbers of characters • Phenetic species concept defines a species as a set of organisms that look similar to each other and distinct from other sets • More formally, it would specify some exact degree of “phenetic similarity”, and similarity would be measured by a phenetic distance statistic • A species is a set of organisms not more than “X” phenetic distance units apart

8. Concerns Regarding the Phenetic Species Concept • Phenetic species concept is an updated, numerical form of the earlier morphological or typological species concept • Phenetic classification lacks a sound philosophical basis; it tends to necessitate only subjective and arbitrary decisions • The neo-Darwinians have dismissed typological classification because there is no reason to suppose that any ideal pattern of morphological types exists in nature

9. The Biological Species Concept • Mayr - “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” • It places the taxonomy of natural species within the conceptual scheme of population genetics • For example, a community of interbreeding organisms is, in population genetic terms, a gene pool (total aggregate of genes in a population)

10. Relationship between Morphological and Biological Species • A justification for defining species morphologically is that the morphological characters shared between individuals are indicators of interbreeding • Problems can, however, arise: • Members of a species are by no means all uniform - biological species are regarded as polytypic - they have many (or perhaps no) morphological types • Also, it is possible for a species to differ reproductively but not morphologically- sibling species

11. Is the Morphological Species still Applicable? A Test Involving Bryozoans • Jackson and Cheetman (1994) addressed whether the fossil morphospecies they had identified were consistent with genetically distinct living bryozoans • Determined that the morphological features used to distinguish bryozoans had a genetic basis • Found unique allozymes in each of the distinguished morphospecies

12. The Recognition Species Concept • According to Patterson (1993), species have aspecific mate recognition system (SMRS) • Species can be defined as a set of organisms with a common method of recognizing mates • Advantages: • SMRSs are easier to observe than interbreeding in nature • The recognition species concept may more accurately represent what happens when a new species originates

13. Limitations of Reproductive Species Concepts • The criterion of interbreeding is useless for asexual populations • Reproductive species concepts cannot be applied to fossils • It is difficult to know whether geographically isolated populations potentially can interbreed

14. The Ecological Species Concept • Populations form the discrete phenetic clusters that we recognize as species because the ecological and evolutionary processes controlling the division of resources tend to produce those clusters • The ecological species concept defines a species as a set of organisms exploiting a single niche (adaptive zone) • The ESC supposes that ecological niches in nature occupy discrete zones, with gaps between them

15. Relevance of the ESC • Consider an array of species exploiting resources that form a single resource axis (e.g., seed size) • An individual suffers intraspecific and interspecific competition for food • Selection within a species might favor individuals at the extremes, as they suffer less competition • This process may lead to character displacement • Character displacement often implies that the 2 species differ more sympatrically than allopatrically • If the characters with respect to which the species differ more in sympatry are related to ecological competition, then character displacement will occur because of the advantage of avoiding competition with a better adapted species where it is present • In a place where only one species exists, selection from interspecific competition is relaxed and the species evolves to exploit a larger niche

16. Contrasting the BSC and the ESC • According to the BSC, species form discrete units because of gene flow • But, the ESC emphasizes selection - selection favors certain forms and removes forms that are intermediate between species

17. The Phylogenetic Species Concept • According to this concept, species are defined as the smallest diagnosable monophyletic group • Any population that forms an independent branch on the phylogeny is recognized as a species • Also, to be called a separate species under the PSC, populations must have been evolutionarily independent for a long enough time for diagnostic traits to emerge

18. Species Concepts and Asexual Species • Gene flow can occur between bacteria taxa whose genomes have diverged up to 16%; in some cases, gene exchange can occur among species of different phyla! • Sexual isolation is not an important criterion for identifying species of bacteria • The primary consequence of gene flow between bacteria is the spread of specific sequences with high fitness advantages • Consequently, it has been proposed that the key to recognizing bacterial species is finding strains that share a large suite of selectively neutral sequences