Estratégias de Reprodução
Autor: Reprodução Animal | Publicado: 2010-11-10T11:06:00-08:00


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 <![endif]-->  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Um dos problemas principais que os&nbsp;organismos&nbsp;vivos&nbsp;tiveram que resolver ao longo do processo&nbsp;evolutivo&nbsp;para tentarem "perpetuar" a espécie&nbsp;foi a da&nbsp;sobrevivência&nbsp;de um número suficiente de&nbsp;descendentes. Para além de eventuais situações de falta de&nbsp;alimentos&nbsp;e da predação, é necessário pensar que os&nbsp;recém-nascidos&nbsp;são geralmente muito mais sensíveis que os&nbsp;adultos&nbsp;às variações do meio&nbsp;ambiente, como a&nbsp;temperatura,&nbsp;ventos,&nbsp;correntes&nbsp;oceânicas, etc. As formas como os organismos resolveram esses problemas designam-se de estratégias reprodutivas. Em geral, os animais "concentraram" as suas atenções na proteção dos&nbsp;óvulos, dos&nbsp;embriões&nbsp;ou das crias. As plantas&nbsp;especializaram-se nas formas de disseminação dos produtos&nbsp;sexuais.

Sirfídeos&nbsp;em cópula durante o voo.
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Uma vez que são "descendentes" das&nbsp;bactérias&nbsp;e dos&nbsp;protistas, os animais começaram como ovíparos, ou seja, o&nbsp;zigoto, com maior ou menor proteção, é lançado no mundo, à sua sorte. Nos animais&nbsp;atuais, a maioria dos&nbsp;invertebrados&nbsp;e dos&nbsp;peixes&nbsp;são&nbsp;ovíparos.As estratégias para a sobrevivência desses zigotos - e dos&nbsp;embriões&nbsp;que deles resultam - incluem:§&nbsp; a produção de um grande número de zigotos;§&nbsp; o desenvolvimento de estados&nbsp;larvares&nbsp;bem adaptados ao&nbsp;meio ambiente; ou§&nbsp; os&nbsp;cuidados parentais&nbsp;- um ou ambos os progenitores cuidam dos ovos até estes&nbsp;eclodirem&nbsp;ou mesmo até as crias atingirem um tamanho que lhes permita sobreviver por si próprias - é o caso da maioria das&nbsp;aves&nbsp;e&nbsp;mamíferos&nbsp;e de alguns peixes.Uma outra forma de proteger os zigotos consiste em deixá-los desenvolverem-se dentro do corpo do&nbsp;mãe. Esta estratégia foi desenvolvida em duas fases:§&nbsp; numa primeira fase, o ovo de fato recebe apenas a proteção física da mãe em relação ao meio ambiente; o ovo tem as suas próprias reservas&nbsp;nutritivas&nbsp;e o embrião desenvolve-se independentemente do&nbsp;metabolismo&nbsp;materno&nbsp;-&nbsp;ovoviviparidade.§&nbsp; numa segunda fase, o corpo materno desenvolve um sistema, não só de proteção, mas também de&nbsp;alimentação&nbsp;do embrião (incluindo a passagem de&nbsp;anticorpos&nbsp;contra eventuais doenças), baseado no seu próprio metabolismo -&nbsp;viviparidade.Uma desvantagem destas últimas estratégias é que o número de zigotos não pode ser muito elevado e a mãe não pode repetir o processo com tanta frequência; por outro lado, os embriões têm maior&nbsp;probabilidade&nbsp;de sobreviverem, enquanto a mãe prossegue a sua vida.Chama-se a este tipo de desenvolvimento evolutivo&nbsp;seleção-K, enquanto que a estratégia de produzir um grande número de zigotos evoluiu segundo um processo de&nbsp;seleção-r.

Fonte: Wikipédia -&nbsp;http://pt.wikipedia.org/wiki/Reprodu%C3%A7%C3%A3o