Population size shapes trade-off dilution and adaptation to a marginal niche unconstrained by sympatric habitual conditions

How does niche expansion occur when the habitual (high-productivity) and
marginal (low-productivity) niches are simultaneously available? Without
spatial structuring, such conditions should impose fitness maintenance in
the former while adapting to the latter. Hence, adaptation to a given
marginal niche should be influenced by the identity of the simultaneously
available habitual niche. This hypothesis remains untested. Similarly, it
is unknown if larger populations, which can access greater variation and
undergo more efficient selection, are generally better at niche expansion.
We tested these hypotheses using a large-scale evolution experiment with
Escherichia coli. While we observed widespread niche expansion, larger
populations consistently adapted to a greater extent to both marginal and
habitual niches. Owing to diverse selection pressures in different
habitual niches (constant versus fluctuating environments; environmental
fluctuations varying in both predictability and speed), fitness in
habitual niches was significantly shaped by their identities.
Surprisingly, despite this diversity in habitual selection pressures,
adaptation to the marginal niche was unconstrained by the habitual niche’s
identity. We show that in terms of fitness, two negatively correlated
habitual niches can still have positive correlations with the marginal
niche. This allows the marginal niche to dilute fitness trade-offs across
habitual niches, thereby allowing costless niche expansion. Our results
provide fundamental insights into sympatric niche expansion.