Reasons Why Evolution is True Part X:
Convergent Evolution

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When you design many objects that perform similar tasks, the logical strategy is to reuse the same design, perhaps with small modifications, for each object. There would be little point in coming up with a new design every time, right!? In nature, however, there are many species that do similar things but have arrived at their method through different designs. This is known as convergent evolution.

Intelligent design, and decent by modification, predict different patterns of similarities and differences between species. Evolutionary theory, which places all living things on a tree of relatedness, leads us to expect that species that are more closely related to each other should tend to be more similar. This is because they have both evolved from a recent ancestor. This ancestor has been ‘modified’ in various ways by natural selection to produce the two (or more) daughter species, but with a shared starting point for these modifications, we expect a fairly similar outcome. Traits that are shared between species due to shared ancestry are known as homologies. Homology has been the basis for determining relatedness between species (phylogeny) for hundreds of years. However, as early taxonomists noted, there are some occasions when species share traits despite the lack of a recent common ancestor. Often these species have reached a similar solution to a shared problem, despite being only very distantly related. This is known as convergence, and the more we look for it in nature, the more we find.

Like homology, convergence is rife within the animal kingdom, and beyond. Convergence can be found in everything from large-scale morphological adaptations right down to the cellular level of enzymatic function and gene sequence. And, time and again, examples of convergent evolution tell the same story; when presented with a certain set of environmental pressures, natural selection will often arrive at similar adaptations. The extent to which convergence is total tells us something about the constraints placed on evolution. It is rare that perfect convergence is possible, and relics of past ancestry are usually evident.

Powered Flight

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The evolution of powered flight is an excellent example of convergent evolution, and the limitations imposed by evolutionary history. Birds and bats have both independently evolved flight. Due to a very distant shared ancestry, both birds and bats have modified the basic pentadactyl (five-fingered) hand to form a wing, however the specifics of these designs are quite different. Bat wings are formed from long flaps of skin stretched across elongated fingers, whilst bird wings are composed of many feathers which cover the arm and fingers. Bird fingers are also reduced both in size and number. Thus both bats and birds have achieved the same end goal: flight, but by slow, generation-by-generation modifications to the body plan they inherited from their ancestors.

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A third independent origin of flight is exemplified by the extinct Pterosaurs of the Late Triassic and Cretaceous (210 – 65.5 mya). Birds and Pterosaurs share many similar adaptations to flying, such as hollow bones, a keelen sternum for flight muscle attachment, and feathers (or wing fibres). However, there are also differences, which reflect the independent evolutionary origins; in birds the wing is supported by an elongated radius and ulna, and modified wrist bones. In contrast, the pterosaur wing was supported by an elongated 4thdigit (pinky finger). Although their shared ancestor did not fly, it may have possessed feathers for thermoregulation, and thus this shared feature may in fact be a homology, rather than an example of convergence. Many insects, too, have evolved powered flight. There are some basic similarities, such as the use of wings, however the pronounced differences reflect the very distant evolutionary relationships. Insects had a totally different body plan from which to evolve flight, and as such the similarities are limited.

Homology and Convergence in Vertebrates

There are examples of convergence within and between these groupings, also. From an early insectivorous ancestor, two groups of bats switched to a diet of fruit; the Phyllostomidae microbats of the New World, and the Pteropodidae megabats of the Old World. In these two independent appearances of a frugivorous diet, the Phyllostomidae and the Pteropodidae have also converged on several aspects of their wing design, which better enable them to access fruit in their environment. Despite being only distantly related, these two groups of bats are more similar in terms of wing allometry, brain size and outer ear function, than they are to other, more closely related, species of bat. Megabats, incidentally, are very similar to birds in terms of wing allometry, despite their separate evolutionary histories.

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Amongst mammals, bats are the only species capable of powered flight. Within bats, only the microbats are capable of sophisticated laryngeal echolocation. Megabats instead rely on vision for navigation and foraging. Although it is shared by all microbats, molecular data suggests that it may have evolved independently in several microbat lineages, although this is still a point of contention amongst specialists. Echolocation appeared around 50mya, which closely coincided with a rapid diversification of bats. Thus, it is difficult to be certain whether echolocating predated this expansion, or occurred multiple times in separate bat lineages. If the latter were the case, this would be an example of parallel evolution, as both lineages evolved the same adaptation in parallel from the same ancestral starting point. While it is still not clear whether echolocation has a single, or several parallel origins, one definite example of convergent evolution occurred between the echolocating bats and the toothed whales. Toothed whales, such as dolphns, porpoises, killer whales and sperm whales, have evolved a form of echolocation, which while serving the same purpose as bat calls, is created using a different mechanism. Toothed whales emit high-frequency clicks by passing air through bone nares to phonic lips, in contrast to the laryngeal involvement in bats. The shape of the whale’s head is key in directing the sound towards its target. For example, the melon is a large, fatty organ on the top of the skull, which is used to modulate the beam of clicks.

And whales share convergence with other species with whom they share a habitat; fish. Fish fins are composed of bony skin-covered spines which protrude from the body. They come in a variety of forms depending upon where they are located on the body, and different forms are adapted to slightly different roles. By contrast, the fins or flippers of whales are modified forelimbs, and show the same pentadactyl limb structure that also forms our hands and the wings of bats and birds. They also have flukes at the end of their tail, which again aid swimming, and strongly resemble the shape of the caudal fins of some fish.

Pollination and the Animals who Pollinate

Outside the animal kingdom, convergence is rife, as well. One nice example of convergent coevolution comes from the pollinators and their plants. Many species of plant are adapted to animal pollination, and their smell, appearance and structure are strongly determined by the pollinators they have coevolved with. Where distantly related species of plant both converge on a similar pollinator, they often also converge on a system for attracting it. Butterfly-pollinated plants, for example, tend to be large and pink or lavendar in colour. Plants that specialise in attracting beetles and flies have convergently evolved the odour of carrion, which is enticing to their pollinators. Both moth- and bat-pollinated plants have to tackle the problem of attracting a nocturnal pollinator. And in response, both are night-flowering, large and white. Likewise, where distantly related species of pollinator have both converged on a similar plant or type of food source, they too have converged on a way of obtaining it. Hawk moths and hummingbirds both hover in front of their food source and there are some stark similarities in their morphology, despite being only extremely very remotely related. To account for the high-energy lifestyle of the pollinator, hummingbird- and hawk moth-pollinated species have also evolved to provide large quantities of nectar.

These are just some of a plethora of examples I could have given, such as the convergent evolution of suspended locomotion (hanging upside down) in two- and three-toed sloths, of slow-wave sleep in birds and mammals, of hind-limb dominance in humans and lemurs, of the spiny exteriors of echidnas, hedgehogs, tenrecs and porcupines, or of silk production in spiders, silk moths, caddis flies and weaver ants. I could have mentioned the camera eyes of cephalopods and vertebrates, or the protective thorns and spines of numereous species of plant. All these examples so far represent functional convergence, where the same end result has been achieved through different morphological adaptations. In fact, convergence can occur at a variety of different levels; functional, anatomical, protein, gene.

Genetic Convergence

Returning to echolocation in bats, a number of genes have now been implicated in its evolution and have revealed some surprisingly insights into convergence. The FOXP2 gene (known to be important in the evolution of language in humans) has been implicated in the evolution of echolocation in bats. Although it is remarkably well conserved across almost all mammals, FOXP2 in bats is highly variable and this variation correlates strongly with differences in call type. Based on this gene alone, echolocation appears to have arisen independently at least twice in the microbat lineages.

Another gene, Prestin, had provided even more intriguing results. Prestin encodes a transmembrane motor protein involved in the motility of the outer hair cells in the cochlea. These hairs act as an amplifier providing much more sensitive hearing, and Prestin is therefore thought to be directly responsible for cochlear sensitivity in the mammalian ear. Sequence variation in Prestin reveals a large divergence between megabats (non-echolocating) and microbats (echolocating), and suggests a single origin of echolocation in all bats, subsequently lost in megabats. When we look outside of bats, Whales also show sequence evolution in the Prestin gene. Most startling of all, they seem to have undergone the same adaptive mutations to their Prestin gene. A phylogeny contructed based upon this gene shows whales and bats as closely related, but this pattern is not seen when only synonymous mutations (mutations which, through the redundancy of the genetic code, have no impact on the resulting protein) are considered. This suggests that, at least in this case, the opportunity was available for near-perfect convergence at the sequence level. That the synonomous mutations do not show this pattern indicates strongly that it is still evolution behind the wheel, however, as it is only the selectively beneficial mutations which have been pushed to converge. One more interesting point, it is only the dolphins and porpoises whose Prestingene converges strongly with bats, sperm whales for example, are more similar to non-echolocating whales. This makes sense when we look at their niches, porpoises and dolphins both emit constant high frequency calls, and thus is it essential for them to be able to have highly sensitive hearing capable of detecting these frequencies. By contrast, sperm whales emit lower frequency calls which do not necessitate exceptional hearing.

This is an extreme example. It is thought to be rare in nature for the appropriate genetic variation and selection pressures to be present to enable sequence convergence in line with morphological convergence. More commonly, at some level or another, differences reveal separate evolutionary origin. Where DNA sequence convergence has not been possible, in many cases enzymatic functional convergence has been, thus creating apparent perfect convergence at the level of the cell. Sugar kinases, which are responsible for breaking down sugars, have evolved independently in different lineages. They can be broadly grouped into three kinase families, and each family has a distinct three-dimensional structure, and yet they all catalyse chemically equivalent reactions on similar or identical substrates. The need to digest sugar has arisen many times in the history of life, and evolution has repeatedly solved the problem, albeit in slightly different ways. Convergent evolution amongst enzymes is actually quite a common phenomenon, and numerous examples have now been identified. At the level of the whole genome, too, convergence is apparent. The process of genomic imprinting, by which the genome is modified through methylation of specific genes, silencing them in the next generation, has independently evolved in both plants and mammals.

Wherever we look in nature, convergence is apparent. In extreme cases, gene sequences can converge in distantly related species subjected to the same environmental pressures. More commonly, convergence is achieved at a cellular, anatomical or just functional level, despite differences at a genetic, enzymatic or even morphological level. Convergent evolution generally acts to solve the same problem but in a slightly different way. When we observe these clear similarities in nature, what is also apparent are the differences. These differences, their nature and their extent, can inform us about the constraints evolution has placed on convergence. The disparity of the starting points can strongly influence how similar a trait in two species can become. The availability of genetic variation in a particular trait may also constrain selection in producing perfect convergence. Sometimes, two solutions can solve the same problem equally well, and differences may reflect slight variations in the problem that is being solved. Nevertheless, the widespread occurrence of convergence throughout the kingdoms of life strongly supports the theory of evolution. It beautifully demonstrates the concept of decent with modification: the “design” we see in extant species is strongly influenced by the “design” of their ancestors. Selection will act to try and solve problems with whatever raw material is available, and this can produce convergence, with varying degrees of success.

What ‘intelligent designer’ would work so hard to modify an old design to fit the purposes of a new model when they could simply scrap it and start again? Or better yet, copy the design they used last time for this same purpose?

Articles in this Series:

  1. Intro: Reasons Why Evolution is True
  2. Part One: The Panda’s Thumb
  3. Part Two: Parasitoid Wasps
  4. Part Three: Ring Species
  5. Part Four: Galapagos Finches
  6. Part Five: The Quirky Human Eye
  7. Part Six: Homology
  8. Part Seven: Coevolution
  9. Part Eight: PreCambrian Rabbits
  10. Part Nine: DIY Evolution
  11. Part Ten: Convergent Evolution

Want to Know More?

Featured image used under a creative commons licence from Wikimedia commons. Image by DiBgd.

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