Missing Link

The phrase missing link is a colloquial term describing a transitional form between taxa in an evolutionary [Page 1602]scheme. Because the primary tenet of evolution is morphological change over time, there is necessarily going to be transitional forms between any two species sharing an ancestor–descendant relationship. Charles Darwin recognized this, and in fact the apparent lack of transitional forms was one of the most significant concerns he expressed in The Origin of the Species. This concern reflected Darwin's belief that evolution occurred primarily through gradual changes. Therefore, most species at any one time in the past would be in a transitional state, and the fossil record should be full of such examples.

During the decade following the publication of The Origin of the Species, this philosophical idea came to underlie many subsequent works on evolution. A strong form of this belief was the development of recapitulation laws by several different researchers, most notably Ernst Haeckel. The law of recapitulation states that evolutionary adaptations proceeded as additions to the development cycle of organisms (ontogeny) and that, therefore, organisms encapsulate the adult morphology of their ancestors during their own growth and development. In other words, ontogeny recapitulates phylogeny. The development of genetics and embryology eventually discredited the simple idea that additional developmental stages are the sole mechanism of evolution, but the idea had a great impact on later research.

During the 1940s, Richard Goldschmidt proposed the idea that evolutionary novelties might have appeared instantaneously, within one generation, through mutations in parts of the genome responsible for developmental changes. This idea became known as the “hopeful monster” hypothesis. Although there are a few limited cases of significant change that can occur between generations (e.g., possibly hairlessness in the naked mole rat), the vast majority of mutations affecting major developmental sequences are lethal or extremely maladaptive. The upshot of the hopeful monster hypothesis is that evolutionary change could occur nearly instantaneously and that transitional forms might not exist at all in some cases. This hypothesis is largely considered incorrect with regard to phyletic change, but modified forms of the hypothesis that focus on fast evolutionary change are still strongly considered for particular evolutionary sequences. Punctuated equilibrium, in particular, was strongly influenced by this idea.

When the term missing link is used, many individuals think of transitional forms between very disparate taxa such as Archaeopteryx, which has been presented as a transitional form between birds and reptiles. However, taxa do not reproduce to create other taxa. Rather, individuals reproduce to create other individuals. Therefore, the only true missing links that can exist are individual organisms that form a distinct lineage. There is no single individual that marks a transition between ancestor and descendant; instead, all conspecifics are links with characteristics that show variation. The limits of the fossil record might seem to showArchaeopteryx as a fantastic chimera of avian and reptilian features, but the individual specimen was simply a member of a variable lineage and was little different from its parents or its offspring. There are rarely hopeful monsters in evolution.

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