The highly productive sciences of biology and paleontology are rapidly bridging the gaps in our knowledge about how disparate life forms diversified to fill all manner of niches on earth. Both in the fossil record and among living biological diversity, biologists and paleontologists are providing myriad colorful examples of ‘transitional’ life forms on a nearly daily basis. These researchers are making life increasingly difficult for bloggers of evolutionary biology like myself. Just how are we supposed to satisfy our obsession of discussing all these finds and still have time in our schedules? Well, now that I’ve ranted, on to the news:
A Fish With Grasshopper Dreams
Terry J. Ord and Tonia Hsieh just published a delightful natural history study about a fish that is very nearly a fully land animal, for it spends the vast majority of its time out of water, and even actively avoids submersion by waves . It is a marvelous living example of what is often referred to as a ‘transitional form': a creature that has characteristics that make it evolutionarily intermediate between two well-established groups or life histories (but beware of the limitations of this term, covered at the end of this post). Why is this so significant? To evolutionary biologists, it is no surprise that evolution repeats itself, resulting in multiple concurrent examples of what look like half-finished projects at various stages of ‘completion’. However, proponents of creationism or intelligent design often point to alleged gaps between major groups of organisms (e.g. land-dwelling vertebrates and fish) in an attempt to support the claim that there is no evidence for the evolution of one group from another. Such evidence may have been less abundant in the days when Darwin published his seminal tome, but after 150 years of research effort, this is no longer the case; not by a longshot.
Ord and Hsieh report in detail the behaviour of a perplexing little fish, the Pacific leaping blenny (Alticus arnoldorum), which, as its name suggests, seems to think it’s a grasshopper. Not only does this highly social colony-breeding fish spend more time out of water than submerged, but it escapes capture by rapidly contorting its body like a discharging spring, propelling it several body-lengths away in a single bound (and making capture for study no easy task). Although the skin and gills of this remarkable little fish must still be periodically moistened, for it breathes through them, it is otherwise entirely terrestrial.
Let that thought really sink in. We have here a land-dwelling fish.
This is a momentous find indeed. However, most people are probably unaware of how many other groups of fish also exhibit adaptations to life out of water to various extents. I review some of these fascinating examples below. Not only do these results catch evolution in the act, providing some of the best examples of living animals that natural selection has moulded into forms transitional between aquatic and land-dwelling creatures. They also lend us a nice view inside the clockwork of evolution, clearly illustrating how it works. Although the first invasion of land by fish during the Devonian Period probably took a different route according to the fossil record, the living examples demonstrate that the transition to land can occur by a number of means. We see several of the ecological factors that can conspire to drive fish onto land.
Most of us are familiar with one of the most unusual fish to leave water (albeit for very short periods of under a minute): flying fish. These creatures propel themselves with their powerful caudal (tail) fin to keep themselves aloft for over 100 m, and up to 400 m, using their enlarged pectoral fins as air foils as they glide just above the surface of the waves at 70 km/h. This flight mechanism facilitates rapid escape from predators. The much lower resistance of air than water makes the atmosphere a much quicker medium through which to move, requiring much less energy to achieve any given speed than through viscous water.
Even though flying fish do not reside out of water for long (and certainly not on land), they illustrate one of the driving forces behind the invasion of land by vertebrates: the escape from natural enemies. Their fast motion also facilitates this escape, just like the hopping habit of the Pacific leaping blenny.
Gouramis, Snakeheads, Blennies and Mudskippers: The Precocious Perciformes
The Pacific leaping blenny belongs to a very large successful group of fish, the order called the Perciformes. Many fish popular with anglers belong to this group, including perches. Interestingly, there are several suborders of fish among the Perciformes that display adaptations for survival out of water. One of the common adaptations is the development of lung-like organs, and this is overwhelmingly in response to poor oxygen conditions in their environment that make breathing by gills alone difficult.
Does a Betta or a kissing gourami inhabit your aquarium? If so, then you have an air-breathing fish. Members of the suborder Anabantoidei, including popular aquarium fish such as gourami and Siamese fighting fish, are known as labyrinth fish. The name refers to an interesting adaptation that the group shares. They possess a highly branched and folded extension of the gills, called the labyrinth organ. This structure allows the fish to gulp air at the surface of their often poorly aerated pools to supplement their oxygen intake. The labyrinth organ is richly supplied with blood vessels to soak up oxygen from the gulped air, rather than from water. Water is not good at carrying oxygen as it is, compared to a gas, but stagnant tropical water holds even less. So the presence of a rich supply of oxygen just above the water’s surface provides a strong selection pressure for a creature to develop a way to tap into it. Interestingly, this group of fish has evolved to depend so highly on the labyrinth organ for oxygen that they would drown if prevented from reaching the surface. So yes, you can drown a fish. These fish, anyway.
Note that the labyrinth organ is an adaptation to breathing air for the express purpose of circumventing the poor oxygenation of their watery environs, but not for life on land. Thus, here is evidence that an adaptation can evolve for a different purpose that that for which it might ultimately be useful. This is called preadaptation. It illustrates that early land animals may have already been breathing air before making their transition to land. This is a very important observation, and it provides a nice retort to the suggestion that major evolutionary changes (e.g. transition from life in the water to life on land) require too many simultaneous adaptations to occur. In fact, many of these transitions probably took place by such ‘baby steps’.
However, some of the labyrinth fish have another land adaptation up their sleeves. The climbing gouramies (family Anabantidae), as their name suggests can climb out of their pools of water and ‘walk’ short distances to other pools, pushing themselves along with their tails and using their gill plates as support. This is a wonderful adaptation because it allows an aquatic creature to not only escape predators that cannot follow it onto land, but it also facilitates escape from drying pools that would ordinarily doom aquatic species, and also allows them to disperse their genes more quickly and over greater distances than swimming alone would allow. In some species, this ability has made them extremely adept at getting around. The snakeheads (suborder Channoidei: family Channidae) are terribly invasive species in North America, for example . They also possess a labyrinth organ, allowing them to stay alive breathing air for up to four days. In this time, they can cross moist ground for distances up to 1/4 mile.
More familiar to most people are mudskippers, bizarre-looking members of the gobies (suborder Gobioidei), a sister group to the blennies. Mudskippers look for all the world like amphibians, with their raised eyestalks and their habit of sitting on rocks and branches above the water line, with only their tails soaking in the water in order to keep their skin moist. Like the Pacific leaping blenny, they are also capable of catapulting themselves great distances (up to 60 cm or 2 feet) into the air. These weird animals breathe air by a number of means: (1) through the moist skin, enriched in fine blood vessels called capillaries, (2) through an especially capillary-rich area in the buccopharyngeal cavity (a region at the back of the mouth and throat), and (3) through their specially adapted gill chambers, which are sealed shut to retain water when the fish moves onto land . The air is swirled around in the gill chamber by rotation of the large eyes, which are used as pumps. Mudskippers also show a wonderful example of how the pectoral fins can be used for locomotion on land. The special elbow-like bends in the fins allow them to be used to prop up the animal and propel it forward.
Electric Eels, Lungfish, Walking Catfish, Bowfins, Eels
Some manner of terrestrial adaptations have arisen in nearly 20 diverse groups of modern fish, including ‘walking’ catfish and eels that can move between bodies of water, breathing air during the journey.
Everyone knows what gives electric eels (Electrophorus electricus) their name — they can deliver 500 watts of electric energy to stun their prey, a lethal dose for humans. But how many of us know that electric eels are obligate air breathers, needing to surface every 10 minutes or so to gulp air for oxygen exchange in an oral cavity . They get about 80% of their oxygen this way!
The air breathing organs of an ancient group of fish called bowfins (order Amiiformes) are derived form the swim bladder, which is a special organ used by bony fishes to maintain neutral buoyancy (float) in the water. In the bowfin, the swim bladder is lined with capillaries and used to help oxygenate their blood because they live in slow moving and oxygen-poor waters. The significance of this adaptation is that the air bladder is homologous with our lungs. In other words, it derives from the same tissues during development, and our lungs are probably evolutionarily derived from swim bladders of early fish.
Lungfish (subclass Dipnoi) take this vascularization (lining with capillaries) of the swim bladder to an even greater extent. Their swim bladder is divided into a great number of smaller sacs, thus considerably increasing the surface area for gas exchange. As their name implies, these structures serve as lungs. Their lungs are an adaptation to their highly marginal or seasonally harsh environment. The lungs help them to survive for months at a time through the dry season during which their bodies of water dry up. During this time, lungfish breathe only air, while they remain entombed in cocoons in the mud with only a breathing tube connecting them to the atmosphere.
The Fossil Record: Tiktaalik, Ventastega and Friends
This post deals mainly with living examples of evolutionary intermediates between aquatic and terrestrial species, and I will cover the fossil record in more detail in a future post (and I have also briefly discussed the fascinating story of the discovery of Tiktaalik, a wonderful example of an intermediate form between fish and amphibians, in the post “Wasp-Infested Dinosaur Eggs and the Science of Paleontology“). Even better examples are still being found, bridging the gap between even Tiktaalik and amphibians. One example is Ventastega, which has shoulder and hip characteristics that place it between Tiktaalik and early amphibians such as Acanthostega .
“God of the Gaps” Meets Achilles and the Tortoise
Perplexingly, despite the accumulating finds of transitional forms that help patch holes in our knowledge of the fossil record, proponents of creationism sometimes view this development as simply an increase in the number of gaps that need to be filled; each gap is simply bisected by a newly discovered transitional form, making two holes out of one and multiplying the burden of proof for scientists! This absurd conclusion is a spin-off of the theological viewpoint popularly referred to as the “God of the Gaps” argument. It is of course a simple logical fallacy, akin to the Achilles versus tortoise paradox, in which the ancient greek runner Achilles appears to be unable to catch up to a tortoise with a head start, but only if the progress of Achilles’ is dissected into incrementally shorter segments delineated by the advance that the tortoise’s makes each time that Achilles catches up to the point occupied by the tortoise on the previous iteration. To escape from the paradox, we must realize that the problem lies in believing that the later shorter segments of advance were given the same temporal importance as the earlier longer ones. Similarly, it is crucial to realize that the closing of the evolutionary distance between established life forms is more important than the number of the gaps between them, because a shorter gap in fact presents a proportionately smaller problem.
Black (and Numerous Shades of Grey) and White
Another complaint that evolutionary biologists face regarding transitional forms is based on the erroneous view that the world is only black and white, that any given transitional form must be viewed as either belonging to one group or another, and that there is no intermediate grey area that any organism can occupy. This view is probably most vocally expressed in discussions of evolutionary anthropology, in which creationists that refuse to accept the evolutionary descent of humans from earlier primates attempt to mask the clearly quantifiable gradient in characteristics between humans and ancestral primates by choosing to classify a hominid fossil as either human or non-human ape (more on this in a later post).
Taxonomy (the science of categorizing and naming creatures) may sometimes appear to try to force a species into one group or another. However, the purpose of taxonomy is mainly to allow us to distinguish species from each other by a list of carefully measured characteristics. However, taxonomy was established as a field of study before the evolutionary relationship between organisms was discovered, and therefore taxonomy lacks a rigorous quantitative estimate of how closely related two groups are to each other. Under the encompassing field of systematics, the hierarchical classification system of taxonomy must strive to maximally reflect the results of a sister branch of study called phylogeny, which attempts to delineate the ancestor/descendent connectivity of different life forms by measuring both the topology (branching pattern) and evolutionary distance between these groups. Phylogenetic and evolutionary studies that show a clear gradual progression in the development of a trait over time (either in the fossil record or in rapidly evolving living systems) highlight a problem that taxonomy by itself is hard-pressed to resolve: it becomes ever more difficult to pigeon-hole a life form into one of two groups if it is evolutionarily intermediate between the two. Taxonomy may be up to the task, but it becomes increasingly impractical as we near the midway point, for the number of characteristics that need to be clearly defined to differentiate the two groups rises, and at some point, arbitrary decisions need to be made regarding, say, the length of a bone in assigning it to one or the other group. Ultimately we can maximally resolve the evolutionary lineage to the point of individuals in the line of descent. Those are the quanta. But how do we know at which individual (had we all of the fossils in our hands even!) to decide to draw the line between fish and amphibians?
This problem is well illustrated by trying to decide at what point a red-to-blue gradient of paint becomes unambiguously blue. Even if we decided to define the border by a particular exact wavelength of the light, our staunch position does not eliminate the fact that the red and blue ends of the gradient are, in fact, connected by a smooth gradient, not a sharp discontinuity. Such is the case with the fossil record. Even if one decides to call one fossil a fish and another an amphibian, it is a misleading distinction if it does not consider the fact that there is a gradual progression of forms that are most parsimoniously explained as expressing gradual evolutionary change.
How Meaningful is the Term “Transitional Form”?
It’s important to add a healthy grain of salt to our discussion of transitional forms. It is reasonable to view every species, especially those from the fossil record, as being transitional; it is statistically unlikely that any given fossil represents the last individual or species in its given line of descent before that particular branch went extinct (and most branches do). The process of fossilization is so sporadic that we are not likely to predictably come across something that is this precisely positioned in the line of descent of a group — except perhaps at a well-defined boundary demarcating a mass extinction beyond which a group is highly unlikely to have survived, (but even some of these boundaries may not have been guillotine sharp with respect to extinction, e.g. some dinosaurs may have survived the asteroid impact 65.5 million years ago by as much as 700,000 years, which I discussed in a previous post, “Dating Dinosaurs“). The only creatures that we can hold in our hands and be certain to be currently at the true terminus of their own lines of descent are living specimens.
But although the vast majority of fossilized organisms are almost certainly transitional between ghosts of evolution that fossilization never preserved, perhaps none of them are truly transitional in a geneological sense between two specific fossils that we can hold in our hands which nonetheless look like the two states of evolution that our specimen bridges. As pointed out in a previous post (“Archaeopteryx: Should Feathers Fly Over Its Fall?“), any given fossil specimen is most likely a member of a short evolutionary branch from the main line of descent in which we’re interested, because it is statistically unlikely in the extreme that the individual preserved for millions of years belonged to the infinitesimally narrow bridge of descent connecting one ancestor with another distant descendent. This situation is a natural result of the incredible bushiness of the tree of life; there are far more terminal twigs than main connective branches.
So, should we even use the term “transitional form” if it is so ambiguous? I have chosen to employ it in this post mainly because it is a familiar term, but I have endeavored to explain its limitations. Nevertheless, semantics aside, the evolutionary intermediacy of organisms between highly different forms and lifestyles is very real, and both the fossil record and the census of living organisms today provide wonderful examples to support this conclusion.
1. Terry J. Ord and S. Tonia Hsieh. (2011) A Highly Social, Land-Dwelling Fish Defends Territories in a Constantly Fluctuating Environment. Ethology, 26 DOI:10.1111/j.1439-0310.2011.01949.x
2. Böhme, Madelaine (May 2004). Migration history of air-breathing fishes reveals Neogene atmospheric circulation patterns. Geology 32 (5): 393–396. doi:10.1130/G20316.1
3. A. Graham JB, ed (1997). Air–breathing Fishes. Evolution, Diversity and Adaptation. San Diego California: Academic Press.
4. Johansen, Kjell (1968). Gas Exchange and Control of Breathing in the Electric Eel, Electrophorus electricus. Z. Vergl. Physiologie (Springer Berlin / Heidelberg) (Volume 61, Number 2 / June, 1968): 137–163.
5. Per E. Ahlberg, Jennifer A. Clack, Ervins Luksevics, Henning Blom, Ivars Zupins. (2008) Ventastega curonica and the origin of tetrapod morphology. Nature 453 (7199): 1199 DOI: 10.1038/nature06991