Phylogenetic inertia or phylogenetic constraint refers to the limitations on the future
evolutionary pathways that have been imposed by previous
adaptations.[1]
Charles Darwin first recognized this phenomenon, though the term was later coined by Huber in 1939.[2] Darwin explained the idea of phylogenetic inertia based on his observations; he spoke about it when explaining the "Law of Conditions of Existence".[3] 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.[4] This is the main concept of phylogenetic inertia.
Evolution of fish to tetrapods. The basic body plan has been phylogenetically constrained.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.[6]
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.[7]
Modes of reproduction
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]
Homologous structures
More specifically than the similarity in body plan, there are
homologous bones across
mammalian taxa. For example, the
pentadactyl limb bone structure observed in the arms of
primates, front legs of equestrians, in the wings of
bats, and the flippers of
seals. The fact that they are homologous is further evidence for phylogenetic inertia; these structures have been modified over time, but they are constrained by common ancestry.[3][4]
Homologous bone structure in forelimbs of four vertebrates.
All vertebrates share a homologous eye structure that effectively have a
blind spot in them where the
optic nerve attaches to the
retina.[10]
Tests for phylogenetic inertia in study systems
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.[11][12][13] 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.[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. It was called Phylogenetic
Eigenvector Regression (PVR), which runs
principal component analyses between species on a pairwise
phylogenetic distance matrix.[11] 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.[13]
^Dawkins, Richard (1982). The Extended Phenotype: The Gene as a Unit of Selection.
Oxford University Press. p. 42.
^Lewin, Roger (1980). "Evolutionary Theory Under Fire". Science.
^Gould, S. J.; Lewontin, R. C. (1979-09-21). "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme". Proceedings of the Royal Society of London B: Biological Sciences. 205 (1161): 581–598.
Bibcode:
1979RSPSB.205..581G.
doi:
10.1098/rspb.1979.0086.
ISSN0962-8452.
PMID42062.
^McKitrick, Mary (1993). "Phylogenetic Constraint in Evolutionary Theory: Has it any Explanatory Power?". Annual Review of Ecology, Evolution, and Systematics. 24: 307–330.
doi:
10.1146/annurev.es.24.110193.001515.
^Blackburn, Daniel; Evans, Howard (1986). "Why are there no Viviparous Birds?". The American Naturalist. 128 (2): 165–190.
doi:
10.1086/284552.