Seasonal timing and population divergence: when to breed, when to migrate

- Timing of reproduction and migration vary within and among populations.
- Timing differences arise from variation in neuroendocrine mechanisms.
- Timing differences (allochrony) can impede gene flow and thus population divergence.
- Seasonally sympatric populations that differ in timing are excellent study systems.
- Environmental change may act on mechanisms to influence biodiversity.

Understanding how populations adapt to constantly changing environments requires approaches drawn from integrative and evolutionary biology as well as population ecology. Timing of reproduction and migration reflect seasonal pulses in resources, are driven by day length, and are also responsive to environmental cues that change with climate. Researchers focusing on birds have discovered accelerated breeding, reductions in migration, and extensive variation in perception, transduction, and response to the environment. We consider situations in which individuals experience the same environment but differ in the timing of the annual cycle. Such scenarios provide exceptional opportunities to study mechanisms related to among-population differences in timing (allochrony) and distribution (sympatry-allopatry-heteropatry), which have the potential either to enhance or reduce population divergence and biodiversity.