Study of fish diversity and the evolutionary relationships among populations, species and higher taxa
Chapter 2
(Helfman, Collette & Facey)

Systematics
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Develop an understanding of patterns of diversity in the context of evolutionary and ecological theory.
– trends in spatial distribution of species
– trends in emergence/extinction of evolutionary groups

Systematics
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Sample questions:
• What has favored/allowed greater diversity of fishes on coral reefs than in lakes?
• What has allowed/favored cypriniforms, siluriforms and characiforms to become so diverse?
• What factors have allowed/favored the persistence of ancient taxa in the Mississippi River basin
(bowfin, gar, paddlefish, etc.)?
• What is the evolutionary (phylogenetic) relationship between salmon and pike?

Subdisciplines in Systematics
• Taxonomy - the theory and practice of describing, identifying and classifying taxa (groups of phylogenetically related organisms)
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Nomenclature - the naming of taxonomic groups
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Classification - organizing taxa into like groupings

Focus of Systematics on Species
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Historically, much of emphasis has been at the species level of classification:
– group of organisms that can reproduce and generate viable offspring
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Today, emphasis is below species level (why?)
– Endangered Species Act:
• applies to distinct population segment of a species which interbreeds when mature

Species Concepts
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Morphological (Linnaeus): the smallest group of individuals that are distinct and distinguishable from all others
– can misclassify based on differences that can be maintained within an interbreeding group
– depends solely on observable morphological differences

Species Concepts
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Biological (Mayr): group of populations of individuals that are similar in form and function and that are reproductively isolated from other populations
– conventional definition until late 1980’s
– includes genetic information
– ignores hybridization
– dependent on geographic isolation to achieve species status

Species Concepts
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Evolutionary : a population or group of populations that shares a common evolutionary fate and historical tendencies
– recognizes more than just genetic and morphological differences
– difficult to determine “evolutionary fate”
– how much diversity is allowed within a common evolutionary fate?

Species Concepts
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Phylogenetic : the smallest biological unit appropriate for phylogenetic analysis
(process that rates traits as ancestral or derived and then looks for groupings based on similarities)
– does not infer modes of speciation
– nothing is arbitrary
– depends on thorough phylogenetic analysis first

Species Concepts
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Usefulness of each concept depends on the use - for Endangered Species Act, use as much evidence as possible:
– morphological, physiological, behavioral
– geographic
– life history & development
– habitat & feeding ecology
– phylogenetics
– evolutionary fate

Determining Relationships
Between Taxa
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Traditional : grade primitive to advanced, link groups based on a few arbitrary traits, generate lineage model based on these limited data

Determining Relationships
Between Taxa
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Phenetics : multivariate statistical approach:
– assemble list of traits
– determine degree of similarity among groups based on number of similar traits
– ignores evolutionary linkage of groups
(convergence could put evolutionarily distinct lines into a single taxon)

Determining Relationships
Between Taxa
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Phylogenetic (cladistic):
– assemble a list of traits
– classify each taxonomic group on basis of presence or absence of each trait
– determine degree of similarity among groups based on shared and unique traits:

Determining Relationships
Between Taxa
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Phylogenetic (cladistic), continued :
– determine degree of similarity among groups based on shared and unique traits:
• shared traits = plesiomorphic traits (ancestral)
• unique traits = apomorphic traits (derived)
• shared unique traits = synapomorphic traits
– monophyletic group of taxa (common origin) = clade

Cladograms
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Phylogenetic relationships expressed in cladograms - branching representation of the evolutionary relationships among taxa based on shared common traits and shared unique traits

Constructing a Cladogram
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Listing of traits
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Coding of each taxon by presence or absence of each trait
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Assemble groupings based on trait conditions
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Use the simplest branching structure possible: principle of parsimony

Speciation
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How do populations become distinct species? - the process whereby gene flow is reduced sufficiently between sister populations to allow each to become different evolutionary lineages
– Allopatric (with geographic isolation)
– Non-allopatric (without geographic isolation)

Speciation
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Allopatric (with geographic isolation) speciation:
– Vicariant - large populations geographically isolated (little inbreeding) (United States)
– Founder - small population becomes geographically isolated and then reproductively isolated via inbreeding, selection, drift
(Gilligan’s Island)
– Reinforcement - early isolation followed by sympatry , but selection against hybrids

Speciation
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Non-allopatric (without geographic isolation)
– Sympatric - sister species evolve within the dispersal range of each other, but adapt to different habitats - habitat-dependent assortative mating
– Parapatric - sister species evolve in segregated habitats across a narrow contact zone - little mixing in spite of proximity

Final synthesis on “species”
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Groupings that are different from each other:
– morphology, behavior, physiology, ecology
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Reproduction is isolated in practice
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Mating systems and mate-recognition systems are important enforcers of isolation