SECTIONAL TRANSACTIONS.— D. 359 



Mr. F. C. Minns. — The experimental approach to sexual-selection 

 ln-55)- 



In an animal of a mixed population the possession of particular character- 

 istics may render mating more probable or less. These characteristics will 

 tend accordingly to become more or less common. When two or more 

 varieties occur together and each shows a preference for its own kind 

 (' assortative mating') they are by this means partially isolated. They will 

 therefore tend to evolve on slightly different lines and new species may 

 result. 



Quantitative experiments on the banana-fly Drosophila have shown 

 assortative mating and other types of sexual selection to occur between the 

 wild type and mutants and also between apparently identical races. 



Mr. E. B. Ford. — Selection in relation to the genie background (12.10). 



Selection can operate only on genotypic variability. This is controlled 

 by genes having multiple effects, and interacting with one another to produce 

 the characters for which they are responsible. They thus form a total gene- 

 complex, alterations within which may affect the operation of any one of its 

 members. 



Two types of selection are thus possible. First, that preserving advan- 

 tageous genes and eliminating disadvantageous ones. Secondly, that 

 tending to alter the effects of a gene, but not the gene itself. This is brought 

 about by selection operating on the gene-complex, so that a genetic constitu- 

 tion is spread through the population which tends to bring out the effects of 

 particular genes to the greatest advantage. 



In this way, varied evolutionary changes may take place within poly- 

 morphic forms, which can yet remain under simple Mendelian control, 

 acting as a switch to determine which group of characters shall be expressed. 

 Fluctuations in numbers allow genes to be tested by selection in different 

 gene-complexes, with some of which they may react in new, and possibly 

 beneficial, ways. Selection acting on the less obvious genie effects may be 

 partly responsible for the recent spread of melanism among the Lepidoptera 

 in industrial areas. 



Prof. H. J. Muller. — Summary of the discussion (12.25). 



Afternoon. 



Prof. W. J. Dakin. — An account of ancient and modem whaling in 

 Australasian seas (2.15). 



At the Plaza Cinema, Manchester Square (4.30) : 



Exhibition of films by kind permission of Gaumont British Instruc- 

 tional Films, Ltd. : 



Hydra. 



Obelia. 



Nursery Island. 



The development of the frog. 



