The principle or mechanism that explains how species change across generations in response to environmental pressures is:

The principle or mechanism that explains how species change across generations in response to environmental pressures is: Microevolution

Explanation:

Evolution is not "moving someplace"; it simply represents variations in inherited traits across time. Hardly this transformation effects in improvements in biological complexity, but to understand this as "improvement" is to mistake the mechanism.

Microevolutionary manners involve variations in the conditions and frequencies of distinct features amongst parts of communities, usually due to ecological manners such as the drift of organisms and varying environmental circumstances as well as communications with parts of various species or the identical species. These methods can, but do not significantly, direct to the development of new species over time .