Zoology 304

The Concept of Species.

The following notes (and the quoted headings) are adapted from Chapter 15 of Mark Ridley's textbook, EVOLUTION, 2nd ed. (1996), Blackwell Science, Inc., Cambridge MA. ISBN 0-86542-495-0.

Brief Outline

• Introduction to species concepts.
• Recognition and phenetic description.
• Phenetic species.
• Biological species.
• Isolating mechanisms.
• Recognition species concept.
• Ecological species concept.
• Selection and gene flow.
• A pluralist species concept.
• Cladistic species concept.
• Nominalism vs. realism in taxonomy.
  • The species category.
  • Supra-specific category.
• Conclusion.

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Introduction to species concepts.

• The concept of species provides background for discussion of speciation.

• Like adaptation, speciation is a central concept in evolution.  Without speciation there would be no diversification, no adaptive radiation, and no phylogeny.

• However, the concept of species has proven to be remarkably difficult to define.  Because our definition of species is closely associated with how we think about species and what questions we ask about the process of speciation, this definition assumes critical importance in evolutionary biology.

• This page introduces basic ideas and difficulties associated with defining and recognizing species.  These ideas also serve to suggest hypotheses concerning speciation.

• The issues are not merely scientific and philosophical ones.  There is also a significant political dimension, associated with the economic consequences of protecting species under laws such as the Endangered Species Act.  Whether a group of organisms must receive protection depends on the legal definition for species.

• Species concepts can be grouped into broad categories.  Several of these are "non-temporal", defining species as they appear at any given moment in time, without regard for how they change over time.  Others are "temporal", defining species by their coherence through time in relation to phylogeny.
   
• Non-temporal species concepts
  • Typological species are defined by similarity to a type specimen or ideal type.  Although this a traditional approach in classical systematics, it lacks any biological justification.  (The world of evolutionary biology is not Platonic; there is no "ideal type".)
  • Phenetic species are defined by clustering of physical characters.
  • Reproductive species are defined by reproductive isolation, including:
    • the biological species concept.
    • the recognition species concept.
  • Ecological species are defined by niche.
  • Genetic species are defined by integrated genetic organization.
    Little is known at present about species-level genomic architecture and patterns of mutation.  It is possible to imagine that such molecular features might turn out to have great significance.  But our current state of ignorance offers little insight.
     
• Temporal species concepts
  • Cladistic species are defined by a segment of a phylogenetic tree.
  • Evolutionary species are defined by vague "tendencies".
    George Gaylord Simpson's evolutionary species concept attempts a plausible synthesis of several species concepts.  A species is "a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies."  Although this sounds good, it begs the question.  "Evolutionary role and tendencies" could be interpreted according to any of the other phenetic, reproductive, ecological, genetic or cladistic concepts, so this definition really means whatever one chooses for it to mean.

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"In practice, species are recognized and defined by phenetic characters."

• One must distinguish between recognizing or characterizing a particular species according to phenetic characters (which is commonplace in practical taxonomy) and defining a species by phenetic characters (which is harder to justify).
• The distinction is between features which are essential, in principle, and those which are useful, in practice.  It is a distinction between an explanation of what a species ought to be and a description of what a species happens to be.

"The phenetic species concept."

• Phenetic species are defined as as populations distinguished by discrete clusters of phenotypic characters.
• The phenetic species concept resembles the practical criteria commonly used to describe species.  A population can be defined as a species by a group of phenetic characters sufficient to distinguish it from other populations.
• The phenetic species concept has no theoretical basis.  Its source is our common intuitive sense that species are, in fact, discernable in nature prior to any theoretical justification.
• Note the relation of the phenetic species concept to the typological species concept, in which a species is defined by resemblance of its members to some ideal type or to a specific type specimen.
• Note also that a phenetic (or typological) species will require a new or modified definition if it undergoes significant evolutionary change without speciation.  This is also true for ecological species but not for reproductive or cladistic species.

"The biological species concept."

• Ernst Mayr's Biological Species Concept, the most widely accepted definition for species, defines species as "groups of interbreeding natural populations that are reproductively isolated from other such groups".
• Most discussion of speciation has been based on this species concept.  Most hypotheses about speciation concern mechanisms which establish reproductive isolation.
• In the Biological Species Concept, species are unified by gene flow within a common gene pool; gene flow involves migration and interbreeding.
• The following section explains some mechanisms which underlie reproductive isolation.

"Interbreeding between species is prevented by isolating mechanisms."

• Isolating mechanisms are features of behavior or morphology or genetics which serve to prevent interbreeding and thereby establish reproductive isolation of populations.
• Isolating mechanisms can be categorized as either premating (prezygotic) or postmating (postzygotic).
  • Premating isolating mechanisms prevent potential mates from two separate populations from getting together.
    • The most basic of premating isolating mechanisms involve one or another variation on allopatry (geographic separation, habitat separation, seasonal or temporal separation).
    • Biologically-based premating mechanisms involve behavior, biomechanics, or gamete interaction to prevent gametes from coming together.
  • Postmating isolating mechanisms involve some form of genetic incompatibility, such as reduced viability or reduced fertility of hybrid individuals.
    • "Haldane's rule" notes that in species with genetic sex determination, reduced viability or fertility is much more common in the heterogametic sex (e.g., the sex that produces different gametes, the XY sex; in mammals the heterogametic sex is male, in birds it is females).
    • Postmating isolating mechanisms comprise a selection pressure that favors premating mechanisms, to minimize the production of less-fit offspring.
• Genetic isolation is not necessarily an absolute either/or trait.  It can also be a continuous variable.  Any isolating mechanism may be "leaky", letting some exceptions occur.
• An isolating mechanism evolve specifically because reproductive isolation is advantageous (see speciation) or may emerge incidentally.

"The recognition species concept."

• The recognition species concept places emphasis not on reproductive isolation per se but on mechanisms for recognizing suitable mates.
• In this concept, a species is defined by mutual recognition of suitable mates among the members of a population.
• "Specific mate recognition system" is the term given to a method of recognizing mates that is shared by all the members of a population.
• In this concept, mate recognition is causal for the origin of species, while reproductive isolation is a secondary consequence of mate recognition.
• The "recognition species concept" is thus associated with a more specific hypothesis concerning speciation than is the more general "biological species concept".

"The ecological species concept."

• The ecological species concept places on emphasis on niche or adaptive zone rather than reproductive isolation.
• This concept draws support from two phenomena.
  • In the phenomenon called competitive exclusion, two or more species cannot occupy the same niche.  Either one will be better and beat out the other in competition, or (in the unlikely event that they are exactly equal) chance drift in relative population size will eventually eliminate one or the other.
  • In the phenomenon called character displacement, similar species which share a common resource will diverge from one another.  This effectively divides the shared niche and reduces competition.
• The ecological species concept is associated with the hypothesis that selection for efficient niche utilization is the primary causal event in speciation.  Reproductive isolation and specific mate recognitions systems are secondarily selected to increase adaptive efficiency.

"Selection and gene flow can both explain the integrity of species."

• Gene flow can keep a species genetically uniform, by mixing alleles that might be selected in different parts of the species range.  :However:   "Selection can produce divergence despite gene flow."  A population can diverge across its range, if selection is stronger than gene flow.
• The opposite is also true.  "Selection can produce uniformity in the absence of gene flow."  Selection to conditions that prevail across the entire species range can maintain uniformity in spite of limited interchange of alleles across the range.

"A pluralistic species concept."

• Whether or not a pluralistic species concept is the best choice is related to the question of how many different ways there are for speciation to occur.
• If species can form by adaptive splitting within a population, and by specific mate recognition systems, and by incidental reproductive isolation (i.e., allopatry), and by reorganization of genomic architecture, then a pluralistic concept might be appropriate.
• But a pluralistic concept raises the question, is there anything essentially the same in the various accounts?  If so, shouldn't that "essence" define species?  And, if not, is the concept of species meaningful?

"The cladistic concept defines species throughout their evolutionary history."

• Cladistic species are segments on a cladogram, joining branch-points on the phylogenetic tree.
• In many situations, this is not only intuitively sensible but coincides with other species concepts.
• This concept begs the several questions addressed by the nontemporal concepts, regarding how such segments are to be recognized and why (or whether) they even exist) in the first place.
• Species formed by hybridization (common among higher plants) do not fit easily into the cladistic species concept, although they present no special difficulty for other concepts.
• The cladistic species concept also introduces awkward nomenclature involving pseudoextinction.  When a new population splits off and diverges from its source, the original population becomes a new species and must be given a new name even if it remains unchanged.

"Taxonomic concepts may be nominalist or realist."

• The relevance of this distinction is the question of whether species as well as other taxonomic categories recognize real distinctions in nature or simply impose arbitrarydistinction for human convenience.
• "The species category."
  • Many (probably most) biologists share a belief that species and at least some higher taxa do indeed reflect real, natural distinctions corresponding to twigs or branches on the phylogenetic tree.
  • But this belief has been remarkably difficult to pin down and justify with rigor.
  • As noted above, the apparent need for a pluralistic species concept raises the question of whether there is indeed anything "real" about species.  If different examples of species require different causes and different definitions, then maybe our intuition that "a species is a species is a species" is misleading.  It may be just an arbitrary convenience after all, and we should be separately distinguishing "phenetic species" and "reproductive isolation species" and "mate recognition species".and "ecological species" and "cladistic species" without any implication that they are all, somehow, the same thing.
• "Categories above the species level."
  • All of the questions which beset species can be asked of higher ("supraspecific") categories, but without quite so sharp a point.

   

Conclusion.  To understand the concept of species we shall need to understand how the intuitive clustering of population characteristics originates.  In other words, we will need to study speciation.

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