Transcript Molecular Systematics

Taxonomic Units
• What is it that we are trying to do?
• Determine relationships among…?
• What do the terminal nodes
represent?
• Often, they are supposed to represent
some taxonomic unit, typically some
group of species.
• “What is a species?”
Taxonomic Units
• Are species really ‘real’?
• Several competing species concepts. All differ in
their assumptions and details but they generally
agree on the following three main points:
1. Species consist of groups of interbreeding
individuals that share a common
ancestry.
2. Species are a fundamental unit of evolution.
3. Species are evolutionarily independent
units. Different species follow different
evolutionary trajectories through time.
Taxonomic Units
• A sampling of species concepts
• 1. Typological - A group of organisms in which
individuals are members of the species if they sufficiently
conform to certain fixed properties.
• ‘type specimens’ and ‘archetypes’
• Clusters of variations or phenotypes within specimens (i.e.
longer and shorter tails) would differentiate =species.
• “Classical" method of determining species, such as with
Linnaeus.
• We now know that different phenotypes do not always
constitute different species (e.g.: a 4-winged Drosophila born
to a 2-winged mother is not a different species).
• Who defines what level of variation is acceptable?
• Doesn’t help define the mechanisms of speciation
(evolution).
Taxonomic Units
• A sampling of species concepts
• 2. Morphological - A population or group of
populations that differs morphologically from other
populations.
• Useful when dealing with fossils
• Morphological criteria are often arbitrary
• Relies on ‘expert opinion’
• What about cryptic species?
• What about sexual dimorphism?
• What about environmentally induced variation?
• Speciation mechanism is lacking.
Taxonomic Units
• A sampling of species concepts
• 3. Biological - “species are groups of actually or potentially
interbreeding natural populations that are reproductively
isolated from other such groups” (Mayr 1963)
• Implies several possible isolation mechanisms and
evolutionary processes.
• That “actually or potentially interbreeding” part bothers some
people.
• How do we test this definition? Artificial conditions?
• Useless for fossils.
• Doesn’t apply to asexual species.
• Difficulties with hybridizing species
• Very popular and useful among vertebrate biologists (tiny
percentage of all life)
Taxonomic Units
• A sampling of species
concepts
• 4. Mate recognition- Similar to
BSC but focuses on prezygotic isolating
mechanisms.
• Often seen as the mirror image of BSC
• Focuses on the negative (isolating) vs.
positive (interbreeding)
• Doesn’t apply to asexual species.
Taxonomic Units
• A sampling of species concepts
• 5. Ecological - A species is a group of organisms that
occupy the same ecological niche
• Niche: The ecological niche of an organism depends not
only on where it lives but also on what it does. By analogy,
it may be said that the habitat is the organism's "address",
and the niche is its "profession", biologically speaking.
(Odum, 1959)
• BUT difficult to recognize, because many organisms
occupy different niches
due to adaptation or developmental changes
• Beierinck – “everything is everywhere…” Does the
ecological species concept provide much resolving power
for microbes?
• A sampling of species concepts
• 6. Evolutionary - “a single lineage of ancestor-descendent populations
which maintains its identity from other such lineages and which has its own
evolutionary tendencies and historical fate”
• Focuses on evolutionary history
• Does not account for genomic hybrids, where genes have passed from one
taxon to another, and the genetic make-up of individuals can be traced to
different phylogenies or genealogies
• Completely sequenced microbial genomes have demonstrated that such
hybrids are common: 5-15% of one bacterial species’ genomes can be
attributed to acquisition from other species, making the evolutionary species
concept practically irrelevant to prokaryotes
• 7. Phylogenetic - A group of organisms that shares an ancestor; a lineage
that maintains its integrity with respect to other lineages through both time
and space.
• A spinoff of the ESC
aka the diagnostic species concept - an irreducible (basal) cluster of
organisms, diagnosably distinct from other such clusters, and within which
there is a parental pattern of ancestry and descent (Cracraft, 1989).
Taxonomic Units
• Many, many others
• Taxospecies
• Genospecies
• Natural
• Phylophenetic
• Genetic
• Cohesion
• Etc…
Taxonomic Units
• Others argue that species are not the important question.
• Evolution doesn’t work on species, it works on
populations
• Evolutionarily significant units (ESU) – a population of
organisms that is considered distinct for purposes of
conservation. Sometimes called Operational Taxonomic
Units (OTU)
• Can apply to any species, subspecies, geographic race, or
population.
• Definitions of an ESU typically include at least one of the
following criteria:
• Current geographic separation
• Genetic differentiation at neutral markers among ESUs
caused by past restriction of gene flow
• Locally adapted phenotypic traits caused by
differences in selection.