The document discusses lactose and lactase and how populations that drink milk have developed more lactase through natural selection. It provides background on how lactose is milk sugar that is normally broken down by the enzyme lactase, but most adults cannot digest milk due to lacking sufficient lactase. The source of the information is from a 1972 Scientific American article by Norman Kretchmer.

1. RST* text mapping *Rhetorical Structure Theory

2. The text: 1) Lactose and Lactase 2) Lactose is milk sugar; 3) the enzyme lactase breaks it down. 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. 6) Norman Kretchmer, Scientific American, page 70, October 1972.

3. The text: 1) Lactose and Lactase 2) Lactose is milk sugar; 3) the enzyme lactase breaks it down. 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. 6) Norman Kretchmer, Scientific American, page 70, October 1972. RST Analysis from the RST web site ( http://www.sfu.ca/rst ) This abstract is nearly half background information. One of the reasons that backgrounding is not signalled by its own distinct signal (such as a conjunction) may be that such a signal would in effect say that the author thought the reader would not know this information. Some readers might find that mildly condescending if it were made explicit. Backgrounding is seldom signalled explicitly.

4. Nucleus:Satellite relations The most frequent structural pattern is that two spans of text (virtually always adjacent, but exceptions can be found) are related such that one of them has a specific role relative to the other. RST says that the nucleus is more essential to the text than the satellite Relation Name Nucleus Satellite Evidence claim evidence for the claim Background text whose understanding is being facilitated text for facilitating understanding Elaboration basic information additional information Preparation text to be presented text which prepares the reader to expect and interpret the text to be presented.

5. Multinuclear relations There are also relations that do not carry a definite selection of one nucleus. Relation Name Span a Span b Contrast Joint List Sequence

6. More RST relations