Biology Reference
In-Depth Information
(Fig. 1.11e). Therefore the population level change can be explained entirely by the
magnitude of the plastic responses of individual females.
Studies of a great tit population in the Hoge Veluwe, The Netherlands, paint a very
different picture (Visser
et al
., 1998; Nussey
et al
., 2005). Here, there has also been a
similar environmental change during the last three decades (1973-2004), with warmer
late spring temperatures and earlier emergence of the tits' caterpillar food supply. However,
there has been no change in the tits' egg laying date, with the result that many of the
Dutch birds are now breeding too late to catch the caterpillar peak for their hungry
offspring. As a result, female lifetime reproductive success has declined over the study
period (in contrast to the Wytham population, which is flourishing). Analysis of the
variation shown by individual females over successive years, showed that (unlike the
Wytham population) females varied in their phenotypic plasticity. Some responded little
to annual variation in temperature whereas others showed a marked response (Fig.
1.10b). Furthermore, the variation in plasticity is heritable. In theory, then, the more
plastic genotypes should now be favoured by natural selection.
Why do the Dutch and British tits differ? One possibility is that females in the two
populations use different proximate cues to time their egg laying (Lyon
et al
., 2008). For
example, if only photoperiod was used as a cue, then individuals would not breed earlier
in warmer springs. By contrast, if both birds and caterpillars responded to temperature,
or some other common environmental cue, then individual tits would automatically
track any yearly variation in caterpillar emergence.
Another possibility is that British and Dutch tits use the same cues but in Britain the
cues are better predictors of the food supply that will be available to nestlings. In the
Hoge Veluwe, over the last three decades there has been little change in early spring
temperatures (when the adult tits are forming food reserves to breed) in contrast to the
markedly warmer late spring temperatures, which influence the caterpillar food
available to nestlings. The adult tits may, therefore, not have been able to predict the
earlier food availability for their offspring (Visser
et al
., 1998).
The conclusion is that we need to understand the proximate mechanisms used to time
egg laying in order to predict how populations will evolve to cope with changing
food supplies.
Differences
between
populations
Behaviour, ecology and evolution
We can now summarize the main themes of this topic.
Firstly, during evolution natural selection will favour individuals who adopt life history
strategies that maximize their gene contribution to future generations. The optimization
of clutch size in great tits provides a convincing quantitative test of this, but we shall see
later in the topic that having offspring is only one of the ways of passing genes on to the
future. Another pathway is by helping close relatives to reproduce. One of the questions
we shall ask is what factors influence which pathway individuals choose.
Secondly, because an individual's success at survival and reproduction depends
critically on its behaviour, selection will tend to design individuals to be efficient at
foraging, avoiding predators, finding mates, parental care and so on. Resources are
limited, so there will always be trade-offs involved, both within and between these
various activities. For example, will an individual avoid predation best by seeking the
Four main themes
