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.