Dollo ”Qanunu” (Fərziyyəsi) Nədir?

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Əvvəlcə, başlıqdakı mürəkkəbliyi də aradan qaldırmaq üçün, dərhal bu şərhi edək: 1893-ci ildə Belçikalı alim Luiz Dollo (Louis Dollo) tərəfindən ortaya atılan bu fərziyyə heç bir zaman qanun və ya elmi adıyla qayda (nəzəriyyə) olmamışdır.

Bəli, ilk zamanlarda bir “qanun” olaraq düşünülüb, elə o cür də adlandırılmışdır. Lakin müasir elmdə “Dollo Fərziyyəsi” olaraq bilinir. Çünki indiki zamanda bu fərziyyəni çürüdən bir çox nümunələr vardır. Hətta bu fərziyyəni çürüdəcək o qədər çox misal var ki, bu fərziyyənin dəstəklənməyi bir yana, Təkamül Biologiyasında “geriyə təkamül”(reverse evolution) (konvergensiya (convergent evolution) ilə birgə düşünüldükdə homeoplasy (morfoloji bənzərlik) deyə bilərik) deyə bir termin vardır. Dollo “Qanunu” daha çox “təbii və statistik fikir” olaraq qəbul edilir. İzah edək:

Dollo fərziyyəsinə görə, təkamülə uğramış bir canlı daha sonra əcdadlarının vəziyyətinə geri dönə bilməz. Məsələn, təkamüldə ortadan yox olan bir orqan yenidən ortaya çıxa bilməz.

Riçard Daukinz (Richard Dawkins) Dollo Fərziyyəsini belə izah edir ki, Dollo Fərziyyəsinə görə təkamülə təsir edən parametrlər o qədər çoxdur ki, baş verən dəyişikliyin təkrarlanması və ya bütünlüklə əvvəlki halına dönməsi statistik olaraq mümkün deyil. Stiven  (Stephen Jay Gould) isə Daukinzə nisbətən məsələyə daha yumşaq yanaşır və Dollo Fərziyyəsini “dönməyən proses” (irreversibility) anlamında ehtimallar kainatından seçilən bir ehtimalın digər ehtimalları məhv etməsi ilə başa düşülür. Yəni qarşınızda A, B, C, D ehtimalları mövcuddursa və siz A’nı seçsəniz digər 3 ehtimal normal olaraq ortadan qalxır. Gould, Dollo’nun fərziyyədə “dönməyən proses”i anlatdığını deyir.
Amma biz Rezonans olaraq Dollo Fərziyyəsini modası keçmiş sayırıq. Və Dawkins və Gould düşüncəsinin ortaq məxrəcini götürməyi düşünürük. Genetika qanunlarını bilməyən Dollonun həyatı boyunca genləri anlamadığını düşünürük. Çünki bəzən genlərimizdə yaranan mutasiya, bir xüsusiyyətin itməsinə və ya yenidən yaranmasına səbəb ola bilər. Və eyni gen illər sonra yenə mutasiyaya uğrayarsa, ata genlərinə geri dönə bilər.

Şimali Amerika’dakı qurbağaların ( Gastrotheca guentheri) alt çənələrindəki dişlərin 200 milyon il sonra yenidən atalarındakı vəziyyətə təkamülləşməsi “geriyə təkamül” ilə izah olunur. Sudan quruya çıxan heyvanların bəzilərinin suya geri dönməsini digər bir misal olaraq göstərə bilərik. Son dediyimiz misalda genetik faktorlardan çox mühit faktoru rol oynayır. Buna görə də geriyə təkamül zamanı sadəcə genlər yox digər faktorlar da nəzərə alınmalıdır.

Dollo Fərziyyəsinə qarşı olan misallardan digəri də tikanbalıqlarıdır (Seattle). Tikanbalıqlarının ataları güclü pulcuqlara, zirehə və üç xətli tikanlara sahib heyvanlar idi. Washington Gölündə yaşayan növlər yüz illər boyu gölün dibindəki lildə gizlənməyə məcbur qaldıqları üçün pulcuqları yumşaqlaşmış və balacalaşmışdır. Bu, 1960-cı ilə qədər belə idi. Göldə görülən işlərdən sonra gölün dibindəki lil təmizləndi və növün populyasiyaları balıqçılarla üzləşməli oldu. Sadəcə 49 il ərzində (1957-2006) növün zəif, zirehsiz, arıq nəsilləri atalarına xas xüsusiyyətlərinə, geriyə təkamülləşdi və yenidən zirehli, güclü pulcuqlara malik oldular. Beləliklə, bu misal ilə gözümüzün önündə meydana gələn tək mikrotəkamül yox, həmçinin geriyə təkamül də elm dünyasına daxil oldu.

Nəticə olaraq, Modern Evolyasiya Biologiyasında Dollo Fərziyyəsinin üzərində çox düşünmək vaxt itkisidir. Əlbəttə, statistik olaraq hansıda xüsusiyyətin tamamilə itirilməsi və ya tamamilə eynisinin qazanılması mümkünsüz görülür. Lakin mühitə və təbii seçməyə görə növlərin milyon illər sonra hansısa xüsusiyyəti itirməsi və ya qazanması yuxarıda çəkdiyimiz bir neçə misalda da sübut edilib. Və bu da bu fərziyyəni çürütməyə yetərlidir. Mühit, sürətlə və təsadüfi dəyişdiyi üçün xüsusiyyətlər də bu istiqamətdə dəyişir. Və bu təsadüfi dəyişikliklər bəzən üst-üstə düşür. Az görülsə də təbiətdə buna rastlamaq mümkündür.

Müəllif: ÇMB, Evrim Ağacı

Tərcümə etdi:Aysel Hüseynli

Mənbə:

  1. Eco-Evo-Devo
  2. NatGeo
  3. Wikipedia

 

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