The Causes of Variation in Tree Seedling Traits: The Roles of Environmental Selection Versus Chance

Abstract

A key aspect of biodiversity is the great quantitative variation in functional traits observed among species. One perspective assertsthat trait values should converge on a single optimum value in a particular selective environment, and consequently trait variation would reflect differences in selective environment, and evolutionary outcomes would be predictable. An alternative perspective asserts that there are likely multiple alternative optima within a particular selective environment, and consequently different lineages would evolve toward different optima due to chance. Because there is evidence for both of these perspectives, there is a long-standing controversy over the relative importance of convergence due to environmental selection versus divergence due to chance in shaping trait variation. Here, I use a model of tree seedling growth and survival to distinguish trait variation associated with multiple alternative optima from variation associated with environmental differences. I show that variation in whole plant traits is best explained by environmental differences, whereas in organ level traits variation is more affected by alternative optima.Consequently, I predict that in nature variation in organ level traits is most closely related to phylogeny, whereas variation in whole plant traits is most closely related to ecology.

Does trait variation within broadly distributed species mirror patterns across species? A case study in Puerto Rico

“…, Fajardo and Siefert ). A further consequence of this is that alternative arrangements of traits might be good solutions for a given environment (Marks and Lechowicz , Marks ).…”

“…For example, if few fundamental trade‐offs operate consistently across species (e.g., biophysical constraints to achieve a given physiological function), then trait variance at the level of individuals and populations (across ecological scales) should be consistent across species. However, alternative explanations propose that co‐occurring species might achieve functional equivalence through different combination of traits resulting in different patterns of variance structure across species (Marks and Lechowicz , Marks ). In this case, traits will be connected through complex networks of trade‐offs that do not necessary result in the same (obligate) trait combinations across species.…”

On the relative importance of CSR ecological strategies and integrative traits to explain species dominance at local scales

“…Thus, these different arrays of traits characterize alternative functional designs (i.e. multiple phenotypes) that may lead to similar responses to a given ecological factor (Marks & Lechowicz ; Marks ; Rosado & de Mattos ; Pivovaroff et al . ).…”

Multiple convergent evolution of arboreal life in oribatid mites indicates the primacy of ecology

“…For many, convergence derives from frequent and independent adaptations and thereby points to the importance of ecological factors during evolution (Sinclair et al 2003;Langerhans & DeWitt 2004;Zhang 2006;Marks 2007). In this view, convergent evolution indicates the limits of potential evolutionary pathways, such that different evolutionary trajectories resulted in similar solutions to the same ecological problem.…”

“…For example, Morris (2003) used convergent evolution as evidence for directed evolution resulting in similar endpoints, although this teleological view has been criticized (Lenski 2003). Convergent evolution may also be a product of chance, as there can be more than one optimum for a trait (Gould & Lewontin 1979;Doolittle 1981;Gould 1989;Zhang & Kumar 1997;Marks 2007). An ecological challenge could have been solved in a similar way by two or more species through chance alone.…”