Settling Basin 



The widening or deepening of a stream so that materials 

 carried in suspension will be deposited. 



Sewall Wright Effect 



The postulate that if a population is subject to cyclical 

 fluctuations in abundance, the evolutionary trend of the 

 species is influenced by the size (population density and 

 area inhabited) of the minimum breeding population, cf. 

 Natural selection. 



Sex Chromosome 



A Chromosome (q. v.) that determines the sex of the 

 offspring of an organism. 



Sex-linkage 



The connection or association of certain Genes or attri- 

 butes of an organism with the Sex chromosome, e.g., certain 

 kinds of color blindness. 



Sex Ratio 



The relationship of the numbers of males and females 

 in a population, approximately 1 : 1 in most kinds of animals. 



Sexual Dimorphism 



The condition in which marked differences in character- 

 istics such as color, size, and form occur between male and 

 female animals in the same species. 



Sexual Reproduction 



The production of offspring resulting from the fusion of 

 sex cells (Gametes, eggs and sperms), cf. Asexual reproduc- 

 tion, Propagate. 



Sexual Selection 



A theory to explain certain kinds of evolution based upon 

 selective mating such as the choosing of a certain mate by a 



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