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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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