Common descent
5259
224978617
2008-07-11T08:00:02Z
ForestDim
5954491
adding link bulgarian
{{evolution3}}
A group of organisms is said to have '''common descent''' if they have a common [[ancestor]]. In modern [[biology]], it is generally accepted that all living [[organism]]s on [[Earth]] are descended from a common ancestor or ancestral gene pool.<ref>The earliest life-like forms probably exchanged genetic material laterally in a manner that is analogous to [[Horizontal gene transfer|lateral gene transfer]] amongst bacteria. For this and other reasons, the most recent common ancestor may have been a genetic pool rather than an organism.</ref>
A theory of '''universal common descent''' based on [[evolution]]ary principles was proposed by [[Charles Darwin]] in his book ''[[On the Origin of Species]]'' (1859), and later in ''[[The Descent of Man and Selection in Relation to Sex|The Descent of Man]]'' (1871).
This theory is now generally accepted by biologists, and the [[last universal ancestor|last universal common ancestor]] (LUCA or LUA), that is, the [[most recent common ancestor]] of all currently living organisms, is believed to have appeared about [[Timeline of evolution|3.9 billion years ago]]. The theory of a common ancestor between all organisms is one of the principles of evolution, although for [[single cell organism]]s and [[virus]]es, single [[phylogeny]] is disputed (see: [[origin of life]]).
== History ==
{{seealso|History of evolutionary thought}}
The first suggestion that all organisms may have had a common ancestor and diverged through random variation and natural selection was made in 1745 by the French mathematician and scientist [[Pierre-Louis Moreau de Maupertuis]] (1698-1759) in his work ''Vénus physique''. Specifically:
:''"Could one not say that, in the fortuitous combinations of the productions of nature, as there must be some characterized by a certain relation of fitness which are able to subsist, it is not to be wondered at that this fitness is present in all the species that are currently in existence? Chance, one would say, produced an innumerable multitude of individuals; a small number found themselves constructed in such a manner that the parts of the animal were able to satisfy its needs; in another infinitely greater number, there was neither fitness nor order: all of these latter have perished. Animals lacking a mouth could not live; others lacking reproductive organs could not perpetuate themselves... The species we see today are but the smallest part of what blind destiny has produced..."''
In 1790, [[Immanuel Kant]] (Königsberg (Kaliningrad) 1724 - 1804), in his ''Kritik der Urtheilskraft'', states that the analogy of animal forms implies a common original type and thus a common parent.
Charles Darwin's grandfather, [[Erasmus Darwin]], hypothesized in 1795 that all warm-blooded animals were descended from a single "living filament":
:''"...would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament, which THE GREAT FIRST CAUSE endued with animality...?"'' (''Zoonomia'', 1795, section 39, "Generation")
In 1859, Charles Darwin's ''The Origin of Species'' was published. The views about common descent expressed therein vary between suggesting that there was a single "first creature" to allowing that there may have been more than one. Here are the relevant quotations from the ''Conclusion'':
:''"[P]robably all of the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."''
:''"The whole history of the world, as at present known, ... will hereafter be recognized as a mere fragment of time, compared with the ages which have elapsed since the first creature, the progenitor of innumerable extinct and living descendants, was created."''
:''"When I view all beings not as [[special creation]]s, but as the lineal descendants of some few beings which lived long before the first bed of the Silurian system was deposited, they seem to me to become ennobled."''
The famous closing sentence describes the "grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one." The phrase "one form" here seems to hark back to the phrase "some few beings"; in any case, the choice of words is remarkable for its consistency with recent ideas about there having been a single ancestral "genetic pool".
== Evidence of universal common descent ==
{{Main|Evidence of common descent}}
=== Common biochemistry and genetic code ===
All known forms of life are based on the same fundamental biochemical organisation: genetic information encoded in [[DNA]], transcribed into RNA, through the effect of protein- and RNA-enzymes, then translated into proteins by (highly similar) ribosomes, with ATP, NADH and others as energy currencies, etc. Furthermore, the [[genetic code]] (the "translation table" according to which DNA information is translated into proteins) is nearly identical for all known lifeforms, from bacteria to humans, with minor local differences. The universality of this code is generally regarded by biologists as definitive evidence in favor of the theory of universal common descent. Analysis of the small differences in the genetic code has also provided support for universal common descent.<ref>{{cite journal | author=Robin Knight et al. | year=2001 | title=Rewiring the keyboard: evolvability of the genetic code | journal=Nature Reviews - Genetics | pages=49–58 | volume=2 | issue=1 | pmid=11253070 | doi=10.1038/35047500}}</ref>
==== Selectively neutral similarities ====
Similarities which have no relevance to evolution and therefore cannot be explained by convergence, tend to be very compelling support for the universal common descent theory.
Such evidence has come from two domains: amino acid sequences and DNA sequences. [[Proteins]] with the same 3-d structure need not have identical [[amino acid]] sequences; any irrelevant similarity between the sequences is evidence for common descent. In certain cases, there are several codons (DNA triplets) that code for the same amino acid. Thus, if two species use the same [[codon]] at the same place to specify an amino acid that can be represented by more than one codon, that is evidence for recency of a common ancestor.
==== Other similarities ====
The universality of many aspects of cellular life is often pointed to as supportive evidence to the more compelling evidence listed above. These similarities include the energy carrier [[adenosine triphosphate|ATP]], and the fact that all [[amino acid]]s found in proteins are left-handed. It is possible that these similarities resulted because of the laws of physics and chemistry, rather than universal common descent and therefore resulted in [[convergent evolution]].
=== Phylogenetic trees ===
[[Image:Phylogenetic tree.svg|thumb|340px|A phylogenetic tree based on rRNA genes.]]
{{seealso|Tree of life (science)}}
Another important piece of evidence is that it is possible to construct detailed [[phylogenetic trees]] (that is, "genealogic trees" of species) mapping out the proposed divisions and common ancestors of all living species. Traditionally, these trees have been built using morphological methods (based on appearance, embryology, etc). Recently, it has been possible to construct these trees using molecular data (based on similarities and differences between genetic and protein sequences). Crucially, all these methods produce essentially similar results, despite the fact that most genetic variation has no influence over external morphology. The fact that phylogenetic trees based on different types of information agree with each other is strong evidence of a real underlying phylogeny - that is, common descent.<ref>Theobald, Douglas, [http://www.talkorigins.org/faqs/comdesc/section1.html 29+ Evidences for Macroevolution], Section 1.3</ref>
== Illustrations of common descent ==
=== Artificial selection ===
[[Artificial selection]] offers remarkable examples of the amount of diversity that can exist between individuals sharing a late common ancestor. To perform artificial selection, one begins with a particular species (following examples include [[wolves]] and [[wild cabbage]]) and then, at every generation, only allow certain individuals to reproduce, based on the degree to which they exhibit certain desirable characteristics. In time, it is expected that these characteristics become increasingly well-developed in successive generations. Many examples of artificial selection, like the ones below, occurred without the guidance of modern scientific insight.
[[Image:IMG013biglittledogFX wb.jpg|thumb|This [[Chihuahua (dog)|Chihuahua]] [[mixed-breed dog|mix]] and [[Great Dane]] show the wide range of dog breed sizes]]
===== Dog breeding =====
{{main|dog breeding}}
An obvious example of the power of artificial selection is the diversity found in various breed in domesticated [[dogs]]. The various breeds of dogs all share common ancestry (being all ultimately descended from [[wolves]]) but were domesticated by humans and then selectively bred in order to enhance various features such as coat color and length or body size. To see the wide range of difference between the many breeds of dogs compare the [[Chihuahua (dog)|Chihuahua]], [[Great Dane]], [[Basset Hound]], [[Pug]], and [[Poodle]]. Also compare this enormous diversity with the relative uniformity of wild wolves.
===== Wild cabbage =====
[[Image:Brassica oleracea0.jpg|thumb|Wild Cabbage plant]]
{{main|Brassica oleracea}}
Early farmers cultivated many popular vegetables from the [[Brassica oleracea]] (common name ''wild cabbage'') by artificially selecting for certain attributes. Common vegetables such as [[cabbage]], [[kale]], [[broccoli]], [[cauliflower]], [[kohlrabi]] and [[Brussels sprouts]] are all descendants of the wild cabbage plant.<ref>Biology of Plants (7th edition) by Raven, Evert, and Eichhorn, 2005, states that these vegetables were "all produced from a single species of plant (Brassica oleraca), a member of the mustard family."</ref> Brussels sprouts were created by artificially selecting for large [[bud]] size. Broccoli was bred by selecting for large [[flower]] stalks. Cabbage was created by selecting for short [[Petiole (botany)|petioles]]. Kale was bred by selecting for large [[leaf|leaves]].
=== Natural selection ===
{{main|natural selection}}
[[Image:Darwin's finches.jpeg|thumb|Darwin's finches]]
Natural selection is the evolutionary process by which favorable traits that are heritable become more common in successive generations of a population of reproducing organisms, and unfavorable traits that are heritable become less common.
===== Darwin's finches =====
{{main|Darwin's finches}}
During Darwin's studies on the [[Galápagos Islands]], Darwin observed 13 species of [[Darwin's finches|finches]] that are closely related and differ most markedly in the shape of their beaks. The [[beak]] of each [[species]] is suited to its preferred food, suggesting that beak shapes evolved by [[natural selection]]. Large beaks were found on the islands where the primary source of food for the finches is nuts and therefore the large beaks allowed the birds to be better equipped for opening the nuts and staying well nourished. Slender beaks were found on the finches which found [[insects]] to be the best source of food on the island they inhabited; their slender beaks allowed the birds to be better equipped for pulling out the insects from their tiny hiding places. The finch is also found on the main land and it is thought that they [[Bird migration|migrated]] to the islands and began [[adaptation|adapting]] to their environment through [[natural selection]].
== See also ==
* [[The Beak of the Finch]]
== References ==
{{reflist}}
==External links==
* [http://www.talkorigins.org/faqs/comdesc/ 29+ Evidences for Macroevolution: the Scientific Case for Common Descent]
* [http://tolweb.org/tree/phylogeny.html The Tree of Life Web Project]
{{origin_of_life}}
[[Category:Evolutionary biology]]
[[Category:Evolution]]
[[bg:Общ произход]]
[[es:LUCA]]
[[fr:Last universal common ancestor]]
[[ja:共通祖先]]
[[pt:Origem comum]]
[[ta:பொது மரபுவழி]]