Why is Lamarck's evolutionary theory considered incorrect?

Question

Many sources postulate without much argumentation that Lamarck's evolutionary theory is "obviously incorrect". A classic example provided in such sources is the one with giraffes, in which Lamarck argued that the necks of these animals became long as a result of continually stretching to reach high foliage. This mechanism for adaptive change in response to the environment is considered wrong.

Now the counterargument to Lamarck's ideas comprises modern findings in genetics. So it is stated that organisms pass down traits through predetermined genetic information, not based on environmental adaptations during their lifetime. Furthermore, genetic research shows that living organisms cannot alter their genetic material as needed.

• Doesn't the latter contradict the evolutionary theory itself, which states that nature (i.e. environment) shapes species through natural selection in the environmental milieu? If it is not the environmental/natural factor that is responsible for genetic evolution, then what is?
• If genetic material were to be unchangeable, then how would adaptation to the environment and consequent evolutionary change would ever be possible? The genotype (following this logic) would remain the same.
• And lastly, maybe giraffes do not develop long necks at such a rapid pace (as Lamarck probably thought) by simply stretching their necks in one generation; however, what, if not neck stretching, is responsible for their consistently longer necks throughout thousands or millions of years? Hadn't the environment gradually shaped their genotype? Similarly, if I run more often, maybe my children will not become more fit, but isn't it just a matter of time (thousands of years) that if all of us consistently run a lot, then our offspring will eventually become more fit? Wouldn't our voluntary efforts eventually enhance our genetics? I do not see any problem with Lamarck's logic at all, except probably the time frame.

Answer 1

In science theories are tested by experiment evidence and, as scientific knowledge progresses, how they can be accommodated in the framework of knowledge. In brief, there is no evidence to support Lamarck’s idea (Lysenko is notorious for his falsification of such evidence during the Soviet era to fit with Stalin’s philosophy), and the subsequent discovery of the molecular basis of heredity provided a requirement for the theory to show how changes in somatic cells could affect the DNA of the germ cells. No such evidence has emerged.

It is true that there are epigenetic effects, but none so far support Lamarckian postulates of the giraffe neck variety.

Finally, a word of advice. Don’t waste your breath on this list with arguments of the sort “Doesn't the latter contradict the evolutionary theory itself”. This is not a philosophical debating society, but community that is concerned with the factual basis of science. If you have never studied molecular sciences, stay away — or ask questions in the course of genuine attempts to acquire knowledge in this area.

Answer 2

I think you misunderstand both Lamarckism and other descriptions of evolution such as the modern synthesis (Darwinism with Mendelian genetics and a few bells and whistles). Both theories accept organisms have heredity with occasional changes. Neither Darwin nor Lamarck knew about genes, so I'll explain what their ideas boil down to in modern language.

In the modern synthesis, frequent changes called mutations may increase fitness (we call these beneficial), preserve fitness (we call these neutral) or decrease fitness (we call these deleterious). Mutations happen because DNA isn't perfectly copied or maintained; there's no bias in favour of beneficial mutations. However, as generations pass beneficial mutations proliferate, deleterious ones disappear (unless continually generated by mutation asymmetries, and even then they'll be rare in the population), and neutral ones' frequency in the population "drift" at random.

In Lamarckism, mutations are preserved based on beneficial acquired characteristics, which would require a way for bodies to identify which acquired traits are useful (injuries aren't!), reverse-engineer the genetic changes that would achieve them (which is basically impossible because DNA is more of a recipe than a blueprint), then update gametes with these changes. There is a lot of evidence that supports Darwinian evolution ("natural selection") but not Lamarckian evolution. For starters, Lamarckism couldn't explain novel biochemistry.

Biologists such as Richard Dawkins (see e.g. Darwin Triumphant, a chapter of A Devil's Chaplain) have pointed out that, even if there were life that could do what Lamarckism requires, it would not only still be susceptible to natural selection, but could only perfect a viable Lamarckism mechanism of discriminating good changes from bad by the action of natural selection. In other words, even if Lamarckism happened it would be an emergent property of Darwinism, not an alternative to it.