452
A phylogenetic tree is a description of evolutionary relationships among organisms or their genes. All living organisms share a common ancestor and are related through the phylogenetic tree of life.
learning outcomes
You should be able to:
Draw and label the parts of a phylogenetic tree and explain the biological interpretation of each part.
Make inferences and predictions about evolutionary groups based on a phylogenetic tree.
Explain how homoplasies (convergences and reversals of character states) are accounted for when reconstructing phylogenetic relationships.
What biological processes are represented in a phylogenetic tree?
Phylogenetic trees can represent any process in which biological lineages diverge, such as speciation (in which case the tree will depict the evolutionary relationships among species), viral replication (in which case the tree will depict the evolutionary relationships among different viral lineages), or gene duplication (in which case the tree will depict the evolutionary relationships among genes). In most cases in this book, phylogenetic trees are used to depict the evolutionary relationships among species or higher groups of organisms.
Why is it important to consider only homologous characters in constructing phylogenetic trees?
Because homologous characters are similar as a result of their common descent. Similarities that result from convergent evolution (the wings of birds and insects, for example) can be misleading about evolutionary relationships if they are mistaken as homologous characters.
What are some reasons that similar traits might arise independently in species that are only distantly related? Can you think of examples among familiar organisms? How do biologists account for these homoplasies in reconstructing phylogenies?
Selection for similar environmental conditions often leads to convergence in traits. For example, fish and dolphins both have fins and are similar in body shape because there is strong selection for these traits in an aquatic environment. But these traits have evolved independently in the two groups. Biologists can usually detect such homoplasies because they conflict with a large number of homologous traits in the groups that are similar as a result of their recent shared ancestry. Biologists can minimize homoplasies using the principle of parsimony.
Phylogenetic analyses have become increasingly important to many types of biological research in recent years, and they are the basis for the comparative nature of biology. For the most part, however, evolutionary history cannot be observed directly. How, then, do biologists reconstruct the past? One way is by using phylogenetic analyses to construct a tree.