254 AN INTRODUCTION TO MODERN GENETICS 



A. CHROMOSOME DIFFERENCES 



1 . Polyploid Species 



The fact of polyploidy may be suspected if an organism is found to 

 have a chromosome number which is an integral multiple of that of a 

 related species. Judged on this criterion alone, polyploids seem to be 

 extremely common among angiosperms; probably a quarter of the 

 known species have multiple chromosome numbers of this kind. It has 

 been suggested that the evolution of angiosperms from lower plants 

 was due to polyploid formation.^ Among the lower plants, the evidence 

 is not extensive, but polyploidy seems to be less important, and indeed 

 in some large groups all the species have the same chromosome number 

 (e.g. nearly all gymnosperms have a haploid number of 12). In animals 

 polyploidy is very rare; haploids are known in Ascaris, and simple 

 tetraploids in Artemia^ and a few other Crustacea^ while triploid and 

 tetraploid Drosophila have been obtained in the laboratory. The 

 chromosome numbers make it possible to suggest that polyploidy has 

 played a part in species formation in hermaphrodite Mollusca and 

 Aimelids, but nothing definite is known about this. This absence of 

 polyploid animals is peculiar;* it may be correlated with a disarrange- 

 ment of the sex determining mechanism, since the few known polyploids 

 are parthenogenetic or hermaphrodite; but on the other hand, it may 

 be that only parthenogenetic or hermaphrodite polyploids can survive . 



Among angiosperms, certain genera, e.g. Antirrhinum, Orchis, have 

 a characteristic chromosome number which is found in all species; in 

 others, e.g. Carex, the species have different chromosome numbers 

 but these do not form a series of multiples ; while in those genera in 

 which polyploidy occurs, the different multiples are usually correlated 

 with differences between species or larger groups, though in a few 

 cases the polyploid forms are classified only as races or sub-species. 



2. The Experimental Production of Polyploid Species 



The general principles of the formation of polyploid species have 

 been most clearly revealed in investigations which have led to the 

 artificial production of polyploids worthy of being ranked as new 

 species. 



Polyploids originate by a partial failure of cell division.^ If the failure 



^ Anderson 1934. ^ Gross 1932. ^ Vandel 19276. * Muller 1925. 

 ^ This was originally suggested on theoretical grounds by Winge 19 17. 



