In the context of AI alignment, the concern is that a base optimizer (e.g., a gradient descent process) may produce a learned model that is itself an optimizer, and that has unexpected and undesirable properties. Even if the gradient descent process is in some sense "trying" to do exactly what human developers want, the resultant mesa-optimizer will not typically be trying to do the exact same thing
Natural selection is an optimization process that optimizes for reproductive fitness. Natural selection produced humans, who are themselves optimizers. Humans are therefore mesa-optimizers of natural selection.
There have been some concerns that the underlying mechanism of in-context learning might be mesa-optimization, a hypothesized situation where models develop an internal optimization algorithm. But In-context learning ability analysis, Anthropic did not observe any evidence of mesa-optimizers.