On the Inheritance of Acquired Traits and the Theory of Use and Disuse.

On the Inheritance of Acquired Traits and the Theory of Use and Disuse.

Charles Darwin’s On the Origin of Species described in great detail a means to explain the theory of evolution through natural selection. Within his work he makes many observations in relation to the heritability of acquired characteristics. As he describes the effects of artificial selection, he dawns on the topic of “The effects of habit and of the use or disuse of parts; correlated variation; inheritance”(Darwin, p10). Darwin makes the observation, when speaking of domesticated mammals, that “not one of our domesticated animals can be named which has not in some country drooping ears...the drooping is due to the disuse of muscles of the ear, from the animals being seldom much alarmed”(Darwin, p10). Darwin argues that the increased use or disuse of parts in an animals lifetime result in heritable variation that can be passed on to their offspring. This variation he argues, is the basis for which animals develop advantageous traits and lose deleterious ones. Darwin bases his theory of heritable variation on the theory of acquired heritability, and the use and disuse principle, which was proposed by Lamarck. Being the predominant idea at the time the theory of Use and disuse states that, “use would cause the structure to increase in size over several generations, whereas disuse would cause it to shrink or even disappear”(Waggoner 1996). His second principle, or the idea of acquired heritability states that, “all such changes were heritable”(Waggoner 1996). Using these principles Darwin attempted to describe the relationships between organisms, the variation that resulted in speciation, and the evolutionary implications of those postulates. Lamarck’s theories provided a mechanism by which Darwin could explain natural selection and evolution, and in this respect they were invaluable. These theories however, are now known to be incorrect. The first principle of use and disuse can be dis-proven by the modern evolutionary principle that natural selection can act only on variability already present within a population. Genetic variation is already present in natural populations and selection acting on this variation results in evolution. The second principle of acquired heritability can be disproved under this same idea. Any particular advantageous trait(s) that an organism may develop during their lifetime, is a result of a genetic basis for that trait already present in the population or that may have arisen through mutation. The fitness increase caused by such a trait would result in the fixation of that trait in a population, so it would seem that an advantageous trait was inherited as a result of the parent organism developing that trait through use or disuse. Without a genetic background to explain heritability Lamarck’s theories made the most sense at the time. Since Darwin’s elaboration on the theory of evolution and heritability, numerous genetic experiments have taken place to attempt to provide a better understanding of the laws of heritability. Darwin makes several assertions within the first chapter of On the Origin of Species that defend his ideas to this day. His idea of correlated variation, or the idea that certain traits are associated with each other is explained by the statement that, “if man goes on selecting, and thus augmenting any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation”(Darwin, p11). What Darwin was referring to, although he didn’t know it, was idea of genetic linkage, or that certain traits are associated with others through genetic linkage on a genome. Darwin used the domestication of the canine as an example of artificial selection to aid in his explanation of heritable variation. He focuses on the idea that while the domestic dog is one species, a significant degree of variation can be seen within that species that has been acquired over time. The large degree of variation seen in domestic dogs can be explained by a domestication experiment conducted on foxes. Through genetic mapping scientist have been able to “identify a locus that is orthologous to, and therefore validates, a genomic region recently implicated in the domestication of dogs”(Kukekova et al., 2011). This locus is significant because selection for a behavioral trait can cause the selection of several phenotypic changes associated with that trait. Because of the prescience of a discrete locus for tame vs. aggressive behavior the canine and other related species were predisposed for domestication by humans. The subsequent phenotypic variation that followed with the domestication of dogs was due to the prescience of alleles associated with the locus for tame behavior. Domestication presents the unique result that, “when subjected to domestication, animals whose evolutionary pathways did not cross, started to evolve in the same direction”(Kukekova et al., 2011). The species of mammals that have been domesticated, show similar morphological changes associated with tame behavioral patterns(see figure 2). Darwin suggested that certain features shared by domestic animals are a result of their domestication, when in actuality it is the reverse effect. Certain features shared by domestic animals are the result of their ancestral varieties being predisposed to domestication because of variation already present in their natural populations. The commonalities shared among domesticates of multiple species rules out the idea that these animals acquired those traits due to environmental factors. Environmental factors are so variable, that it would be impossible if not highly improbable that the majority of domesticated mammals would develop such similar characteristics. In addition, “it seems unlikely that these similar trends of morphological and physiological transformation of different domestic animals depend on homology-independent mutations of structural homologous genes”(Trut et al., 2009). The association between a docile nature, and distinct morphological changes seen across multiple species suggests that these animals share a similar genetic structure and share a common ancestor. A shared, relatively recent common ancestor would explain why highly divergent species under selection for a particular trait would evolve similar characteristics. Another observation that Darwin poses as evidence for acquired heritability, and one of the most well know is that of the giraffe and its long neck. Darwin presents that, “the giraffe, by its lofty stature, much elongated neck, forelegs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the reach of the other Ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during dearths”(Darwin, p104).
Darwin goes on to suggest that over time as giraffes stretched their necks farther, those that could reach the highest branches and used their long necks to the greatest advantage would pass those traits on to their offspring. Studies have shown however, that this observation is flawed because of multiple disadvantages that giraffes suffer as a result of their long necks, and behavioral tendencies of the species that suggest it provides no particular advantage during dearths. The first flaw with Darwin’s observations of the giraffe stem from the fact that if giraffes possessed a truly significant advantage over other ungulata, then why haven’t more long necked species evolved? In addition, why haven’t shorter species of ungulata suffered greatly or gone extinct in prolonged periods of dearth? Evidence suggests that, “animals frequently feed at shoulder level during winter bottlenecks, when their neck should assist them in gaining a feeding height advantage”(Simmons and Altwegg, 2009). Feeding at lower heights rules out the idea that the giraffe’s long neck provides a significant advantage over other species during dearth. Many studies have suggested that the long necks have evolved through sexual selection, as males use their necks in mating displays, as well as for male-male competition for females. The males with larger necks tend to be preferred by oestrous females and more commonly win contests for access to females(Simmons and Altwegg, 2009). If one looks at physiology of a giraffe there are many disadvantages to their elongated neck and bone structure. As the neck grows longer more energy is required to pump blood to the brain and there is a greater probability that an organism will sustain injury due to elongated limbs(Mitchell and Skinner, 2003). When giraffes drink water they must spread their forelimbs in an awkward position that takes time and increases their vulnerability to predators. A disadvantage such as their inability to reach the ground from standing position suggests that giraffes most likely evolved their long necks from sexual selection pressure, which can select for traits that could potentially decrease lifespan in return for increased reproductive success. The disadvantages to the giraffes long neck coupled with its relatively limited advantages rule out the possibility that they gained this trait through acquired heritability, or even natural selection. While Darwin was mistaken about the mechanism by which organisms acquired advantageous traits, he made many accurate observations and was able to make many accurate predictions from those observations that still hold true today. Regardless of his lack of knowledge of the laws of heritability, Darwin was able to describe correlated variation in great detail, and elaborate on its importance to the heritability of traits. Darwin provided a logical analysis of his observations and provided a solid basis for which other scientists could test and elaborate on his theories. Lastly, Darwin’s observations were accurate and logical however, because he lacked a mechanism for which to describe how traits were inherited, he based his assumptions on the theories of acquired heritability and use/disuse, which we know now to be incorrect.

(from Trut. et al. 2009)
References:
1. Darwin, Charles. (1859). On the Origin of Species. London: John Murray. 2. Waggoner, B. (1996). Jean-Baptiste Lamarck (1744-1829). UCMP Berkeley. http://www.ucmp.berkeley.edu/history/lamarck.html 3. Kukokeva, A.V. et al.(2011). Mapping Loci for Fox Domestication: Deconstruction/Reconstruction of a Behavioral Phenotype. Behav Genet, 41: 593-606. 4. Trut, L. et al.(2009). Animal Evolution During Domestication: the domesticated fox as a model. BioEssays, 31: 349-360. 5. Simmons, R.E., Altwegg, R.(2009). Necks-for-sex or Competing Browsers? A Critique of Ideas on the Evolution of the Giraffe. Journal of Zoology, 282: 6-12. 6. Mitchell, G., Skinner, J.D.(2003). On the Origin, Evolution and Phylogeny of Giraffes Giraffa camelopardalis. Transactions of the Royal Society of South Africa, 58(1): 51-73.

References: 1. Darwin, Charles. (1859). On the Origin of Species. London: John Murray. 2. Waggoner, B. (1996). Jean-Baptiste Lamarck (1744-1829). UCMP Berkeley. http://www.ucmp.berkeley.edu/history/lamarck.html 3. Kukokeva, A.V. et al.(2011). Mapping Loci for Fox Domestication: Deconstruction/Reconstruction of a Behavioral Phenotype. Behav Genet, 41: 593-606. 4. Trut, L. et al.(2009). Animal Evolution During Domestication: the domesticated fox as a model. BioEssays, 31: 349-360. 5. Simmons, R.E., Altwegg, R.(2009). Necks-for-sex or Competing Browsers? A Critique of Ideas on the Evolution of the Giraffe. Journal of Zoology, 282: 6-12. 6. Mitchell, G., Skinner, J.D.(2003). On the Origin, Evolution and Phylogeny of Giraffes Giraffa camelopardalis. Transactions of the Royal Society of South Africa, 58(1): 51-73.