Ch. 3—Key concepts
• A biological species is defined as a group of
potentially interbreeding populations that are
reproductively isolated from other such groups
under natural conditions.
• It is impossible for paleontologists to utilize the
biological species concept, because of the inability
to observe reproductive behavior in fossils.
• In practice, paleontologists recognize fossil
species as discrete groups of individuals that are
separated from other such groups by morphologic
gaps.
Fossils & Evolution—Chapter 3
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Ch. 3—Key terms
• Species
– Biological species concept
– Morphological species concept
•
•
•
•
•
Sympatric / Allopatric populations
Allopatric speciation
Cline / chronocline / chronospecies
Anagenesis (phyletic gradualism)
Punctuated equilibrium
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Biological species concept
• A species is a group of potentially
interbreeding populations that are
reproductively isolated from other such
groups
– Most species are separated geographically into
local breeding populations
– Populations are reproductively isolated only if
interbreeding would not occur if they lived in the
same area
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Biological species
a-d all discrete
• Most biologic species
are distinct because
they belong to evolving
lineages that have been
reproductively isolated
for a long time
• Biologists encounter
difficulty only when a
lineage is branching at
the present time!
Fossils & Evolution—Chapter 3
morphology
4
Biologic methods of species
discrimination
• How do biologists establish that two
populations represent two species?
– Reproductive isolation is difficult to document
• Experience shows that distinct populations cannot live
sympatrically unless they belong to different species
• Therefore, if two distinct populations overlap in their
geographic range, they probably represent two species
– Mainly, biologists rely on morphologic differences
(occasionally with the benefit of biogeographic info)
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Sympatric geographic
ranges: partial overlap
Allopatric geographic
ranges: no overlap
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The “paleontologic species
problem”
• It is not possible to apply
the biologic species
definition to fossils
• In practice, a
paleontologist includes in
a fossil species those
specimens that he/she
believes would have
formed a biologic species
had they lived together at
the same time
Fossils & Evolution—Chapter 3
Are a and f discrete?
morphology
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Origin of species
• Speciation vs. anagenesis
– Speciation = the splitting of a lineage resulting
in an increase in the number of species
– Anagenesis = gradual evolution within a
lineage whereby one species changes into
another (without an increase in the number of
species)
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Chronocline
(produced by anagenesis)
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Speciation vs. anagenesis
Time 2
Species B
Species B
Arbitrary boundary
“pseudoextinction”
Time 1
speciation
event
Species A
Species A
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Natural selection and anagenesis
• All populations are variable
– Variation in the gene pool is expressed outwardly as
variation in the phenotypes of individuals
– Sources of variation are
• Point mutations
• Genetic shuffling from sexual reproduction
– As time passes, natural selection operates on
phenotypic variation
• Certain kinds of individuals will have greater reproductive
success; these kinds will become more common in the
population after many generations
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Natural selection and anagenesis
• Natural selection is a sorting of individuals
– So that a population becomes progressively better
adapted to a constant environment, or
– So that a population adapts to changing environmental
conditions
• In a chronologic series of populations, there is a point at
which the accumulated differences between a population
and the starting population are so great that the two would
be reproductively isolated if they had been living together
at the same time—at this point, a new species has been
formed by anagenesis
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Allopatric speciation
• If two or more breeding populations of a given
species are geographically isolated from one
another, they may undergo anagenesis
independently until reproductive isolation occurs
– One species becomes two (geographic speciation has
occurred)
– In nature, speciation occurs rapidly in small peripheral
populations that have become isolated from the larger
parent population (i.e., allopatric speciation)
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Allopatric speciation
Time 4: secondary sympatry,
but with reproductive isolation
Time 3: geographic & reproductive isolation
Time 2: migration & geographic variation
Time 1: homogeneous population
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Clines and ring species
• Gene flow between adjacent populations of a
species may be restricted by partial geographic
barriers
• If so, then subtle genetic differences among
populations may develop (genetic gradient)
– Cline = series of populations whose gene pools differ
slightly along a genetic gradient
• Populations at the extreme edges of a species’
range may be reproductively isolated even
though they are connected by a chain of
interbreeding populations
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Cline
Fossils & Evolution—Chapter 3
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Ring
species
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Rates of evolution
• Most biologists believe that well-established
species evolve slowly, because:
– Genotypes are so complex and highly integrated that
random changes are unlikely to be advantageous
– Changes may not “take” in a large, thoroughly
interbreeding population
• Because large populations resist change, most
evolution probably is concentrated in relatively
rapid speciation events in peripherally isolated
populations
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Punctuated equilibrium
• Eldridge and Gould 1972
• Once a species is established, it undergoes
no significant, directional change: gene pool
reaches a state of equilibrium (stasis)
• New species arise suddenly through
allopatric speciation, then themselves
become static entities
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Punctuated equilibrium
• “Paleontologic species
problem” is alleviated
somewhat if anagenesis
is rare and most
evolutionary change is
associated with
allopatric speciation
events
• In reality, both
anagenesis and
speciation occur
S.E. = speciation event
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Paleontologic species problem
• Conclusion: There is no way to overcome
the PSP
• In practice, fossil species are recognized by
the morphologic gaps between one another
• Where there is no clearcut gap, there is
uncertainty!
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Morphologic clusters
9
8
width
7
6
5
4
3
3
4
5
6
7
8
9
10
11
12
13
14
15
length
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