LOWER-LEVEL COMPLEXITY LOSS. “A hypothesis has been advanced recently predicting that, in evolution, as higher-level entities arise from associations of lower-level organisms, and as these entities acquire the ability to feed, reproduce, defend themselves, and so on, the lower-level organisms will tend to lose much of their internal complexity.”
http://www.bioone.org/doi/abs/10.1554/0014-3820(2002)056%5B0441:ACDOCI%5D2.0.CO%3B2