Organisms in nature evolve within complex, species-rich communities shaped by ongoing ecological interactions. Yet theoretical frameworks remain fragmented—ecology often ignores evolution, while evolutionary theory assumes isolated populations. I will present our efforts to bridge this gap by developing a unified theory of eco-evolutionary dynamics in highly diverse communities. We begin by treating new mutants as invaders and constructing a theory of ecological invasions that incorporates feedbacks from community restructuring. This leads to the concept of dressed invasion fitness, which extends classical fitness measures to account for extinction cascades and community shifts. Surprisingly, we find that mutants and parents can frequently coexist—contrary to traditional models—often due to the extinction of low-abundance species. These predictions are borne out in experimental plant and microbial ecosystems. Building on this, we generalize Kimura’s fixation probability to complex communities, revealing how ecological interactions reshape evolutionary outcomes. Together, these results lay the groundwork for a general theory of eco-evolution, offering new insight into how evolution proceeds in the tangled web of life.