The Mutation Theory and the Species-Concept

1. Hongyu Guo March, 2007 The Mutation Theory and the Species-Concept

2. Background • Darwin’s Origin of Species and Darwinism • 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.

3. Background • Mutationism (Mendelism) • In 1900, Mendelian inheritance was “rediscovered”. • Geneticists learned that discontinuousvariations 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.

4. The Mutation Theory and the Species-Concept -R. R Gates, 1917 Maianthemum Canadense Smilacina (Maianthemum) Maianthemumdilatatum Maianthemum Maianthemumbifolium (M. dilatatum)

5. The Mutation Theory and the Species-Concept-R. R Gates, 1917 Kruhsea strepotoides (Streptopusstreptopoides) Platystemon Streptopus brevipes (Streptopusstreptopoides) Platystigma Different in flower size Different in pistils

6. The Mutation Theory and the Species-Concept-R. R Gates, 1917 paler and smaller Distribution of North American screech owl

7. The Mutation Theory and the Species-Concept-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.

8. The Mutation Theory and the Species-Concept-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.

9. The Mutation Theory and the Species-Concept-R. R Gates, 1917 Darwinism Mendelism Gates

10. 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.

11. 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.

12. Modern Evolutionary Synthesis Evolution in Mendelian Populations -Sewall Wright, 1930 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.

13. Modern Evolutionary Synthesis Genetics and the Origin of Species -Theodosius Dobzhansky, 1937 • 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.

14. Modern Evolutionary Synthesis Systematics and the Origin of Species -Ernst Mayr, 1942 • 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.

15. Modern Evolutionary Synthesis Modern Evolutionary Synthesis described by D. J. Futuyma, 1986 • “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

16. Molecular era of Speciation Molecular Origin of Species – M. Nei & J. Zhang, 1998 • 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. Prezygotic isolation If so, what is the evolutionary mechanism of reproductive isolation? Postzygotic isolation Ai Ai Bi Bi Aim Bim Making new species, two ways Ai+1 Bi+1

17. Epigenetics and Evolution Timescales 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? )

18. Species Problem How 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) • Diversity of species definition: Typological species Morphological species Biological / Isolation species  Biological / reproductive species Mate-recognition species Ecological species Phenetic species Recognition species Microspecies Cohesion species Phylogenetic (Cladistic)/ Evolutionary / Darwinian species

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