In a study of the phylogeny of Old World monkeys (Hayasaka...

In a study of the phylogeny of Old World monkeys (Hayasaka et al. 1996 ), the three individual rhesus macaques that were studied did not form a monophyletic group. Instead, the mtDNA of one of the rhesus macaques was more similar to the mtDNA of Japanese and Taiwanese macaques (which are different species) than it was to the other rhesus macaques. How might this have happened? (There are at least two possibilities.) With this result as background, explain why it is useful that the phylogeny in Figure 20.4 includes several individuals from each species.

Key Concepts

Monophyly

Monophyly refers to a group in a phylogenetic tree that consists of a common ancestor and all of its descendants. In phylogenetic studies, identifying monophyletic groups is essential for accurately placing species in an evolutionary context and understanding their relationships.

Mitochondrial DNA (mtDNA)

Mitochondrial DNA is a genetic marker that is inherited maternally and typically presents a rapid rate of evolution. Its use in phylogenetic studies provides insights into the maternal lineage and can reveal relationships and divergences among closely related species or populations.

Hybridization and Introgression

Hybridization occurs when individuals from different species interbreed, potentially leading to the transfer of genetic material from one species to another (introgression). This process can result in unusual phylogenetic patterns, such as one individual's mtDNA being more similar to that of another species, thereby complicating the interpretation of evolutionary relationships.

Incomplete Lineage Sorting

Incomplete lineage sorting is a phenomenon where ancestral genetic diversity is not completely separated into descendant species following a speciation event. This can lead to gene trees (such as those based on mtDNA) that differ from the actual species tree, as some individuals may retain ancestral genetic variants that have persisted across species boundaries.

Sampling Multiple Individuals per Species

Including several individuals from each species in phylogenetic analyses helps capture the range of genetic variation within species. This comprehensive sampling is critical for detecting processes like hybridization, introgression, and incomplete lineage sorting. It ensures that patterns observed in the tree are reflective of overall species diversity rather than anomalies from single individuals, thereby providing a more robust and accurate depiction of evolutionary relationships.

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