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Aunque autores han querido ver en el pensamiento de Lamarck una especie de vitalismo que sería el que propiciaría ese aumento de la complejidad: «Esa fuerza innata postulada por él no ha sido validada por la ciencia experimental y no constituye una contribución científica empírica».<ref>Antonio Fontdevila, Andrés Moya, 2003, p. 30.</ref>
=== La herencia de los caracteres adquiridos ===
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{{cita|Superficialmente, las generaciones sucesivas de los cuerpos de insectos-palo parecen constituir un linaje de réplicas. Pero si se modifica experimentalmente a un miembro de dicho linaje (por ejemplo, quitándole una pata), el cambio no pasa a la siguiente generación.|Dawkins, ''El gen egoísta''.}}
Estrictamente no puede hablarse de «la herencia de los caracteres adquiridos» ya que en aquel tiempo se desconocían los mecanismos de la herencia biológica, y porque el postulado de Lamarck, exactamente, fue que las transformaciones que operan las diferentes circunstancias a las que están sometidos los organismos, los medios de la Naturaleza se encarga de fijarlas en el grupo que se encuentra sometido a estas circunstancias, con tal de que se mantengan en el tiempo. En ningún caso puede considerarse lamarckismo que una característica contraída en vida por un organismo, pase directamente a su descendiente. Es obvio que si se circuncida a alguien, esto no significará que sus descendientes nazcan circuncidados. Lamarck habló de ciertos fluidos que se encargarían de fijar esas adaptaciones,<ref>{{cita| Solamente nos habla de «fluidos internos» que actúan sobre «los constituyentes plásticos del animal» para trazar canales, desplazar masas y construir incluso órganos. En pocas palabras, para modelar poco a poco la forma de los cuerpos.|Jacob, ''Lógica de lo viviente'', Cap. 3 - ''El tiempo''.}}</ref> lo que permite pensar que suponía en los organismos la capacidad de fijar evolutivamente las soluciones encontradas a los problemas planteados por las circunstancias. «Todo cambio de un ser que lo transforma en otro ser, implica […] satisfacer una necesidad concreta, una mayor capacidad de respuesta a las exigencias vitales».<ref>Jacob citando a Lamarck, ''Lógica de lo viviente'', Cap. 3 - ''El tiempo''.</ref> Quizá, más correcto sería decir que si durante generaciones todos o la mayoría de los individuos de un grupo se viesen sometidos a un traumatismo como la amputación de un miembro, con el transcurrir de su evolución, tal circunstancia sería menos traumática y las soluciones encontradas para sobrevivir en tales circunstancias, con el paso del tiempo, podrían encontrase presentes en el pool genético del grupo. O, también, si hubiesen logrado eludir la amputación, estandarizándose una estrategia, igualmente se habría fijado. Hoy en día esto no es aceptado; pero, para resolver casos parecidos a éste, en los que queda demostrado una respuesta de la especie al ambiente, se acepta como posible solución el “efecto Baldwin”, mediante el cual, la selección natural fijaría esas respuestas adaptativas.<ref>{{cita|He aquí lo que se preguntó a sí mismo [Balwin]: ¿cómo puede ser que los animales individuales, resolviendo problemas durante toda su vida, puedan cambiar las condiciones de competición para sus propios descendientes, haciendo que estos problemas sean más fáciles de resolver en el futuro?|Dennett, ''La peligrosa idea de Darwin'', p.1177.}}</ref>
«Tomemos dos ejemplos chocantes: el alcohol y la leche. La habilidad para digerir grandes cantidades de alcohol depende en cierta medida de la excesiva producción de unas enzimas llamadas alcohol deshidrogenasas llevada a cabo por un conjunto de genes del cromosoma 4».<ref>Matt Ridley, ''Genoma'', p. 220.</ref> Existe una relativa tolerancia al alcohol en los individuos de las sociedades occidentales que debe suponerse producto de su contacto con el alcohol y que no se da en los nativos de América del norte y de Australia. Desde el lamarckismo, la explicación sería que siendo el alcohol una circunstancia a la que han estado sometidos los organismos, con el tiempo, estos organismos habrían adquirido una mayor tolerancia. los organismos se habrían habituado al consumo de alcohol y esta característica, con el tiempo iría fijándose en el grupo de individuos que ha estado en contacto con el alcohol. La explicación darviniana sería que en el transcurso de los años que hemos estado separados genéticamente (entre 35.000 – 15.000 años),<ref>{{cita| La ocupación de las dos américas, cuya fecha todavía no está clara, pero puede situarse casi con seguridad hace entre 35.000 y 15.000 años.|Luca y Francesco Cavali-Storza, ''¿Quiénes somos? Historia de la diversidad humana'', p. 143.}}</ref> aproximadamente 800 generaciones, los individuos con mayor tolerancia al alcohol habrían adquirido la suficiente ventaja biológica como para imponer en el pool genético esa característica de su genoma. Cabe también suponer, dado que en occidente la tolerancia al alcohol puede representarse mediante una [[campana de Gauss]], que se han producido sucesivos errores genéticos en el sentido de aumentar la tolerancia al alcohol que restaurase la variabilidad inicial, desplazando la campana a posiciones de mayor tolerancia. Como desde el darwinismo no se puede suponer tal direccionalidad, el "efecto Baldwin" vendría a auxiliar a la selección natural para salvar este escollo, la selección natural habría favorecido los hábitos adquiridos.
{{cita| Algo similar ocurre con un gen del cromosoma 1, el gen de la lactasa. Esta enzima es necesaria para la digestión de la lactosa, un azúcar que abunda en la leche. Cuando nacemos todos tenemos este gen activado en nuestro sistema digestivo, pero en gran parte de los mamíferos —y por lo tanto en gran parte de las personas— se desactiva durante la infancia. […] Sin embargo, de vez en cuando el gen que controla la desactivación del gen de la lactasa sufre una mutación y la producción de lactasa no se detiene al final de la infancia. Esta mutación permite a su portador beber y digerir la leche a lo largo de toda su vida. Los hechos indican que, en primer lugar, tales personas emprendieron una vida de pastoreo y posteriormente desarrollaron una capacidad para digerir la leche en respuesta a ella, no que emprendieran una vida de pastoreo porque estuvieran genéticamente dotadas para ello.<ref>{{cita|De diez mil años acá el hombre empezó a producir su propio alimento, cultivando vegetales y criando animales.[…] durante las 400 o 500 generaciones que se han sucedido en este período, la población mundial ha aumentado mil veces o más.|Luca y Francesco Cavali-Storza, ¿Quiénes somos? Historia de la diversidad humana, p. 143.}}</ref> Este es un descubrimiento significativo, un ejemplo de cambio cultural que conduce a un cambio evolutivo y biológico. Se pueden inducir cambios en los genes voluntaria y conscientemente. Al emprender el estilo de vida razonable de los pastores de ganado lechero, los seres humanos crearon sus propias fuerzas evolutivas. Suena casi como la gran herejía lamarckiana que confundió el estudio de la evolución durante tanto tiempo: la idea de que un herrero que a lo largo de su vida ha adquirido unos brazos musculosos tenía hijos con brazos musculosos. No es eso, pero es un ejemplo de cómo la acción voluntaria y consciente puede modificar las fuerzas evolutivas sobre una especie, concretamente sobre nuestra especie. |Matt Ridley, Genoma, pp.220-221.}}
Lamarck formuló, como mecanismo por el que las especies se adaptaban a las circunstancias, la transferencia de las modificaciones experimentadas por los organismos en vida al conjunto de la especie mediante un proceso lento y sostenido. Sostuvo que si una «raza» (esto es, un grupo de organismos) estaba sometida a las mismas condiciones ambientales y estas condiciones se prolongaran durante mucho tiempo, se transformarían adaptándose a ese ambiente. Como mecanismo, propuso que las trasformaciones que los organismos experimentamos en vida sometidos a los diferentes ambientes, con el tiempo se fijarían en su descendencia, lo que hoy conocemos como transferencia horizontal. Advirtió que ese proceso es un proceso tan lento que desde nuestra capacidad de observación pasaría inadvertido. Según las dos leyes que formuló, los cambios se producen no en el individuo sino en la población (no en uno o varios individuos, sino en el conjunto de individuos del grupo) y no son cambios inmediatos sino que se fijarían a lo largo de un prolongado proceso.<ref>{{cita| Para que esta serie de transformaciones haya podido efectuarse, tiene que haberse desarrollado a lo largo de períodos muy largos. Todo lo que se encuentra sobre la superficie del globo cambia progresivamente de forma y estado. Todos los cuerpos de este planeta sufren «mutaciones» más o menos rápidas de acuerdo con su naturaleza y según las fuerzas ejercidas sobre ellos. La estabilidad que ve el hombre en la naturaleza es sólo aparente. La razón de esto es que el ser humano tiende a referir todos los eventos a la duración de su propia vida, de manera que unos cuantos miles de años le parecen un periodo inmenso, cuando en realidad no permiten contemplar más que estados estacionarios, intervalos entre los cambios que afectan al mundo vivo. Sin embargo, aunque las modificaciones que sufren los seres nos resulten imperceptibles, aunque las formas que encontrarnos en el Egipto de hoy no se diferencien en nada de las que vivían hace tres mil años, la lentitud del proceso de transformación es compensada por su enorme duración. Para que pueda nacer toda la diversidad del mundo vivo a partir de eventos tan lentos, es necesario y suficiente que se acumule el tiempo. «En comparación con las duraciones que se nos antojan grandes en nuestros cálculos ordinarios, sin duda fue necesario un tiempo enorme y una variación considerable de las circunstancias que se han sucedido, para que la naturaleza haya podido llevar la organización animal al grado de complicación y desarrollo que observamos.»|Jacob, Lógica de lo viviente, Cap. 3-El tiempo.}}</ref>
{{cita|Las variaciones en las circunstancias para los seres vivientes, y sobre todo para los animales, producen cambios en sus necesidades, en sus hábitos y en el modo de existir, y si estos cambios dan lugar a modificaciones o desarrollos en los órganos o en la forma de sus partes, se debe inducir que insensiblemente todo cuerpo viviente cualquiera debe variar en sus formas o sus caracteres exteriores, aunque semejantes variaciones no llegasen a ser sensibles más que después de un tiempo considerable. […] En cada lugar donde los seres pueden habitar, las circunstancias que establecen en él un orden de cosas permanecen largo tiempo siendo las mismas y no cambian en realidad más que con una lentitud tan grande, que el hombre no puede advertirlas directamente. Está obligado a consultar los monumentos para reconocer que en cada uno de aquellos lugares el orden de cosas establecido no ha sido siempre el mismo. Las razas de animales que viven en cada uno de ellos deben conservar en él largo tiempo sus hábitos. De aquí para nosotros la aparente constancia de las razas que llamamos especies, constancia que hizo nacer en nuestro pensamiento la idea de que las razas son tan antiguas como la Naturaleza.|Lamarck, ''Filosofía zoológica'', pp. 69, 174.}}
Lamarck, para explicar esa consolidación de las modificaciones que experimentaban los organismos, habló de una especie de “fluido” presente en ellos. No existiendo en aquellos momentos ningún tipo de conocimiento sobre los mecanismos de la herencia, sobre genética; esta parte de la teoría carece de interés; y podría decirse que quedó inexplicado como podría transferirse a la herencia tales caracteres adquiridos.
El problema para aceptar la herencia de los caracteres adquiridos sigue siendo el mismo, ofreciendo aún más dificultades desde la genética de [[Mendel]] y desde el actual paradigma genético, donde un carácter sería la expresión de un gen o un conjunto de genes. Esa transferencia difusa que operaría durante largo tiempo, no se contempla. La herencia de los caracteres de Lamarck supondría para las especies la respuesta que con el tiempo se daría en la [[Amazonía]] como consecuencia de la combustión de miles y miles de automóviles funcionando en [[Europa]], simultáneamente y de forma sostenida en el tiempo. Estos automóviles, por sí solos no modificarían sustancialmente las condiciones en la Amazonía, pero el efecto conjunto y sostenido sí se haría sentir en ella. Desde nuestro actual paradigma genético esto es imposible, no se contempla esta acción difusa. Habría que recurrir a explicaciones [[holismo|holísticas]] como la esbozada por [[Casilda Rodrigáñez]] basándose en la [[Simbiogénesis]] postulada por [[Lynn Margulis]] y que existiese la posibilidad de que en esas interacciones participaran los mecanismos de la herencia, posibilidad no aceptada en la actualidad desde el estamento académico:
{{cita|El proceso evolutivo que dio lugar a los organismos vivos complejos se ha llamado simbiogénesis.<ref>Margulis, L, Sagan, D. ''¿Qué es la vida?, Tusquets, 1996.</ref> Una simbiosis quiere decir que dos formas de vida autónomas, con su capacidad de autorregulación y su ecosistema propio, se unen promoviendo una autorregulación conjunta y un ecosistema común en el que ambas quedan englobadas, manteniendo cada cual su propio funcionamiento autorregulador. Esto explica que una célula no deja de ser una célula porque forme parte de un tejido. Es decir, la simbiogénesis explica que en un organismo hipercomplejo cada sistema que lo forma tiene una capacidad de autorregulación propia, y por eso se dice que es un ‘sistema cerrado’ en su organización, y al mismo tiempo “abierto” en su relación ‘informacional’ con los otros sistemas. Tan importante es que se mantenga el cierre organizativo de un sistema (su autorregulación), como su apertura informacional y relación con los demás (autorregulación común). Los sistemas orgánicos están continuamente enviándose señales entre sí y en todos los sentidos y direcciones, y cada vez se encuentran más enzimas, fijadores, moduladores, etc. que forman parte de los procesos, así como los ‘receptores’ de las señales…|[[Casilda Rodrigáñez]], ''[http://www.elciudadano.cl/2009/10/08/sobre-la-funcion-organica-y-social-de-la-sexualidad/ Sobre la función orgánica y social de la sexualidad.]''}}
No siendo aceptable la simplificación de la herencia de los caracteres en «el brazo del herrero», no se corresponde con el mecanismo formulado por Lamarck; tampoco este mecanismo puede considerarse viable desde el actual paradigma de la [[genética]].<ref name=mae>{{cita|Los efectos del uso y desuso significan simplemente que, si el organismo hace uso habitual de una parte de su cuerpo, esa parte se desarrollará y funcionará mejor; por el contrario, toda parte subutilizada se atrofiará o encogerá. Por ejemplo, las personas que se entrenan para correr maratones tendrán fuertes músculos en las piernas y no se fatigan fácilmente. De modo inverso, los astronautas no pueden tolerar por mucho tiempo bajas condiciones de gravedad sin que sus músculos comiencen a sufrir una degeneración debida al desuso. Los efectos del uso y desuso se encuentran bien documentados en la actualidad; pero no se sabe todavía si sus efectos se heredan.|Mae-Wan Ho, 1988, ''Genetic engineering''}}</ref>
=== Adaptación al medio ===
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