2.3.1.3 Evolutionary barriers to switches of habit
The Melampsora story raises two questions: (1) whether the
influence of pathogens serves to maintain a habit, or drives switches
from one to the other; and (2) if the latter, in what direction? It was
long ago proposed (e.g. Holttum 1953; Axelrod 1966) that the deciduous
habit arose as a defence against seasonal stresses. But, once
established, it may persist, in a state of ‘biotic lock-in’, due to
factors like the behaviour of Melampsora rusts. Answers to both
questions, however, depend on the evolutionary hurdles facing the
respective switches.
The evolutionary hurdles for a shift in habit will differ between the
directions of switch. A related question is what factors, in what
combinations, may either drive a switch or prevent one? Another question
involves relationships between individual fitness and population
fitness. An evolutionary switch from a deciduous to an evergreen habit
might be expected to be favoured by a mild climate, imposing no
stringent requirements for anatomical and physiological adjustments to
abiotic factors. However, it would likely require adjustment to biotic
factors. These might include enhanced morphological, anatomical or
chemical defences against herbivores, or evolution of greater pathogen
resistance. A mild climate, if humid, tends to favour various fungal or
oomycete pathogens, creating a need for greater resistance. Thus, an
evolutionary inertia may work against a switch to an evergreen habit. A
potential corollary to that is restriction of the host’s geographic
range to where deciduousness does not incur a decisive competitive
disadvantage.
In a co-adapted relationship between a temperate-zone evergreen host and
a pathogen, the likely gains from becoming deciduous in avoiding
infection pressure appear minor, especially compared with the selective
and energetic costs of acquiring the habit. With a new pathogen
arriving, the comparative fitness advantages and selective costs can
change radically, as with Melampsora . This risk is much elevated
by increasing human influences, from long-distance trade and travel.