Lecture 10Species Concepts

1) Why are there species?

2) Species concepts: retrospective vs prospective,

categories vs processes

3) PSC vs BSC: examples

4) Evolution of reciprocal monophyly: sorting of

ancestral variation

5) Barcoding: species discovery and species

identification

What are species?

Why are organisms grouped in clusters separated by phenotypic gaps?

Darwin sums it up:

"Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of species being, as we see them, well defined?” - Charles Darwin, The Origin of Species

Why are there species?

Discrete units (species) are an inevitable consequence of:

Why are there species?

Species Concepts A-Z:Agamospecies ConceptBiological Species ConceptCladistic Species ConceptCohesion Species ConceptComposite Species ConceptEcological Species ConceptEvolutionary Significant UnitEvolutionary Species ConceptGenealogical ConcordanceGenetic Species ConceptGenotypic Cluster ConceptHennigian Species Concept

Internodal Species ConceptMorphological Species ConceptNon-dimensional Species ConceptPhenetic Species ConceptPhylogenetic Species Concept IPhylogenetic Species Concept IIPhylogenetic Species Concept IIIPolythetic Species ConceptRecognition Species ConceptReproductive CompetitionSuccessional Species ConceptTaxonomic Species Concept

How do we recognize species?

“It is clear that arguments [about species concepts] will persist for years to come, but equally clear that, like barnacles on a whale, their main effect is to retard slightly the progress of the field.” Coyne (1992)

“Given the recent pace of new proposals, each evolutionary biologist may soon have his or her own definition of species.” Schemske (2000)

“It is as if on one hand we know just what ‘species’ means, and on the other hand, we have no idea what it means.” Hey (2001)

Why do we spend so much time on this?

Why do we recognize species? (Hey 2001 TREE)

The Typological Species Concept(TSC, Linnaeus)

Group of inds differ from other groups by constant diagnostic characters “Type specimen”

How do we recognize species?

Problems with the TSC: 

How do we recognize species?

Species Concepts A-Z:Agamospecies ConceptBiological Species ConceptCladistic Species ConceptCohesion Species ConceptComposite Species ConceptEcological Species ConceptEvolutionary Significant UnitEvolutionary Species ConceptGenealogical ConcordanceGenetic Species ConceptGenotypic Cluster ConceptHennigian Species Concept

Internodal Species ConceptMorphological Species ConceptNon-dimensional Species ConceptPhenetic Species ConceptPhylogenetic Species Concept IPhylogenetic Species Concept IIPhylogenetic Species Concept IIIPolythetic Species ConceptRecognition Species ConceptReproductive CompetitionSuccessional Species ConceptTaxonomic Species Concept

How do we recognize species?

BSC: ProspectivePSC: Retrospective

Emphasis on: Emphasis on:

 Darwin:-Explanation of how new species arise

Modern Synthesis:

Dobzhansky (1937) Mayr (1942)Genetics and the Origins of Species Systematics and the Origins of Species

Species Concepts

Biological Species Concept (BSC)

Ernst Mayr

Every naturalist and every evolutionist is awed by the diversity of living nature. One cannot help asking oneself why there are so many species. Indeed, why are there species at all? Why is not the organic world a single continuity? Why has nature, and more precisely natural selection, favored the discontinuities among the species? What is the meaning of species? The answer the evolutionist gives to these questions is the Biological Species Concept.

Why interbreeding?

Biological Species Concept (BSC)

Ernst Mayr

South America

North America

BSC Example: Geminate Species

Pacific Ocean

Atlantic Ocean

Gradual formation of the Central American Landbridge

Snapping shrimp(Alpheus)

Nancy KnowltonSmithsonianTropical ResearchInstitute

Geminate species

P = PacificC = Caribbean

Knowlton et al. 1993 Science

1) Pre-mating isolating mechanisms a) Temporal isolation. Individuals of different species do not mate because they are active at different times of day or in different seasons. b) Ecological isolation. Individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences. c) Behavioral isolation. Potential mates meet, but choose members of their own species. d) Mechanical isolation. Copulation is attempted, but transfer of sperm does not take place.2) Post-mating isolating mechanisms a) Gametic incompatibility. Sperm transfer takes place, but egg is not fertilized. b) Zygotic mortality. Egg is fertilized, but zygote does not develop. c) Hybrid inviability. Hybrid embryo forms, but of reduced viability. d) Hybrid sterility. Hybrid is viable, but resulting adult is sterile. e) Hybrid breakdown. First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2 and backcrosses) may be inviable or sterile.Alternative: contrast pre-zygotic isolation (items 1+ 2a above) with post-zygotic isolation (2b-2e)

HybridizationMicrobes and fossils

Biological Species Concept (BSC)

Phylogenetic Species Concept (PSC)

Joel Cracraft

A “cosmopolitan” copepod, Eurytemora affinis

Phylogenetic analyses: at least 8 cryptic species

Lee (2000) Evolution

Phylogenetic Species Concept (PSC)

Gene trees versus species trees (?)Expensive (?)

Discovered in 1986 (S. Chisholm)

Very small (<1 µm), Ps picoplankton

Probably most abundant autotroph on earth

100,000 cells per drop of sw

50% of atmosph O2

Prochlorococcus

ProchlorococcusWhy so successful?

Light harvesting capabilities

Gulf Stream Sargasso Sea

Flow cytometry signatures(chlorophyll fluorescence)

Ch

loro

phy

ll flu

ore

scen

ce

Forward angle light scatter

Moore et al. 1998 Science

Moore et al. 1998 Science

Moore et al. 1998 Science

16S rRNA

Low-light lineages

Not monophyletic

Monophyletic

Lee (2000) Evolution

DNA Barcoding

Application of the PSC to

DNA Barcoding

Use of organelle genomes

Animals:

-mtDNA: cytochrome c oxidase-1 (CO1)

Plants:

-plastid: rbcL, matK, trn-psbA

Two rules for species discovery

DNA Barcoding

Hickerson et al. 2006 Systematic Biology

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