# data[].attributes.body
Through natural selection, whales started to slowly migrate into the water over millions of years.
offspring.evolution.berkeley.edu/evolibrary/article/evograms_03

# data[].attributes.attachment_link.description
The first thing to notice on this evogram is that hippos are the closest living relatives of whales, but they are not the ancestors of whales. In fact, none of the individual animals on the evogram is the direct ancestor of any other, as far as we know.

# data[].attributes.body
sexual selection targets pelvic bones.
https://www.ncbi.nlm.nih.gov/pubmed/25186496

# data[].attributes.attachment_link.readable_text
It was previously considered that the pelvis is useless in dolphins and whales. But the pelvic bone still retained its important, even when the animals shifted from walking to swimming.

According to a new paper, the pelvis is not useful for walking but it is useful for successful reproduction in mammals living in water. The muscle of a male whale that controls the penis attach directly to the pelvic bone, providing the mammal larger control of its reproductive organs.

"People that really know the reproductive biology of whales and dolphins already know and have known that these pelvic bones are an anchor point for reproductive organs", study author Jim Dines told the Washington Post.

To provide new support to what vintage anatomy texts already declared, Dr. Dines and his co-author Matthew Dean, assistant professor at the University of Southern California scanned hundreds of bones and examined them based on size and shape. They compared the two data sets and they found that the bigger the penis, the bigger the pelvic bone.

Whale sex is especially enigmatic thing and it remains unobserved. As it is known that in all animals, bigger testes and penises evolve when there's more sexual competition. If the pelvis evolves to help in reproduction, then the researchers assumed that it would get bigger support for a larger reproductive organ. Researchers used 3D scanner to analyze the bones.

Many studies have shown that sexual competition supports the evolution of larger reproductive organs. But researchers have concluded that pelvises in marine mammals such as whales and dolphins have grown in size, facilitating control of increasingly large testes and penis.

The research provides the evidence that the pelvis is not useless but it is important if a male whale needs to maintain control of his growing appendage.

# data[].attributes.attachment_link.title
Pelvic Bone Important for Reproduction in dolphins, whales

# data[].attributes.attachment_link.description
It was previously considered that the pelvis is useless in dolphins and whales. But the pelvic bone still retained its important, even when the animals shifted from walking to swimming. According to a new paper, the pelvis is not useful for walking but it is useful for successful reproduction in mammals living in water.

# data[].attributes.body
The flipper of a whale, the wing of a bat, and the leg of a cat are all very similar to the human arm.
https://www.thoughtco.com/about-homologous-structures-1224763

# data[].attributes.attachment_link.description
OBJECTIVE: to examine the similarities in homologous structures The forelimbs of mammals are an example of homologous structures - they are similar in form but have different functions. When scientists compare the forelimb bones in different animals, they look for similarities in the detailed structures of the bones.

# data[].attributes.body
The placentals and marsupials, ran out of land space back when there was dinosaurs, which caused them to start to shift more into the water.
https://www.sciencedaily.com/terms/parallel_evolution.htm

# data[].attributes.attachment_link.readable_text
Many of the world's great natural history museums devote at least one hall to creatures that no longer exist today. In the old tradition, in order to keep any young upstarts from getting any ideas about evolution, skeletons or parts of skeletons were grouped by the functions they performed, a visitor being likely to find the wing of a bat and the wing of the bird in the same display case even though the two animals extremely distantly related. Newer layouts, conversely, have largely ignored the end-function of one line or another to group animals together by homology and their shared characters, the most well-known example being the remodeled 4th Floor of the American Museum of Natural History in New York City which has attempted to arrange its fossil collections as a walk-through cladistic diagram.

Still, the generally discarded of grouping animals by their adaptations to general habitats or niches is not without it's charms. Over and over again, evolution has produced forms that seem to converge on certain body plans, varying habitats making some traits advantageous and others a liability, helping to adapt different organisms to their local ecologies. Flight has independently evolved several times (and the ability to glide an even greater number of times), as well as adaptations to marine environments, saber-like canine teeth, immense sails along the spine, and slicing premolar teeth, although each time such familiar features seem to arise it shows that there is more than one way to solve an evolutionary problem from any given point in an organism's natural history. Not everything can be chalked up to convergence of form in order to carry out particular functions, however. Parallel evolution, although sometimes difficult to determine, also allows relatively closely related forms to take the same evolutionary paths, showing many of the same anatomical characters even though they diverged from a common ancestor at some point in the past and occupy at least two different lines of descent. In fact, it is often these weird and wonderful creatures that are forgotten or overlooked, more people recognizing the term "saber-toothed cat" (or, loathe as I am to say it, "saber-toothed tiger") or the genus Smilodon than the term "Nimravid" or the genus Dinictis. The following entry, therefore, will be an attempt to navigate through the somewhat "entangled bank" of evolutionary relationships among animals that appear to be shaped in similar ways by the environment but constrained by their species' history, showing us that there is more than one way to make a saber-toothed cat.
Back into the pool: Of Ichthyosaurs, Sharks, and Cetaceans
Perhaps one of the most well-known (or at least widely cited) examples of evolutionary convergence has been that of the similar body shapes of sharks, ichthyosaurs, and cetaceans. It's difficult to see these three distinct groups of creatures side by side and not recognize the similarities, but why are they similar in the first place? If they belong to groups that are distantly-related branches of the evolutionary "bush," why should they have developed similar body forms?

One of the most well-known examples of evolutionary convergence; (From Top to Bottom) An ichthyosaur Ophthalmosaurus icenicus, a Porpoise, and a Spiny Dogfish ( Squalus acanthias)

From the 1925 creationist bookThe Predicament of Evolution by George McReady Price.

Creationists have been quick to seize upon the idea of convergence as if it were one of evolution's weak points. In 1926, George McCready Price wrote the following in one of the more well-known early American anti-evolution texts, The Predicament of Evolution;

For instance, we have the shark, the ichthyosaur (an extinct kind of fish-shaped reptile), and the dolphin (a true warmblooded mammal, and not a fish at all), all of which greatly resemble each other in external shape and general appearance. Each has the same long, sharp snout, the same powerful tail, the same general fishlike shape. And yet the first of these is a true fish, the second was just as true a reptile, while the third is a mam-mal, bringing forth its young alive and feeding them by milk, just as does a cow or a horse, though it lives in the sea.

Here the evolutionists have to say that this peculiar shape and general form has been evolved separately and independently in each of these three instances. Indeed, Henry Fairfield Osborn, President of the American Museum of Natural History, New York City, declares that a very similar shape and form has been independently evolved "at least twenty-four times."-"Encyc. Brit.," Vol. XX, p. 578...

From this large group of facts we become convinced that these many similar or identical structures, which must have been evolved quite independently (if evolved at all), make too great a draft on our credulity. At least, these hundreds of examples of "parallel evolution" greatly weaken our confidence in homology, or similarity of parts and organs, as a proof of blood relationship.

Such arguments have become traditional amongst creationist apologists, suggesting that if convergent evolution does occur then we must throw homology out the window as similar structures will only mislead us as to the true affinities of the creatures being studied. As we will later see with Cuvier's Ptero-dactyl, this can be a danger for scientists who are unwary and wish to shoehorn creatures into existing taxonomic categories, but not for those who actually look beyond superficial appearances.

The reason why the shark, the ichthyosaur, and the porpoise should all look vaguely the same is because they live(d) in the same environment; the ocean. An organism that is suspended in a fluid that is much denser than air can be adapted in various ways to such an "alien" environment, but physics does dictate what shapes can be taken based upon life history. It is possible to be a floating filter feeder, exhibiting a round shape, but such a strategy is essentially out of the question for animals that need to move quickly and to hunt for food. What is required is not only a powerful propulsive organ to keep the organism moving forward, but also extra appendages to allow for the control of movement and a streamlined shape to reduce drag (and hence reduce energy costs for moving through the water).

One of Charles R. Knight's renditions of an ichthyosaur.

In fact, sharks as a whole provide a good model for various forms of ichthyosaurs. While ichthyosaurs are generally presented as already being streamlined and possessing a large caudal fin with two equally long lobes, we would be loathe to forget that they too are products of evolution and many fossils show us that they were not always an Euryapsid (thank you, johannes) answer to modern-day Lamnid sharks. Early ichtyhosaurs actually had more of a "bump" towards the back of their tail rather than a full-blown caudal fin, their overall body shape and lack of a large propulsive surface keeping them from moving too quickly through the water. A similar tail type/form can be seen in many modern day sharks like the Nurse Shark, which generally live along the bottom feeding on crustaceans and inhabitants that can be sucked out of coral crevices. Being that ichthyosaurs lack gills, it is unlikely that their early representatives were bottom-dwellers, instead preferring shallow areas, which can be especially productive in terms of food.

Modification of the "tail kink" (which was at first thought to be a taphonomic feature, early reconstructions showing "amphibious" ichthyosaurs with straight tails) seen in early forms allowed for the eventual evolution of a crescent-moon shaped tail, as well as adaptations in the skull and of the limbs into fins (the addition of digits and the addition of bones in the digits being quite common in the latest forms). This more-familiar shape would allow ichthyosaurs maximum propulsion with their caudal fin (the spine going downwards instead of upwards, as in sharks) while they would be able to exert control over their motions with their pectoral fins and would be kept from rolling in the water by their dorsal fins. The evolution of large eyes and other features aside, the overall shape and basic skeletal structure of ichthyosaurs seems to be an optimal design for medium-to-large, fast-moving, oceanic predators (although mosasaurs, pliosaurs, and plesiosaurs took different evolutionary routes).

What allowed ichthyosaurs to develop an effective side-to-side motion of the tail would not work for cetaceans, however. Ichthyosaurs developed their mode of propulsion by side-to-side motions of the spine, perhaps swimming in a mode similar to eels or cat sharks at first, a common form of locomotion in modern reptiles. This sort of motion is usually accomplished on land via a sprawling gait, the limbs being held out to the sides and the animal exhibiting a bit of a side-to-side motion as it moves along.

Whether the ancestors of icthyosaurs were sprawlers (to a greater or lesser extent, predisposing them to side-to-side motions of the tail and body) or not, cetaceans evolved much more recently in evolutionary history, and developed from ancestors that carried their legs directly underneath their body. The plasticity of early archaeocetes and their artiodactyl ancestors was greatly diminished, their hip and spine structure adapted to up-and-down undulations rather than the side-to-side motion seen in the video of the salamander. This sort of constraint has not stopped mammals from becoming adapted to the water, however, and clues to the evolution of cetacean movement can be seen in living animals like Giant River Otters;

In the water, undulations of the spine accompanied with some propulsion from the limbs proves to be very effective, and it's not hard to imagine an archaeocete like Ambulocetus, as my friend Neil so aptly described, as a " sexy otter." Once undulation of the spine became established as a method of moving through the water, the eventual addition of a tail fluke would do for cetaceans what the crescent-shaped tail of tuna, sharks, and icthyosaurs acheived in terms of speed and power, the body being adapted towards a streamlined appearance with (again) the pectoral fins providing lift/control and the dorsal fin preventing rolling. Larger forms of whales, namely the Mysticetes or Baleen Whales, grew to immense size and gave up some of the features that seem to be convergent with sharks and the smaller ichthyosaurs (some, in fact, did acheive whale-size), but they are derived from more predatory designs and their niche as massive, far-ranging suspension feeders free them from some constrains while imposing some new ones.

A painting of leaping ichthyosaurs by Heinrich Harder (circa 1916)

Human engineering has recognized similar constraints for motion in the water and even in the air; planes and submarines most closely resemble sharks and dolphins in overall shape, the placement and size of the wings on a 747 having much the same function as the large pectoral fins of far-ranging pelagic fish like the Blue Shark. Life in the water adapted all three groups of animals towards the same shape because there does not seem to be any other way to be a fast-moving, medium-to-large sized marine predator; speed and some degree of maneuverability are paramount. Some other lines have diverged from this shape, as noted before, but the sharks, dolphins, and (I don't think it's too much of a stretch to say) ichthyosaurs all occupied essentially the same niche and therefore were adapted in a particular fashion.

Do not think, however, that the convergence of three lines towards one body plan gives credence to a kind of "orthogenesis" or progressive force driving evolution. There was no sort of supernatural or external force manipulating the genetic material of these groups with the shape of a dolphin or shark in mind. Rather, the environment and local ecology determined what form would be favored through time, and even though the three groups may look the same and have significant convergences, they also have many traits in common with their ancestors, allowing us to trace their evolutionary history (which is why no one is arguing that dolphins, sharks, and ichthyosaurs are closely related or form a small monophyletic grouping).
A marsupial you wouldn't want to meet
Living members of the Carnivora (bears, cats, dogs, civets, weasels, etc.) have always caught my attention, but there was an entire group of carnivorous mammals, now extinct, that have left no living representatives. The last known member of this group was named Thylacoleo carnifex by Richard Owen, and it has some of the strangest dentition ever seen in a marsupial. Marsupial mammals are well-known in Australia, creatures like kangaroos, koalas, and wombats coming most immediately to mind out of living extant taxa. There was a much more diverse population of marsupials during the Pleistocene, however, and the "marsupial lion" was likely a formidable predator.

In order to understand why Thylacoleo is relevant to our discussion of convergence we need to first understand what makes living placental Carnivores so special. Many carnivores, especially cats, have a rather specialized dentition, certain molars and premolars making up what is known as the "carnissal shear." These teeth are pointed and act like scissors, easily cutting up flesh or crushing bone. The molars behind the shear are often reduced (some groups have retained their molars in order to incorporate a more generalized diet, like dogs and bears), the dental specialization perhaps being one of the keys to the success of this group. Earlier predators of now-extinct lines like Mesonychids lacked such specialized cutting teeth, and the teeth behind the canines of the large Andrewsarchus show that their oral tool-kit was a bit more blunted.

Thylacoleo, a carnivorous marsupial not descended from the Miacids that gave rise to living carnivores, also developed something of a "carnissal shear" but in a different way. Rather than a battery of teeth that became sharpened, one of the upper and lower premolars of Thylacoleo became elongated and blade-like, and the cleaver-like teeth helped to sharpen each other as they moved past each other when opening or closing the jaw. Thylacoleo also had a terrible bite, the attachments for the muscles that opened and shut the jaw were massive, somewhat constricting the amount of space the brain could take up, but giving Thylacoleo what was perhaps the most powerful bite forces amongst mammalian predators, especially given it's relatively small body size (it was only about four feet long and 220 pounds).

These eminent authors received the support, in reference to objections to my conclusions, of the (then) Curator of the Australian Museum, Sydney, Mr. GERARD KREFFT, who, in his contribution to the 'Annals and Magazine of Natural History,' series 3, vol. 18, 1866, p. 148, records his opinion that "the famous marsupial Lion was not much more carnivorous than the Phalangers of the present time."

The species of carnivorous Phalanger is not named. No evidence of such by fossil specimens has reached me, nor have I found such exceptional habit of an existing species of Phalangista elsewhere noted.

Even so, Thylacoleo carnifex and its relatives represent a branch of marsupials that became fairly specialized predators, and given the plasticity of tooth structure, it's not hard to see how sharp premolars could be adapted into a blade to cut flesh. While it may be easy to draw connections between this animal and living carnivores, however, perhaps we should be more measured in our descriptions; both groups met the same challenges in similar ways, but the differences are far more striking and important in this example of convergence on a particular niche.
On what day were the Ptero-Bats created?
Before there were natural history museums, there were motley assortments of organic odds and ends known as curiosity cabinets, and in the cabinet of Karl Theodor there would eventually come to be a petrified treasure. Although it was probably collected around 1767, the first known pterosaur fossil was not described until 1784, when the appointed caretaker of the collection, Cosimo Alessandro Collini, attempted to determine the nature of the strange creature that came to him from the limestone of Bavaria (the same deposits that later yeilded Archaeopteryx). Although certain that he was the remains of an animal from an earlier time, Collini was agnostic about what kind of animal he had come to possess. Years later, the famed anatomist Georges Cuvier investigated Collini's paper and illustrations, noting that the creature was certainly a reptile. Still, the fossil would remain without a proper name until Cuvier would write a more detailed analysis in 1809, dubbing the fossil "Ptero-dactyle."

Not everyone agreed with the analysis of Cuvier, however, especially since Cuvier did not get to see the fossil himself and had to work from the drawings in Collini's paper. Samuel Thomas von Soemmerring, of the Bavarian Academy of Science, thought that the pterosaur was some unknown type of bat, a view that would remain entrenched in the minds of some scientists for many years. Indeed, one restoration by Edward Newmann in 1843 (and "re-drawn" for Gosse's work Omphalos, as shown below), depicted the two known types of pterodactyl known at that time as fuzzy bats, complete with cute little ears. It is clear from the drawing that pterosaurs do not make good bats, although this didn't stop many German paleontologists from taking such a stance through the first half of the 19th century.

Newmann's "marsupial bats", conspicuously missing their ears, from Gosse's Omphalos. It's likely that Gosse recognized the reptilian nature of these Pterodactyl by the time he wrote his book, so Newmann's work was copied minus the more mammalian aspects.

But why was there such confusion? It is likely because there is something familiar about pterosaurs that had been seen in living bats; the extension of digits to hold a membraneous wing. While the first fossil, despite wonderful preservation, did not preserve a membrane impression, it is hard to look at it and not recognize the superficially similar structure of a bat's wing, which also carries a membrane to enable flight. In fact, birds seem a bit unusual in developing feathers for flight; many varieties of gliding and flying creatures have taken to the air (regardless of whether they engage in powered flight or glide) by the use of membranes. Indeed, gliding may often precede powered flight, and once an animal has developed a membrane that can be stretched between its limbs to glide, the extension of the digits at the point(s) of attachment can help to expand the wing size. Such changes likely occur as a result of changes in development, natural selection favoring the invasion of a new niche based upon variations that exist in a population, although in the case of pterosaurs we can no longer test to see if this is correct.

As we just saw with Thylacoleo, however, the convergences of pterosaurs and bats are rather slight, overall. While both acheived flight on membraneous wings attached to extended digits (many more in the case of bats) and have relatively compressed bodies, pterosaurs had a much greater diversity in shape and size than modern bats. Likewise, they did not elongate the rest of their fingers, suggesting that there was some situation (be it climbing or hanging on to a perch) that the pterosaurs still needed their other fingers for (although bats can climb pretty well with their thumbs, and some have even evolved suction disks). Still, it can be said that both took to the air by similar means and had to deal with similar constraints, but their evolutionary paths are far more divergent than that of the aforementioned sharks, ichthyosaurs, and cetaceans.
It doesn't look like much of a planet-eater to me

A female Gharial at the National Zoo in Washington, D.C.

The most notable difference are the complex bones at the back of the throat of Deinosuchus which are arranged to slide past each other as the jaw opened and closed. No such feature is seen in the giant phytosaur. Still, even after the phytosaurs died out, crocodilians did not return to the water until about the Cretaceous period, many forms being absolutely terrifying land predators that have also long been extinct. One of the early forms was Protosuchus, a small true crocodilian that represented a line that changed little during its tenure on the earth.
Saber-toothed Nimravid doesn't sound quite the same...
Many museums have cases devoted to the great saber-toothed cats of epochs long gone, but it would take someone with more than just a cursory understanding of paleontology to sort out what is really being displayed. Saber-teeth, or elongated canines, have evolved many times over in the course of mammalian history, showing up in herbivores like the living Musk Deer as well as extinct groups like the gorgonopsids. Animals as different as a Musk Deer and Inostrancevia are fairly easy to tell apart, even for the non-specialist, but what about nimravids and the "true" saber-toothed cats?

A diagram of the three ideas of Nimravid/Felid evolution.

James Whitcomb Riley is purported to have once written "When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck." Unfortunately, this argument is quite popular (even being utilized by the likes of prominent Intelligent Design advocate Michael Behe) despite being very superficial and even vapid. Needless to say, it doesn't apply to our discussion of Nimravids and true felid saber-toothed cats, but in decades past the two groups were lumped together.

<img src=" " alt="Skulls" />

Despite these differences, it has often been difficult to differentiate the groups (and debate still continues). The diagram above, based upon one in Robert Hunt's 1987 paper " Evolution of the aeluroid Carnivora. Significance of auditory structure in the nimravid cat Dinictis," offers three simplified versions of the hypotheses about the relationships of nimravids and felids. Initially it was thought that there was a progressive evolution from ancestor to descendant in a straight line, the nimravids being the direct ancestors to the saber-toothed cats. This view does not represent how evolution truly works, however, and was found to be incorrect. In its place came a view that nimravids and saber-toothed cats diverged from a common ancestor at about the same time, going off in separate directions. This is better, and is more consonant with the data, but again it suggests that the line representing the common ancestor went extinct, either in becoming nimravids or saber-toothed cats. What seems to be the case based upon current data is that the nimravids split off from a common ancestor somewhat before the saber-toothed cats, the line containing their common ancestor continuing its own evolution as both groups evolved. Such a branching pattern is not unusual, and should even be expected, especially since there are living primates like tarsiers and lemurs that represent the overall kind of animal our ancestors once were, but still quite different and undergoing their own evolution alongside our own lineage.

To complicate things even further, the skull or skeleton of the marsupial Thylacosmilus is also often thrown into the mix. Although totally unrelated to nimravids or felid saber-toothed cats, the South-American Thylacosmilus converges closely on the appearance of the placental predators, although there are some important differences. As can be seen from the above photographs, the eye of Thylacosmilus is entirely enclosed by bone on the side of the head, while in many felids and nimravids the eye socket is not entirely ringed-in by bone as if someone had bored a hole in the skull (compare the skull of Thylacosmilus with that of Thylacoleo, above). Further, the teeth of Thylacosmilus have very deep roots, going back in the skull almost over the eye. Originally it was thought that the teeth faced outward, but this was based upon a distorted skull and later finds showed the true position of the long canines.

Now that we have elimated Thylacosmilus from the running as another case of marsupial convergence, we must ask why the nimravids and felid saber-toothed cats are so close to each other in appearance. While many of the instances I've discussed previously have been instances of convergence, be it throughout the entire body or merely certain aspects of it, the nimravid-felid connection is a wonerful example of parallel evolution. W.E. le Gros Clark provides an excellent summation of understanding the difference in his 1959 book The Antecedents of Man;

From what has already been said, it is clear that, in assessing degrees of phylogenetic affinity, it is always necessary to take into account the factors of parallelism and convergence in the evolutionary development of related or unrelated groups. These processes can lead to structural similarities, which, taken by themselves, may be misleading. The term convergence is applied to the occasion in general proportions or in the development of analogous adaptations in response to similar functional needs. But such similarities are superficial and easily distinguishable by a detailed comparative study of the animal as a whole. For example, the resemblance in general appearance, even in a number of morphological features, of the Tasmanian wolf to a dog does not obscure the fact that in fundamental details of their anatomical construction they belong to quite different mammalian groups. On the other hand, the potentialities of parallelism seem often to have been much overestimated by some anatomists, for this phenomenon has sometimes been invoked in support of extreme claims for independant evolution of groups which are almost certainly quite closely related. We can agree with G.G. Simpson that the whole basis of parallelism depends on an initial similarity of structure and the inheritance of a common potentiality for reproducing homologous mutations, and that, this being so, the initial similarity and the homology of mutations themselves imply an evolutionary relationship. Expressed in another way, it may be said that convergence increases resemblances (which are, however, no more than superficial), while parallelism does not so much increase resemblances as maintain and perpetuate (by development 'in parallel' so to speak) similarities which have already existed ab initio in the genetic make-up of related types. Thus, 'closeness of parallelism tends to be proportional to closeness of affinity.'

There are a few problems with this reasoning, namely that it seems to give credence to an almost pre-determined genetic course for the lines to evolve in parallel, although le Gros Clark makes it clear in the work that he does not support in any way the notion of orthogenesis. Still, the passage makes the important distinction that in order to undergo parallel evolution groups need to be somewhat closely related and already bear similar structures, evolution preserving many of the similar traits instead of working to the same end from two disparate points. In the case of the nimravids and the felids it seems that they evolved from a common ancestor which was probably taken to carnivory. Nimravids branched off earlier, being more "primitive," while the felids came off the same line (or a very similar one) after it had accumulated a few more evolutionary changes. Indeed, even if form seems to be static or change little, it's hard for me to believe that designs are not slightly adapted this way or that as if the creature was an already perfect creation not influenced by changing ecological circumstances. Still, it seems that the nimravids and felids were adapted in similar ways, their ancestral lines probably possessing at least semi-retractable claws, long and sharp canines (although not long to the extreme like its descendants), a shortened face, and a developed carnissal shear. It is really not that difficult to change a civet-like creature (or in the case of our hypothetical common ancestor, a creature a bit closer to a cat) into a saber-toothed Smilodon, the changes being modifications of existing structures more than the creation of something entirely new out of nowhere. In fact, the vertebrate tetrapod skeleton has proven to be quite versatile, and most of the major bones in any vertebrate skeleton can be found to correspond with those in another vertebrate, allowing us to compare rhinos with ceratopsians, dromeosaurs with birds, cats with dogs, ichthyosaurs with cetaceans, and humans with primates.
Of constraints and convergence
I hope that is has become clear why convergence is such a strong theme in the evolution of vertebrates. At this point in the history of evolution, vertebrates have had a chance to fill nearly every niche imaginable in a large variety of habitats over millions of years, and so common themes are bound to arise. When groups return to the ocean, the environmental constraints shape them in ways peculiar to their new way of life that would not be advantageous in other situtations (i.e. being such a large aquatic animal that you'd be crushed by your own weight if you came onto land). When mammals become adapted to be predators, their dentition and morphology must be altered if they are to be successful hunters, carnivores past and present showing some suprising similarities despite being only distantly related. Even when taking to the air, laws of physics still apply, and natural selection often works through physical and chemical constraints to produce new forms.

It is of little doubt that the tetrapod design is a versatile one, retaining its overall character through the various changes that it has endured. Indeed, even when a lineage dies out and may seem gone forever, there is no law that says a similar situtation in the future will not produce forms that may be strikingly familiar, even if such organisms are not directly related to the last group that filled their new niche. Evolution has produced "endless forms most beautiful and most wonderful" and will continue to do so long after I am gone, but random mutation/natural selection do not work in isolation from the rest of the natural world. Evolution has produced so many amazing creatures precisely because ecology, physics, and chemistry have offered up both opportunities and challenges, and I only regret that I will not be able to witness the familiar and unfamiliar about what is swimming in the seas, flying in the air, or stalking the land 500 million years from now.

# data[].attributes.attachment_link.description
Many of the world's great natural history museums devote at least one hall to creatures that no longer exist today. In the old tradition, in order to keep any young upstarts from getting any ideas about evolution, skeletons or parts of skeletons were grouped by the functions they performed, a visitor being likely to find...

# data[].attributes.body
The ability of bats and toothed whales to echolocate is a case of convergent evolution.
http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1002788

# data[].attributes.attachment_link.title
In evolution, why did mammals' (whales/dolphins) legs turn back into fins/tails, but their lungs didn't turn back into gills?

# data[].attributes.attachment_link.description
Answer (1 of 13): To add to some of what has been already said with respect to the lungs, so far posters such as Joshua Engel have most of the story. The metabolism of an endothermic animal requires a much greater amount of oxygen than that of an ectothermic animal such as a fish, this is indeed ...

# data[].attributes.body
Whales, cats, humans, and bats clearly cannot interbreed and are very different species, but the similar bone structure in the forelimbs indicate they once diverged from a common ancestor.

https://www.thoughtco.com/what-is-divergent-evolution-1224810

# data[].attributes.attachment_link.readable_text
In 1878 the above sequence of images was taken by Eadweard Muybridge to settle a debate among horse racers--when a horse was galloping full speed, did they ever have all four legs off the ground? The human eye alone was not fast enough to resolve the dispute. Muybridge, the photographer who was called in, set up a sequence of cameras that would be set off by trip wires and found that horses are in fact completely air born at certain points within a full gallop. ( Watch the video of this horse here.)

Based on these images alone, though, how do we know that the horse was actually running? Couldn't the pictures have been of different horses? Couldn't they have been jumping and not running? This is more than just a thought experiment. We're asking if it's justified to tie different pieces of information together to form a progression, just like we do in evolution. Below we will explore several examples of biological progressions and ask what evidence there is that they represent evolutionary change over time.

Claim: Species have changed over time--descent with modification

Prediction: If evolution is true and species have changed over time, then we would expect there to be examples of this preserved in rock strata, across geography, in a lab, or in recorded history.

Falsification: Progressions of change across time do not exist. Species are shown to be unchangeable.

Observations: Fossil progressions, microevolution, biogeography (see examples below).

Corroboration: Dating techniques confirms the progression's sequence. Multiple traits are held in common by the organisms showing relatedness.

Inference: Provided with this gradual gradation in a trait we can infer that the species are related (though not necessarily direct descendants--evolution tends to meander).
Examples:

Fossil progressions

The fossil record demonstrates not only large scale changes across time, phyla, but also within individual species. Three particularly important features are represented within the fossil record. Firstly, the species represented within the fossil record are often vastly different from the life of today. Secondly, is that the the fossils are separately and distinctly layered. Meaning, that we don't find rabbit fossils in the Pre-Cambrian layers, nor dinosaurs mixed in with mammoths. Lastly, both on the macro and micro scale of the fossil record we see trends of change that progress through time.

Whale evolution Individual traits such as leg/pelvis position, nostril location, and ankle bones can also be traced back to the early artiodactyls that led to modern day whales. The transition from toothed whale to baleen whale also has left transition fossils. This can be seen in the pattern of blood vessels on the roof of Aetiocetus weltoni's mouth that are highly similar to those used today to nourish the areas that produce the keratin baleen. Horse Tetrapod We can even track individual traits, like skull bones and limb structure. Humans Bird Brachiopods Diatom Foraminifera
Trilobites

Turtles Elephant Microevolution - There have been many instances where the observed change within a species is fast enough that it can be tracked by researchers. Extending these trend lines into the future we can see how major adaptive changes can add up over time. Soapberry bugs Three-spine stickleback fish Salmon Bacteria: Because of their short generation time, bacteria are excellent subjects to use for evolution experiments. The longest running experiment is Richard Lenski's E. coli experiment. For some 60,000 generations the bacteria have been grown in a laboratory environment under controlled conditions and given very restrictive diets of sugars and nutrients. At regular intervals the bacteria are sampled, frozen, and DNA sequenced. Population growth, fitness, and body size are tracked over time. Their average size also increase remarkably: One of the most exciting finds of the study was the explosion of growth in one strain of bacteria that had multiple mutations that enable them to digest citrate in the presence of oxygen. This ability resulted in the doubling of their population within their beaker. Flat periwinkle: The European green crab was introduced to the North Eastern coast of North America in the early 1800's. These crabs started feeding on flat periwinkles by crushing them with their claws. By comparing shells from museum specimens prior to the crab introduction to those of current day periwinkles a thickening of the shell was observed (conceivably due to the crab predation). Subsequently, the natural selection process of thin shelled periwinkles was replicated in an experimental setting. Samoan Blue Moon Butterfly ( Hypolimnas bolina): A parasite common to arthropods called Wolbachia is passed through the female line in the egg. To increase the parasite's chances of getting into the next generation of butterflies the parasite was killing off males. This resulted in a drastically skewed population distribution with 99% of adults captured being female. Over 4 years and 10 generations of butterfly the population re-established equilibrium with a 50:50 ratio of female to male. Researchers believe this is due to the evolution of a repressor gene that inhibits the Wolbachia's biochemical attack. HIV: At this time the most common treatment for HIV are drug "cocktails," which are the combination of multiple drugs that combat HIV. Isn't that a waste to use multiple drugs to fight one condition? Not when evolution is taken into account. HIV evolves incredibly fast for two reasons. 1) It's high population within a human, numbering far into the trillions per individual. 2) It's use of a error prone reverse transcriptase to copy its genes. This essentially is the perfect storm of drug resistance evolution. That being the case, the current strategy is to overwhelm the HIV virus making it necessary for it to evolve multiple adaptations at the same time--a feat that is difficult but not impossible and the drugs only currently delay the process of evolution. Great lists of observed evolution events: http://www.wired.com/wiredscience/2008/12/evolutionexampl/http://listverse.com/2011/11/19/8-examples-of-evolution-in-action/http://phylointelligence.com/observed.html Artificial selection Dogs Wild mustard Corn Biogeography: Because migration can be difficult and take time, biogeography can spatial represent evolutionary change over time. This phenomena is most obvious in 'ring species' that start spreading slowly over many generations around geographic obstacles, like mountain ranges. The population diverges into two as it spreads around the geographic obstacle and by the time the species meets again on the other side of the obstacle so much time, mutation, genetic drift, and selection has taken place that the two species can no longer interbreed. Even more interesting is that the two separate species can interbreed with those geographically closest to them from the direction that they migrated. Ensatina salamander of California - The species spread from the north and traveled around the Central Valley and diverged into species that can not mate with each other in the south, due to their divergence, but can mate with the species in the direction that they migrated from. The Greenish Warbler ( Phylloscopus nitidus) historically spread around the Himalayas and can interbreed with those from the direction they spread but not those that long ago diverged from them and spread around the opposite side of the mountain range.

The Larus Gulls of the Northern Hemisphere: Notice the darkening of the feathers in a clockwise direction starting at the bottom. Gull species near each other can interbreed, except between the Herring Gull and the Lesser Black-Backed Gull that diverged from each other long the longest ago. This inability to breed tells the story of how the species likely spread overtime around the world and met back up, now no longer able to interbreed.

# data[].attributes.attachment_link.keywords[]
evolutionevidence.org: a new method for teaching evolution evidence

# data[].attributes.attachment_link.description
In 1878 the above sequence of images was taken by Eadweard Muybridge to settle a debate among horse racers--when a horse was galloping full speed, did they ever have all four legs off the ground? The human eye alone was not fast enough to resolve the dispute.

# data[].attributes.headline
Ontogeny Recapitulates phylogeny of a whale

# data[].attributes.subject
Ontogeny Recapitulates phylogeny of a whale

# data[].attributes.body
DNA suggests that whales come from hoofed animals.
https://evolutionforskeptics.wordpress.com/2017/01/22/ontogeny-recapitulates-phylogeny-fetal-whales-have-hindlimb-buds/

# data[].attributes.attachment_link.readable_text
All whales and dolphins are descended from terrestrial mammals, ancient creatures that were very similar to the modern hippopotamus. Now, a fascinating new genetics study shows the incredible evolutionary changes these animals had to experience to become the perfectly adapted marine animals we see today.

Biologists aren't entirely sure which creature modern cetaceans ( dolphins, whales, and porpoises) are descended from. The traditional theory suggests mesonychids, an extinct order of carnivorous ungulates (hoofed animals) which resembled wolves. But more recent genetic analysis points to artiodactyls, a hippo-like creature. Regardless, all cetaceans were land mammals at one point in their evolutionary history - and they had to undergo some rather remarkable changes to adapt to underwater life.

Everyone knows that dolphins are smarter than most other creatures -- but now we have the genetic...
Read more

Recently, researchers from Korea Institute of Ocean Science and Technology, Korea Genome Research Foundation, BGI, and other institutes, performed a comprehensive analysis of the genomes of several cetaceans, including the minke whale, fin whale, bottlenose dolphin, and a finless porpoise. They did so to improve their understanding of the evolutionary changes required for terrestrial mammals to adapt to the ocean - but at the level of the genome.

Lack of Oxygen, Excess Salt

First and foremost, the adaptation to ocean life was marked by resistance to physiological stresses caused by a lack of oxygen and high salt levels. The researchers were able to identify several whale-specific genes strongly associated with these crucial adaptations, including the peroxiredoxin (PRDX) family, O-linked N-acetylglucosaminylation (O-GlcNAcylation). Basically, adaptations to the water favored those mutations that were associated with stress-responsive proteins and anaerobic metabolism.

Deep Dives

But the researchers say the most dramatic environmental adaptation for whales is the ability to dive deep underwater, a feat that would normally induce hypoxia. Typically, when a terrestrial mammals dives too deep, its body produces more reactive oxygen species (ROS) - harmful compounds that can damage DNA. But the body can also produce glutathione, a a well-known antioxidant that prevents damage to important cellular components by ROS. So not surprisingly, the researchers found an increased ratio of reduced glutathione/glutathione disulfide when whales experience hypoxic or oxidative stress.

Other studies have shown how whales can hold their breath for up to an hour.

Unlike humans, many whales can go on hour-long dives without needing to breathe. But no one has...
Read more

From Teeth to Baleen

Lastly, in the case of minke whales, the researchers uncovered the mechanisms required for the evolutionary transition from teeth to baleen. The study affirmed previous research showing connections between several genes - or pseudogenes - that play a role in tooth enamel formation and biomineralization (namely the genes ENAM, MMP, and AMEL).

The researchers also found that the gene families related to whale's body hair and sensory receptors had shrunk, including Keratin-related gene families associated with hair formation and several Hox genes that play an important role in the body plan and embryonic development.

Read the entire study at Nature: " Minke whale genome and aquatic adaptation in cetaceans".

Images: Killer whale breaching, Gerard Lacz/Animals Animals - Earth Scenes via National Geographic.

# data[].attributes.attachment_link.title
How whales made the dramatic evolutionary shift from land to the sea

# data[].attributes.attachment_link.description
All whales and dolphins are descended from terrestrial mammals, ancient creatures that were very similar to the modern hippopotamus. Now, a fascinating new genetics study shows the incredible evolutionary changes these animals had to experience to become the perfectly adapted marine animals we see today.

# data[].attributes.body
Vestigial organs in the whale include the femur and the pelvis
http://homepages.abdn.ac.uk/zoohons/struthers/hind_limbs.hti

# data[].attributes.attachment_link.description
Struthers lived in an exciting time for biologists. Charles Darwin's ideas on the evolution of species by natural selection was still the subject of intense debate and there was a great need for hard evidence that species really did change with time. Vestigial, apparently functionless organs were offered as one kind of evidence.

# data[].attributes.body
Scientists have found that a genetic mutation in the eyes of right whales that hampers their ability to see in bright light may make them more susceptible to fatal entanglements in fishing gear
https://www.sciencedaily.com/releases/2016/10/161024171845.htm

# data[].attributes.attachment_link.readable_text
Posted on Tuesday, October 25th, 2016 at 1:12 pm
Scientists have found that a genetic mutation in the eyes of right whales that hampers their ability to see in bright light may make them more susceptible to fatal entanglements in fishing gear, one of the major causes of death for this critically endangered mammal.
(From Phys.org)- The study of this whale species, which numbers less than 500 individuals remaining in the Western Atlantic Ocean, may also help scientists better understand how vision works in other mammals, including people.

Florida Institute of Technology doctoral student Lorian Schweikert and her adviser, Michael Grace, professor of neuroscience and senior associate dean of science, worked with Jeffry Fasick, an assistant professor of biology at the University of Tampa, to characterize this newly discovered mutation in Northern right whales and Bowhead whales. Their results suggest that this mutation may seriously harm the whales' ability to visually avoid entanglement.

According to their new study, "Evolutionary Loss of Cone Photoreception in Balaenid Whales Reveals Circuit Stability in the Mammalian Retina," published this month in the Journal of Comparative Neurology, the normal light-detecting proteins in cone photoreceptor cells are missing in these whales, demonstrating for the first time the complete loss of cone-based light detection in any mammal.
Read the full article here: http://phys.org/news/2016-10-genetic-mutation-whale-eyes-mortality.html#jCp

# data[].attributes.attachment_link.title
Genetic Mutation In Whale Eyes May Increase Mortality Risks

# data[].attributes.attachment_link.description
Scientists have found that a genetic mutation in the eyes of right whales that hampers their ability to see in bright light may make them more susceptible to fatal entanglements in fishing gear, one of the major causes of death for this critically endangered mammal.

# data[].attributes.body
Natural Selection: The process whereby organisms better adapted to their environment tend to survive and produce more offspring.

Mutation: The changing of the structure of a gene

Migration: Movement from one part of something to another.

Gene flow: The transfer of genetic variation from one population to another.

Divergent Evolution: The accumulation of differences between groups which can lead to the formation of new species

Parallel Evolution:  The independent evolution of similar traits, starting from a similar ancestral condition.

Convergent Evolution:  The process whereby organisms not closely related, independently evolve similar traits as a result of having to adapt to similar environments or ecological niches.

Sexual selection: Natural selection arising through preference by one sex for certain characteristics in individuals of the other sex.

Genetic drift: Variation in the relative frequency of different genotypes in a small population.

Homologous structures:  When very different animals have bones that appear very similar in form or function and seem to be related.

Vestigial Organs:  A rudimentary structure in humans corresponding to a functional structure or organ in ancestral animals.

Ontogeny Recapitulates phylogeny: The development of an organism expresses all the intermediate forms of its ancestors throughout evolution.

Molecular Evolution:  The process of change in the sequence composition of cellular molecules such as DNA, RNA, and proteins across generations.