Introduction to evolution: Difference between revisions

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[[File:DNA Overview.png|thumb|upright=0.5|A section of [[DNA]]]]

Every living organism (with the possible exception of [[RNA]] [[virus]]es) contains molecules of DNA, which carries genetic information. Genes are the pieces of DNA that carry this information, and they influence the properties of an organism. Genes determine an individual's general appearance and to some extent their behaviour. If two organisms are closely related, their DNA will be very similar.<ref name="NAS">[[#NAS 1998|NAS 1998]], [http://www.nap.edu/openbook.php?record_id=5787&page=27 "Evolution and the Nature of Science"]</ref> On the other hand, the more distantly related two organisms are, the more differences they will have. For example, brothers are closely related and have very similar DNA, while cousins share a more distant relationship and have far more differences in their DNA. Similarities in DNA are used to determine the relationships between species in much the same manner as they are used to show relationships between individuals. For example, comparing chimpanzees with gorillas and humans shows that there is as much as a 96&nbsp;percent similarity between the DNA of humans and chimps. Comparisons of DNA indicate that humans and chimpanzees are more closely related to each other than either species is to gorillas.<ref>{{cite news |last=Lovgren |first=Stefan |date=August 31, 2005 |title=Chimps, Humans 96 Percent the Same, Gene Study Finds |url=http://news.nationalgeographic.com/news/2005/08/0831_050831_chimp_genes.html |archive-url=https://web.archive.org/web/20050905010617/http://news.nationalgeographic.com/news/2005/08/0831_050831_chimp_genes.html |url-status=dead |archive-date=September 5, 2005 |work=National Geographic News |___location=Washington, D.C. |publisher=National Geographic Society |access-date=2007-12-23}}</ref><ref>{{harvnb|Carroll|Grenier|Weatherbee|2005|p=}}</ref><ref>{{cite web |url=http://genome.ucsc.edu/cgi-bin/hgTrackUi?db=hg19&g=cons46way |title=Cons 46-Way Track Settings |website=[[UCSC Genome Browser|UCSC Genome Bioinformatics]] |___location=Santa Cruz, CA |publisher=[[University of California, Santa Cruz]] |access-date=2015-01-10}}</ref>

The field of [[molecular phylogenetics|molecular systematics]] focuses on measuring the similarities in these molecules and using this information to work out how different types of organisms are related through evolution. These comparisons have allowed biologists to build a ''relationship tree'' of the evolution of life on Earth.<ref>{{cite journal |last1=Ciccarelli |first1=Francesca D. |last2=Doerks |first2=Tobias |last3=von Mering |first3=Christian |last4=Creevey |first4=Christopher J. |last5=Snel |first5=Berend |last6=Bork |first6=Peer |author-link6=Peer Bork |date=March 3, 2006 |title=Toward Automatic Reconstruction of a Highly Resolved Tree of Life |journal=[[Science (journal)|Science]] |volume=311 |issue=5765 |pages=1283–1287 |bibcode=2006Sci...311.1283C |doi=10.1126/science.1123061 |pmid=16513982 |display-authors=3 |citeseerx=10.1.1.381.9514 |s2cid=1615592 }}</ref> They have even allowed scientists to unravel the relationships between organisms whose common ancestors lived such a long time ago that no real similarities remain in the appearance of the organisms.

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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}}</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>