The study of geometric morphometrics in anthropology has made a major impact on the field of morphometrics by aiding in some of the technological and methodological advancements. Geometric morphometrics is an approach that studies shape using Cartesian landmark and semilandmark coordinates that are capable of capturing morphologically distinct shape variables. The landmarks can be analyzed using various statistical techniques separate from size, position, and orientation so that the only variables being observed are based on morphology. Geometric morphometrics is used to observe variation in numerous formats, especially those pertaining to evolutionary and biological processes, which can be used to help explore the answers to a lot of questions in physical anthropology. Geometric morphometrics is part of a larger subfield in anthropology, which has more recently been named virtual anthropology. Virtual anthropology looks at virtual morphology, the use of virtual copies of specimens to perform various quantitative analyses on shape (such as geometric morphometrics).
Background The field of geometric morphometrics grew out of the accumulation of improvements of methods and approaches over several decades beginning with Francis Galton (1822-1911). Galton was a polymath and the president of the Anthropological Institute of Great Britain. In 1907 he invented a way to quantify facial shapes using a base-line registration approach for shape comparisons. This was later adapted by Fred Bookstein and termed "two-point coordinates" or "Bookstein-shape coordinates". In the 1940s, D'Arcy Wentworth Thompson (biologist and mathematician, 1860-1948) looked at ways to quantify that could be attached to biological shape based on developmental and evolutionary theories. This led to the first branch of multivariate morphometrics, which emphasized matrix manipulations involving variables. In the late 1970s and early 1980s, Fred Bookstein (currently a professor of Anthropology at the University of Vienna) began using Cartesian transformations and David George Kendall (statistician, 1918-2007) showed that figures that hold the same shape can be treated as separate points in a geometric space. Finally, in 1996, Leslie Marcus (paleontologist, 1930-2002) convinced colleagues to use morphometrics on the famous Ötzi skeleton, which helped expose the importance of the applications of these methods.
Traditional morphometrics Traditional morphometrics is the study of morphological variations between or within groups using multivariate statistical tools. Shape is defined by collecting and analyzing length measurements, counts, ratios, and angles. The statistical tools are able to quantify the covariation within and between samples. Some of the typical statistical tools used for traditional morphometrics are: principal components, factor analysis, canonical variate, and discriminant function analysis. It is also possible to study allometry, which is the observed change in shape when there is change in size. However, there are problems pertaining to size correction since linear distance is highly correlated with size. There have been multiple methods put forth to correct for this correlation, but these methods disagree and can end up with different results using the same dataset. Another problem is linear distances are not always defined by the same landmarks making it difficult to use for comparative purposes. For shape analysis itself, which is the goal of morphometrics, the biggest downside to traditional morphometrics is that it does not capture the complete variation of shape in space, which is what the measurements are supposed to be based on. For example, if one tried to compare the length and width for an oval and tear drop shape with the same dimensions they would be deemed as the same using traditional morphometrics. Geometric morphometrics tries to correct these problems by capturing more variability in shape.
Steps in a geometric morphometric study There is a basic structure to successfully performing and completing every geometric morphometric study:
Design Study: what is your objective/hypothesis? what morphology must you capture to explore this? Collect Data: choose your landmark set and method of collection Standardize Data: make your landmarks comparable across all specimens (superimposition) Analyze Data: choose a statistical approach depending on your original question and how you designed the study Interpret Results: take the outcome of your statistical analysis and reflect it back to the context of your original specimens
Data collection methods
Landmarks The first step is to define your landmark set. Landmarks have to be anatomically recognizable and the same for all specimens in the study. Landmarks should be selected to properly capture the shape trying to be observed and capable of being replicated. The sample size should be roughly three times the amount of landmarks chosen and they must be recorded in the same order for every specimen.
Semilandmarks
… excerpt ends here. Continue reading the full article.
