Species Concepts

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 controversyis theoretical, not practical

• Under most circumstances, there are no practical problemswhen defining species

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 arephenotypically intermediate

• Variation poses most of the practical problems of speciesrecognition 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

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 evolutionarybiology lies elsewhere

• What we want to determine is whether there is some deepertheoretical concept beyond individual characters that can beused to recognize individual species

Morphospecies Concept• Species traditionally have been described and identified on the

basis of morphological criteria, a classification system referred to asthe morphological or typological species concept

• Species are groups of individuals that are morphologically similarand clearly distinguishable from individuals of other groups

• Species had traditionally been defined by reference to amorphological type

• Usually any geographic variation among members of the group wasnot detected or simply ignored

Concerns Regarding the Morphospecies Concept

• It became apparent that what appeared to be distinctmorphological species at the local level were merely one in aseries of morphologically intergrading populations on a broadergeographic scale

• Geographic variation became commonplace, and species wereviewed as multi-populational systems distributed over a broadgeographic range

• Emphasis shifted from characterizing individuals from localpopulations to describing populational systems

Phenetic Species Concept• A quantitative approach to

systematics that seeks to classifyorganisms on the basis of their overallsimilarity

• Based on numerical taxonomy,which measures and recordssimilarities for large numbers ofcharacters

• Phenetic species concept defines aspecies as a set of organisms that looksimilar to each other and distinct fromother sets

• More formally, it would specify someexact degree of “phenetic similarity”,and similarity would be measured bya phenetic distance statistic

• A species is a set of organisms notmore than “X” phenetic distance unitsapart

Concerns Regarding the Phenetic Species Concept

• Phenetic species concept is an updated, numerical form of theearlier morphological or typological species concept

• Phenetic classification lacks a sound philosophical basis; it tends tonecessitate only subjective and arbitrary decisions

• The neo-Darwinians have dismissed typological classificationbecause there is no reason to suppose that any ideal pattern ofmorphological types exists in nature

The Biological Species Concept

• Mayr - “Species are groups of interbreeding naturalpopulations that are reproductively isolated from othersuch groups.”

• It places the taxonomy of natural species within theconceptual scheme of population genetics

• For example, a community of interbreeding organisms is,in population genetic terms, a gene pool (total aggregateof genes in a population)

Relationship between Morphologicaland Biological Species

• A justification for defining species morphologically isthat the morphological characters shared betweenindividuals 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 perhapsno) morphological types

– Also, it is possible for a species to differ reproductively but notmorphologically- sibling species

Is the Morphological Species still Applicable? ATest Involving Bryozoans

• Jackson and Cheetman (1994) addressed whether the fossilmorphospecies they had identified were consistent withgenetically distinct living bryozoans

• Determined that themorphologicalfeatures used todistinguish bryozoanshad a genetic basis• Found uniqueallozymes in each ofthe distinguishedmorphospecies

The Recognition Species Concept• 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

Limitations of “Reproductive” SpeciesConcepts• 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

The Ecological Species Concept

• The ecological species conceptdefines a species as a set oforganisms exploiting a singleniche (adaptive zone)

• The ESC supposes thatecological niches in natureoccupy discrete zones, withgaps between them

Populations form the discrete phenetic clusters that we recognizeas species because the ecological and evolutionary processescontrolling the division of resources tend to produce those clusters

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

• If the characters with respect to which the species differ more insympatry are related to ecological competition, then characterdisplacement will occur because of the advantage of avoidingcompetition with a better adapted species where it is present• In a place where only one species exists, selection from interspecificcompetition is relaxed and the species evolves to exploit a larger niche

This process may lead to characterdisplacement• Character displacement oftenimplies that the 2 species differmore sympatrically thanallopatrically

Contrasting the BSC and the ESC

• According to the BSC, species form discreteunits because of gene flow

• But, the ESC emphasizes selection - selectionfavors certain forms and removes forms that areintermediate between species

The Phylogenetic Species Concept• Species are defined

as the smallestdiagnosablemonophyletic group

• Any population thatforms an independentbranch on thephylogeny isrecognized as aspecies

• To be recognized as a species, populations must havebeen evolutionarily independent for a long enough timefor diagnostic traits to emerge

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 ofdifferent phyla!

• Sexual isolation is not an important criterion for identifying species ofbacteria

• The primary consequence of gene flow between bacteria is the spread ofspecific sequences with high fitness advantages

• Consequently, it has been proposed that the key to recognizing bacterialspecies is finding strains that share a large suite of selectively neutralsequences