Recently I’ve been reading Robert Asher’s new book Evolution and Belief: Confessions of a Religious Paleontologist. This book provides an interesting lay-level explanation of evolution. Robert Asher, Curator of Vertebrates at the Museum of Zoology, Cambridge, is not an atheist; he does not rule out the existence of the supernatural or spiritual. He is, as he describes himself, a religious paleontologist. He is not evangelical, and like many he explicitly disavows the designation.
Chapter three of Evolution and Belief is entitled Characters and Common Descent. In this chapter Asher looks at evolutionary trees, intermediates, and transitional features concentrating on currently living species. One common argument brought up to cast doubt on the theory of evolution is the absence or paucity of intermediate or transitional forms. The following video gives one rather notorious example of the argument.
Now I think that Kirk Cameron was playing to the camera here, building an audience for the debate later that night. He is a comedian and knows how to get a reaction. Nonetheless the point is serious, if evolution is true there should be transitional forms in the fossil record. When I’ve asked for arguments people found convincing or would like to see addressed in future posts this issue of transitional forms, or related questions of fossils and evolutionary change, come up repeatedly. But Asher leads us to think hard about what is meant by intermediate or transitional forms in biology. Evolution is not inherently a purposeful progression from simple to complex.
… animals are not really ever in “transition.” Living things are not trying to become something else. Apes from the early Miocene did not anticipate that some of their descendent would evolve into habitual bipeds. …
Terms such as “transition” and “intermediate” are useful because they convey the real sense in which both living and fossil animals mix anatomical and molecular attributes from various parts of the Tree of Life. (p. 47)
Many animals, even those that seem to us to perfectly “normal” exhibit what might be called transitional or intermediate features. In part they are normal because evolution is not like water flowing in a river from point A to B, say the Mississippi from Itasca to the Gulf of Mexico, but more like Lake Shasta filling all available space as the water level rises. The progress of evolution enables access to new areas in the feasibility space for biological life.
The Platypus. The tree of life shown to the right is adapted from Figure 3.1 (p. 44) and illustrates the relationship between some of the species Asher discusses as “intermediate” or “transitional”. The platypus, for example, “lays eggs and has multiple bones in its shoulder skeleton (like a crocodile), but provides milk for its young, shows a single bone in its jaw, and has three ear bones (like a kangaroo).” (p. 45) The echidna is another less well known egg-laying mammal found in Australia and New Guinea.
Their long evolutionary past shows that neither echidna nor platypus is simply a throwback to some 160-million-year-old animal. However, it is equally clear that these animals mix anatomical features otherwise found in reptiles and mammals in just the way one would expect if Darwinian natural selection was the mechanism behind their evolution. (p. 54)
The bandicoot provides a different kind of example of evolution. Although it clearly belongs to the family of marsupials it possesses a placenta that is more like that of nonmarsupial mammals. Paraphrasing from Asher p. 56: All amniotes, animals not requiring direct access to standing water for reproduction, have four amniotic membranes. The amnion surrounds the embryo, the chorion lines the egg, the allantois stores water and waste and the yolk sac stores nutrients. Most marsupials have a placenta that combines chorion and yolk sac. In contrast placental mammals have a placenta that combines chorion and allantois. Bandicoots, unlike other marsupials, have a placenta that combines chorion and allantois like mice, apes, and other placental mammals. This feature of the placenta of the bandicoot illustrates the way that similar anatomical features can evolve independently. Asher gives some plausible explanations for why the combination of chorion and allantois doesn’t dominate in marsupials – but that is secondary to the main point.
The Tarsiers. The tarsier is a small primate found today in the Philippines, Borneo, Sulawesi and Sumatra, but found in fossil forms over a far larger range. This small primate is characterized by a small nose and large eyes. Although it is not apparent from the picture, the tarsiers share a number of features with the lemurs and galagos despite being haplorhines, primates that possess small dry noses and are more visually oriented. In fact, the tarsiers, with their large eyes, are among the most visually oriented haplorhines.
Tarsiers have a jaw structure consistent with the galagos and lemurs and other strepsirhine primates.For example …
the two halves of its jaw, loosely connected to one another in front, not solidly fused into a single, horseshoe-shaped bone as they are in an anthropoid. (p. 59)
But other features are more like the anthropoids (monkeys, apes, humans):
On the other hand, tarsiers resemble anthropoids in having a bony wall in the back of their orbit …, multiple enclosed spaces within their middle ear, a right angle defining the connection of its astralus to its fibula (comprising the ankle), and in not having the capacity to make their own ascorbic acid, or vitamin C. (p. 59)
And perhaps most interesting, like the anthropoids the tarsier lacks a tapetum, a reflective structure within the eyeball. The large eyes of this nocturnal creature compensate for the lack of the tapetum. The galagos has the tapetum and has a larger nose and smaller eyes, although also nocturnal occupying a similar ecological niche. The shiny eye in the picture of a galagos or bush baby to the left is a consequence of the tapetum. The “eyeshine” you see from a deer or raccoon caught in the headlights is also a result of the tapetum, which is common to many mammals.
To wrap it up. This is not an exhaustive list of all “transitional” or “intermediate” forms observed among living creatures. It is only a very small selection of the cases that have been studied and documented. And some of the similarities and differences are only apparent when comparing bones, metabolism, or placental membranes. Others are only apparent when studying the DNA sequences themselves.
Anti-evolutionists can complain that there are unsolved questions and uncertainties, and indeed a careful search of the literature will find qualifications to the generally accepted ideas concerning adaptation and evolutionary relationships that I’ve summarized above. This is the nature of any vibrant scientific field. Nevertheless, the reality is that the Darwinian process of natural selection is reasonable demonstrable as the major explanatory factor in all of the above cases. … Calling such an animal an act of “design” or “creation” simply repeats the fact that they exist. We knew that. No such claim does the hard work of specifying a mechanism by which their particular suite of characters came about in an individual, living species. (p. 62)
Asher misses part of the point here. The one who calls such an animal an act of design or creation is simply stating that God created each in largely the form they currently display. There is no reason to do harder work of specifying a mechanism. But the reason that evolutionary theory is the uncontested basis of all of modern biology is that it works to explain the broad sweep of life. It accounts for the intermediate forms that exist, for the hidden and apparently unnecessary features in many creatures, and for the variety of ways that different animals occupy their various niches. Evolution over long periods of time is not seriously disputed. That natural selection is the mechanism may need some refinement. Natural selection is unquestionably one of the major mechanisms resulting in biological diversity, but it need not be the only mechanism at play.
There is another important point here though. Evolution is not a red in tooth and claw survival of the fittest. As far as any individual animal is concerned, they simply live from day-to-day, and eventually die. The past was much like the present, with changes imperceptible to the average cat, fish, or dinosaur. Evolution works at the level of populations, and gradually populations differentiate and fill different niches in the biosphere. Even here the “intermediate” forms can and do often persist to the present. We have no crocoduck, but we do have the platypus and the echidna.
Do these creatures, for example the platypus and the tarsier, answer some of the questions about transitional and intermediate forms?
What do you expect from intermediate or transitional forms?
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