Greater agreement exists on the enhancement of plant facilitation with elevation,
documented in floral communities across the globe (Callaway et al. 2002). Together
with productivity, variation in extrinsic mortality related to interactions is a major
driver of organisms’ life strategies, influencing patterns of growth, development,
age and size at maturity, allocation in self-maintenance and parental care, as
detailed further in the text.
11.3 The Process of Life-History Evolution
11.3.1 Mechanisms
Life-history variation among mountain populations can be explained by both
phenotypic plasticity and local adaptation. The former, by means of which
organisms with the same genetic constitution adjust their development to the current conditions, is generally considered poorly efficient for coping with extreme
environments (Grime 1974; DeWitt et al. 1998). Some studies have shown that
plastic genotypes bear a cost of low performance in unfavourable alpine habitats
when compared to locally adapted genotypes (Emery et al. 1994; Stöcklin et al.
2009; Fischer and Karl 2010). However, purely plastic life-history responses to
changes in resource availability among elevations have been described (Dobson and
Murie 1987; Blanckenhorn 1997; Sears and Angilletta 2003; Yeh and Price 2004).
More frequently, selection for stress tolerance induces ecotypic differentiation in
the form of local adaptations, exemplified by significant non-additive gene–environment interactions and populations that show genetic differences, and performances, corresponding to the conditions met along the gradient (Törang et al. 2015;
Muir et al. 2014). Since environmental variation occurs at small spatial scales
across elevations, strong local selection and limited gene flow are required to
promote local adaptation sensu strictu (i.e. demonstrated by comparing performances after reciprocal transplants; Kawecki and Ebert 2004), and population size
may also matter (Leimu and Fischer 2008). Local adaptation also results in gene–
environment covariation and non-random distributions of genotypes along the
gradient, an evolutionary pathway fairly well documented in mountain species. If,
for instance, slow-growing genotypes are favoured at high elevations, where low
temperatures also act to slow organisms’ growth rates, then genetic and environmental influences on phenotypic expression covary positively. This process of
co-gradient selection explains size reduction in alpine plants, with genotypes for
small size found primarily on uplands where the environment also hampers somatic
growth (Aarssen and Clauss 1992; Byars et al. 2007). The same environmental
context may however select for genetically rapid growth and development to
compensate for environmental conditions that slow down these processes (Conover
and Schultz 1995), a countergradient pattern often adopted by ectotherms in cool
environments.
11 Life-History Responses to the Altitudinal Gradient
257
documented in floral communities across the globe (Callaway et al. 2002). Together
with productivity, variation in extrinsic mortality related to interactions is a major
driver of organisms’ life strategies, influencing patterns of growth, development,
age and size at maturity, allocation in self-maintenance and parental care, as
detailed further in the text.
11.3 The Process of Life-History Evolution
11.3.1 Mechanisms
Life-history variation among mountain populations can be explained by both
phenotypic plasticity and local adaptation. The former, by means of which
organisms with the same genetic constitution adjust their development to the current conditions, is generally considered poorly efficient for coping with extreme
environments (Grime 1974; DeWitt et al. 1998). Some studies have shown that
plastic genotypes bear a cost of low performance in unfavourable alpine habitats
when compared to locally adapted genotypes (Emery et al. 1994; Stöcklin et al.
2009; Fischer and Karl 2010). However, purely plastic life-history responses to
changes in resource availability among elevations have been described (Dobson and
Murie 1987; Blanckenhorn 1997; Sears and Angilletta 2003; Yeh and Price 2004).
More frequently, selection for stress tolerance induces ecotypic differentiation in
the form of local adaptations, exemplified by significant non-additive gene–environment interactions and populations that show genetic differences, and performances, corresponding to the conditions met along the gradient (Törang et al. 2015;
Muir et al. 2014). Since environmental variation occurs at small spatial scales
across elevations, strong local selection and limited gene flow are required to
promote local adaptation sensu strictu (i.e. demonstrated by comparing performances after reciprocal transplants; Kawecki and Ebert 2004), and population size
may also matter (Leimu and Fischer 2008). Local adaptation also results in gene–
environment covariation and non-random distributions of genotypes along the
gradient, an evolutionary pathway fairly well documented in mountain species. If,
for instance, slow-growing genotypes are favoured at high elevations, where low
temperatures also act to slow organisms’ growth rates, then genetic and environmental influences on phenotypic expression covary positively. This process of
co-gradient selection explains size reduction in alpine plants, with genotypes for
small size found primarily on uplands where the environment also hampers somatic
growth (Aarssen and Clauss 1992; Byars et al. 2007). The same environmental
context may however select for genetically rapid growth and development to
compensate for environmental conditions that slow down these processes (Conover
and Schultz 1995), a countergradient pattern often adopted by ectotherms in cool
environments.
11 Life-History Responses to the Altitudinal Gradient
257