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Molecules, Morphology and Species Concepts
Speciation Occurs at Widely Differing Rates
Horseshoe crabs (Limulus), the same as fossils 300 Mya
Darwin’s Finches: 13 species within 100,000 years.
Speciation Rates
Generalists, like the horseshoe crab, tend to remain as
stable species.
Specialists, like the Galapagos finch,
tend to be unstable as species.
Speciation also becomes rapid when, as
occurred with Galapagos finches, new
niches become available.
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Process-oriented taxonomy
Speciation is a universal process.
Speciation is the core of taxonomy
because
speciation processes provide the most
plausible criteria for species definition.
Microevolution
leads to changes
within populations (often
genotypic)
genotypic)
Macroevolution
leads to phenotypic
changes resulting in
distinctive lineages
(e.g. species)
Evolution creates (and destroys) new species, but …
What is a species?
Its not as straightforward a question as most believe.
Eastern Meadowlark
Western Meadowlark
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Determining what is and what isn’t a distinct species
has big consequences
ENDANGERED
Northern spotted owl
Not endangered
Barred Owl
What is a Species?
And these are all members of a single species.
What is a Species?
There is only one extant (existing) human species.
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Diversity of Life
• 1.4 million species described
• ca. 10 million estimated
• many extinct species: >90% of all species that every lived!
Large diversity of species - speciation must be
a common event
Understanding how this diversity arose is a
central question in evolutionary biology
Diversity of Life
• All cultures recognize different forms
(species) and name them
• But what “are” species?
• The challenge to scientists is to name
and classify species in a testable,
replicable way
Taxon units are working
hypotheses
• Systematics depends
on species
• Species may be flawed
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Species A
Genetic Divergence
Two species of Tuna!
Diversity of Life
Species = basic unit
Continuous lineage - information
passed through genes
Speciation - rise of new species
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“A flock of species
concepts”
Species
• Smallest independently evolving unit
• Independent evolution occurs when mutation,
selection, gene flow, and drift operate independently
in different populations
• Therefore, these species will follow independent
evolutionary trajectories
• The essence of speciation is lack of gene flow
• All modern species “concepts” really fall under a
lineage concept
• The different species “concepts” are really criterion
for determining when you have lineage formation
• Defining species is extraordinarily contentious
• Potentially 23 different species concepts
Species concepts
!
ore
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Phenetic (morphological) species
an
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are
concept
e
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Ecological species
e t concept
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cou species concept
Phylogenetic
Of
Biological species concept
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Biological species concept
Species are groups of actually or
potentially interbreeding natural
populations, which are reproductively
isolated from other such groups.
(Mayr, 1942)
Problems with Biological Species
• Fossils - how do we know if
they interbreed?
• “Good” species can
sometimes produce viable
hybrids
• hummingbirds
• Asexual organisms?
• Parthenogenic organisms??
Morphological species
Species defined by measurable
morphological characteristics
(quantitative morphometric evidence,
statistically tested)
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Morphological Species Concept sometimes gets it wrong
New Zealand moa (extinct): females
much bigger than males!
Ecological species concept
A species is a lineage (or closely related
set of lineages) which occupies an
adaptive zone [ecological niche]
minimally different from that of any other
lineages in its range and which evolves
separately from all lineages outside its
range.
(Van Valen, 1976)
Problems with Ecological Species
• Niches are very hard to define in a testable
way
• Most species don’t respect scientists ideas
about niches!! (e.g. invasive species)
• If a species exploits a new niche, it does not
necessarily become a new species, or
become isolated from populations in the
original niche
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Evolutionary species
concept
• Evolutionary species concept: A species is a
single lineage of ancestor-descendant
populations which maintain its identity from
other such lineages and which has it own
evolutionary tendencies and historical fate
(Simpson, 1961; Wiley, 1981).
• Difficult to apply, so we use the following:
Phylogenetic species
concept
• A species is the smallest diagnosable
cluster of individual organisms within
which there is a parental pattern of
ancestry and descent (Cracraft 1983).
Phylogenetic Species Concept
Species =
lineage between
successive
speciation
events
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Phylogenetic Species Concept
Two-step process to define species
• 1. Grouping taxa (regardless of rank) using
the criterion of monophyly, or at least
potential monophyly… I.e. build a tree
• 2. Ranking groups by the criterion of
smallest diagnosable monophyletic groups
Problems:
Almost any
population could be
diagnosed
Not all monophyletic
groups are species!
But wait, there’s more…
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Recognition Species
Concept
• Two individuals that recognise each other as
potential mates are the same species!
• The “Leave them
to themselves”
approach to biology
To each its own?
Each of the species concepts has certain
advantages and disadvantages which
makes it the favourite of one or the other
discipline of biology.
Even more important some concepts
have features which prevent their use in
some disciplines.
What they can’t do…
Biological SC: asexual, fossils
Ecological SC: much (esp. botany)
Morphological SC: things without
whole populations
Recognition SC: farmers
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Speciation
The species “concept” problem arises,
because the species is not the central
issue
The central issue of interest is speciation
Speciation
The process of speciation combines micromacroevolution; molecules and morphology
All data is important
Taxon Sampling
•Dependent on preservation and
specimen availability
•Dependent on access to specimen
material
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Conclusions
The pattern (sum, end result) of all
speciation processes identify a species
A fixed definition of a species is only
possible after it ceased to exist (??)
Species are hypotheses about
relatedness and relationships
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