analysis question

[Aliza Yahav]

Hello,
I'm just beginning to get acquainted with RSTanaysis, and have been trying to follow the examples provided on the site. I'd like to know if the following analysis is a possible alternative to the one appearing on p. 4 of the Introduction.
The example: 1. Lactose and Lactase (preparation)
2. Lactose is milk sugar; 3. the enzyme lactase breaks it down (elaboration)
4. For want of lactase most adults cannot digest milk. 5. In populations that drink milk the adults have more lactase, perhaps through natural selection. (contrast)

If we're looking at writer's purpose, is it possible that the relation between spans 4 and 5 is non-volitional result? The situation in which most adults can't digest milk has resulted in a process of natural selection in milk drinking populations. The 'contrast' relation doesn't seem to express the implicit connection which gives these two spans coherence.
Thank you,
Aliza Yahav 
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