Convergent Evolution

Convergent evolution describes the independent development of similar features in species of different lineages. Two species from unrelated lines can develop the same traits if they live in similar habitats, and have to develop solutions to the same kind of problems. Similar structures among species are either ‘homologous’ (derived from a common ancestors), or, as in the case of convergent evolution, ‘analogous’ (independent adaptations to similar conditions).

The wing is a classic example of convergent evolution in action. Flying insects, birds, and bats have all evolved the capacity of flight independently. They have ‘converged’ on this useful trait. All wings have functional similarities: they are thin and strong, with a wide surface area, and can be mechanically moved in a regular way so as to create lift. However, in each case the wings evolved separately, so their form reflects certain physical necessities.

The ancestors of both bats and birds were terrestrial quadrupeds, and each has independently evolved powered flight via adaptations of their forelimbs. Although both forelimb adaptations are superficially ‘wing-shaped,’ they are substantially dissimilar in construction. The bat wing is a membrane stretched across four extremely elongated fingers, while the airfoil of the bird wing is made of feathers, which are strongly attached to the forearm and the highly fused bones of the wrist and hand, with only tiny remnants of two fingers remaining, each anchoring a single feather. (Both bats and birds have retained the thumb for specialized functions.) So, while the wings of bats and birds are functionally convergent, they are not anatomically convergent. Similarly, the extinct pterosaur also shows an independent evolution of vertebrate forelimb to wing. An even more distantly related group with wings is the insects, they not only evolved separately as wings, but from totally different organs, starting from a fundamentally different bodyplan.

One of the most famous examples of convergent evolution is the camera eye of cephalopods (e.g. squid), vertebrates (e.g. mammals), and cnidaria (e.g. box jellies). Their last common ancestor had a simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye. The similarity of the structures in most respects, despite the complex nature of the organ, illustrates how there may be some biological challenges which have an optimal solution.

The morphology of large, fast-moving aquatic animals tends towards a torpedo shape: tuna, sharks, dolphins, killer whales, ichthyosaurs all have a similar shape. This streamline reduces drag as they move through the water. Fins of some (ichthyosaurs, sharks) occur in the same places on the body. They have arrived at this shape from very different starting points, indicating that they are optimal locations. The enzymology of proteases provides some of the clearest examples of convergent evolution. These examples reflect the intrinsic chemical constraints on enzymes, leading evolution to independently converge on equivalent solutions repeatedly.

Convergence has been associated with Darwinian evolution in the popular imagination since at least the 1940s. For example, Elbert A. Rogers argued that ‘if we lean toward the theories of Darwin might we not assume that man was [just as] apt to have developed in one continent as another?’ The degree to which convergence affects the products of evolution is the subject of a popular controversy. In his book, ‘Wonderful Life,’ biologist Stephen Jay Gould argues that if the tape of life were re-wound and played back, life would have taken a very different course. English paleontologist Simon Conway Morris counters this argument, arguing that convergence is a dominant force in evolution and that since the same environmental and physical constraints act on all life, there is an ‘optimum’ body plan that life will inevitably evolve toward, with evolution bound to stumble upon intelligence, a trait of primates, corvids, and cetaceans, at some point.

In some cases, it is difficult to tell whether a trait has been lost then re-evolved convergently, or whether a gene has simply been ‘switched off’ and then re-enabled later. Such a re-emerged trait is called an ‘atavism’ (the tendency to revert to ancestral type). From a mathematical standpoint, an unused gene (selectively neutral) has a steadily decreasing probability of retaining potential functionality over time. However, the time scale of this process varies greatly in different phylogenies; in mammals and birds, there is a reasonable probability of remaining in the genome in a potentially functional state for around 6 million years.

Convergent evolution can be compared to ‘parallel evolution,’ the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades (i.e. they share a common ancestor that belongs to their own group, and are more closely related to one another than to any other clade—but are very similar forms that evolved in isolation). When the ancestral forms are unspecified or unknown, or the range of traits considered is not clearly specified, the distinction between parallel and convergent evolution becomes more subjective. For instance, the striking example of similar placental and marsupial forms is described by Richard Dawkins in ‘The Blind Watchmaker’ as a case of convergent evolution, because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences.

Stephen Jay Gould describes many of the same examples as parallel evolution starting from the common ancestor of all marsupials and placentals. Many evolved similarities can be described in concept as parallel evolution from a remote ancestor, with the exception of different structures that were co-opted to a similar function. For example, consider Mixotricha paradoxa, a microbe that has assembled a system of rows of apparent cilia and basal bodies closely resembling that of ciliates but that are actually smaller symbiont micro-organisms; or the differently oriented tails of fish and whales. On the converse, any case in which lineages do not evolve together at the same time in the same ecospace might be described as convergent evolution at some point in time.

A related question occurs considering the homology of morphological structures. For example, many insects possess two pairs of flying wings. In beetles, the first pair of wings is hardened into wing covers with little role in flight, while in flies the second pair of wings is condensed into small halteres used for balance. If the two pairs of wings are considered as interchangeable, homologous structures, this may be described as a parallel reduction in the number of wings, but otherwise the two changes are each divergent changes in one pair of wings.

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