Evidence of Evolution
Evolution is descent with modification
Modification - Evolutionary Processes:
Natural Selection: Most educated people have learned about Charles Darwin's Theory of Evolution by Natural Selection, as least in high school, so we needn't belabor it here. Quick review of Darwin's tenets:
- Diversity: Diversity of form among individuals in a population is a genuine natural property. In contrast, "ideal types" and "Aristotelian essences" - Archetypes representing the pure ideal of a species - are illusory human constructs. At least some of the elements that account for this diversity are inheritable characteristics.
- Selection: Not all characteristics are equally beneficial to organisms seeking to live and reproduce. They may be:
- The recombination of genes through sexual reproduction
- The occasional mutation
The 20th century New Synthesis - the marriage of evolution with the sciences of genetics and embryology made us aware of other mechanisms of evolutionary change, including:
- Genetic drift: The role of random chance.
- Genetic linkage: The effect of the close proximity of genes on a chromosome
- Pleiotropy: The tendency of a single gene to code for more than one phenotypic trait
- Sexual selection: The role of ate selection preferences
- Heterchrony: The role of changes in developmental schedule
Descent - Evolutionary Patterns:
Although all of this can be demonstrated in the laboratory, the fact that it is hard to see on a human time scale has limited the public's willingness to accept the reality of the phenomenon at all.
Perhaps the real demonstration of of evolution lies in recognizing the pattern that it generates in living and ancient creatures. For example:
A cladogram (stick-figure evolutionary tree) of the familiar land vertebrates. It shows:
- The pattern of common ancestry: Crocodilians and birds are one another's closest relatives, being descended from the relatively recent last common ancestor of Archosauria to which they both belong. Members of Lepidosauria are more distantly related.
- The pattern of shared evolutionary novelties: The tuatara and lizards and snakes inherited a modified mid-dorsal scale row from their last common ancestor. This feature helps us identify members of Lepidosauria. It doesn't matter that some members have secondarily lost the novelty.
Cladistics: The science of Systematics - the ordering of the diversity of life - employs repeatable "cladistic" algorithms to identify the most Parsimonious potential arrangements of the tree of evolution that minimize the number of evolutionary novelties necessary to account for their distribution. (The rise of cladistics was an intellectual revolution in systematics that occurred during the late 20th century. More on this in BSCI106.) Thus, cladograms are hypotheses of evolutionary history that can be tested by the addition of new data.
Sources of evidence: The physical evidence that informs these parsimony analyses mostly comes from three sources:
- Morphology: The study of the anatomical features of living creatures at their various life stages. Occasionally, the behavior of living creatures illuminates oru interpretation of their morphology.
- Molecules: With the sequencing of many parts of the genome, information about the base-pair sequence in DNA, and other molecular information, can be used. E.G. the presence of an "A" in two creatures in a base-pair position that ancestrally held a "G" would be an evolutionary novelty suggestive of recent common ancestry.
- Fossils: Morphology and molecules provide copious information, but they represent only one time slice. What if evolutionary relations have been concealed by later evolutionary events? Fossils provide a necessary window into deep time with which to anchor our hypotheses. Note:
- Fossils tend to be fragmentary but may reveal crucial information
- Fossil evidence is usually morphological, but may sometimes provide molecular evidence.
- The behaviors associated with fossil morphologies can be difficult to interpret.
- Because morphology and molecules are fundamentally different classes of data, there is no professional consensus over what their relative weight should be.
- Each data source has its professional advocates and detractors.
- Different sources of data sometimes yield different results that are difficult to reconcile.
Evolutionary Time: Assessing the tempo of evolutionary change
First, some background on the chronology of fossils:
- Igneous - solidified from molten material
- Metamorphic - recrystallized by heat and/or pressure.
- Sedimentary rock - rock composed of the transported remains of pre-existing rocks.
Sedimentary Juniata Formation (Ordovician) - LaVale, MD
Stratigraphy - Turning the sedimentary record into history: Sedimentary rocks have the advantage that their relative ages can be determined without complex technology.
- Superposition. In undisturbed strata, older layers lie beneath younger ones because they must have already been present for the younger ones to be deposited on top of them.
- Cross-cutting relations. If one structure cuts across another, then the one that is being cut must be oldest.
- Incusions. If pieces of one type of material are included in a layer, then the material of which the pieces are made must be older than the layer.
Quaternary alluvium overlies Point Loma Formation (Cretaceous) - San Diego, CA
So fossils tell us something about the age of rocks, but what do rocks tell us about the age of fossils? Prior to the 20th century researchers really didn't have any idea of the numeric ages of the rocks they studied. That changed with the discovery of:
- Radioactivity: The discovery that radioactive isotopes decayed into their stable daughter products with fixed probabilities that could be described by half-lives enabled geologists to determine absolute ages by comparing the concentrations of radioactive isotopes and their stable daughter products.
- Geomagnetic reversals: The pattern produced by the irregular reversals of Earth's magnetic field.
Taken together, the evidence of the rock record enables us to establish the chronology of the history of life with reasonable fidelity.
Fossils and Paleontology: Fossils: Are any record of past life incorporated into the rock record. Paleontology is the study of the fossil record. We encounter fossils in sedimentary rocks because, unlike other rocks, sedimentary rocks form where life lives.
My purpose today is to provide examples of the reciprocal-illumination of these sources of data in the interpretation of some major evolutionary transformations in vertebrate history.
Compelling case studies I: Whales Learn to Swim
Evolutionary novelties of living whales:
- Accoustic isolation of the middle ear to optimize hearing underwater. Achieved by the enclosure of the middle ear inside a bony capsule (or Auditory bulla) that is only loosely attached to the rest of the skull.
- Telescoping of the bones of the skull to strengthen the region between the eye-sockets. (Compare a gray fox and bottlenose dolphin
- Retraction of the nostrils to the top of the head.
- Simplification of tooth morphology
- Shortening and reduction of cervical vertebrae (= reduction of the neck)
- Modification of tail and trunk for axial swimming using dorsoventral (up and down) flexion. (Increase in number of lumbar and caudal (tail) vertebrae.)
- Reduction of hindlimbs to vestigial state or complete absence.
Early artiodactyl Diacodexis from The Fossil Forum
Eocene whale Dorudon atrox from The Lord Geekington
- Differentiation of its teeth
- Incomplete retraction of nostrils and telescoping of mid-skull
- Retention of small hind limbs
During the early 1990s, paleontologists were very happy with their progress, as the evolutionary history of early whales was being filled in quite nicely by new discoveries, including the skull of Pakicetus, which seemed to be near the terrestrial aquatic transition. (Link to an early 90s style Pakicetus reconstruction.)
The mesonychid Mesonyx by Charles Knight colored by Jennarotancrede
Mesonychids were impressive carnivorous hoofed mammals of the early Paleogene, and included Andrewsarchus, the largest known terrestrial mammalian predator. The anatomical similarity of the skulls of early fossil whales know in the late 20th century to those of mesonychids led paleontologists to assume that whales were ultimately derived from them. The similarity of their teeth was particularly convincing to paleomammologists who were accustomed to squeezing information out of the complex teeth of mammals. Mesonychids, themselves, were close to the ancestry of artiodactyls, but clearly outside of it. Therefore so were whales - the 1990s paleontological consensus.
The "whippo" hypothesis: During the mid-1990s, repeated molecular phylogenetic analyses yielded very robust support for a close relationship between whales and, specifically, hippopotamuses. To paleontologists, this seemed bizarre:
- Traditionally, hippos were considered closely related to pigs and peccaries, and no thorough phylogenetic analysis had been performed to contradict that notion.
- Hippos are herbivores who seemed to show no particular similarity to fossil whales.
- The mesonychid hypothesis seemed strong.
- Sometimes, results from different parts of the genome contradict one another. Maybe this result reflected some unusual feature of the genes that had been sequenced.
Landmarks in cetacean evolution: Watch for the following trends:
- Modification of tooth row
- Retraction of nostrils and telescoping of skull
- Lengthening of torso and tail
- Reduction of hindlimbs
The cetacean Pakicetus
- an elongate snout in front of the nostrils, with incisors and canines arrayed in a V when viewed from below.
- Incisors and canines are simple cones, while post-canine teeth remain complex (in typical mammalian fashion).
The cetacean Ambulocetus from Research Casting International
Ankles of Rhodocetus and Antilocapra (pronghorn) in proximal view.
- In all placental mammals, the astragalus (a tarsal - in humans it is called the "talus") articulates with the shin across a spool shaped surface. This is the cylindrical joint around which the foot rotates. Only in artiodactyls, the distal (outer) surface of the astragalus is also spool-shaped, giving the foot a double-cylindrical articulation with the shin. This greatly extends the range of rotation of the artiodactyl ankle. Rhodocetus clearly displays this double-spool astragalus shape.
- Moreover, the foot is large and the ankle has considerable leverage (compare with the very cursorial pronghorn (Antilocapra). Rhodocetus used its hindlimbs.
And this was the huge news. Whales are definitely some kind of artiodactyl. During the same interval, cladistic analyses were reexamining the relations of extant and fossil artiodactyls. The simple result when new data was included:
- Hippos and whales are, in fact, closest relatives, based not only on the molecules, but also on newly identified characteristics of the middle and inner ear.
The Whippomorph Hypothesis has won the day by:
- Examining a new source of data
- Provoking reexamination of existing data
Indohyus indirae from The Guardian.
Hard to say as our record of ancient hippos is poor, but we do have this:
- Indohyus: The closest relatives of cetaceans include small herbivores like Indohyus. Not especially whale-like except:
- The v-shaped array of incisors
- Incipient cetacean specializations of the middle and inner ear.
Daeodon shoshonensis from Trailside Museum of Natural History.
- Entelodontidae: The sister taxon of whales and hippos together includes creatures with a totally different appearance - scary terrestrial omnivores. And yet, the connection between them and hippos is plausible.
Compelling case studies II: The Origin of Bird Flight
Left: Allosaurus a "dinosaur" by Camus Altamirano Right: Malachite kingfisher, a "bird"
Bird origins and bird flight: The idea that birds are a kind of dinosaur that evolved the power of flight is not new. It was advocated in the late 19th century by "Darwin's bulldog," Thomas Henry Huxley, but fell into disfavor in the early 20th century. Starting in the 1964 with John Ostrom's description of the "raptor" Deinonychus, the possible dinosaurian origin of birds roared back to life as the center of a true academic psychodrama. By dumb luck, the issue had popped up just in time to become the poster-child for larger disputes about:
- The efficacy of the cladistic approach to reconstructing evolutionary history
- Whether dinosaurs were warm or cold-blooded
- How the ancestors of birds got off the ground
The Berlin specimen of Archaeopteryx lithographica from Biologypop
The first breakthrough was the 1860s discovery of Archaeopteryx, the first known feathered fossil. Thus, for over a century it has been a fundamental benchmark in paleontology - "the first bird." For roughly a century, no more fossils were recognized as being close to birds. But there were problems. We can call it a bird, but if we were to see a living one, it would seem immediately slightly non-birdy with its:
- Long, slender unfused tail skeleton
- Unfused three-fingered hands
- Toothy mouth
- small weak breast-bone
The second breakthrough came in 1964 - roughly a century after the discovery of Archaeopteryx when John Ostrom published his description of the dromaeosaurid ("raptor") Deinonychus which possessed many bird-like features (especially of the hand and wrist.)
In a totally separate issues, Ostrom raised the question of whether a creature that looked like Deinonychus could possibly be cold-blooded, questioning the prevailing dogma of dinosaur paleobiology. During the following decade, cladistic methods began to be applied to extinct vertebrates, supporting close relationships between birds and theropod (meat-eating) dinosaurs like Deinonychus.
The pigeon-sized deinonychosaurian Anchiornis huxleyi reconstructed with true colors
by Michael DiGiorgio from Grrlscientist
- Thanks to the exceptional preservation conditions of rock units like the Yixian Formation of Liaoning Province, China, we now know that feather-like fuzz occurred in both small and large theropods.
- Proper contour feathers occur in theropods like oviraptorosaurs and dromaeosaurids.
- Birdy characteristics have been identified in other theropods where they were previously overlooked, like the furcula (wish-bone) of Allosaurus
- A wide range of similar feathered, arm-flapping theropods has become known.
The current consensus is that contour feathers and a range of other birdy characteristics characterize the theropod group Eumaniraptora. Basal members of the major eumaniraptoran groups were broadly similar:
- Deinonychosauria (including "Raptors", Middle Jurassic - Latest Cretaceous): Microraptor (reconstruction). From such beginnings, deinonychosaurs, evolved into larger terrestrial predators like Velociraptor.
- Avialae (including birds, Early Cretaceous - Recent): Jeholornis (reconstruction) - a chicken-sized omnivore found with both seeds and fish in its gut.
But could any of them fly?
Their forelimb mechanics make this much clear: They could definitely flap. Biomechanically, Archaeopteryx is considered to be at or near the threshold of flight. Its wings are very short, its breast-bone is not large enough to serve as the origin for powerful flight muscles, and its skeleton was not strengthened and fused in the manner of modern birds. If it could fly, it did so weakly and over short distances. So why did it waste metabolic energy growing and supporting big feathered forelimbs? Indeed, what did Caudipteryx do with its dinky "wings?"
Turkey-sized oviraptorosaur Caudipteryx (left) and deinonychosaur Mei long
- "Trees Down" (also called the Arboreal Model)
- Ancestors of birds were tree-dwellers (arboreal)
- Powered flight evolved from gliding/parachuting:
- "Ground Up" (also called the Cursorial Model)
- Ancestors of birds were ground running animals (cursorial)
- Powered flight evolved from activity useful to runners, outside of the context of a tree-dwelling phase
- Evolution of the wing stroke evolved in some non-flight context (possibly food capture; possibly as a speed-aid or an aid for leaping and jumping)
- Through enlargement of the proto-wing in the non-flight context, the forelimbs became large enough and developed enough to begin to carry the animal through the air
- Birds (or their close relatives) only got into the trees after having developed the early phases of flight
This issue became a proxy for the bigger issue of bird origins. By the turn of the century, a majority of paleontologists were won over to the idea that birds were a sub-group within theropod dinosaurs, but an active minority continued to oppose the idea, seeking bird origins among other (arboreal) fossil reptiles including drepanosaurids and coelurosauravids, and regarding cladistic methods with suspicion.
- Dinosaurophile cladists were drawn to the "ground-up" hypothesis because their analyses indicated that the closest relatives of birds were long-legged terrestrial runners.
- Their opponents were drawn to the "trees-down" hypothesis because they sought arboreal non-dinosaurian bird-ancestors.
The discovery of creatures like Caudipteryx with its dinky feathered arms added color to this debate.
In 2003 research by Ken Dial of the University of Montana's Flight Laboratory revealed a locomotory behavior in modern birds not previously realized. Birds were discovered to run vertically up surfaces, aided by flapping their wings back and forth in order to generate traction against the surface. They called this behavior Wing Assisted Incline Running, or WAIR for short.
Note that these birds are NOT climbing in the typical sense: they are literally running up the sides of trees. Dial and his team studied the ontogenetic (growth) changes in the ability for birds to use this behavior, determining that WAIR is useful even in individuals with little dinky wings useless for flight. Sound familiar? We finally have a compelling evolutionary scenario for the evolution of wings from small theropod forelimbs.
Human-sized deinonychosaurian Deininychus
contemplates eating or befriending a wren by E Willoughby
Oddly, of the two major Eunamiraptoran groups, only Avialae stepped across the threshold of proper flight while deinonychosaurs backed away from it to become large flightless predators (right).
The take-home message:
Both bird and whale evolution document profound transitions in body form and ecology. For decades, these morphological gaps were matched by:
- embarrassing gaps in our knowledge the fossil record
- frustrating apparent contradictions between different sources of data.
By its nature, the fossil record will always consist mostly of gaps. The wonder and glory is the real strength of the trend, over the last two-three decades, toward the narrowing of those gaps. In the case of whales, the emerging evidence is so good that it was cited by U.C. Berkeley paleontologist Kevin Padian as a case study of a compelling evolutionary story - a "poster-child" of evolution. Similar citations could be made to the developing stories of: