Biology Reference
In-Depth Information
5
Initial clutch size laid
8
4
3
7
2
6
1
5
4
5
6
7
8
Experimental brood size
Fig. 1.9
Experiments on clutch size in magpies. Pairs that had initially laid 5, 6, 7 or
8 eggs were given experimentally reduced or enlarged broods. Pairs that had naturally
laid large clutches did better with large broods and those naturally laying small clutches
did better with small broods. From Högstedt (1980). Reprinted with permission from
AAAS.
Phenotypic plasticity: climate change
and breeding times
The ability of a single genotype to alter its phenotype in response to environmental
conditions is termed
phenotypic plasticity
. For example, we have just seen that clutch size
is a phenotypically plastic trait which varies with season and food availability. When
the phenotypic variation is continuous, the relationship between phenotype and the
environment for each genotype is called a
reaction norm
(Fig. 1.10). There may be
genetic variation in both the elevation of the line (the trait value) and its slope (the way
the trait value changes in response to the environment). Recent studies of the earlier
breeding of songbirds in response to climate change provide a good example of
phenotypic plasticity. They also show how useful it is to study both proximate and
ultimate explanations of behaviour together, hand in hand.
Over 47 years (1961-2007), the mean egg laying date of female great tits in the
Wytham Wood population (near Oxford, UK) has advanced by about 14 days
(Fig. 1.11a). The main changes have been from the mid 1970s, since when there has
been a marked increase in spring temperatures (Fig. 1.11b). This has led to the earlier
Reaction norms
Warmer springs
and earlier
breeding in great
tits
