Lecture 04: Species concepts/Speciation Flashcards
Do species exist? - Mayr 1982
“The so-called species problem can be reduced to a simple choice between two alternatives: Are species realities of nature or are they simply theoretical constructs of the human mind?” (Mayr 1982, The growth of biological thought. Harvard University Press)
Do species exist? - Hendry et al. 2000
Species are no more real than any other hierarchical level in the tree of life. They should be replaced by genotypic clusters described according to their genetic divergence from other clusters. (Hendry et al. 2000, Questioning species realities, Conservation Genetics)
Typological species concept- Linne
- Morphology concept -> distinguish through phenotyp (example: five petals different species then three petals
-> A species is a group of organisms conforming to a common morphological plan - Individuals do not stand in any special relation to each other, being merely expressions of the same type
- Variation is the result of imperfect manifestations of the idea implicit in each species
“(According) to Linnaeus, species were: 1. Distinct and monotypic. 2. Immutable and created as such. 3. Breeding true.” (Ramsbottom 1938, Proc. Linnean Soc. London)
Biological species concept (BSC)
Species are a reproductively isolated population:
“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” (Mayr, 1942; 1969:26)
(1) “Interbreeding: => a genetic unit
- species are gene pools: a coadapted gene complex
- conspecifics (members of the same species) resemble each other because they are related (have common ancestors)
(2) ** “Natural Populations => an ecological unit **
- Population thinking: Organisms derive their properties from the group.
- Variation among individuals is important.
(3) “Reproductively isolated” => a reproductive unit
- Reproductive Isolating Mechanisms (RIMs): prevent mating outside the species
- Species Recognition Mechanisms (SRMs): allow recognition of potential mates
Criterion of reproductive isolation can be tested, observed, or inferred:
=> BSC suggests a research program (the mark of a good theory)
(1) Experiment: a test cross
A and B interbreed, but neither with C =>A & B are conspecific, C is separate
(2) ** Field observations: a species gap exists**
Populations maintain identity when sympatric (occuring in the same place) & synchronic (occuring at the same time)
(3) **Inference: forms look different **
Statistical analyses show significant differences
Genetic studies indicate no gene flow between forms
Difficulties with application of the BSC
(1) Reproductive isolation is a “typological criterion” (an either / or rule).
- Does one successful hybridization invalidate species distinction?
(2) Reproductive isolation evolves gradually: species distinction somewhat arbitrary (willkürlich)
(3) Reproductive criterion does not work with asexual species
(4) Genetic differences may not indicate reproductive isolation, in particular in allopathy
Phylogenetic species concept (PSC):
- Species are monophyla
- Time is important -> historical point of view
- Genetic very important
- “A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent.” (Cracraft 1983, Species Concepts and Speciation Analysis. Plenum Press)
How species form
- Initial separation of populations leading to reduced gene flow
- Divergence in traits of separated populations
- Reproductive isolation (-> secondary contact)
=> Often allopatric
Speciation – Evolution of reproductive isolation
Prezygotic / premating isolating mechanisms:
- Ecological isolation – populations in different habitats in the same region
- Seasonal or temporal isolation – different mating / flowering times
- Sexual or behavioral isolation – weak mutual attraction between ♀ and ♂ of different species
- Mechanical isolation – physical incompatibility of sex organs
- Isolation by different pollinators – in flowering plants
- Gametic:
– external fertilization: ♀ and ♂ gametes do not attract each other
– internal fertilization: gametes are inviable in sexual ducts of other species
Dobzhansky 1970
Speciation – Evolution of reproductive isolation
Postzygotic / postmating isolation mechanisms
- Hybrid inviability – hybrid zygotes have reduced viability or are inviable (unfruchtbar)
- Hybrid sterility – F1 hybrids of one or both sexes do not produce functional gametes
- Hybrid breakdown – F2 backcross hybrids have reduced viability or fertility
Sterilität ist die Unfähigkeit zur Empfängnis, und Unfruchtbarkeit die U
Dobzhansky 1970
The Dobzhansky-Muller theory of postzygotic isolation
- Postzygotic isolation is easily achieved if genetic compatibility is the consequence of epistastic interaction
- a model of the evolution of genetic incompatibility, important in understanding the evolution of reproductive isolation during speciation and the role of natural selection in bringing it about.
Haldane’s rule (1922)
”When in the F1 offspring of the two different animal races one sex is absent, rare or sterile, that sex is the heteroyzgous one.”
wenn bei Arthybriden ein Geschlecht nicht vorkommt, seltener ist oder häufiger steril ist, dann ist es das heterogametische Geschlecht (das mit zwei verschiedenen Geschlechtschromosomen).
The “dominance theory” of Haldane’s rule:
* Haldane’s rule is in part explained by the Dobzhansky-Muller theory
* Lower fitness of F1 hybrid of heterogametic sex caused by mutations that produced recesive allele in a sex chr acting epistatically (negative interaction) with alleles in other parts of genome.
Allopatric speciation
- speciation that happens when two populations of the same species become isolated from each other due to geographic changes
Example: Amphipods in New Zealand –> were split into several populations by the separation of the two islands when the sea level rose after the last ice age, and when ocean currents changed
Peripatric speciation
- special version of the allopatric speciation mode and happens when one of the isolated populations has very few individuals.
Example: Chestnut-tailed antbird in Amazonia -> lives in continuous forest in Bolivia to the border of Brazil but can be found 350ha isolated forest “island” on a plateau
Parapatric speciation
- a species is spread out over a large geographic area. Although it is possible for any member of the species to mate with another member, individuals only mate with those in their own geographic region.
- Instead of being separated by a physical barrier, the species are separated by differences in the same environment.
Example: Weakly electric fish Pollimyrus castelnaui & P. Marianne -> in the south of Africa in the Okavangoand the Upper Zambezi River systems
Sympatic speciation
occurs when there are no physical barriers preventing any members of a species from mating with another, and all members are in close proximity to one another
* Controversial: Often micro allopatric, because hard to prove so often not used
Example: Nicaragua Cichlids -> Eco-morphologically distinct; Morphometrics: 2 body types; Pharyngeal jaw differed (associated to food processing); Stomach content = different diet
