20.1 Organizing Life on Earth
Scientists continually gain new information that helps understand the evolutionary history of life on Earth. Each group of organisms went through its own evolutionary journey, or its phylogeny. Each organism shares relatedness with others, and based on morphologic and genetic evidence, scientists attempt to map the evolutionary pathways of all life on Earth. Historically, scientists organized organisms into a taxonomic classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary relationships.
20.2 Determining Evolutionary Relationships
To build phylogenetic trees, scientists must collect accurate information that allows them to make evolutionary connections between organisms. Using morphologic and molecular data, scientists work to identify homologous characteristics and genes. Similarities between organisms can stem either from shared evolutionary history (homologies) or from separate evolutionary paths (analogies). Scientists can use newer technologies to help distinguish homologies from analogies. After identifying homologous information, scientists use cladistics to organize these events as a means to determine an evolutionary timeline. They then apply the concept of maximum parsimony, which states that the order of events probably occurred in the most obvious and simple way with the least amount of steps. For evolutionary events, this would be the path with the least number of major divergences that correlate with the evidence.
20.3 Perspectives on the Phylogenetic Tree
The phylogenetic tree, which Darwin first used, is the classic “tree of life” model describing phylogenetic relationships among species, and the most common model that scientists use today. New ideas about HGT and genome fusion have caused some to suggest revising the model to resemble webs or rings.