The Mutation Theory The Mutation Theory and and

the Species-Conceptthe Species-Concept

Hongyu GuoHongyu GuoMarch, 2007March, 2007

Darwin’s Darwin’s Origin of SpeciesOrigin of Species and Darwinism and Darwinism BackgroundBackground

In 1859, Darwin published his pivotal work On the Origin of Species by Means of Natural Selection. All organisms have descended with modification from a common ancestor. He introduced the theory that populations evolve over the course of generations through a process of natural selection In the view of Darwinism, hereditary variation arises by continuous “fluctuation”, and evolutionary change accumulates automatically in infinitesimal increments as selection preserves fluctuations in the favorable direction.

BackgroundBackground Mutationism (Mendelism) In 1900, Mendelian inheritance was “rediscovered”. Geneticists learned that discontinuous variations could arise by mutation and be transmitted to offspring by Mendel’s laws. In the view of Mutationism, the infinitesimal heritable variation could not be taken for granted. As a result, evolution was seen as a two-step process: the chance occurrence of a mutation, followed by its persistence or elimination, it is also called “jumping” evolution.

The Mutation Theory and the The Mutation Theory and the Species-ConceptSpecies-Concept --R. R Gates, 1917R. R Gates, 1917

Smilacina (Maianthemum)

Maianthemum

Maianthemum dilatatum

Maianthemum Canadense

Maianthemum bifolium (M. dilatatum)

The Mutation Theory and the The Mutation Theory and the Species-ConceptSpecies-Concept -R. R Gates, 1917-R. R Gates, 1917

Platystemon

Platystigma

Streptopus brevipes (Streptopus stre

ptopoides)

Kruhsea strepotoides (Streptopus st

reptopoides)

Different in flower size Different in pistils

The Mutation Theory and the The Mutation Theory and the Species-ConceptSpecies-Concept -R. R Gates, 1917-R. R Gates, 1917

Distribution of North American screech owl

paler and smaller

The Mutation Theory and the The Mutation Theory and the Species-ConceptSpecies-Concept -R. R Gates, 1917-R. R Gates, 1917

Distribution of Colaptes auratus Distribution of Colaptes cafer

Gates suggested that these species differ remarkably in their color makings, they can not be reasonable supposed to have developed through gradual adaptation.

The Mutation Theory and the The Mutation Theory and the Species-ConceptSpecies-Concept -R. R Gates, 1917-R. R Gates, 1917 Conclusion

In plant and animal species there are two distinct types of variability:

• Discontinuous, independent of environmental or functional influence, and has given rise to many specific and generic characters.

• Continuous and apparently represents the results of the stress of the environment of the species in its dispersal.

The Mutation Theory and the The Mutation Theory and the Species-ConceptSpecies-Concept -R. R Gates, 1917-R. R Gates, 1917

DarwinismMendelism

GatesGates

Modern Evolutionary Synthesis

The Correlation Between Relatives on the Supposition of Mendelian Inheritance -R. A. Fisher, 1918

The starting point of Modern Evolutionary Synthesis Fisher provide a rigorous statistical model for Mendelian inheritance. He showed using the model how continuous variation could be the result of the action of many discrete loci, satisfying both Mendelism and Darwinism.

Modern Evolutionary Synthesis

A Mathematical Theory of Natural and Artificial Selection

-J.B.S. Haldane, 1924

Haldane outlined the first mathematical models for many cases of evolution due to selection.

He Showed that selection of a given intensity is most effective when amphimixis does not affect the character selected, e.g. in complete inbreeding or homogamy. Selection is very ineffective on autosomal recessive characters so long as they are rare.

Modern Evolutionary Synthesis

Evolution in Mendelian Populations

Wright using gene frequency model showed that:

The differing statistical situations to be expected among natural species are adequate to account for the different sorts of evolutionary processes.

Conditions in nature are often such as to bring about the state of poise among opposing tendencies on which an indefinitely continuing evolutionary process depends.

-Sewall Wright, 1930

Modern Evolutionary Synthesis

Genetics and the Origin of Species

A landmark book in Modern Evolutionary Synthesis

He suggested that mutations crop up naturally all the time. Some mutations are harmful in certain circumstances, but a surprising number have no effect one way or the other. These neutral changes appear in different populations and linger, creating variability that is far greater than anyone had previously imagined.

-Theodosius Dobzhansky, 1937

Modern Evolutionary Synthesis

Systematics and the Origin of Species

New species definition: A species is not just a group of morphologically similar individuals, but a group that can breed only among themselves, excluding all others. (Biological Species)

Allopatric speciation theory: When populations of organisms get isolated, the sub-populations will start to differ by genetic drift and natural selection over a period of time, and thereby evolve into new species.

-Ernst Mayr, 1942

Modern Evolutionary Synthesis

“The major tenets of the evolutionary synthesis, then, were that populations contain genetic variation that arises by random (i.e.. not adaptively directed) mutation and recombination; that populations evolve by changes in gene frequency brought about by random genetic drift, gene flow, and especially natural selection; that most adaptive genetic variants have individually slight phenotypic effects so that phenotypic changes are gradual (although some alleles with discrete effects may be advantageous, as in certain color polymorphisms); that diversification comes about by speciation, which normally entails the gradual evolution of reproductive isolation among populations; and that these processes, continued for sufficiently long, give rise to changes of such great magnitude as to warrant the designation of higher taxonomic levels (genera, families, and so forth).” - D. J. Futuyma

Modern Evolutionary Synthesis described by D. J. Futuyma, 1986

Molecular era of SpeciationMolecular Origin of Species – M. Nei & J. Zhang, 1998

Making new species, two ways

Prezygotic isolation

Postzygotic isolation

Reproductive isolation between different species appears to be caused by the incompatibility of alleles at two or more loci that control mating, spermiogenesis and development.

If so, what is the evolutionary mechanism of reproductive isolation?

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Epigenetics and EvolutionEpigenetics and EvolutionTimescales of Genetic and Epigenetic Inheritance –O. J. Rando & K. J. Verstrepen, 2007

Organisms have evolved epigenetic mechanisms to influence the timing or genomic location of heritable variability.

Hypervariable contingency loci and epigenetic switches increase the variability of specific phenotypes. Error-prone DNA replicases produce bursts of variability in times of stress. Interestingly, these mechanisms seem to tune the variability of a given phenotype to match the variability of the acting selective pressure.

These suggest that selection and variability are less independent than once thought. ( Nonrandom evolution? )

Species ProblemSpecies ProblemHow best to define "species" ?

Mayr’ s species definition: a species is a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups. (assuming sexual reproduction)

Phylogenetic (Cladistic)/ Evolutionary / Darwinian species

Diversity of species definition:

Ecological species Phenetic species Recognition species Microspecies Cohesion species

Typological species Morphological species Biological / Isolation species  Biological / reproductive species Mate-recognition species

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