Lamarckism

Chevalier de Lamarck (1844–1829) was a French naturalist and invertebrate zoologist. He is best known for a theory of evolution developed in his book, Philosophie zoologique, published in 1809. This theory, known today as lamarckism, is based on the so-called “inheritance of acquired traits,” meaning that characteristics that an organism may develop during its lifetime are heritable, and can be passed on to its progeny.

The anatomical, biochemical, and behavioral characteristics that an individual organism displays as its develops through life is known as its phenotype. The phenotype that an individual actually develops is somewhat conditional, based on two key factors: (1) the fixed genetic potential of the organism, that is, its genotype, and (2) the environmental conditions which an organism experiences as it grows. For example, an individual plant (which has a particular genotype) that is well supplied with nutrients, moisture, and light throughout its life will grow larger and will produce more seeds than if it did not experience such beneficial conditions. Today, the effects of environment on genotypic expression are known as phenotypic plasticity.

However, at the time of Lamarck and other biologists of the late eighteenth and nineteenth centuries, the mechanisms of inheritance were not known. Nor were they known to Lamarck’s great successors, Charles Darwin and Alfred Russel Wallace, independent discoverers of the theory of evolution by natural selection, first published in 1859. Scientists thought that the developmental contingencies of individual organisms (which they called “acquired traits”) could somehow become incorporated into the genetic make-up of individuals and so be passed along to offspring. By this means, they thought, evolution could occur. For example, if the ancestors of giraffes had to stretch vigorously to reach tree foliage high in the canopy, this act might have caused the individual animals to develop somewhat longer necks, and this acquired trait somehow have been inherited by their descendants. Eventually, this mechanism could have resulted in the appearance of the modern, extremely long-necked giraffe.

Modern biologists, however, have a good understanding of the biochemical nature of inheritance. They know that phenotypic plasticity is limited by the fixed genetic potential that exists in all individuals. Therefore, the idea of the inheritance of acquired traits is no longer influential in evolutionary science. Instead, biologists believe that evolution largely proceeds through the differential survival and reproduction of individuals whose genetic complement favors these characters in particular environments, compared with other, “less-fit” individuals of their population. If the phenotypic advantages of the “more-fit” individuals are due to genetically fixed traits, they will be passed on to their offspring. This results in genetic change at the population level, which is the definition of evolution. This is, essentially, the theory of evolution by natural selection, first proposed by Darwin and Wallace in 1859. Darwin believed that Lamarckian inheritance of acquired characters was a secondary contributor to evolution, but was mistaken.