Transcript Fish Systematics

Fish Systematics:
How does this stuff work??
Study of fish diversity and the evolutionary
relationships among populations, species and
higher taxa
Chapter 2
(Helfman, Collette & Facey)
Why Systematics?
•
•
•
•
•
Organization
Basis for identification
Discrimination
Understanding relationships
Common language!
Systematics
• Understand patterns of diversity
– How? ...in the context of evolutionary and
ecological theory.
– trends in where fish groups are found (spatial
distribution)
– trends in emergence/extinction of evolutionary
groups
Systematics
• 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)
– Taxonomy can be predictive!!
• Nomenclature - the naming of taxonomic groups
• Classification - organizing taxa into like groupings
What is a Species?
• C. Tate. Regan (1926) (20th Century)
“A community, or a number of related
communities whose distinctive
morphologyical characters are, in the
opinion of a competent systematist,
sufficiently definite to entitle it, or them to a
specific name.”
Focus of Systematics on Species
• Historically, understanding species* most
common:
*group of organisms that can reproduce and generate
viable offspring
• Today, emphasis is below species level (why?)
– Endangered Species Act:
• applies to distinct population segment of a species which
interbreeds when mature
Species Concepts
• Morphological (Linnaeus): the smallest
group of individuals that look different from
each other.
– can misclassify based on differences that can be
maintained within an interbreeding group
– depends only on observable morphological
differences
Species Concepts
• 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
• Evolutionary (Wiley): 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?
– Nelson 1999 Reviews in Fish Biology and Fisheries
Species Concepts
• Phylogenetic: the smallest biological unit
appropriate for phylogenetic analysis
(process that rates traits as ancestral
(plesiomorphes) or derived (apomorphies)
and then looks for groupings based on
similarities (shared, synapomorphies)
– does not infer modes of speciation
– nothing is arbitrary
– depends on thorough phylogenetic analysis first
Species Concepts
• 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
• Traditional: examine and list primitive to
advanced, link groups based on a few
arbitrary traits, generate lineage model
based on these limited data
Determining Relationships
Between Taxa
• 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
• 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
• Phylogenetic (cladistic), continued:
– determine degree of similarity among groups
based on shared and unique traits:
1. shared traits = plesiomorphic traits (ancestral)
2. unique traits = apomorphic traits (derived)
3. shared unique traits = synapomorphic traits
– monophyletic group of taxa (common origin) =
clade
Cladograms
• 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
•
•
•
•
Listing of traits
Coding of each taxon by presence/absence of each trait
Assemble groupings based on trait conditions
Use the simplest branching structure possible: principle
of parsimony
More on traits...
• Meritic-count it!
• Morphometric-measurable shape
fin length
eye shape
ratios between such measures...
anatomical characteristics
molecular characteristics
head length
Which traits do I use?
Speciation
• 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
• 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
• Non-allopatric (without geographic
isolation)
– Sympatric - sister species evolve within the
dispersal range of each other, but adapt to
different habitats - habitat-dependent assortive
mating (tribes)
– Parapatric - sister species evolve in segregated
habitats across a narrow contact zone - little
mixing in spite of proximity
Final synthesis on “species”
• Groupings that are different from each
other:
– morphology, behavior, physiology, ecology
• Reproduction is isolated in practice
• Mating systems and mate-recognition
systems are important enforcers of isolation