The Species Concept in the Light of Cytology and Genetics

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Abstract
Apart from certain cytoplasmic differences, variations arise mostly in the chromosomes. An understanding of species depends upon an analysis of how chromosomes change. Such changes are (1) in number, (2) in structure. A gene represents a difference which has arisen at a locus of a chromosome. Genie changes are of various kinds. Some dominant gene mutations are duplications, some recessives are minute deletions, but other types of genic change probably occur. In Drosophila spp. numerous parallel mutations and numerous chromatin rearrangements have taken place. A breed of goats which have stiff legs when frightened are regarded as having undergone a mutation of the nervous system parallel to that of death-feigning insects. The shaker mutation in mice is cited as parallel to the similar one in Drosophila. Various mutations must begin to arise with a certain frequency at some period in the life of a species, and over only a part of its distribution. Neutral mutations will only spread in the species according to the mutation rate. Natural selection hastens the spread of others. Much interspecific differentiation has arisen through genic mutations. Changes in chromosome numbers as well as chromatin rearrangements lead to inter-sterility, which is thus often the beginning rather than the end point in the formation of new species. The extensive phenomena of self-sterility in plants, and the many other forms of sterility which exist, make intersterility an uncertain guide regarding the nearness of relationship between types. Cases of amphidiploidy and some cases of autotetraploidy produce forms which have all the criteria of true species. The term species cannot be defined. The conceptions associated with the terms jordanon, linneon and syngameon are useful, and the Onagra section of Oenothera is an evolving syngameon.

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