Competition among species, or inter-specific competition, can have an even greater effect on selection than competition within species (intra-specific competition). This is often the case in lower density populations. Different species can have positive, neutral, or negative effects on each other's fitness, and the effect species 1 has on species 2 is not necessarily the same that 2 has on 1. The effects that cohabiting species have on each other shapes evolution the same way that selective pressures from within a species or the physical environment shapes it.