Natural Selection, Ecology and Behaviour - An Introduction to Behavioural Ecology - page 19

(a) (b)

Spring temperature Spring temperature

Fig. 1.10 Phenotypic plasticity in laying dates in response to spring temperatures.
Dashed lines represent examples of reaction norms for different individual females, who
may differ in their average laying date (elevation) or in their plasticity in response to
spring temperatures (slope). In Wytham Woods, UK, the great tits respond as in
(a), with no significant variation between females in plasticity and a strong average
population response to temperature (solid line). In the Hoge Veluwe, The Netherlands,
the great tits respond as in (b), with no significant average population response (solid
line) but significant variation in individual female plasticity. After Charmantier et al .
(2008). Reprinted with permission from AAAS.

emergence of oak leaves ( Quercus robur ) and of winter moth caterpillars ( Operophtera
brumata ), which feed on the oak leaves and are a key food for nestling tits. The rates of
change of egg laying date with temperature (Fig. 1.11c) and of caterpillar emergence
with temperature (Fig. 1.11d) are similar, so the tits have closely tracked the temporal
changes in food availability over almost five decades.
How have the tits managed to do this? For temperate breeding birds, an increasing
photoperiod in the spring is the primary proximate cue that initiates gonadal growth
and the hormonal changes involved in breeding. However, the response can be fine-
tuned by other cues, such as temperature, food availability and social stimulation
(Dawson, 2008). One possible explanation for the earlier breeding is that there has been
micro-evolutionary change in the tit population, with selection favouring new
genotypes with different thresholds of response to these proximate cues (e.g. breeding at
shorter lengths of day). The other possibility is that earlier breeding has simply arisen
through phenotypic plasticity, with no need for any genetic change.
Anne Charmantier, Ben Sheldon and colleagues have shown that this second
hypothesis explains the response to climate change by the Wytham great tits. They
analysed the laying dates of 644 individual females who had bred in three or more
years. They found no significant variation among these individual responses to spring
temperature, so all females had similar reaction norms (Fig. 1.10a). Furthermore, the
slope of these individual responses was similar to that for the population as a whole

Genetic change
or phenotypic
plasticity?