Biologists today may choose among and sometimes combine several systems of classification. Increasingly, systematic taxono-mists agree that an organism's classification should reflect phylogeny. Recall that phylogeny is the evolutionary history of a species or a taxon. Thus, modern taxonomists are often involved in phylogenetics, the analysis of the evolutionary or ancestral relationships among taxa.

Systematists usually use several types of evidence to hypothesize about phylogenetics. Often, they compare the visible similarities among currently living species or fossils from extinct organisms. Biologists may also compare patterns of embryonic development and the ways in which the embryos of different species express similar genes. Furthermore, they may compare similar chromosomes and macromolecules, such as DNA or RNA, from different species to deduce phylogenetic relationships.

Systematists often represent their hypotheses in the form of a phylogenetic diagram, also called a phylogenetic tree. This type of diagram looks like a family tree and has a branching pattern that indicates how closely related a subset of taxa are thought to be. As with all hypotheses, phylogenetic diagrams may change whenever new discoveries and investigations cause scientists to revise their hypotheses.

Evidence of Shared Ancestry

Biologists use fossils as important clues for the timing of evolutionary changes and divergence. The use of fossils in the study of evolutionary relationships is critical, but the fossil record may lack evidence about many kinds of organisms, especially small and soft-bodied organisms, such as worms, fungi, or bacteria. The fossil record can help provide the framework of a phylogenetic diagram, but a systematist would test inferred relationships with additional evidence.

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