Evidence Supporting Biological Evolution - Science and Creationism - NCBI Bookshelf
Homology involves the theory that macroevolutionary relationships can be By arranging or classifying large sets of anatomical structures according to the .. theory of bird evolution suggests that they evolved from dinosaurs or other reptiles. Darwin was well aware of this difficulty and attempted to preempt his .. the evolutionary history of the birds, and their relationship to dinosaurs. A phylogenetic tree is a diagram that represents evolutionary relationships among organisms. For example, the phylogenetic tree below represents relationships between . (shape/appearance), internal anatomy, behaviors, biochemical pathways, How do we know which kinds of dinosaurs were most closely related?.
His apparent lack of interest in the London specimen is part of a pattern of interpretation that, I will now suggest, is made comprehensible by carefully scrutinizing his views on biological classification. Darwin on species and varieties Understanding Darwin's views on species can be challenging. There is fairly widespread agreement GhiselinBeattyEreshefsky that he rejected the reality of the species category, meaning he regarded the Linnaean rank of species as a useful heuristic rather than a genuine division in the natural world.
Passages such as the following excerpt from an letter to Joseph Hooker have been of particular importance in the debate on this issue: Darwin Here, Darwin seems to be suggesting that the dream of determining the necessary and sufficient conditions for membership in the species category is unrealizable. Regardless of whether he did or did not believe in the reality of the species category, remarks such as these are important for our purposes because they provide valuable insight into his views about the process of biological classification.
Crucially, however, he did not believe that the process of determining whether a particular specimen should be ranked as a species or a variety is entirely haphazard: Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.
Darwinpp. That is, when we attempt to determine whether a specimen belongs to a particular species, the judgment we reach will likely be based on morphological comparisons with previously described members of the species. If the investigator believes that the unidentified specimen is sufficiently similar to known varieties, it will be classified as a member of the described species.
If it is not deemed sufficiently similar, it will be treated as a new species. With this in mind, we can turn our attention back to transitional forms and, in particular, how Darwin would go about classifying them.
Darwin on transitional forms Casually speaking, a transitional or intermediate form is a species in which a quantitative trait of interest represents an approximate morphological median between two other species. If a hypothetical transitional form is labeled B, we should expect it to be similar to a species A in some regards, but in other ways, it should resemble a species C.
This is the way transitional forms were understood during the nineteenth century and, arguably, the same way we tend to conceive them today. The fact that transitional forms should have been coveted by Darwin appears obvious, in part because he held that evolutionary change is a very gradual process. As Eldredge has recently discussed, Darwin believed that species initially form continuous populations.
Over time, however, populations begin to diverge geographically, leading to subpopulations being subjected to different selective pressures.
Specialist forms with extreme morphologies are favored over generalist ancestral types, and as time passes, the latter are eventually driven to extinction. If, as Darwin suggested, evolutionary change occurs in this gradual manner rather than through rapid saltations, the gaps we observe in the fossil record must have formerly been filled by morphological intermediates. Because his detractors repeatedly criticized him for failing to produce evidence that transitional forms once existed, why did he not attempt to rebut these charges by pointing to the existence of an animal such as Archaeopteryx?
One reason is that it would be extremely difficult for someone with his views about classification to declare that a particular specimen is transitional.
Darwin's thoughts on intermediate forms are most fully developed in the following selection from the Origin, which deserves to be quoted at length: It is all-important to remember that naturalists have no golden rule by which to distinguish species and varieties; they grant some little variability to each species, but when they meet with a somewhat greater amount of difference between any two forms, they rank both as species, unless they are enabled to connect them together by close intermediate gradations.
And this from the reasons just assigned we can seldom hope to effect in any one geological section.
Evidence of common descent
Supposing B and C to be two species, and a third, A, to be found in an underlying bed; even if A were strictly intermediate between B and C, it would simply be ranked as a third and distinct species, unless at the same time it could be most closely connected with either one or both forms by intermediate varieties. Nor should it be forgotten, as before explained, that A might be the actual progenitor of B and C, and yet might not at all necessarily be strictly intermediate between them in all points of structure.
So that we might obtain the parent-species and its several modified descendants from the lower and upper beds of a formation, and unless we obtained numerous transitional gradations, we should not recognise their relationship, and should consequently be compelled to rank them all as distinct species.
Again, this means that if the organism under consideration cannot be connected to previously known varieties, it will be classified as a new species.
If we follow Darwin and accept that there is no touchstone standard by which species can be distinguished from varieties, we are forced to maintain that most classifications are made in the manner just described. However, if systematic analyses are carried out exclusively by means of comparison with known forms, the task of classifying paleontological specimens becomes very difficult. As Darwin mentioned in conjunction with his pessimistic arguments about the geological record, fine gradations of form are not found because a fossilization is extremely rare and b phenomena such as erosion and the movement of the Earth's crust are constantly destroying the geological strata.
To understand why the incompleteness of the geological record made the identification of transitional forms difficult for Darwin, it is necessary to note that he regarded well-marked varieties as incipient species. Given this equivalence principle, the process of determining whether a particular fossil should be classified as a transitional form is similar to the procedure by which a specimen is judged to be a species or a variety.
According to Darwin, the transitional designation can only be conferred upon a specimen if it can be connected to two seemingly distinct lineages by means of a series of intermediate varieties.
Given that such varieties are not preserved in the fossil record, he was unable to class Archaeopteryx, Compsognathus, or any of the other fossils that have been discussed as transitional forms.
Just as an unidentified specimen of contemporary origin that cannot be linked to known varieties will be classified as a new species, a paleontological specimen that cannot be linked to purported ancestral and descendent types by means of varieties will be treated as a new monotypic lineage. Therefore, it can be said that the incompleteness of the geological record and Darwin's views about species and classification conspired to prevent him from supporting his theory by citing the existence of certain well-known transitional-form candidates.
Positive paleontological evidence At this point, enough has been said about Darwin's treatment of the transitional-form candidates see figure 2 for a summary of the relevant eventsbut one very important question remains unanswered: There is some indication that the positive evidence Darwin desired was a graded succession of forms.
Evidence of this type would have done little to appease a persistent critic who relishes in playing the missing-link game, and, of course, Darwin himself did not need to be persuaded of the truth of his theory. However, such a discovery would have undoubtedly helped to sway objective researchers who had previously been unconvinced by the data and accompanying discussion in the Origin.
A timeline of events, — Portraits courtesy of Wikimedia Commons.
View large Download slide A timeline of events, — The fact that Darwin hoped paleontological evidence would be discovered is shown in his praise of the American paleontologist Othniel Marsh's work on horses and Cretaceous toothed birds Sequence comparison is considered a measure robust enough to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. For example, neutral human DNA sequences are approximately 1.
The analysis by Carl Woese resulted in the three-domain systemarguing for two major splits in the early evolution of life. The first split led to modern Bacteria and the subsequent split led to modern Archaea and Eukaryotes. Some DNA sequences are shared by very different organisms. It has been predicted by the theory of evolution that the differences in such DNA sequences between two organisms should roughly resemble both the biological difference between them according to their anatomy and the time that had passed since these two organisms have separated in the course of evolution, as seen in fossil evidence.
The rate of accumulating such changes should be low for some sequences, namely those that code for critical RNA or proteinsand high for others that code for less critical RNA or proteins; but for every specific sequence, the rate of change should be roughly constant over time.
These results have been experimentally confirmed. Two examples are DNA sequences coding for rRNAwhich is highly conserved, and DNA sequences coding for fibrinopeptides amino acid chains that are discarded during the formation of fibrinwhich are highly non-conserved. Vital proteinssuch as the ribosomeDNA polymeraseand RNA polymeraseare found in everything from the most primitive bacteria to the most complex mammals. The core part of the protein is conserved across all lineages of life, serving similar functions.
Higher organisms have evolved additional protein subunitslargely affecting the regulation and protein-protein interaction of the core. Other overarching similarities between all lineages of extant organisms, such as DNARNAamino acids, and the lipid bilayergive support to the theory of common descent.
Phylogenetic analyses of protein sequences from various organisms produce similar trees of relationship between all organisms. As there is no functional advantage to right- or left-handed molecular chirality, the simplest hypothesis is that the choice was made randomly by early organisms and passed on to all extant life through common descent.
Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogenes"dead" genes that steadily accumulate mutations. A pseudogene can be produced when a coding gene accumulates mutations that prevent it from being transcribed, making it non-functional. Non-functional pseudogenes may be passed on to later species, thereby labeling the later species as descended from the earlier species. Other mechanisms[ edit ] A large body of molecular evidence supports a variety of mechanisms for large evolutionary changes, including: The endosymbiotic theory explains the origin of mitochondria and plastids including chloroplastswhich are organelles of eukaryotic cells, as the incorporation of an ancient prokaryotic cell into ancient eukaryotic cell.
Rather than evolving eukaryotic organelles slowly, this theory offers a mechanism for a sudden evolutionary leap by incorporating the genetic material and biochemical composition of a separate species. Evidence supporting this mechanism has been found in the protist Hatena: Many lineages diverged when new metabolic processes appeared, and it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor or by detecting their physical manifestations.
As an example, the appearance of oxygen in the earth's atmosphere is linked to the evolution of photosynthesis. Specific examples from comparative physiology and biochemistry[ edit ] Chromosome 2 in humans[ edit ] Further information: Fusion of ancestral chromosomes left distinctive remnants of telomeres, and a vestigial centromere Evidence for the evolution of Homo sapiens from a common ancestor with chimpanzees is found in the number of chromosomes in humans as compared to all other members of Hominidae.
All hominidae have 24 pairs of chromosomes, except humans, who have only 23 pairs. Human chromosome 2 is a result of an end-to-end fusion of two ancestral chromosomes. The correspondence of chromosome 2 to two ape chromosomes. The closest human relative, the common chimpanzeehas near-identical DNA sequences to human chromosome 2, but they are found in two separate chromosomes.
The same is true of the more distant gorilla and orangutan. Normally a chromosome has just one centromere, but in chromosome 2 there are remnants of a second centromere. These are normally found only at the ends of a chromosome, but in chromosome 2 there are additional telomere sequences in the middle.
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Phylogenetic trees | Evolutionary tree (article) | Khan Academy
Cytochrome c A classic example of biochemical evidence for evolution is the variance of the ubiquitous i. The variance of cytochrome c of different organisms is measured in the number of differing amino acids, each differing amino acid being a result of a base pair substitution, a mutation. If each differing amino acid is assumed the result of one base pair substitution, it can be calculated how long ago the two species diverged by multiplying the number of base pair substitutions by the estimated time it takes for a substituted base pair of the cytochrome c gene to be successfully passed on.