  Consider a population evolving from year to year through three seasons:
spring, summer and winter. Every spring starts with $N$ dormant individuals
waking up independently of each other according to a given distribution. Once
an individual is awake, it starts reproducing at a constant rate. By the end of
spring, all individuals are awake and continue reproducing independently as
Yule processes during the whole summer. In the winter, $N$ individuals chosen
uniformly at random go to sleep until the next spring, and the other
individuals die. We show that because an individual that wakes up unusually
early can have a large number of surviving descendants, for some choices of
model parameters the genealogy of the population will be described by a
$\Lambda$-coalescent. In particular, the beta coalescent can describe the
genealogy when the rate at which individuals wake up increases exponentially
over time. We also characterize the set of all $\Lambda$-coalescents that can
arise in this framework.
