Introduction to evolution: Difference between revisions

Revision as of 18:38, 16 June 2022

Revision as of 18:47, 27 July 2022

Line 53:

Evolution is not a random process. Although mutations in DNA are random, natural selection is not a process of chance: the environment determines the probability of reproductive success. Evolution is an inevitable result of imperfectly copying, self-replicating organisms reproducing over billions of years under the selective pressure of the environment. The outcome of evolution is not a perfectly designed organism. The end products of natural selection are organisms that are adapted to their present environments. Natural selection does not involve [[orthogenesis|progress towards an ultimate goal]]. Evolution does not strive for [[Evolution of biological complexity|more advanced]], more intelligent, or more sophisticated life forms.<ref>{{harvnb|Gould|1980|p=24}}</ref> For example, [[flea]]s (wingless parasites) are descended from a winged, ancestral [[Mecoptera|scorpionfly]], and [[snake]]s are [[lizard]]s that no longer require limbs—although [[Python (genus)|pythons]] still grow tiny structures that are the remains of their ancestor's hind legs.<ref>{{cite journal |last1=Bejder |first1=Lars |last2=Hall |first2=Brian K. |author-link2=Brian K. Hall |date=November 2002 |title=Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss |journal=[[Evolution & Development]] |volume=4 |issue=6 |pages=445–458 |doi=10.1046/j.1525-142X.2002.02033.x |pmid=12492145 |s2cid=8448387 }}</ref><ref>{{cite journal |last1=Boughner |first1=Julia C. |last2=Buchtová |first2=Marcela |last3=Fu |first3=Katherine |last4=Diewert |first4=Virginia |last5=Hallgrímsson |first5=Benedikt |last6=Richman |first6=Joy M. |date=June 25, 2007 |title=Embryonic development of ''Python sebae'' – I: Staging criteria and macroscopic skeletal morphogenesis of the head and limbs |journal=Zoology |volume=110 |issue=3 |pages=212–230 |doi=10.1016/j.zool.2007.01.005 |pmid=17499493 |display-authors=3 }}</ref> Organisms are merely the outcome of variations that succeed or fail, dependent upon the environmental conditions at the time.

Rapid environmental changes typically cause extinctions.<ref name="PBS_Evolution_library">{{cite web |url=https://www.pbs.org/wgbh/evolution/library/faq/cat03.html |title=Frequently Asked Questions About Evolution |year=2001 |website=Evolution Library |series=[[Evolution (TV series)|Evolution]] |publisher=[[WGBH Educational Foundation]]; Clear Blue Sky Productions, Inc. |___location=Boston, MA |type=Web resource |oclc=48165595 |access-date=2008-01-23}}</ref> Of all species that have existed on Earth, 99.9&nbsp;percent are now extinct.<ref name="PBS_Evolution_extintion">{{cite web |url=https://www.pbs.org/wgbh/evolution/extinction/massext/index.html |title=A Modern Mass Extinction? |year=2001 |website=Evolution Library |series=Evolution |publisher=WGBH Educational Foundation; Clear Blue Sky Productions, Inc. |___location=Boston, MA |type=Web resource |oclc=48165595 |access-date=2008-01-23}}</ref> Since life began on Earth, five major mass extinctions have led to large and sudden drops in the variety of species. The most recent, the [[Cretaceous–Paleogene extinction event]], occurred 66&nbsp;million years ago.<ref>{{cite journal |last1=Bambach |first1=Richard K. |last2=Knoll |first2=Andrew H. |author-link2=Andrew H. Knoll |last3=Wang |first3=Steve C. |date=December 2004 |title=Origination, extinction, and mass depletions of marine diversity |url=http://biology.kenyon.edu/courses/biol241/extinction%20and%20marine%20diversity%20banbach%202004.pdf |journal=[[Paleobiology (journal)|Paleobiology]] |volume=30 |issue=4 |pages=522–542 |doi=10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2|s2cid=17279135 }}</ref>

== Genetic drift ==