Selezione diversificante

Rappresentazione diagrammatica dei tre principali tipi di selezione naturale:
*selezione diversificante
*selezione stabilizzante
*selezione direzionale

Nell'ambito della selezione naturale, si parla di selezione diversificante o divergente (in inglese disruptive selection) quando vengono favoriti i fenotipi che esprimono alleli posti agli estremi della distribuzione dei caratteri, a scapito dei caratteri al centro della distribuzione. Questa condizione si viene a creare quando la popolazione vive in un ambiente non uniforme, nel quale un fenotipo può essere favorito in una determinata nicchia ecologica, mentre l'altro è più adatto in un'altra nicchia. In questo modo, entrambi i fenotipi aumenteranno in frequenza nel corso delle generazioni, la varianza dei caratteri aumenterà e la curva assumerà un andamento bimodale. Questo tipo di selezione ha una notevole importanza perché determina un aumento della diversità genica all'interno delle popolazioni e, di conseguenza, promuove la speciazione.[1][2]

In genere questo tipo di selezione favorisce fenotipi diversi in sottoambienti con caratteristiche ambientali diverse. Se la selezione è sufficientemente efficiente (se non c'è eccessivo flusso genico o deriva genetica, ad esempio) questo fenomeno porta alla speciazione simpatrica.

La selezione diversificante è più comune in quelle specie che hanno il loro areale in zone eterogenee e ricche di nicchie ecologiche vuote. Questa strategia, inoltre, consente la sopravvivenza della specie qualora alcune popolazioni si estinguano in seguito ad eventi che possono risparmiare altre popolazioni con adattamenti diversi.

Esempi

Un esempio di selezione diversificante è costituito dalla farfalla africana Papilio dardanus Brown, 1776 (Papilionidae), le cui femmine, in zone differenti, si sono evolute, per forma e schemi di colorazione, in popolazioni che risultano estremamente somiglianti ad altre specie di farfalle che i predatori evitano perché non appetibili (mimetismo batesiano).[3]
Un altro caso emblematico è rappresentato dalla pianta Agrostis capillaris L. (Poaceae), che ha sviluppato, in corrispondenza di vecchie miniere di rame abbandonate, dei genotipi in grado di resistere a questo elemento chimico, altrimenti molto nocivo.[4][5]

Note

  1. ^ (EN) Sinervo, Barry. 1997. Disruptive Selection [1] Archiviato il 24 giugno 2010 in Internet Archive. in Adaptation and Selection. 13 April 2010.
  2. ^ (EN) Lemmon, Alan R. 2000. EvoTutor. Natural Selection: Modes of Selection [2]. 13 April 2010.
  3. ^ Nijhout H.F, Polymorphic mimicry in Papilio dardanus: mosaic dominance, big effects, and origins (PDF), in Evolution and Development 2003; 5(6): 579–592.
  4. ^ McNeilly T., The evolution of copper tolerance in Agrostis tenuis Sibth. Ph.D. Thesis, University of Wales., 1966.
  5. ^ (EN) McNeilly T., Evolution in closely adjacent plant populations III. Agrostis tenuis on a small copper mine. (PDF), in Heredity, vol. 23, 1968, pp. 99-108, DOI:10.1038/hdy.1968.8. URL consultato il 23 ottobre 2011.

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