Speciation
Process by which species originate
Origin of diversity we observe in nature
Outline
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Species concepts
Species numbers through time
What drives speciation? Two examples
Discussion
Species Concepts
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Typological
Biological
Evolutionary
Genealogical
Ecological
Cohesive
Typological Species Concept
A species is a set of organisms that resemble one another
and is distinct from other sets (Linnaeus) -Type
• Do large differences in phenotypes always reflect large
differences in relatedness among organisms?
• How well are we able to discern small but significant
differences?
Biological Species Concept
Species are groups of actually or potentially interbreeding
natural populations that are reproductively isolated
from each other (Mayr 1942; Dobzhansky 1935).
•How do we evaluate “potentially interbreeding” for
populations that are geographically separated?
•How much reproductive isolation is needed?
•How does this apply to asexual organisms?
Evolutionary Species Concept
A species is a single lineage of organisms that maintains
its identity from other such lineages and has its own
evolutionary tendencies and historical fate (Wiley 1978)
•How much identity is needed?
•How can the historical fate of a population be determined?
•What criteria is used determine the “evolutionary tendency
of a population?
Genealogical Species Concept
A species is the smallest monophyletic group of
common ancestry (de Queiroz & Donoghue 1990).
•Will a currently monophyletic “group” remain
monophyletic?
•Genetic polymorphisms can be shared for a long period
of time, does this alone prevent speciation?
Ecological Species Concept
A species is a lineage that occupies a niche minimally
different from that of any other lineage in its range
and which evolves separately from all lineages
outside its range (Van Valen 1976).
•Might many different genotypes converge upon the
phenotype allowing survival in this range?
•How can it be known if a lineage will evolve separately
in the future?
Cohesion Species Concept
“The most inclusive population of individuals having
the potential for phenotypic cohesion through intrinsic
mechanisms (genetic and/or demographic
exchangeability)” Templeton 1989.
Increase in number of marine genera
over time (Sepkoski 1997)
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Mimulus cardinalis and Mimulus lewisii: A case study of
prezygotic isolation leading to speciation.
Top: M. cardinalis
Bottom: M. lewisii
Mimulus cardinalis and Mimulus lewisii: A case study of
prezygotic isolation leading to speciation.
M. cardinalis-hummingbird
pollinated, high in anthocyanins
M. lewisii-bee polinated, low in
anthocyanin and carotenoid pigments
Top: M. cardinalis
Bottom: M. lewisii
Mimulus relationships
M. lewisii (A), an F1 hybrid (B),
M. cardinalis (C), and examples
of variation in floral traits found
in F2 hybrids (D–L).
Schemske and Bradshaw PNAS October 12, 1999 vol. 96 no. 21 p11911
Results from Mimulus Studies
•Pollinators of the two species have strict
fidelity
• 99.915% occurrence of parental M. lewisii and
M. cardinalis in seeds collected from a
sympatric population in the Sierra Nevada
• Floral traits associated with pollination are
under relatively simple control
• Adaptive shift in pollinator preference may be
initiated by a single major mutation in the YUP
allele
Effects of YUP allele substitution
M. lewisii NIL’s with the M. cardinalis
yup allele had yellow-orange flowers
and received 68 times more
hummingbird visits than wild type
M. cardinalis NIL’s with the M. lewisii
yup allele had dark pink flowers and
received 74 times more bee visits than
wild type
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a, b, M. lewisii; c, d, M. cardinalis.The wildtype allele at the YUP locus (a, c) has
been substituted by introgression with the
allele from the other species (b, d).
Flowers in each NIL pair (a and b, c and
d) are full siblings.
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Bradshaw and Schemske Nature 2003
Feeding behavior driving
morphological differences
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• Differential expression of the
gene Bmp4 accounts for big
morphological differences
• Micro- vs Macroevolution
different mechanisms or
different scales?
Top: Metriaclima zebra
Bottom: Labeotropheus fuelleborni
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r os se r pmo ce d ) de ss e rpm oc nU ( FFIT
. e rut ci p si ht ee s o t d ed ee n e ra
Quagga: the missing link
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Quagga (Equus quagga quagga) in Londoner Zoo, 1870
Progression?
Questions
•What defines a species?
•Are there clear examples of Macroevolution?
•Is the species concept equivocal across all taxa?
(are evolutionary processes similar across taxa?)
•Why is it that the issue of formation of new species is
so contentious (and so important to demonstrate and elucidate)?