Plant biomechanics
From Biomch-W
Contents |
The Comparative Approach
Introduction
Under Construction
Phylogenetic Approach
The phylogenetic approach is a useful tool for organizing cross-species observations regarding the functional morphology of specialization and conservation. Closely related to systematics, the field of cladistics was founded by Willi Hennig (1966) in an attempt to explain biological diversity through logical data structures such as trees. This "tree of life" approach has fared quite well and provides logical consistency with Darwinian theory, although in some biomechanical circumstances other relational structures provide a more parsimonius explaination of the data. Cladistic theory will provide us with a number of rules and assumptions (quite independent of the relational structure used) for inferring how traits have evolved over time given their current distribution in related taxa.
Definitions
- Taxa
- Unit of analysis. This is usually a species, but can also be a morphologically specialized population within a species.
- Trait
- A single feature or function, such as a leaf or a trunk stabilization. Traits can be further divided into states. States (at an interval or ordinal level of measurement) are particularly important for the inferrence of evolutionary hypotheses.
- Specialization
- A trait that arises de novo in a particular lineage and is not directly associated with other lineages. Can also be referred to as a derived trait.
- Conservation
- A trait that is retained across multiple lineages. A conserved trait is assumed to have a common ancestral origin, regardless of how many lineages it is associated with.
- Synapomorphy
- A shared and derived trait, found within a single lineage.
- Apomorphy
- A derived trait present in only a single taxa.
- Outgroup
- A taxa or group of taxa that are used to polarize the phylogeny. Typically, polarization is used to determine which state of the trait is ancestral. They are also used to order various states of a trait. Taxa that are far removed phylogenetically are typically used so as to ensure that any traits shared in common with the taxa being analyzed are ancestral forms.
- Clade
- A distinct lineage that shares common trait states among themselves and can be clustered into a subgroup based on this evidence.
- Sympleisiomorphy
- Technical name for a conserved trait. In this case, the trait has remained in an ancestral state in most, if not all, of the taxa under analysis.
- Homology
- The holy grail of comparative work. A homology is a trait of the same or closely related states distributed across related taxa. When a trait is homologous, it is usually evidence of modification through descent. Therefore, homologous traits point to evoluitionary divergence.
- Homoplasy
- The type II error of comparative work. Homoplasy can be equated with convergent evolution, or a lack of evolutionary divergence between two taxa even though they share traits with closely related states.
Sources
Hennig, W. (1966). Phylogenetic Systematics. University of Illinois Press.
Developmental Approach
Another way to evaluate current biomechanical diversity is to probe the biomechanical performance of organisms during development. This is useful in two ways:
- Traits that appear at different stages in development can determine the future morphology of the organism. Koehl (1990) and Koehl et al (2000) argue that morphogenesis during development provides a template upon which further morphological and biomechanical change can take place.
- This approach conforms to the old idea of "phylogeny recapitulating ontogeny" (see Gould, 1977) -- that is, forms of a trait that appear early in development are indiciative of an organism's phylogenetic history.
Sources
Experimental Approach
Under Construction
Sources
Model Systems
Under Construction
