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

Niche expansion remains significantly understudied in sympatric scenarios where marginal and habitual niches are simultaneously available. Devoid of spatial constraints, such conditions impose selection to maintain fitness in habitual (high-productivity) niches while adapting to marginal (low-productivity) niches. Hence, habitual niche composition should constrain adaptation to marginal niches. This remains untested. Similarly, it is unknown if larger populations expand their niches better. We tested these hypotheses using experimental evolution with Escherichia coli and found that larger populations consistently adapted better to both marginal and habitual niches. Whereas the habitual niche composition (constant versus fluctuating environments; environmental fluctuations varying in both predictability and speed) significantly shaped fitness in habitual niches, surprisingly, it failed to constrain adaptation to the marginal niche. Curiously, two negatively correlated habitual niches can still each be positively correlated with the marginal niche. This allows the marginal niche to dilute trade-offs between habitual niches, thereby enabling costless niche expansion.