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Chevalier de Lamarck was a French naturalist and invertebrate zoologist who lived from 1844-1829. 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 socalled "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. However, the phenotype that an individual actually develops is somewhat conditional, and is based on two key factors: (1) the fixed genetic potential of the org anism (or its genotype; this refers to the specific qualities of its genetic material, or DNA [deoxyribonucleic acid]); and (2) the environmental conditions which an organism experiences as it grows. For example, an individual plant (with a particular, fixed genotype) that is well supplied with nutrients, moisture, and light throughout its life will grow larger and will produce more seeds than if that same plant did not experience such beneficial conditions. Conditional developmental possibilities as these are now known to be due to differing expressions of the genetic potenti al of the individual (biologists refer to the variable expression of the genome of an organism, as influenced by environmental conditions encountered during its development, 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 (this includes Charles Darwin and Alfred Russel Wallace, the co-discoverers of the theory of evolution by natural selection, first published in 1859). These scientists thought that the developmental contingencies of individual organisms (which they called "acquired traits") were not initially fixed genetically, but that they could somehow become incorporated into the genetic make-up of individuals, and thereby be passed along to their offspring, so that evolution could occur. For example, if the ancestors of giraffes has to stretch vigorously to reach their food of tree foliage high in the canopy, this physical act might somehow have caused the individual animals to develop somewhat longer necks. This "acquired" trait somehow became fixed in the genetic complement of those individuals, to be passed on to their offspring, who then also had longer necks. Eventually, this presumed mechanism of evolution 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 only a reflection of the variable, but strongly fixed genetic potential that exists in all individuals. Therefore, the idea of the inheritance of acquired traits is no longer influential in evolutionary s cience. 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.

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