Saç rənglərinin təkamülü

79

Yalnız tünd saç rəngi xüsusiyyəti olan insanların olduğu bir dünyanı xəyal edin. Qabaqcıl insanlardan, müasir insanların ortaya çıxdığı dövrlərə qədər dünya buna bənzəyirdi. İlk hominidlərin Afrikada yaşadıqları bilinir. Afrikanın ekvator ətrafındakı bölgələri il ərzində birbaşa günəş işığı qəbul edir. Bu da insan təkamülünə təsir edər və insan piqmentlerinin ola bildiyi qədər tünd olmasını təmin edən təbii seçim işləməkdə idi. Melanin kimi tünd piqmentlər, zərərli ultrabənövşəyi işığının saçlar və dəridən keçərək bədənə təsir etməsinə mane olar. Daha tünd dəri və ya saç, fərdi günəş işığından daha yaxşı qoruyacaq.

 

İnsanlar, dünyanın digər bölgələrinə köç etdikcə, tünd rəngin seçim istiqamətindəki təzyiq azaldıqca daha açıq dəri və saç rəngləri artmışdır. Əslində, insanların Qərbi Avropa və Skandinaviya bölgələrinə köç etməsiylə, insanlar kafi səviyyədə D vitamini ehtiyacını qarşılaya bilmək üçün daha çox açıq rəngli hala gəldilər. Dəri və saçlardakı tünd rəngli piqmentlər, istənməyən zərərli ultrabənövşəyi şüalarına mane olarkən, eyni zamanda günəş işığının həyatta qalmaq üçün əhəmiyyətli olan digər komponentlərinə də maneə olurlar. Ekvator yaxınlığında olan və günəş işığının birbaşa qəbul edilən bölgələrində, D vitamini əldə etmək bir problem deyil. Ancaq, insanlar ekvatorun şimalına (və ya cənubuna) köç etdikcə, günəş işığı miqdarı il ərzində dəyişiklik göstərir. Qışda, insanların çölə çıxıb lazımlı quruluş maddələrini təmin edəcəyi günəş işığının saatları çox azdır.

 

Bu cəmiyyətlərin, köç edən ataları daha soyuq iqlimlərin olduğu bölgələrə yerləşmişlər, dəridəki və saçlardakı piqmentlər solğunlaşmağa və yeni rəng birləşmələri yaratmağa imkan verir. Saçların rəngi poligenik bir xüsusiyyət olduğu üçün insanlardakı saç rənginin fenotipinə bir çox gen nəzarət edər. Bu da dünyadakı fərqli cəmiyyətlərdə görülən çox fərqli rəng tonlarının meydana gəlməsinin səbəbidir. Dəri rəngi və və saç rəngi bir şəkildə əlaqəli olsa belə, müxtəlif birləşmələrin meydana gəlməsinə mane olacaq qədər əlaqəli deyil. Dünyada bu fərqli tonlar və rənglər görülməyə başlayınca, bu xüsusiyyətlər üzərindəki təbii seçimin təsiri azaldı və cinsi seçimin təsiri artdı.

 

Bu mövzuda edilən araşdırmalar, bir populyasiyanın gen hovuzunda daha az olan saç rəngi xüsusiyyəti sahiblərini daha cəlb edici hala gətirmək olduğunu göstərir. Bu da Şimal bölgələrdəki daha çox D vitamini əldə etmək üçün fayda təmin edən sarı saç xüsusiyyətinin çox sürətli bir şəkildə çoxalmasını təmin etmişdir. Fərdlərdə sarı saç rəngi müşahidə edilməyə başladığı zaman, ətrafdakı tünd saç rənginə sahib fərdlərə görə cinsi olaraq daha cazibədar hala gəldilər. Nəsillər sonra, sarı saç daha intensivləşmişdir və tez çoxalmış oldu. Daha sonra şimaldakı insanlar köç etməyə başladı və digər bölgələrdən insanlarla evlənərək fərqli saç rəng tonlarının meydana gəlməsini təmin etdilər.

 

Qırmızı saç isə, bu müddətdə formalaşan bir DNT mutasiyası nəticəsində ortaya çıxmışdır. Neandertallar, Homo sapiens əmisi uşaqlarına görə daha açıq saç rənglərinə sahib olduqları bilinir. Avropada bu iki fərqli növ arasında gen axışı və uyğunlaşma olduğu düşünülür. Bu da daha fərqli tonlarda saç rənglərinin meydana gəlməsinə qatqı təmin etmiş ola bilər.

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