Abstract
Analyses of morphological disparity have been used to characterize and investigate the evolution of variation in the anatomy, function and ecology of organisms since the 1980s. While a diversity of methods have been employed, it is unclear whether they provide equivalent insights. Here, we review the most commonly used approaches for characterizing and analysing morphological disparity, all of which have associated limitations that, if ignored, can lead to misinterpretation. We propose best practice guidelines for disparity analyses, while noting that there can be no 'one-size-fits-all' approach. The available tools should always be used in the context of a specific biological question that will determine data and method selection at every stage of the analysis.
Highlights
Clades of organisms are characterized by variation in both numbers of species and range of phenotypes through time
Clades may be exceptionally rich in species and phenotypic diversity, species-rich but disparity-poor, species-poor but rich in disparity or depauperate in both species diversity and disparity. These phenomena suggest that taxonomic diversity and phenotypic disparity are not inextricably linked, raising important questions, such as: how does disparity evolve? Are some morphologies more common than others? Is anatomical evolution unbounded or are some anatomies impossible to achieve? What role does ecology play in structuring disparity? Analyses of species diversity have a venerable history, but those of disparity are comparatively more recent
Methods to capture disparity are based on multidimensional spaces where each dimension represents an aspect of morphological variation and biological observations can be placed in this space based on their trait values
Summary
Clades of organisms are characterized by variation in both numbers of species and range of phenotypes through time. Patterns of disparity have been used successfully to compare modules of evolution in various groups [8,9], allowing researchers to link variation in shape to a group’s evolutionary or developmental constraints [10] This approach investigates how the morphologies of organisms have changed over time, by focusing on the disparity of taxa in particular time intervals or slices. One of the most important insights is the discovery that morphological disparity is often greatest early in the evolutionary history of clades [21,22,23], indicating that capacity for evolutionary innovation wanes as clades age, which some have argued reflects the evolutionary assembly of gene regulatory networks that constrain later fundamental change [22,23] This example highlights one of the greatest challenges confronting researchers who are attempting, increasingly, to obtain general insights from multiple independent studies: can the insights gained from studies using a diversity of methods, approaches and data types be considered equivalent?. Throughout, it is important to remember that these tools should always be used in the context of a specific scientific question, as this will drive data and methodological choices at every stage of the process
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