Phylogenetic inertia

Phylogenetic inertia or phylogenetic constraint refers to the limitations on the future evolutionary pathways that have been imposed by previous adaptations.

Charles Darwin first recognized this phenomenon, though the term was later coined by Huber in 1939. Darwin explained the idea of phylogenetic inertia based on his observations; he spoke about it when explaining the "Law of Conditions of Existence". Darwin also suggested that, after speciation, the organisms do not start over from scratch, but have characteristics that are built upon already existing ones that were inherited from their ancestors; and these characteristics likely limit the amount of evolution seen in that new taxa. This is the main concept of phylogenetic inertia.

Richard Dawkins also explained these constraints by likening natural selection to a river in his 1982 book The Extended Phenotype.

Birds are the only speciose group of vertebrates that are exclusively oviparous. It has been suggested that birds are phylogenetically constrained, as being derived from reptiles, and likely have not overcome this constraint or diverged far enough away to develop viviparity.

There have been several studies that have been able to effectively test for phylogenetic inertia when looking into shared traits; predominantly with a comparative methods approach. Some have used comparative methods and found evidence for certain traits attributed to adaptation, and some to phylogeny; there were also numerous traits that could be attributed to both. Another study developed a new method of comparative examination that showed to be a powerful predictor of phylogenetic inertia in a variety of situations. It was called Phylogenetic Eigenvector Regression (PVR), which runs principal component analyses between species on a pairwise phylogenetic distance matrix. In another, different study, the authors described methods for measuring phylogenetic inertia, looked at effectiveness of various comparative methods, and found that different methods can reveal different aspects of drivers. Autoregression and PVR showed good results with morphological traits.

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