  Natural environments can present diverse challenges, but some genotypes
remain fit across many environments. Such `generalists' can be hard to evolve,
out-competed by specialists fitter in any particular environment. Here,
inspired by the search for broadly-neutralising antibodies during B-cell
affinity maturation, we demonstrate that environmental changes on an
intermediate timescale can reliably evolve generalists, even when faster or
slower environmental changes are unable to do so. We find that changing
environments on timescales comparable to evolutionary transients in a
population enhances the rate of evolving generalists from specialists, without
enhancing the reverse process. The yield of generalists is further increased in
more complex dynamic environments, such as a `chirp' of increasing frequency.
Our work offers design principles for how non-equilibrium fitness `seascapes'
can dynamically funnel populations to genotypes unobtainable in static
environments.
