This document discusses several concepts of what defines a biological species. It describes how species were traditionally defined based on morphology (morphospecies concept) but this posed problems with geographic variation. The biological species concept defines a species as groups that interbreed, but this has limitations for asexual groups. Other concepts discussed include the phenetic concept using overall similarity, ecological concept based on niche exploitation, and phylogenetic concept as the smallest diagnosable group on the evolutionary tree. Later concepts try to address problems with geographic variation and application to asexual groups.

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 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

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 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

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
• 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
 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

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
• 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
 This process may lead to character
displacement
• Character displacement often
implies that the 2 species differ
more sympatrically than
allopatrically

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
• 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

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