Phylogenetic inertia

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

[5] Birds are the only speciose group of vertebrates that are exclusively oviparous, or egg laying.

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, or live birth.

[8][9] 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.

[12] Another study developed a new method of comparative examination that showed to be a powerful predictor of phylogenetic inertia in a variety of situations.