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Background: The year 2009 is the 200th anniversary of the publication of Jean-Bapteste Lamarck's Philosophie Zoologique and the 150th anniversary of Charles Darwin's On the Origin of Species. Lamarck believed that evolution is driven primarily by non-randomly acquired, beneficial phenotypic changes, in particular, those directly affected by the use of organs, which Lamarck believed to be inheritable. In contrast, Darwin assigned a greater importance to random, undirected change that provided material for natural selection.
The concept: The classic Lamarckian scheme appears untenable owing to the non-existence of mechanisms for direct reverse engineering of adaptive phenotypic characters acquired by an individual during its life span into the genome. However, various evolutionary phenomena that came to fore in the last few years, seem to fit a more broadly interpreted (quasi)Lamarckian paradigm. The prokaryotic CRISPR-Cas system of defense against mobile elements seems to function via a bona fide Lamarckian mechanism, namely, by integrating small segments of viral or plasmid DNA into specific loci in the host prokaryote genome and then utilizing the respective transcripts to destroy the cognate mobile element DNA (or RNA). A similar principle seems to be employed in the piRNA branch of RNA interference which is involved in defense against transposable elements in the animal germ line. Horizontal gene transfer (HGT), a dominant evolutionary process, at least, in prokaryotes, appears to be a form of (quasi)Lamarckian inheritance. The rate of HGT and the nature of acquired genes depend on the environment of the recipient organism and, in some cases, the transferred genes confer a selective advantage for growth in that environment, meeting the Lamarckian criteria. Various forms of stress-induced mutagenesis are tightly regulated and comprise a universal adaptive response to environmental stress in cellular life forms. Stress-induced mutagenesis can be construed as a quasi-Lamarckian phenomenon because the induced genomic changes, although random, are triggered by environmental factors and are beneficial to the organism.
Conclusion: Both Darwinian and Lamarckian modalities of evolution appear to be important, and reflect different aspects of the interaction between populations and the environment.
The celebrations of Darwin's 200 years jubilee and the 150 anniversary of On the Origin of Species  in 2009, to a large extent, overshadowed another anniversary: JeanBapteste Lamarck's magnum opus, Philosophie Zoologique , was published in 1809, the year of Darwin's birth . Arguably, Lamarck's book was the first published manifesto of biological evolution as fittingly pronounced by Darwin himself in the later editions of the Origin [4-6]. Lamarck's concept of evolution was limited in scope: in particular, he did not believe in extinction of species but rather thought that species are gradually transformed into other species via phyletic modification. Lamarck also believed in the innate tendency of organisms to progress toward perfection down the succession of generations. In line with this idea, Lamarck speculated on an extremely simple and straightforward mechanism of evolutionary change whereby the use of a particular organ would lead to its gradual functional improvement that would be passed through generations (the example of the giraffe's neck is, probably, one of the most notorious "just so stories" in the history of biology). Later, a generalization of Lamarck's hypothetical mechanism became known as inheritance of acquired characters (characteristics) (IAC) to emphasize a key aspect of this mechanism, namely, the direct feedback between phenotypic changes and the (what is now known as) the genotype (genome). However, it should be stressed that the phrase "inheritance of acquired characters" is substantially imprecise in that Lamarck and his followers were very particular about adaptive (beneficial, useful) not just any acquired traits being inherited. Furthermore, inheritance of acquired characters certainly is not Lamarck's original idea; rather, it appears to have been "folk wisdom" in Lamarck's day . Hereinafter we use the acronym IAC with this implicit understanding.