this theory. This theory examines how individual personality differences are based on natural selection. Through natural selection organisms change over time through adaptation and selection. Traits are developed and certain genes come into expression based on an organism's environment and how these traits aid in an organism's survival and reproduction. Polymorphisms, such as sex and blood type, are forms of diversity which evolve to benefit a species as a whole. The theory of evolution has wide-ranging implications on personality psychology. Personality viewed through the lens of evolutionary biology places a great deal of emphasis on specific traits that are most likely to aid in survival and reproduction, such as conscientiousness, sociability, emotional stability, and dominance. The social aspects of personality can be seen through an evolutionary perspective. Specific character traits develop and are selected for because they play an important and complex role in the social hierarchy of organisms. Such characteristics of this social hierarchy include the sharing of important resources, family and mating interactions, and the harm or help organisms can bestow upon one another. Drive theories In the 1930s, John Dollard and Neal Elgar Miller met at Yale University, and began an attempt to integrate drives (see Drive theory), into a theory of personality, basing themselves on the work of Clark Hull. They began with the premise that personality could be equated with the habitual responses exhibited by an individual – their habits. From there, they determined that these habitual responses were built on secondary, or acquired drives. Secondary drives are internal needs directing the behaviour of an individual that results from learning. Acquired drives are learned, by and large in the manner described by classical conditioning. When we are in a certain environment and experience a strong response to a stimulus, we internalize cues from the said environment. When we find ourselves in an environment with similar cues, we begin to act in anticipation of a similar stimulus. Thus, we are likely to experience anxiety in an environment with cues similar to one where we have experienced pain or fear – such as the dentist's office. Secondary drives are built on primary drives, which are biologically driven, and motivate us to act with no prior learning process – such as hunger, thirst or the need for sexual activity. However, secondary drives are thought to represent more specific elaborations of primary drives, behind which the functions of the