{"id":18717,"date":"2017-11-01T15:58:34","date_gmt":"2017-11-01T12:58:34","guid":{"rendered":"http:\/\/109.232.216.219\/~bilimvegelecek\/?p=18717"},"modified":"2018-02-08T16:19:38","modified_gmt":"2018-02-08T13:19:38","slug":"yasasin-fosil","status":"publish","type":"post","link":"https:\/\/bilimvegelecek.com.tr\/index.php\/2017\/11\/01\/yasasin-fosil","title":{"rendered":"Ya\u015fas\u0131n fosil!"},"content":{"rendered":"

Coelacanth\u2019\u0131n fosil formu ile ya\u015fayan formu aras\u0131ndaki y\u00fcksek benzerlik, evrimsel bak\u0131\u015f \u00f6l\u00e7e\u011fi olarak genom d\u00fczeyine inildi\u011finde, evrimle\u015fme h\u0131z\u0131 bak\u0131m\u0131ndan anlam kazan\u0131r ve Coelacanth genomu sudan karaya ge\u00e7i\u015fe dair \u00e7ok kritik bilgiler verir. Coelacanth genomu, evrimsel de\u011fi\u015fim ge\u00e7irmeyen canl\u0131 olmad\u0131\u011f\u0131n\u0131n a\u00e7\u0131k bir \u00f6rne\u011fini olu\u015fturur ve evrimin canl\u0131ya ve ya\u015fad\u0131\u011f\u0131 ortama g\u00f6re farkl\u0131 h\u0131zlarda ger\u00e7ekle\u015fmi\u015f ve ger\u00e7ekle\u015fen tarihsel bir s\u00fcre\u00e7 oldu\u011funu g\u00f6zler \u00f6n\u00fcne serer.<\/em><\/p>\n

Ex Africa semper aliquid novi<\/em>. Afrika\u2019dan her zaman yeni \u015fey \u00e7\u0131kar ya da Afrika\u2019dan \u00e7\u0131kan her \u015fey yenidir anlam\u0131ndaki bu yayg\u0131n ve k\u00f6kleri antik d\u00f6nemin b\u00fcy\u00fck ansiklopedisti Plinius1<\/sup> ve hatta Aristoteles\u2019e2<\/sup> dek uzanan deyi\u015f, 1938\u2019de G\u00fcney Afrika\u2019da East London a\u00e7\u0131klar\u0131nda yakalanan, \u201cya\u015fayan fosil\u201d tabir edilen Coelacanth t\u00fcr\u00fcn\u00fcn bilim d\u00fcnyas\u0131na heyecanla duyuruldu\u011fu makalenin ilk c\u00fcmlesini olu\u015fturur.3<\/sup> Yaln\u0131zca fosil \u00f6rneklerinden bilinen ve yakaland\u0131\u011f\u0131 g\u00fcne kadar yakla\u015f\u0131k 300 milyon y\u0131ld\u0131r ortal\u0131kta g\u00f6z\u00fckmemi\u015f bulunan bu acayip g\u00f6r\u00fcn\u00fc\u015fl\u00fc, etli loblu y\u00fczge\u00e7leri karasal canl\u0131lar\u0131n ayaklar\u0131n\u0131 and\u0131ran 1 metreyi a\u015fan uzunluktaki \u201ccanavar\u201d ilk g\u00fcnden itibaren hem bilim d\u00fcnyas\u0131n\u0131n hem de her fosilin alt\u0131nda kendi kaderinden izler bulmaya mahk\u00fbm evrim kar\u015f\u0131tl\u0131\u011f\u0131n\u0131n ilgi nesnesi olmay\u0131 s\u00fcrd\u00fcrm\u00fc\u015ft\u00fcr. Ka\u015fifinin nam\u0131na Latimeria chalumnae<\/em> ad\u0131 verilen bu \u201carkaik\u201d t\u00fcr\u00fcn yaln\u0131z olmad\u0131\u011f\u0131 ise, 1997\u2019de Endonezya\u2019da ke\u015ffedilen ikinci bir soyda\u015f\u0131n\u0131n, Latimeria menadoensis<\/em>\u2019in varl\u0131\u011f\u0131yla anla\u015f\u0131lm\u0131\u015ft\u0131r.4<\/sup><\/p>\n

Her iki t\u00fcr\u00fc de s\u0131n\u0131rl\u0131 alanlarda ya\u015fayan bu tuhaf g\u00f6r\u00fcn\u00fcml\u00fc bal\u0131klar, lob y\u00fczge\u00e7li bal\u0131klar s\u0131n\u0131f\u0131na d\u00e2hil edilirler ve hem biyolog hem de uzman olmayan g\u00f6zlemci a\u00e7\u0131s\u0131ndan en \u00e7arp\u0131c\u0131 \u00f6zellikleri, her ikisinin de 300 milyon y\u0131l kadar \u00f6nceki fosil formlar\u0131na olan yak\u0131n morfolojik benzerlikleridir. Fosil kay\u0131ttaki muhtemel atalar\u0131n\u0131n en son 300 milyon y\u0131l kadar \u00f6nce g\u00f6r\u00fclmesi, bir ba\u015fka deyi\u015fle, sonraki d\u00f6nemlerde ba\u015fka bir akraba \u00fcye i\u00e7ermeyi\u015fleri ve ya\u015fayan \u00fcyelerin de ge\u00e7mi\u015fin izlerini bu denli netlikle sergilemesi, bu iki Coelacanth t\u00fcr\u00fcn\u00fcn \u201cya\u015fayan fosiller\u201d olarak da pop\u00fclerize edilmelerinin nedenidir.<\/p>\n

\u2018Ya\u015fayan fosil\u2019 denmesinin nedeni, yava\u015f evrimle\u015fme<\/em><\/strong><\/p>\n

Bu yak\u0131n benzerlik, hemen evrim kar\u015f\u0131tl\u0131\u011f\u0131n\u0131n \u201cde\u011fi\u015fmezlik\u201d dogmas\u0131n\u0131n motorunun di\u015flilerinden biri olmu\u015f olmakla birlikte, normal bilim icraat\u0131 a\u00e7\u0131s\u0131ndan, ge\u00e7mi\u015fle g\u00fcn\u00fcm\u00fcz aras\u0131nda yer tuttu\u011fu evrimsel \u00f6nem bak\u0131m\u0131ndan yo\u011fun ilgi konusudur ve \u00f6zellikle son 4 y\u0131lda yay\u0131mlanan ve ayr\u0131nt\u0131l\u0131 genomik analizlerin sonu\u00e7lar\u0131n\u0131 i\u00e7eren yay\u0131nlarla, \u00f6zellikle sudan karaya \u00e7\u0131k\u0131\u015f ve karasal adaptasyonlara dair \u00e7ok \u00e7arp\u0131c\u0131 bilgiler bilim d\u00fcnyas\u0131na sunulmu\u015ftur. Bu t\u00fcr kapsaml\u0131 bir \u00e7al\u0131\u015fmada, \u00f6ncelikle, akci\u011ferli bal\u0131klar\u0131n m\u0131 yoksa coelacanth\u2019\u0131n m\u0131 tetrapodlar\u0131n (d\u00f6rt uzuv \u00fczerinde hareket eden, genellikle karasal canl\u0131lar) atas\u0131 oldu\u011funa dair me\u015fakkatli sorun, Latimeria chalumnae<\/em>\u2019nin genomik varyasyonunun akci\u011ferli bal\u0131kla kar\u015f\u0131la\u015ft\u0131r\u0131lmas\u0131yla \u00e7\u00f6z\u00fclm\u00fc\u015ft\u00fcr.5<\/sup> Bu \u00e7al\u0131\u015fman\u0131n da g\u00f6sterdi\u011fi gibi, Coelacanth, tarihsel olarak, akci\u011ferli bal\u0131klara yak\u0131n olmakla onlardan \u00f6ncedir ve akci\u011ferli bal\u0131klar karasal hayvan formlar\u0131n\u0131n hemen \u00f6ncesinde yer al\u0131rlar.<\/p>\n

\"\"
1938\u2019de ke\u015ffedilen ve Latimeria chalumnae ad\u0131 verilen ilk \u00f6rnek (3\u2019nolu kaynaktan)<\/figcaption><\/figure>\n

S\u00f6z konusu \u00e7al\u0131\u015fman\u0131n bir di\u011fer \u00e7arp\u0131c\u0131 sonucu, Coelacanth\u2019\u0131n hem akci\u011ferleri bal\u0131klara hem de tetrapodlara oranla protein evrimle\u015fme h\u0131zlar\u0131n\u0131n \u00f6nemli d\u00fczeyde d\u00fc\u015f\u00fck olmas\u0131d\u0131r: Coelacanth proteinlerinin akci\u011ferli bal\u0131k, tavukgiller ve memelilerle olan kar\u015f\u0131la\u015ft\u0131rmalar\u0131 proteinlerinin yava\u015f evrimle\u015fti\u011fini g\u00f6stermektedir. Ayr\u0131ca, Coelacanth genomunda b\u00fcy\u00fck \u00e7apl\u0131, kromozom seviyesinde meydana gelen de\u011fi\u015fimlerin (yap\u0131sal kromozom d\u00fczenlenmeleri) h\u0131z\u0131 da yava\u015ft\u0131r. Yine, Latimeria chalumnae<\/em>\u2019n\u0131n Endonezya\u2019daki kuzeni Latimeria menadoensis<\/em> ile genetik kar\u015f\u0131la\u015ft\u0131r\u0131lmas\u0131, her iki t\u00fcr aras\u0131ndaki genetik benzerlik d\u00fczeyinin, \u015fempanze ile insan aras\u0131ndaki gibi, % 98 civar\u0131nda oldu\u011funu da g\u00f6stermektedir. \u00d6yle g\u00f6r\u00fcl\u00fcyor ki, Coelacanth\u2019\u0131n \u201cya\u015fayan fosil\u201d olarak da adland\u0131r\u0131lmas\u0131n\u0131n \u00f6n\u00fcn\u00fc a\u00e7an ge\u00e7mi\u015f ile g\u00fcn\u00fcm\u00fcz formlar\u0131 aras\u0131ndaki keskin benzerli\u011fi, di\u011fer canl\u0131lara g\u00f6re hissedilir oranda yava\u015f evrimle\u015fiyor olmas\u0131d\u0131r.<\/p>\n

Sudan karaya ge\u00e7i\u015fe dair kritik bilgiler<\/em><\/strong><\/p>\n

\u00dczerinde durdu\u011fumuz modern (genomik) \u00e7al\u0131\u015fman\u0131n belki de en \u00f6nemli sonu\u00e7lar\u0131, sucul ya\u015famdan karasal ya\u015fama ge\u00e7i\u015f hakk\u0131nda s\u00f6yledikleridir. Karasal omurgal\u0131lar\u0131n evrimindeki \u00f6nemli noktalardan biri de gen kayb\u0131d\u0131r: karadaki ya\u015famla beraber (ve karasal ya\u015fama ge\u00e7i\u015f s\u00fcrecini bir par\u00e7as\u0131 olarak), bu ya\u015fam bi\u00e7imi i\u00e7in gereksiz konuma d\u00fc\u015fen pek \u00e7ok gen kaybedilmi\u015f durumdad\u0131r ve Coelacanth genomu, karasal omurgal\u0131lar\u0131n en yak\u0131n ortak sucul atas\u0131na (akci\u011ferli bal\u0131k) ili\u015fkin bu bak\u0131mdan da genetik bilgi sa\u011flamaktad\u0131r. Coelacanth genomuna bak\u0131ld\u0131\u011f\u0131nda, karasal ya\u015fam i\u00e7in gerekli olmad\u0131\u011f\u0131 a\u015fik\u00e2r pek \u00e7ok genin, akci\u011ferli bal\u0131klar \u00fczerinden, tetrapodlarda kaybedildi\u011fini g\u00f6stermektedir. Bu genler aras\u0131nda y\u00fczge\u00e7 geli\u015fimi, b\u00f6brek geli\u015fimi, i\u015fitme ta\u015f\u0131 ve kulak geli\u015fimi, g\u00f6vde-somit (ense halkas\u0131)-kuyruk geli\u015fimi, g\u00f6z ve beyin geli\u015fimi ile ilgili genler yer almaktad\u0131r. Tetrapodlarda yer almayan bu genlerin, karasal ya\u015fama ge\u00e7i\u015fte \u00f6nemli yenilikleri tetikledi\u011fi ku\u015fkusuzudur.<\/p>\n

Sudan karaya ge\u00e7i\u015fteki \u00f6nemli yenilikler aras\u0131nda, hi\u00e7 ku\u015fkusuz, \u00fcrenin v\u00fccuttan at\u0131lmas\u0131na ili\u015fkin, \u00fcre d\u00f6ng\u00fcs\u00fcn\u00fcn de\u011fi\u015fimleri yer almaktad\u0131r. Bilindi\u011fi gibi, sucul ortamlarda, ba\u015fl\u0131ca azotlu metabolik at\u0131k \u00fcr\u00fcn amonyakt\u0131r ve bu \u00fcr\u00fcn v\u00fccuttan at\u0131ld\u0131\u011f\u0131nda su ile seyreltilerek zarars\u0131z hale gelir. Ancak karasal ortamlarda, daha az zararl\u0131 \u00fcre ve \u00fcrik asit \u015feklinde bir \u00fcr\u00fcn meydana getirilir. Pek \u00e7ok amfibide ve kimi kaplumba\u011falarda ve memelilerde \u00fcrenin azotlu metabolik at\u0131k olmas\u0131, azotlu metabolik at\u0131k olarak \u00fcrenin kullan\u0131lmas\u0131n\u0131n sudan karaya ge\u00e7i\u015fteki \u00f6nemli bir yenilik oldu\u011funu d\u00fc\u015f\u00fcnd\u00fcrmektedir. Coelacanth genomu, bu a\u00e7\u0131dan da \u00f6nemli bilgiler vermektedir: yap\u0131lan kar\u015f\u0131la\u015ft\u0131rmalar sonucunda, karaci\u011fer men\u015feli \u00fcre d\u00f6ng\u00fcs\u00fcnde kritik \u00f6nemi olan h\u0131z-s\u0131n\u0131rlay\u0131c\u0131 nitelikteki \u00f6nemli bir enzimi (Karbomoil fosfat sentaz 1, CS1) kodlayan gendeki \u00f6nemli bir do\u011fal se\u00e7ilim h\u0131z\u0131n\u0131n varl\u0131\u011f\u0131 saptanm\u0131\u015ft\u0131r. Tetrapodlara ve amniyotlara uzanan evrimsel soy hatlar\u0131nda bu enzim h\u0131zl\u0131 bi\u00e7imde do\u011fal se\u00e7ilimle evrimle\u015fiyor g\u00f6z\u00fckmektedir. Coelacanth ile tetrapodlar aras\u0131nda, bu enzim a\u00e7\u0131s\u0131ndan 5 aminoasitlik temel bir fark bulunmaktad\u0131r ve bu farklardan \u00fc\u00e7\u00fc enzimin yap\u0131sal-konformasyonel a\u00e7\u0131dan kritik b\u00f6lgelerine (2 tanesi ATP ba\u011flanma b\u00f6lgesi, bir tanesi altbirim etkile\u015fim b\u00f6lgesi olmak \u00fczere) d\u00e2hildir.<\/p>\n

Anla\u015f\u0131laca\u011f\u0131 \u00fczere, Coelacanth\u2019\u0131n fosil formu ile ya\u015fayan formu aras\u0131ndaki y\u00fcksek benzerlik, evrimsel bak\u0131\u015f \u00f6l\u00e7e\u011fi olarak genom d\u00fczeyine inildi\u011finde, evrimle\u015fme h\u0131z\u0131 bak\u0131m\u0131ndan anlam kazanmakta ve Coelacanth genomu sudan karaya ge\u00e7i\u015fe dair \u00e7ok kritik bilgiler vermektedir. Coelacanth genomu, evrimsel de\u011fi\u015fim ge\u00e7irmeyen canl\u0131 olmad\u0131\u011f\u0131n\u0131n a\u00e7\u0131k bir \u00f6rne\u011fini olu\u015fturmakta ve evrimin canl\u0131ya ve ya\u015fad\u0131\u011f\u0131 ortama g\u00f6re farkl\u0131 h\u0131zlarda ger\u00e7ekle\u015fmi\u015f ve ger\u00e7ekle\u015fen tarihsel bir s\u00fcre\u00e7 oldu\u011funu bir kez daha g\u00f6zler \u00f6n\u00fcne sermektedir. \u00d6n\u00fcm\u00fczdeki ay bu \u201cya\u015fayan fosil\u201d konusunu Coelacanth ile birlikte di\u011fer \u00e7arp\u0131c\u0131 \u00f6rnek t\u00fcrler e\u015fli\u011finde irdelemeye devam edece\u011fiz.<\/p>\n

Kaynaklar<\/strong><\/p>\n

1) Pliny, Natural History<\/em>, Book VIII, 17.42., Loeb Classical Library, Harvard University Press, 2006 (son bas\u0131m).<\/p>\n

2) Aristotle, History of Animals<\/em>, 606b 20., Loeb Classical Library, Harvard University Press, 1991.<\/p>\n

3) J. L. B. Smith (1939), A living fish of Mesozoic type, Nature<\/em>, 143: 455-456.<\/p>\n

4) M. V. Erdmann, R. L. Caldwell ve M. Kasim Moosa (1998); Indonesian \u201cking of the sea\u201d discovered, Nature<\/em>, 395: 335.<\/p>\n

5) C. T. Amemiya ve ark. (2013), The African coelacanth genome provides<\/p>\n

insights into tetrapod evolution, Nature<\/em>, 496: 311-316.<\/p>\n","protected":false},"excerpt":{"rendered":"

Coelacanth\u2019\u0131n fosil formu ile ya\u015fayan formu aras\u0131ndaki y\u00fcksek benzerlik, evrimsel bak\u0131\u015f \u00f6l\u00e7e\u011fi olarak genom d\u00fczeyine inildi\u011finde, evrimle\u015fme h\u0131z\u0131 bak\u0131m\u0131ndan anlam kazan\u0131r ve Coelacanth genomu sudan karaya ge\u00e7i\u015fe dair \u00e7ok kritik bilgiler verir. Coelacanth genomu, evrimsel de\u011fi\u015fim ge\u00e7irmeyen canl\u0131 olmad\u0131\u011f\u0131n\u0131n a\u00e7\u0131k bir \u00f6rne\u011fini olu\u015fturur ve evrimin canl\u0131ya ve ya\u015fad\u0131\u011f\u0131 ortama g\u00f6re farkl\u0131 h\u0131zlarda ger\u00e7ekle\u015fmi\u015f ve ger\u00e7ekle\u015fen […]<\/p>\n","protected":false},"author":67,"featured_media":18718,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1131,1475,21,32],"tags":[200,816,2204],"class_list":["post-18717","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-165-sayi","category-bilim-ve-tarih","category-biyoloji","category-evrim","tag-evrim","tag-fosil","tag-yasayan-fosiller"],"acf":[],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t