Editing Speciation (section)

== Rates ==
[[File:Punctuated-equilibrium.svg|thumb|upright=1.15|[[Phyletic gradualism]], above, consists of relatively slow change over geological time. [[Punctuated equilibrium]], bottom, consists of [[Morphology (biology)|morphological]] stability and rare, relatively rapid bursts of [[evolution]]ary change.]]

There is debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant and gradual over time (known as "Phyletic gradualism" – see diagram), some [[Paleontology|palaeontologist]]s such as [[Niles Eldredge]] and [[Stephen Jay Gould]]<ref name="Gould&Eldredge1977">{{cite journal |last1=Gould |first1=Stephen Jay |last2=Eldredge |first2=Niles |author-link2=Niles Eldredge |date=Spring 1977 |title=Punctuated equilibria: the tempo and mode of evolution reconsidered |url=http://www.nileseldredge.com/pdf_files/Punctuated_Equilibria_Gould_Eldredge_1977.pdf |journal=[[Paleobiology (journal)|Paleobiology]] |volume=3 |issue=2 |pages=115–151 |jstor=2400177 |archive-url=https://web.archive.org/web/20140624060126/http://www.nileseldredge.com/pdf_files/Punctuated_Equilibria_Gould_Eldredge_1977.pdf |archive-date=2014-06-24 |access-date=2015-09-15 |doi=10.1017/s0094837300005224|bibcode=1977Pbio....3..115G |s2cid=83492071 }}</ref> have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as ''[[punctuated equilibrium]]''. (See diagram, and [[Speciation#Darwin's_dilemma:_why_species_exist|Darwin's dilemma]].) {{Citation needed|date=November 2023}}

=== Punctuated evolution ===
{{Main|Punctuated equilibrium|Rate of evolution}}

Evolution can be extremely rapid, as shown in the creation of domesticated animals and plants in a very short geological space of time, spanning only a few tens of thousands of years. [[Maize#Origin|Maize]] (''Zea mays''), for instance, was [[Zea (plant)#Origin of maize and interaction with teosintes|created]] in [[Mexico]] in only a few thousand years, starting about 7,000 to 12,000 years ago.<ref>{{harvnb|Laws|2010|pp=210–215}}</ref> This raises the question of why the long term rate of evolution is far slower than is theoretically possible.<ref name="williams 1992b">{{harvnb|Williams|1992|loc=chpt. 9}}</ref><ref name="EldredgeN">{{harvnb|Eldredge|Gould|1972|loc=[http://www.blackwellpublishing.com/ridley/classictexts/eldredge.asp chpt. 5]}}</ref><ref name="mayr 1954">{{harvnb|Mayr|1954|pp=[https://www.blackwellpublishing.com/ridley/classictexts/mayr.asp 157–180]}}</ref><ref>{{harvnb|Maynard Smith|1989|p=281}}</ref>

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| header = [[Domestic plants|Plants]] and [[domestic animal]]s  can differ markedly from their wild ancestors
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| image1 = Maize-teosinte.jpg
| caption1 = Top: wild [[teosinte]]; middle: maize-teosinte hybrid; bottom: [[maize]]}}
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| image1 = Brassica_oleracea0.jpg
| caption1 = [[Brassica oleracea|Ancestral wild cabbage]] 
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| caption2 = [[Cauliflower|Domesticated cauliflower]]}}
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| image1 = Carassius_gibelio_2008_G2.jpg
| caption1 = [[Prussian carp|Ancestral Prussian carp]]
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|caption2 = [[Goldfish|Domestic goldfish]]
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| image1 = Ovis_orientalis_LC0267.jpg
| caption1 = [[Mouflon|Ancestral mouflon]]
| image2 = Pair_of_Icelandic_Sheep.jpg
|caption2 = [[Sheep|Domestic sheep]]
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Evolution is imposed on species or groups. It is not planned or striven for in some [[Lamarckism|Lamarckist]] way.<ref name="GouldHopefulMonster">{{harvnb|Gould|1980|loc=[https://web.archive.org/web/20190509061859/http://www.stephenjaygould.org/library/gould_hopeful-monsters.html pt. 4, chpt. 18]}}</ref> The mutations on which the process depends are random events, and, except for the "[[silent mutation]]s" which do not affect the functionality or appearance of the carrier, are thus usually disadvantageous, and their chance of proving to be useful in the future is vanishingly small. Therefore, while a species or group might benefit from being able to adapt to a new environment by accumulating a wide range of genetic variation, this is to the detriment of the ''individuals'' who have to carry these mutations until a small, unpredictable minority of them ultimately contributes to such an adaptation. Thus, the ''capability'' to evolve would require [[group selection]], a concept discredited by (for example) [[George C. Williams (biologist)|George C. Williams]],<ref>{{harvnb|Williams|1974}}</ref> [[John Maynard Smith]]<ref>{{cite journal |last=Maynard Smith |first=John |author-link=John Maynard Smith |date=March 14, 1964 |title=Group Selection and Kin Selection |journal=Nature |volume=201 |issue=4924 |pages=1145–1147 |doi=10.1038/2011145a0 |bibcode=1964Natur.201.1145S|s2cid=4177102 }}</ref> and [[Richard Dawkins]]<ref name="River_Out_of_Eden">{{harvnb|Dawkins|1995|loc=[[God's utility function|chpt. 4]]}}</ref><ref name=dawkins>{{cite journal |last1=Dawkins |first1=Richard |author-link=Richard Dawkins |date=December 1994 |title=Burying the Vehicle |url=http://www.simonyi.ox.ac.uk/dawkins/WorldOfDawkins-archive/Dawkins/Work/Articles/1994burying_the_vehicle.shtml |journal=[[Behavioral and Brain Sciences]] |volume=17 |issue=4 |pages=616–617 |doi=10.1017/S0140525X00036207 |s2cid=143378724 |issn=0140-525X |archive-url=https://web.archive.org/web/20060915085010/http://www.simonyi.ox.ac.uk/dawkins/WorldOfDawkins-archive/Dawkins/Work/Articles/1994burying_the_vehicle.shtml |archive-date=2006-09-15 |access-date=2015-09-15}} "Remarks on an earlier article by [[Elliott Sober|&#91;Elliot&#93; Sober]] [sic] and [[David Sloan Wilson]], who made a more extended argument in their recent book ''Unto Others : The Evolution and Psychology of Unselfish Behavior''"</ref><ref name="dennet">{{cite journal |last=Dennett |first=Daniel C. |author-link=Daniel Dennett |date=December 1994 |title=E Pluribus Unum? |journal=Behavioral and Brain Sciences |volume=17 |issue=4 |pages=617–618 |doi=10.1017/S0140525X00036219 |s2cid=146359497 |url=http://cogprints.org/281/0/wilsonso.htm |archive-url=https://web.archive.org/web/20071227065311/http://cogprints.org/281/0/wilsonso.htm |archive-date=2007-12-27}} "Commentary on Wilson & Sober: Group Selection."</ref><ref name="pinker">{{cite web |last=Pinker |first=Steven |date=June 18, 2012 |url=http://edge.org/conversation/the-false-allure-of-group-selection |title=The False Allure of Group Selection |author-link=Steven Pinker |website=edge.org |publisher=[[Edge.org|Edge Foundation, Inc.]] |access-date=2015-09-15}}</ref> as selectively disadvantageous to the individual.

The resolution to Darwin's second dilemma might thus come about as follows:

If sexual individuals are disadvantaged by passing mutations on to their offspring, they will avoid mutant mates with strange or unusual characteristics.<ref name="Langlois & Roggman, 1990"/><ref name="Koeslag, 1990"/><ref name="Koeslag, 1995"/><ref name="miller"/> Mutations that affect the external appearance of their carriers will then rarely be passed on to the next and subsequent generations. They would therefore seldom be tested by natural selection. Evolution is, therefore, effectively halted or slowed down considerably. The only mutations that can accumulate in a population, on this punctuated equilibrium view, are ones that have no noticeable effect on the outward appearance and functionality of their bearers (i.e., they are "silent" or "[[neutral theory of molecular evolution|neutral mutation]]s", which can be, and are, used to trace the [[history of molecular evolution|relatedness and age of populations and species]].<ref name="Koeslag, 1990"/><ref name="Campbell">{{harvnb|Campbell|1990|pp=450–451, 487–490, 499–501}}</ref>)

This argument implies that evolution can only occur if mutant mates cannot be avoided, as a result of a severe scarcity of potential mates. This is most likely to occur in [[small population size|small, isolated communities]]. These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves,<ref name="ayala">{{harvnb|Ayala|1982|pp=73–83, 182–190, 198–215}}</ref> or during the aftermath of a [[Extinction event|mass extinction]].<ref name="Campbell"/> Under these circumstances, not only is the choice of mates severely restricted but population bottlenecks, founder effects, genetic drift and inbreeding cause rapid, random changes in the isolated population's genetic composition.<ref name="ayala"/> Furthermore, hybridization with a related species trapped in the same isolate might introduce additional genetic changes. If an isolated population such as this [[Toba catastrophe theory#Genetic bottleneck hypothesis|survives its genetic upheavals]], and subsequently expands into an unoccupied niche, or into a niche in which it has an advantage over its competitors, a new species, or subspecies, will have come into being. In geological terms, this will be an abrupt event. A resumption of avoiding mutant mates will thereafter result, once again, in evolutionary stagnation.<ref name="Gould&Eldredge1977"/><ref name="EldredgeN"/>

In apparent confirmation of this punctuated equilibrium view of evolution, the [[Fossil#Dating|fossil record]] of an evolutionary progression typically consists of species that suddenly appear, and ultimately disappear, hundreds of thousands or millions of years later, without any change in external appearance.<ref name="Gould&Eldredge1977"/><ref name="Campbell"/><ref name="McCarthy">{{harvnb|McCarthy|Rubidge|2005}}</ref> Graphically, these fossil species are represented by lines parallel with the time axis, whose lengths depict how long each of them existed. The fact that the lines remain parallel with the time axis illustrates the unchanging appearance of each of the fossil species depicted on the graph. During each species' existence new species appear at random intervals, each also lasting many hundreds of thousands of years before disappearing without a change in appearance. The exact relatedness of these concurrent species is generally impossible to determine. This is illustrated in the diagram depicting the [[Punctuated equilibrium#Evidence from the fossil record|distribution of hominin species through time]] since the [[Hominini|hominins]] separated from the line that led to the evolution of their closest living primate relatives, the chimpanzees.<ref name="McCarthy"/>

For similar evolutionary time lines see, for instance, the paleontological list of [[List of African dinosaurs#Timeline|African dinosaurs]], [[List of Asian dinosaurs#Timeline|Asian dinosaurs]], the [[Lampriformes#Timeline of genera|Lampriformes]] and [[Amiiformes#Timeline of genera|Amiiformes]]. {{Citation needed|date=November 2023}}