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Common descent is a general descriptive theory that proposes to explain the origins of living organisms. Because it is so well supported scientifically, macroevolution is often called the "fact of evolution" by biologists. The theory specifically postulates that all of the earth's known biota are genealogically related, much in the same way that siblings or cousins are related to one another. Thus, macroevolutionary processes necessarily entail the transformation of one species into another and, consequently, the origin of higher taxa.
Universal common descent is the hypothesis that all living organisms are the lineal descendants of one original living species. All the diversity of life, both past and present, was originated by normal reproductive processes observable today. Thus, all extant species are related in a strict genealogical sense. More specifically, macroevolution is proposed to occur on a geological timescale and in a gradual manner. "Gradualness" has little to do with the rate or tempo of evolution; it is a mode of change that is dependent on population phenomena. The truth of macroevolution is not assumed a priori in this discussion. Simply put, the hypothesis of common descent, combined with modern biological knowledge, is used to deduce predictions; these predictions are then compared to the real world in order see how the hypothesis fairs in light of the observable evidence. Without assuming the truth of universal common descent, it is highly probable that the hypothesis will indeed fail for most of these predictions -- and this is exactly why many of these predictions are such strong evidence for common descent.
Descent from a common ancestor entails a process of branching and divergence of species, in common with any genealogical process. The macroevolutionary prediction of a unique, historical universal phylogenetic tree (the "Tree of Life") is the most important, powerful, and basic conclusion from the hypothesis of universal common descent. If modern species have descended from ancestral ones in this tree-like, branching manner, a rigorous classification of species should reflect their divergence and it should be possible to infer the true historical tree that traces their paths of descent. Cladistics is a method used to determine the standard phylogenetic tree based on morphology by classifying organisms according to their shared derived characters (proposed by taxonomist Willi Hennig in 1950).
According to the theory of common descent, modern living organisms, with all their incredible differences, are the progeny of one single species in the distant past. In spite of the extensive variation of form and function among organisms, several fundamental criteria characterize all life: (1) replication, (2) information flow in continuity of kind, (3) catalysis, and (4) energy utilization (metabolism). These four functions are required to generate a physical historical process that can be described by a phylogenetic tree.
A basic prediction of the genealogical relatedness of all life, combined with the constraint of gradualism, is that organisms should be very similar in the particular mechanisms and structures that execute these four basic life processes. All known living things use polymers to perform these four basic functions: polynucleotides, polypeptides, and polysaccharides. All known life uses the same polymer, polynucleotide (DNA or RNA), for storing species specific information. All known organisms base replication on the duplication of this molecule. In all known organisms, enzymatic catalysis is based on the abilities provided by protein molecules which are constructed with the same subset of 22 amino acids (even though there are 293 naturally occurring amino acids). All known organisms, with extremely rare exceptions, use the same genetic code for transmitting information from the genetic material to the catalytic material. All known organisms use extremely similar, if not the same, metabolic pathways and metabolic enzymes in processing energy-containing molecules.
If there is one historical phylogenetic tree which unites all species in an objective genealogy, all separate lines of evidence should converge on the same tree. And indeed, independently derived phylogenetic trees of all organisms match each other with an extremely high degree of statistical significance. There are over 1041 different possible ways to arrange the 30 major taxa represented into a phylogenetic tree. Speaking quantitatively, independent morphological and molecular measurements have determined the standard phylogenetic tree to better than 41 decimal places, which is a much greater precision and accuracy than that of even the most well-determined physical constants. For comparison, the charge of the electron is known to only seven decimal places, the Planck constant is known to only eight decimal places, the mass of the neutron, proton, and electron are all known to only nine decimal places, and the universal gravitational constant has been determined to only three decimal places.
Any fossilized animals found should conform to the standard phylogenetic tree. If all organisms are united by descent from a common ancestor, then there is one single true historical phylogeny for all organisms, just like there is one single true historical genealogy for any individual human. It directly follows that if there is a unique universal phylogeny, then all organisms fit in that phylogeny uniquely. We have found a quite complete set of dinosaur (reptile)-to-bird transitional fossils with no morphological gaps, represented by Eoraptor, Herrerasaurus, Ceratosaurus, Allosaurus, Compsognathus, Sinosauropteryx, Protarchaeopteryx, Caudipteryx, Velociraptor, Sinovenator, Beipiaosaurus, Sinornithosaurus, Microraptor, Archaeopteryx, Rahonavis, Confuciusornis, Sinornis, Patagopteryx, Hesperornis, Apsaravis, Ichthyornis, and Columba, among many others. We also have an exquisitely complete series of fossils for the reptile-to-mammal intermediates, ranging from the pelycosauria, therapsida, cynodonta, up to primitive mammalia.
Based upon the consensus of numerous phylogenetic analyses, Pan troglodytes (the chimpanzee) is the closest living relative of humans. Thus, we expect that organisms lived in the past which were intermediate in morphology between humans and chimpanzees. Over the past century, many spectacular paleontological finds have identified such transitional hominid fossils. Another impressive example of incontrovertible transitional forms predicted to exist by evolutionary biologists is the collection of land mammal-to-whale fossil intermediates. Whales, of course, are sea animals with flippers. Since they are also mammals, the consensus phylogeny indicates that whales and dolphins evolved from land mammals with legs. In recent years, we have found several transitional forms of whales with legs, both capable and incapable of terrestrial locomotion (see Transitional Fossils FAQ).
The reptile-bird intermediates date from the Upper Jurassic and Lower Cretaceous (about 150 million years ago), whereas pelycosauria and therapsida (reptile-mammal intermediates) are older and date from the Carboniferous and the Permian (about 250 to 350 million years ago). This is precisely what should be observed if the fossil record matches the standard phylogenetic tree. The most scientifically rigorous method of confirming this is to demonstrate a positive correlation between phylogeny and stratigraphy (the strata or rocks of the geologic column where fossils are found throughout the world), i.e. a positive correlation between the order of taxa in a phylogenetic tree and the geological order in which those taxa first appear and last appear (whether for living or extinct intermediates).
Within the error inherent in the fossil record, prokaryotes should appear first, followed by simple multicellular animals like sponges and starfish, then lampreys, fish, amphibians, reptiles, then mammals, etc. Studies from the past ten years addressing this very issue have confirmed that there is indeed a positive correlation between phylogeny and stratigraphy, with statistical significance. Using three different measures of phylogeny-stratigraphy correlation [the RCI, GER, and SCI], a high positive correlation was found between the standard phylogenetic tree and the stratigraphic range of the same taxa, with very high statistical significance (P < 0.0001).
It would be highly inconsistent if the chronological order were reversed in the reptile-bird and reptile-mammal example. More generally, the strongest falsification of this prediction would be the finding that there was a negative correlation between stratigraphy and the phylogenetic tree that describes the genealogical relatedness of all living organisms.
Some of the more renowned evidences for evolution are the various nonfunctional or rudimentary vestigial characters, both anatomical and molecular, which are found throughout biology. During macroevolutionary history, functions necessarily have been gained and lost. Thus, from common descent and the constraint of gradualism, we predict that many organisms should display vestigial structures, which are structural remnants of lost functions.
There are many examples of rudimentary and nonfunctional characters carried by organisms, and these can very often be explained in terms of evolutionary histories: (1) snakes such as pythons (which are legless snakes) carry vestigial pelvises hidden beneath their skin; (2) some lizards carry rudimentary, nonfunctional legs underneath their skin, undetectable from the outside; (3) many cave dwelling animals, such as the fish Astyanax mexicanus (the Mexican tetra) and the salamander species Typhlotriton spelaeus and Proteus anguinus, are blind yet have rudimentary, vestigial eyes; (4) dandelions reproduce without reproduction (a condition known as apomixis), yet they retain flowers and produce pollen (which are useless); (5) over 90% of all adult humans develop third molars (otherwise known as wisdom teeth), and in one-third they are malformed and impacted (these useless structures point to our ancestors who were herbivorous, and molar teeth were required for chewing and grinding plant material, but now in human beings can cause significant pain and increased risk for injury, etc); (6) there are many examples of flightless beetles (such as the weevils of the genus Lucanidae) which retain perfectly formed wings housed underneath fused wing covers. All of these examples can be explained in terms of the beneficial functions and structures of the organisms' predicted ancestors.
Anatomical atavisms are closely related conceptually to vestigial structures. An atavism is the reappearance of a lost character specific to a remote evolutionary ancestor and not observed in the parents or recent ancestors of the organism displaying the atavistic character. As with vestigial structures, no organism can have an atavistic structure that was not previously found in one of its ancestors.
Probably the most well known case of atavism is found in the whales. According to the standard phylogenetic tree, whales are known to be the descendants of terrestrial mammals that had hindlimbs. Thus, we expect the possibility that rare mutant whales might occasionally develop atavistic hindlimbs. In fact, there are many cases where whales have been found with rudimentary atavistic hindlimbs in the wild: hindlimbs have been found in baleen whales, humpback whales, and in many specimens of sperm whales. Most of these examples are of whales with femurs, tibia, and fibulae; however, some even include feet with complete digits.
Other famous examples of atavisms exist, including (1) rare formation of extra toes in horses (2nd and 4th digits), similar to what is seen in the archaic horses Mesohippus and Merychippus; (2) atavistic thigh muscles in birds and sparrows (Passeriform); (3) hyoid muscles in dogs; (4) wings in earwigs (normally wingless); (5) atavistic fibulae in birds (the fibulae are normally extremely reduced); (6) extra toes in guinea pigs and salamanders; (6) the atavistic dew claw in many dogs; and (7) various atavisms in humans -- such as the "true human tail." Concerning the latter, more than 100 cases of human tails have been reported in the medical literature and less than one-third of these are medically known as "pseudo-tails" (which are not true tails). True human tails are complex structures which have muscle, blood vessels, occasional vertebrae and cartilage, can move and contract, and they are occasionally inherited.
Vestigial characters are also found at the molecular level: (1) the L-gulano-g-lactone oxidase gene, the gene required for Vitamin C synthesis, was found in humans and guinea pigs, and in other primates (chimpanzees, orangutans, and macaques), exactly as predicted by evolutionary theory (it exists as a pseudogene, present but incapable of functioning); (2) multiple odorant receptor genes; (3) the RT6 protein gene; (4) the galactosyl transferase gene; and (5) the tyrosinase-related gene (TYRL). We share these vestigial genes with other primates, and the mutations that made these genes nonfunctional are also shared with several other primates.
Embryology and developmental biology have provided some fascinating insights into evolutionary pathways. Since the cladistic morphological classification of species is generally based on derived characters of adult organisms, embryology and developmental studies provide a nearly independent body of evidence. From embryological studies it is known that two bones of a developing reptile eventually form the quadrate and the articular bones in the hinge of the adult reptilian jaw. Accordingly, there is a very complete series of fossil intermediates in which these structures are clearly modified from the reptilian jaw to the mammalian ear.
Early in development, mammalian embryos temporarily have pharyngeal pouches, which are morphologically indistinguishable from aquatic vertebrate gill pouches. This evolutionary relic reflects the fact that mammalian ancestors were once aquatic gill-breathing vertebrates. The arches between the gills, called branchial arches, were present in jawless fish and some of these branchial arches later evolved into the bones of the jaw, and, eventually, into the bones of the inner ear.
Many species of snakes and legless lizards (such as the "slow worm") initially develop limb buds in their embryonic development, only to reabsorb them before hatching. Similarly, modern adult whales, dolphins, and porpoises have no hind legs. Even so, hind legs, complete with various leg bones, nerves, and blood vessels, temporarily appear in the cetacean fetus and subsequently degenerate before birth.
Mammals evolved from a reptile-like ancestor, and placental mammals (like humans and dogs) have lost the egg-tooth and caruncle (and eggshell). However, monotremes, such as the platypus and echidna, are primitive mammals that have both an egg-tooth and a caruncle, even though the monotreme eggshell is thin and leathery. Most strikingly, during marsupial development, an eggshell forms transiently and then is reabsorbed before live birth. Though they have no need for it, several marsupial newborns (such as baby Brushtail possums, koalas, and bandicoots) retain a vestigial caruncle as a clear indicator of their reptilian, oviparous ancestry.
The fossil record has confirmed that birds once had teeth, as demonstrated by the fossils of many birds with teeth including Archaeopteryx. Furthermore, this predicted possibility has been confirmed experimentally in a modern bird, the chicken. Kollar and Fisher transplanted a small piece of mammalian mesenchymal tissue (which forms teeth) underneath the beak-forming epithelial layer of a developing chick. Intriguingly, they observed that the chicken epithelium secreted dental enamel and directed the adjacent mesenchyme to form teeth. This would have been impossible unless the chicken still retained the genes and developmental pathway for making teeth. Thus, chickens have not yet completely lost the genes coding for tooth development (two of Stephen Jay Gould's popular books are titled Hen's Teeth and Horse's Toes and The Panda's Thumb which explain some of this past evolutionary history).
Common ancestors originate in a particular geographical location. Thus, the spatial and geographical distribution of species should be consistent with their predicted genealogical relationships. The standard phylogenetic tree predicts that new species must originate close to the older species from which they are derived. Closely related contemporary species should be close geographically, regardless of their habitat or specific adaptations (if not, there should be a good explanation, such as extreme mobility in the case of birds, sea animals, or human intervention).
Examples of present biogeography supporting evolutionary theory are (1) marsupials (kangaroos, etc) which only inhabit Australia (exceptions such as some South American species and the opossum are explained by continental drift); (2) conversely, placental mammals are virtually absent on Australia, despite the fact that many would flourish there (humans introduced most of the few placentals found on Australia); (3) the southern reaches of South America and Africa and all of Australia share lungfishes, ostrich-like birds (ratite birds), and leptodactylid frogs -- all of which occur nowhere else; (4) alligators, some related species of giant salamander, and magnolias only occur in Eastern North America and East Asia (which were once spatially close in the Laurasian continent); (5) indigenous Cacti (Cactus plant) only inhabit the Americas, while Saharan and Australian vegetation is very distantly related (mostly Euphorbiaceae); (6) members of the closely related pineapple family inhabit many diverse habitats (such as rainforest, alpine, and desert areas), but only in the American tropics, not African or Asian tropics, etc.
As for past biogeography, we find the earliest marsupial fossils (e.g. Alphadon) from the Late Cretaceous, when South America, Antarctica, and Australia were still connected. Additionally, the earliest ancestors of modern marsupials are actually found on North America. The obvious paleontological deduction is that extinct marsupial fossil organisms should be found on South America and Antarctica, since marsupials must have traversed these continents to reach their present day location in Australia. Interestingly, we have found marsupial fossils on both South America and on Antarctica. This is an astounding macroevolutionary confirmation, given that no marsupials live on Antarctica now.
The Equidae (i.e. horse) fossil record is very complete (though extremely complex) and makes very good geographical sense, without any large spatial jumps between intermediates. Every single one of the fossil ancestors of the modern horse are found on the North American continent. Finally, the theory of common descent predicts that we may find early hominid fossils on the African continent. Numerous transitional fossils between humans and the great apes have been found in southern and eastern Africa. Examples include Ardipithecus ramidus, Australopithecus anamensis, Australopithecus afarensis, Australopithecus garhi, Kenyanthropus platyops, Kenyanthropus rudolfensis, Homo habilis, and a host of other transitionals thought to be less related to Homo sapiens, such as the robust australopithicenes.
The principle of evolutionary opportunism is closely related to evolutionary history and to the effects of contingency. Descent with gradual modification means that new organisms can only use and modify what they initially are given; they are slaves to their history. New structures and functions must be recruited from previous, older structures. One major consequence of the constraint of gradualism is the predicted existence of "paralogy": similarity of structure despite difference in function.
There are countless examples of paralogy in living and extinct species -- the same bones in the same relative positions are used in primate hands, bat wings, bird wings, pterosaur wings, whale and penguin flippers, horse legs, the digging forelimbs of moles, and webbed amphibian legs. All of these characters have similar structures that perform various different functions. The standard phylogenetic tree shows why these species have these same structures, i.e. they have common ancestors that had these structures. The fossil record shows a general chronological progression of intermediate forms between theropod dinosaurs and modern birds, in which theropod structures were modified into modern bird structures.
On the molecular level, the existence of paralogy is quite impressive. Many proteins of very different function have strikingly similar amino acid sequences and three-dimensional structures. A frequently cited example is lysozyme and a-lactalbumin. A-Lactalbumin is very similar structurally to lysozyme, even though its function is very different (it is involved in mammalian lactose synthesis in the mammary gland). On a grander scale, a stunning confirmation of these evolutionary predictions has come from an analysis of Saccharomyces cerevisiae (baker's yeast) and Caenorhabditis elegans (a worm). The genes used by the yeast, a unicellular organism, are mostly genes dealing directly with core biochemical functions that all organisms must perform. From an evolutionary perspective, we would expect these genes to be ancient. Thus it was expected and shown that the worm contains a great majority of these genes. In contrast, the extra genes used by the worm, which deal with multicellularity, should be more recently evolved. Phylogenetic analysis has shown that this is exactly the case. An even larger study of the known eukaryotic genomes has further demonstrated that paralogy is rampant in nature, and that true structural innovation is relatively rare ("Comparative Genomics of the Eukaryotes" [2000] Science 287: 2204-2218).
A corollary of the principle of evolutionary opportunism is analogy. Analogy is the case where different structures perform the same or similar functions in different species. Two distinct species have different histories and different structures; if both species evolve the same new function, they may recruit different structures to perform this new function. Analogy also must conform to the principle of structural continuity; analogy must be explained in terms of the structures of predicted ancestors. There are many anatomical examples of functional analogy. One case is the vertebrate eye and the cephalopod eye. Another is the case of American and Saharan desert plants, which use different structures for the same functions needed to live in dry, arid regions. By contrast, we would not expect newly discovered species of dolphins, whales, penguins, or any close mammalian relatives to have gills (a possible analogy with fish), since their immediate ancestors lacked gills or gill-like structures from which they could be derived.
A familiar molecular example is the case of the three proteases subtilisin, carboxy peptidase II, and chymotrypsin. These three proteins are all serine proteases (i.e. they degrade other proteins in digestion). They have the same function, the same catalytic residues in their active sites, and they have the same catalytic mechanism. Yet they have no sequence or structural similarity. Another molecular example is that of DNA polymerases. Rat polymerase ß has obviously evolved from nucleotidyl transferases by mutating to catalyze several nucleotide additions instead of just one -- which nicely illustrates why analogy is ultimately also paralogy.
Another consequence of evolutionary opportunism is the existence of apparent suboptimal function. This does not refer to a structure functioning poorly. It simply means that a structure with a more efficient design (usually with less superfluous complexity), could perform the same final function equally as well. Structures with suboptimal function should have a gradualistic historical evolutionary explanation, based on the opportunistic recruitment of ancestral structures.
For example, the mammalian gastrointestinal tract crosses the respiratory system. Functionally, this is suboptimal; it would be beneficial if we could breathe and swallow simultaneously. However, there is a good historical evolutionary reason for this arrangement. The Osteolepiformes (Devonian lungfish), from which mammals evolved, swallowed air to breathe. Only later did the ancestors of mammals recruit the olfactory nares of fish for the function of breathing on land. Another anatomical example of suboptimal function is the inverted mammalian retina, with its blind spot. In order to deal with the many problems inherent in an inverted retina, the vertebrate eye utilizes various complex compensatory structures and mechanisms. In contrast with mammalian eyes, cephalopod eyes have very different underlying retinal structures (e.g. they are verted, not inverted), and they have no blind spots. This strongly suggests that mammals also could have eyes without blind spots.
With the recent sequencing of the human genome, we have found that less than 2% of the DNA in the human genome is used for making proteins (International Human Genome Sequencing Consortium 2001). A full 45% of our genome is composed of transposons, which serve no known function for the individual (except to cause a significant fraction of genetic illnesses and cancers). Twenty percent of the human genome are pseudogenes. They also serve no function for the individual. A remarkable example is the glyceraldehyde-3-phosphate dehydrogenase (GDPH) gene. In humans, there is one functional GDPH gene, but there are at least twenty GDPH pseudogenes. In mice, there are approximately 200 GDPH pseudogenes, none of which are necessary. In addition to one or two functional copies, there are between 20 and 30 pseudogenes of cytochrome c in both humans and the rat.
A lot of wasted energy is expended in dealing with this useless DNA; however, all these molecular examples also have convincing explanations based on evolutionary histories.
The molecular sequence evidence gives the most impressive and irrefutable evidence for the genealogical relatedness of all life. The nature of molecular sequences allows for extremely impressive probability calculations that demonstrate how well the predictions of common descent with modification actually match empirical observation. There are several categories and independent lines of molecular sequence evidence useful for determining phylogenetic relationships. Studies of functional elements include ribosomal RNA, ubiquitous proteins, and mitochondrial DNA comparisons; studies of nonfunctional elements include comparisons of pseudogenes, endogenous retroviral genes, and mobile genetic elements (such as introns, transposons, or retroelements).
Cytochrome c is an essential and ubiquitous protein found in all organisms, including eukaryotes and bacteria. The mitochondria of cells contain cytochrome c, where it transports electrons in the fundamental metabolic process of oxidative phosphorylation. The oxygen we breathe is used to generate energy in this process. Using an ubiquitous gene such as cytochrome c, there is no reason to assume that two different organisms should have the same protein sequence or even similar protein sequences, unless the two organisms are genealogically related. Hubert Yockey has done a careful study in which he calculated that there are a minimum of 2.3 x 1093 possible functional cytochrome c protein sequences, based on genetic mutational analyses (Yockey, H.P. [Cambridge Univ Press, 1992] Information Theory and Molecular Biology, Chapter 6). For perspective, the number 1093 is about one billion times larger than the number of atoms in the visible universe. Thus, functional cytochrome c sequences are virtually unlimited in number, and there is no a priori reason for two different species to have the same, or even mildly similar, cytochrome c protein sequences.
From the theory of common descent and our standard phylogenetic tree we know that humans and chimpanzees are quite closely related. We therefore predict, in spite of the odds, that human and chimpanzee cytochrome c sequences should be much more similar than, say, human and yeast cytochrome c -- simply due to inheritance. This has been confirmed: Humans and chimpanzees have the exact same cytochrome c protein sequence. In the absence of common descent, the chance of this occurrence is conservatively less than 10-93 (1 out of 1093). Thus, the high degree of similarity in these proteins is a spectacular corroboration of the theory of common descent. Furthermore, human and chimpanzee cytochrome c proteins differ by about 10 amino acids from all other mammals. The chance of this occurring in the absence of a hereditary mechanism is less than 10-29.
Further, bat cytochrome c is much more similar to human cytochrome c than to hummingbird cytochrome c; porpoise cytochrome c is much more similar to human cytochrome c than to shark cytochrome c. The phylogenetic tree constructed from the cytochrome c data exactly recapitulates the relationships of major taxa as determined by the completely independent morphological data. Why would two organisms have such similar ubiquitous proteins when the odds are astronomically against it? We know of only one reason for why two organisms would have two similar protein sequences in the absence of functional necessity: heredity. Thus, in such cases we can confidently deduce that the two organisms are genealogically related.
Like protein sequence similarity, the DNA sequence similarity of two ubiquitous genes also implies common ancestry. If chimps and humans are truly genealogically related, we predict that the difference between their respective cytochrome c gene DNA sequences should be less than 3% -- probably even much less, due to the essential function of the cytochrome c gene. As mentioned above, the cytochrome c proteins in chimps and humans are exactly identical. The clincher is that the two DNA sequences that code for cytochrome c in humans and chimps differ by only one base (a 0.3% difference), even though there are 1049 different sequences that could code for this protein. The combined effects of DNA coding redundancy and protein sequence redundancy make DNA sequence comparisons doubly redundant; DNA sequences of ubiquitous proteins are completely uncorrelated with phenotype, but they are strongly causally correlated with heredity. This is why DNA sequence phylogenies are considered so robust.
Other nonfunctional molecular examples that provide evidence of common ancestry are pseudogenes. Pseudogenes are very closely related to their functional counterparts (in primary sequence and often in chromosomal location), except that either they have faulty regulatory sequences or they have internal stops that keep the protein from being made. They are functionless and do not affect an organism's phenotype when deleted. Finding the same pseudogene in the same chromosomal location in two species is strong evidence of common ancestry.
This also has been confirmed: there are very many examples of shared pseudogenes between primates and humans. One is the ψη-globin gene, a hemoglobin pseudogene. It is shared among the primates only, in the exact chromosomal location, with the same mutations that render it nonfunctional. Another example is the steroid 21-hydroxylase gene. Humans have two copies of the steroid 21-hydroxylase gene, a functional one and a nonfunctional pseudogene. Chimps and humans both share the same eight bp deletion in this pseudogene that renders it nonfunctional.
These previous points are all evidence of macroevolution alone; the evidence and the conclusion are independent of any specific gradualistic explanatory mechanisms for the origin and evolution of macroevolutionary adaptations and variation. This is why scientists call universal common descent the "fact of evolution." None of the evidence above assumes that natural selection is true or that it is sufficient for generating adaptations or the differences between species and other taxa. Thus, the macroevolutionary conclusion stands, regardless of the mechanism.