why is Lamarck's theory disproved?

Answer 1

Lamarck thought that characteristics gained by an organism during its life could be passed on to its offspring. For example, he thought if you started out scrawny but lifted weights diligently, you could pass on your newly acquired muscularity to your kids.

It would be nice, wouldn't it?

Unfortunately, we now know that this is impossible - Lamarck's theory is disproven by the mechanism of heredity. We now know that DNA is responsible for heredity and we know an immense amount about how this occurs. Lamarck's theory would require that characteristics acquired during life directly alter DNA - and that is impossible! When you work out and subsequently build muscle, it does not alter the DNA of your gametes at all. The only way DNA could be changed (historically) was by chance mutations.

Answer 2

Lamarck's theory was disproved by a guy named Weismann (if I remember correctly - not sure about the spelling). He proceeded to cut the tails of a number of generations of mice only to find that their offspring were always born with a tail intact. This ties into the 'use/disuse' theory proposed by Lamarck. Had Lamarck been correct, changes that occurred in the parent should have been passed onto the offspring. I.E. - Giraffes necks (according to Lamarck) became longer as each parent stretched to reach leaves further up on the trees. The stretching produced slightly longer necks, that were then passed on to the next generation. Over many generations, giraffes developed the long neck we see today. (Don't discount Lamarck completely - he did propose one of the first theories of evolution).

Answer 3

It is disproven by contrary evidence:
For example, characteristics acquired during the lifetime of a parent are not passed to the children. An athlete who develops a large muscle mass through weight training does not have kids who already have this large muscle mass.

Lamarck's theory was based upon the idea that if an organism developed a characteristic feature through adapting to a new way of life during its lifetime, it would pass this on to its offspring. Although it is not technically "adaptive" (like a giraffe stretching its neck for leaves high in the trees), chracteristics that are acquired during a lifetime of a parent CAN, in fact be passed to offspring. This is the whole idea of mutation of genes. If those mutations increase the survival of the offspring by making then more competitive and ultilmately more reproductively successful, then they tend to be perpetuated. This is the essense of Darawinian microevolution.