Speciation

Process by which species originate

Origin of diversity we observe in nature

Outline

• Species concepts

• Species numbers through time

• What drives speciation? Two examples

• Discussion

Species Concepts

• Typological

• Biological

• Evolutionary

• Genealogical

• Ecological

• Cohesive

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?

Typological 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?

Biological 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?

Evolutionary 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?

Genealogical 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?

Ecological Species Concept

“The most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic mechanisms (genetic and/or demographic exchangeability)” Templeton 1989.

Cohesion Species Concept

Increase in number of marine genera over time (Sepkoski 1997)

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Top: M. cardinalis Bottom: M. lewisii

Mimulus cardinalis and Mimulus lewisii: A case study of prezygotic isolation leading to speciation.

Top: M. cardinalis Bottom: M. lewisii

M. cardinalis-hummingbird pollinated, high in anthocyanins

Mimulus cardinalis and Mimulus lewisii: A case study of prezygotic isolation leading to speciation.

M. lewisii-bee polinated, low in anthocyanin and carotenoid pigments

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

•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

Results from Mimulus Studies

Effects of YUP allele substitution

• a, b, M. lewisii; c, d, M. cardinalis.The wild-type 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.

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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Bradshaw and Schemske Nature 2003

Feeding behavior driving morphological differences

• Differential expression of the gene Bmp4 accounts for big morphological differences

• Micro- vs Macroevolution different mechanisms or different scales?

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Top: Metriaclima zebraBottom: Labeotropheus fuelleborni

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)?