page_title: Phylogenetic inertia
section_title: Body plan
hierarchical_section_title: Phylogenetic inertia / Examples of phylogenetic inertia / Body plan
caption_attribution_description: English: A cladogram of the evolution of tetrapods showing the best-known transitional fossils. From bottom to top: Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega, Ichthyostega, Pederpes. Español: Cladograma de la evolución de los tetrápodos, donde se muestran los fósiles de transición más conocidos.
context_page_description: 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.
context_section_description: Most terrestrial vertebrates have a body plan that consist of four limbs. The phylogenetic inertia hypothesis suggests that this body plan is observed, not because it happens to be optimal, but because tetrapods are derived from a clade of fishes (Sarcopterygii) which also have four limbs. Four limbs happened to be suitable means of locomotion, and so this body plan has not been selected against.
  Humans do not have optimal structure for bipedalism, because much of the human body plan originally evolved under quadrupedal locomotion, and has since been constrained because of phylogenetic inertia.
caption: Evolution of fish to tetrapods.  The basic body plan has been phylogenetically constrained.
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