Zoology 510, Class Notes for Ridley, Chapter 14
Evolution and Classification.

No written assignment, but you should try to answer the Study and Review Questions at the end of the chapter (pp. 396-397).

Brief Outline

• Introduction to Chapter 14. Evolution and Classification.
• Checklist of important concepts and terms
• Section-by-section comments
  • 14.1. A hierarchy of nested groups.
  • 14.2. Principles of classification.
  • 14.3. Schools of classification.
  • 14.4. A method for judging merit.
  • 14.5. Distance measures and cluster statistics.
  • 14.6. Phylogenetic relations.
    • 14.6.1. Hennig's cladism.
    • 14.6.2. Monophyletic, paraphyletic, and polyphyletic groups.
    • 14.6.3. Multitude of levels.
  • 14.7. Evolutionary clssification.
  • Box 14.1. Adaptive Classification.
  • 14.8. Principle of divergence.
  • 14.9. Strengths and weakness summarized.

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Introduction to Chapter 14.

• This chapter introduces the basic principles of classification, the major schools (i.e., competing camps) of classification, and two guidelines for weighing the merits of each.  Conflict arises between the need for an immediately practical method for classifying organisms and the need for evolutionary principles to justify the resulting classification.

• The two basic principles for classifying organisms are
  • phenetic similarity, and
  • phylogenetic relationships.

• The three major schools of classification are
  • Phenetics, or Numerical Taxonomy, associated with Sokal and Sneath;
  • Cladistics, founded by Hennig, and
  • Evolutionary classification, advocated by Mayr, Simpson, and Dobzhansky.
• The guidelines for weighing merit are:
  • Objectivity,
  • Naturalness, and
  • Practical utililty.

• The three schools measure up, roughly, in the following way:
  • Phenetics is practical (at least in principle) but not objective.
  • Cladistics is objective and natural but (often) not practical.
  • Evolutionary classification is a somewhat muddled combination, traditionally useful but not rigorous by any strict criterion.

CHECK LIST of important TERMS

• Hierarchical classification / nested sets / Linnaean hierarchy
• Phenetic
• Phylogenetic
• Phenetics / phenetic classification / numerical taxonomy
• Cladistics / phylogenetic systematics
• Evolutionary taxonomy
• Objective vs. subjective methods
• Natural vs. artificial methods
• Distance measures and cluster statistics
• Sister species
• Analogy, convergence, parallelism (homoplasy)
• Homology
  • Derived (apomorphic) character
  • Ancestral (plesiomorphic) character
  • Shared derived homology (synapomorphy)
  • Shared ancestral homology (symplesiomorphy)
• Monophyletic group / "clade"
• Paraphyletic group / polyphyletic group
• Adaptive breakthrough, "grade" (Box 14.1, p. 391)

Chapter 14, Section-by-Section Comments

14.1. "Biologists classify species into a hierarchy of groups."

• The hierarchy of biological groups was obvious long before Darwin, and formed the basis for Linnaean classification.
• Darwin offered an explanation for why this pattern exists.
• How particular groupings should be recognized and named is the problem of classification.

14.2. "There are phenetic and phylogenetic principles of classification."

• The Greek root for phenetic (and for "phenotype") means "to show" and refers to visible bodily characteristics.
• The word phylogenetic comes from Greek "phylo", referring to tribe or race, and "genesis", or origin.  Phylogenesis is thus the origin of groups (such as species, genera, families, orders, etc.)
• In other words, organisms can be classified according to bodily (phenetic) similarity or to ancestral (phylogenetic) relationships.
• Sometimes both phenetic similarity and phylogenetic relationships suggest coinciding groups.  But such a pleasant outcome is not guaranteed by any principle of evolution.

14.3. "There are phenetic, cladistic, and evolutionary schools of classification."

• These schools will each be discussed in turn.  Notice how each one utilizes different principles and offers different advantages.

14.4. "A method is needed to judge the merit of a school of classification."

• The suggested criteria are:
  • Objective is preferable to subjective.
  • Natural is preferable to artificial.
  • Theoretical justification is also meritorious.
  • So is practicality.

• "Objective" grouping means that, for all species, the criteria for grouping depend only upon features intrinsic to the species being studied.  "Subjective" grouping implies that incidental events or choices by the researcher may determine the groups.
• "Natural" grouping means that species in each group share characteristics beyond those which were initially used to define the groups.  In other words, the groups exist in nature.  "Artificial" groups are arbitrary sets which happen to share some defining feature.  For example, organisms grouped solely by size or by color (objective but artificial criteria) would not be expected to share other common characteristics besides size or color.
• Any classification is unlikely to be either objective or natural if it depends on one or a small set of arbitrarily chosen characteristics.

14.5. "Phenetic classification uses distance measures and cluster statistics."

• Phenetic classification does not depend on any knowledge or hypothesis of phylogeny.
• Phenetic classification presumes that the organisms, when adequately measured, will tell us how they should be classified.
• Phenetic classification thus does not concern itself with naturalness.  If all characters are utilized in the classification, the distinction between "natural" and "artificial" groupings is meaningless.
• Phenetic classification attempts to address the objectivity criterion by utilizing (measuring) very many characteristics (so that subjectivity in character choice would be eliminated).  But in practice it remains necessary to choose which and how many characters to measure.  Furthermore, there has been no agreement on which statistical tools should be used to identify groupings.  So choice of statistic is also subjective (and different statistics can yield different groupings).
• Phenetic classification also offers no insight into anything besides the classification itself; it does not represent phylogeny or any other pattern.

14.6. "Phylogenetic classification uses inferred phylogenetic relations."

• 14.6.1 "Hennig's cladism classifies species by their phylogenetic branching relations."
  • The term cladistics is based on the term clade, a synonym for monophyletic group, defined as the entire set of species descended from a common ancestor.
  • The great advantage of phylogenetic systematics / cladistics lies in its rationale.  In cladistics, there is one true classification which is based upon the objective, natural reality of phylogeny.  Unfortunately, since phylogeny itself remains largely unknown, so does the basis for cladistic classification.
  • Phylogenetic systematics must therefore be carried out by inferring phylogeny.  But once we have a cladistic classification, we also have a hypothesis of phylogeny.
  • The principle tool for inferring phylogeny is the recognition of shared-derived characters (synapomorphies).  These are homologies which originated in the most recent common ancestor for a group of species and serve to distinguish all descendents of that ancestor from all other species (see Figure 14.6, p. 382).
  • Shared-derived homologies must be distinguished from convergent characters (analogies) and inconsistent ancestral homologies which arise when characters are altered within a group.  These other bases for similarity (homoplasy) can confuse interpretation.

• 14.6.2 "Cladists distinguish monophyletic, paraphyletic, and polyphyletic groups."
  • A monophyletic group consists of an ancestral species and all of its descendent species.  Only monophyletic groups are allowed in cladistic classification.  Monophyletic groups are sometimes called clades.
    • Operationally, a monophyletic group is defined by a suite of shared-derived characters.
  • A paraphyletic group consists of an ancestral species and some but not all of its descendents.
    • Operationally, a paraphyletic group is defined by a suite of ancestral traits which have been modified or lost in the excluded species.
  • A polyphyletic group consists of species whose common ancestor is not included within the group.
    • Operationally, a polyphyletic group is defined by convergent traits (analogies).
  • For pictorial summary, see Fig. 14.6, p. 382.
  • Consistent application of cladistic principles requires that a species ancestral to a split be named differently from either descendent species.  This is so even if one of the descendent species remains unchanged from the ancestral condition.  Otherwise, the species which has remained unaltered from its ancestral condition would constitute the beginning of a paraphyletic group.

• 14.6.3 "A strictly cladistic classification could theoretically have an impractically large number of levels."
  • The phylogenetic rational for cladistics yields no expectation for tidy levels like genera, families, orders, classes, etc.
  • Every pair of sister taxa comprises a cladistic group.
  • Therefore, a complete cladistic classification would have a named group for every branching event in phylogeny, a result that quickly becomes unwieldy (e.g., a minimum of twenty levels below "class" would be needed to handle a million species of insects, and this would also require a million named taxa at supra-specific levels.)  That's why Hennig proposed a simple numbering scheme rather than named levels.
  • So even a classification founded on phylogeny may assign named levels according to subjective (arbitrary) criteria.

• To compare groupings allowed by cladistics with those allowed by phenetics and evolutionary classification, see Table 14.1, p. 381.
• By utilizing only appropriate (shared-derived) characters, which can be justified on the basis of evolutionary theory, cladistic classification attempts to be objective and natural.  But it is not easy to apply cladistic principles in practice.  Some attempts yield results which are no less subjective than those of phenetics.

14.7. "Evolutionary classification is a synthesis of the phenetic and phylogenetic principles."

• Evolutionary classification attempts to represent major features of evolutionary history.  This includes not only phylogeny but also major phenetic innovations (specifically, "adaptive breakthroughs", see Box 14.1).
• To accomplish this, evolutionary classification recognizes not only many monophyletic groups but also many paraphyletic groups.  The paraphyletic groups are distinguished by exclusion of one or more derived groups which have undergone significant divergences from the shared common ancestor.
• Thus a group can be defined either by shared-derived homologies OR by ancestral homologies which have been lost or modified by an excluded subgroup that shares the same common ancestor.
• Evolutionary classification does not have a consistent relationship either to phylogeny or to phenetic similarity.  In cladistics, knowing how an organism is classified is equivalent to knowing how it is ancestrally related to all other organisms.  In phenetics, knowing how an organism is classified is equivalent to knowing how similar it is to other organisms.  In evolutionary classification, knowing how an organism is classified is insufficient for either purpose.

Box 14.1. "Adaptationist Classification."

• When a suite of derived characters (like legs and lungs for tetrapod vertebrates, wings with feathers for birds, language capacity for humans) leads to occupation of a previously unexploited adaptive zone (such as terrestrial life, flight, or complex cultural adaptation, for the lungs, wings and language examples), the event can be termed an adaptive breakthrough.
• Ernst Mayr has proposed the term grade (as distinct from "clade") for groups distinguished by adaptive breakthroughs
• Grades of adaptive organization can be seen as justifying the paraphyletic groups recognized by evolutionary classification.

14.8. "The principle of divergence explains why phylogeny is hierarchical."

• This section asks the question, why do any of these methods of classification work?  Why do organisms come in nested sets?
• Phylogeny itself is part of the answer.  But if descendents remained too much like their ancestors, all descendent groups would overlap.  And if descendents departed too dramatically from their ancestors, there would be no similarity within groups.
• Gradual divergence offers an explanation.  The reasons for divergence (competition, adaptive radiation, and drift) assure that descendents will come to differ from one another.  The fact that evolutionary change is necessarily gradual assures similarity among descendents of a common ancestor.

14.9. "Conclusion."

• Here Ridley summarizes the strengths and weakness of the three principle schools of classification.  You don't need to choose among them, but you should be aware of how they differ.

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