Incomplete lineage sorting

Incomplete Lineage Sorting

Incomplete lineage sorting (ILS) is a phenomenon in evolutionary biology where the genealogical history of a set of alleles does not match the species tree. This occurs when ancestral genetic lineages fail to coalesce (or "sort") into a single lineage before the speciation events that separate the descendant species. As a result, the gene tree may differ from the species tree, leading to discordant phylogenetic signals.

Background

In the context of phylogenetics, the species tree represents the evolutionary relationships among a group of species, while a gene tree represents the evolutionary history of a particular gene or genetic locus. Ideally, these trees should be congruent, but due to processes like incomplete lineage sorting, they can differ.

ILS is particularly common in rapidly radiating lineages, where multiple speciation events occur in quick succession. This rapid divergence does not allow sufficient time for ancestral alleles to coalesce into a single lineage, resulting in a mismatch between gene trees and the species tree.

Mechanism

The process of incomplete lineage sorting can be understood through the concept of coalescent theory. Coalescent theory models the genealogical history of alleles in a population, tracing them back to a common ancestor. In a large ancestral population, multiple alleles can persist through several speciation events before coalescing. This persistence leads to the retention of ancestral polymorphisms across descendant species.

When a speciation event occurs, the alleles present in the ancestral population are randomly sorted into the descendant lineages. If the alleles do not coalesce before the speciation event, different descendant species may inherit different alleles, resulting in a gene tree that does not match the species tree.

Implications

Incomplete lineage sorting has significant implications for phylogenetic analysis and the reconstruction of evolutionary histories. It can lead to incorrect inferences about the relationships among species if not properly accounted for. Researchers must use methods that can distinguish between true species relationships and those confounded by ILS.

One approach to address ILS is the use of multispecies coalescent models, which explicitly model the process of lineage sorting across multiple species. These models can help infer the species tree by integrating information from multiple gene trees, accounting for the discordance caused by ILS.

Examples

ILS has been observed in various taxa, including primates, birds, and plants. In primates, for example, the evolutionary relationships among humans, chimpanzees, and gorillas have been complicated by ILS, as some gene trees suggest closer relationships between humans and gorillas, while others support a closer relationship between humans and chimpanzees.

In birds, rapid radiations such as those seen in the Passeriformes have also been affected by ILS, complicating the resolution of their phylogenetic relationships.

Conclusion

Incomplete lineage sorting is a common and important phenomenon in evolutionary biology that highlights the complexity of reconstructing evolutionary histories. Understanding and accounting for ILS is crucial for accurate phylogenetic inference and for unraveling the true evolutionary relationships among species.

See Also

• Phylogenetics
• Coalescent theory
• Gene tree
• Species tree
• Multispecies coalescent model

References

• Degnan, J. H., & Rosenberg, N. A. (2009). Gene tree discordance, phylogenetic inference and the multispecies coalescent. Trends in Ecology & Evolution, 24(6), 332-340.
• Maddison, W. P. (1997). Gene trees in species trees. Systematic Biology, 46(3), 523-536.