• 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

• 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

• 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

• Species traditionally have been described and identified on the basis of morphological criteria, a classification system referred to as the morphological or typological 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

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

• 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

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

• 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)

• 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

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

• According to Patterson (1993), species have a specific 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

• 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

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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

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
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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

• 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

• Species are defined as the smallest diagnosable monophyletic group
• Any population that forms an independent branch on the phylogeny is recognized as a species
• To be recognized as a species, populations must have been evolutionarily independent for a long enough time for diagnostic traits to emerge

• 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