Introduction to evolution: Difference between revisions

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[[File:Haeckel Orchidae.jpg|thumb|left|upright|Darwin noted that [[Orchidaceae|orchids]] have complex [[adaptation]]s to ensure pollination, all derived from basic floral parts.]]

Darwin gained extensive experience as he collected and studied the natural history of life forms from distant places. Through his studies, he formulated the idea that each species had developed from ancestors with similar features. In 1838, he described how a process he called natural selection would make this happen.<ref name="Confessions">{{cite journal |last=Eldredge |first=Niles |author-link=Niles Eldredge |date=Spring 2006 |title=Confessions of a Darwinist |url=http://www.vqronline.org/vqr-portfolio/confessions-darwinist |journal=[[Virginia Quarterly Review]] |___location=Charlottesville, VA |publisher=[[University of Virginia]] |volume=82 |issue=2 |pages=32–53 |access-date=2015-01-07 |archive-date=2015-09-06 |archive-url=https://web.archive.org/web/20150906090012/http://www.vqronline.org/vqr-portfolio/confessions-darwinist |url-status=dead }}</ref>

The size of a population depends on how much and how many resources are able to support it. For the population to remain the same size year after year, there must be an equilibrium or balance between the population size and available resources. Since organisms produce more offspring than their environment can support, not all individuals can survive out of each generation. There must be a competitive struggle for resources that aid in survival. As a result, Darwin realised that it was not chance alone that determined survival. Instead, survival of an organism depends on the differences of each individual organism, or "traits," that aid or hinder survival and reproduction. Well-adapted individuals are likely to leave more offspring than their less well-adapted competitors. Traits that hinder survival and reproduction would ''disappear'' over generations. Traits that help an organism survive and reproduce would ''accumulate'' over generations. Darwin realised that the unequal ability of individuals to survive and reproduce could cause gradual changes in the population and used the term ''natural selection'' to describe this process.<ref name="Geographic" /><ref name="wyhe" />

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Darwin was still researching and experimenting with his ideas on natural selection when he received a letter from [[Alfred Russel Wallace]] describing a theory very similar to his own. This led to an immediate joint publication of both theories. Both Wallace and Darwin saw the history of life like a [[family tree]], with each fork in the tree's limbs being a common ancestor. The tips of the limbs represented modern species and the branches represented the common ancestors that are shared amongst many different species. To explain these relationships, Darwin said that all living things were related, and this meant that all life must be descended from a few forms, or even from a single common ancestor. He called this process ''descent with modification''.<ref name="wyhe">{{cite web |url=http://darwin-online.org.uk/darwin.html |title=Charles Darwin: gentleman naturalist |last=van Wyhe |first=John |author-link=John van Wyhe |year=2002 |work=[[The Complete Works of Charles Darwin Online]] |oclc=74272908 |access-date=2008-01-16}}</ref>

Darwin published his theory of evolution by natural selection in ''On the Origin of Species'' in 1859.{{sfn|Darwin|1859}} His theory means that all life, including [[human]]ity, is a product of continuing natural processes. The implication that all life on Earth has a common ancestor has met with [[Objections to evolution|objections]] from some [[Creation–evolution controversy|religious groups]]. Their objections are in contrast to the level of support for the theory by [[level of support for evolution|more than 99 percent]] of those within the [[scientific community]] today.<ref name="delgado">{{cite journal |last=Delgado |first=Cynthia |date=July 28, 2006 |title=Finding the Evolution in Medicine |url=http://nihrecord.nih.gov/newsletters/2006/07_28_2006/story03.htm |journal=NIH Record |volume=58 |issue=15 |pages=1, 8–9 |access-date=January 9, 2015 |archive-date=September 24, 2015 |archive-url=https://web.archive.org/web/20150924003836/https://nihrecord.nih.gov/newsletters/2006/07_28_2006/story03.htm |url-status=dead }}</ref>

Natural selection is commonly equated with ''survival of the fittest'', but this expression originated in [[Herbert Spencer]]'s ''Principles of Biology'' in 1864, five years after Charles Darwin published his original works. ''Survival of the fittest'' describes the process of natural selection incorrectly, because natural selection is not only about survival and it is not always the fittest that survives.<ref>{{harvnb|Futuyma|2005b|pp=93–98}}</ref>{{Clear}}

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Given the right circumstances, and enough time, evolution leads to the emergence of new species. Scientists have struggled to find a precise and all-inclusive definition of ''species''. Ernst Mayr defined a species as a population or group of populations whose members have the potential to interbreed naturally with one another to produce viable, fertile offspring. (The members of a species cannot produce viable, fertile offspring with members of ''other'' species).<ref>{{harvnb|Mayr|2001|pp=165–169}}</ref> Mayr's definition has gained wide acceptance among biologists, but does not apply to organisms such as [[bacteria]], which reproduce [[Asexual reproduction|asexually]].

Speciation is the lineage-splitting event that results in two separate species forming from a single common ancestral population.<ref name="Geographic" /> A widely accepted method of speciation is called ''[[allopatric speciation]]''. Allopatric speciation begins when a population becomes geographically separated.<ref name="PBS_Evolution_glossary" /> Geological processes, such as the emergence of mountain ranges, the formation of canyons, or the flooding of land bridges by changes in sea level may result in separate populations. For speciation to occur, separation must be substantial, so that genetic exchange between the two populations is completely disrupted. In their separate environments, the genetically isolated groups follow their own unique evolutionary pathways. Each group will accumulate different mutations as well as be subjected to different selective pressures. The accumulated genetic changes may result in separated populations that can no longer interbreed if they are reunited.<ref name="Geographic">{{cite journal |last=Quammen |first=David |author-link=David Quammen |date=November 2004 |title=Was Darwin Wrong? |url=http://ngm.nationalgeographic.com/ngm/0411/feature1/fulltext.html |archive-url=https://web.archive.org/web/20071215203824/http://ngm.nationalgeographic.com/ngm/0411/feature1/fulltext.html |url-status=dead |archive-date=December 15, 2007 |journal=[[National Geographic (magazine)|National Geographic]] |type=Online extra |___location=Washington, D.C. |publisher=[[National Geographic Society]] |access-date=2007-12-23}}</ref> Barriers that prevent interbreeding are either ''prezygotic'' (prevent mating or fertilisation) or ''postzygotic'' (barriers that occur after fertilisation). If interbreeding is no longer possible, then they will be considered different species.<ref>{{cite journal |last=Sulloway |first=Frank J. |author-link=Frank Sulloway |title=The Evolution of Charles Darwin |url=http://www.smithsonianmag.com/science-nature/the-evolution-of-charles-darwin-110234034/ |date=December 2005 |journal=[[Smithsonian (magazine)|Smithsonian]] |___location=Washington, D.C. |publisher=Smithsonian Institution |access-date=2015-01-11}}</ref> The result of four billion years of evolution is the diversity of life around us, with an estimated 1.75&nbsp;million different species in existence today.<ref name="Cavalier-Smith">{{cite journal |last=Cavalier-Smith |first=Thomas |author-link=Thomas Cavalier-Smith |date=June 29, 2006 |title=Cell evolution and Earth history: stasis and revolution |journal=[[Philosophical Transactions of the Royal Society B|Philosophical Transactions of the Royal Society B: Biological Sciences]] |volume=361 |issue=1470 |pages=969–1006 |doi=10.1098/rstb.2006.1842 |pmc=1578732 |pmid=16754610}}</ref><ref>{{cite web |url=http://enviroliteracy.org/article.php/58.html |title=How many species are there? |date=June 17, 2008 |website=Enviroliteracy.org |publisher=Environmental Literacy Council |___location=Washington, D.C. |access-date=2015-01-11 |archive-date=2015-01-07 |archive-url=https://web.archive.org/web/20150107131752/http://enviroliteracy.org/article.php/58.html |url-status=dead }}</ref>

Usually the process of speciation is slow, occurring over very long time spans; thus direct observations within human life-spans are rare. However speciation has been observed in present-day organisms, and past speciation events are recorded in fossils.<ref>{{cite journal |last1=Jiggins |first1=Chris D. |last2=Bridle |first2=Jon R. |date=March 2004 |title=Speciation in the apple maggot fly: a blend of vintages? |journal=[[Trends (journals)|Trends in Ecology & Evolution]] |volume=19 |issue=3 |pages=111–114 |doi=10.1016/j.tree.2003.12.008 |pmid=16701238 }}</ref><ref>{{cite web |url=http://www.talkorigins.org/faqs/faq-speciation.html |title=Observed Instances of Speciation |last=Boxhorn |first=Joseph |date=September 1, 1995 |website=TalkOrigins Archive |publisher=The TalkOrigins Foundation, Inc. |___location=Houston, TX |access-date=2007-05-10}}</ref><ref>{{cite journal |last1=Weinberg |first1=James R. |last2=Starczak |first2=Victoria R. |last3=Jörg |first3=Daniele |date=August 1992 |title=Evidence for Rapid Speciation Following a Founder Event in the Laboratory |journal=[[Evolution (journal)|Evolution]] |volume=46 |issue=4 |pages=1214–1220 |doi=10.2307/2409766 |jstor=2409766 |pmid=28564398}}</ref> Scientists have documented the formation of five new species of cichlid fishes from a single common ancestor that was isolated fewer than 5,000&nbsp;years ago from the parent stock in Lake Nagubago.<ref>{{harvnb|Mayr|1970|p=348}}</ref> The evidence for speciation in this case was morphology (physical appearance) and lack of natural interbreeding. These fish have complex mating rituals and a variety of colorations; the slight modifications introduced in the new species have changed the mate selection process and the five forms that arose could not be convinced to interbreed.<ref>{{harvnb|Mayr|1970|p=}}</ref>