Table 2.

What mechanisms could have produced the response to selection?

Hypothesis	Explanation
Enrichment hypothesis	A single genotype satisfied the selection criterion when grown alone in a microcosm.
Selective amplification	A single genotype satisfied the selection criterion, but only when a nonspecific background of other genotypes was also present.
Additive interactions	The response to selection could be accounted for by summing the response of individual species grown in isolation.
Nonadditive interactions	The response to selection could be reproduced only by growing all species together in the same microcosm. these species interacted to produce a response of greater magnitude than the individual contributions (synergy).

Note: There are four possible hypotheses about the possible changes in species composition as a result of a response to artificial ecosystem selection. These hypotheses relate only to the relative contribution of different “species” to the response (Williams and Lenton 2007). It is likely that a comparison of the metagenomes of responding lines will show differences in the functional gene categories present in each line.