Experiments have shown that if the moths lived in an environment without these predatory bats for millions of years, their “ears” would likely desensitize, and they would become less receptive to high frequency sounds (Fullard, 1994). Sensitivity would decrease as environmental selective pressures were removed (Fullard, Ratcliffe, & Hofstede, 2006). Let’s explore which evolutionary mechanisms are responsible for such a change, and which are irrelevant: Mutation, Natural Selection, Genetic Drift, and Gene Flow.
Mutation occurs when during when mistakes occur in meiosis, mitosis, or DNA repair (University of California Museum of Paleontology [UCMP], n.d. f). However, only mutations in meiosis, called “germ line” mutations, can be passed on from generation to generation (UCMP, n.d. d). In such cases of meiosis, random mistakes occur in replication, causing the new DNA to be slightly different than the parent DNA (UCMP, n.d. f). Sometimes, external factors, such as exposure to radiation and harmful chemicals, cause DNA to break down (UCMP, n.d. f). When the DNA repairs itself, random mistakes can be made, also causing the DNA to be mutated (UCMP, n.d. f).
In the case of the noctuid moths, a series of mutations during reproduction may occur over the course of a million years, causing the moths to lose functionality in their high frequency sound sensory organs. Learn about why these traits are passed on in Natural Selection.
Natural selection is the evolutionary mechanism which describes that if a trait in one organism makes it more likely to reproduce than its peers, that trait will consequently become more frequent in future generations UCMP, n.d. e. Thus, although mutations are random, those that are beneficial can be passed on, while those that are harmful are not UCMP, n.d. e.
In the case of the noctuid moth, when a random mutation occurs that makes one moth deafer than the others, that moth expends less energy than its peers, because it does not have to put as much energy into operating this organ (Fuller, 1994). It can use this energy for such tasks as eating more food or evading predators, and therefore, it is more likely to reproduce than its peers with fully functional organs (Fuller, 1994). Because there is no natural selective pressure from the predatory bats, the mutation causing deafness is more likely to be passed on by natural selection than the gene producing a fully functional ear (Fuller, 1994). Learn about how this process is aided and inhibited in Genetic Drift.
Genetic drift is the evolutionary mechanism which describes random fluctuation of gene frequency from generation to generation (UCMP, n.d. c). Certain organisms reproduce more than others by chance, and some are removed from the gene pool by chance (UCMP, n.d. c). Therefore, the next generation would have a different frequency of each gene which was randomly increased or decreased in frequency (UCMP, n.d. a). Due to its random nature, genetic drift tends to decrease population diversity (UCMP, n.d. a).
Genetic drift is always occurring in populations (UCMP, n.d. c). It tends to increase the frequency of the prevalent gene, while decreasing the frequency of the less occurring one (UCMP, n.d. a). Thus, in the case of noctuid bats, natural selection and genetic drift oppose each other until the population of bats without fully functional ears is greater than the population with them. Genetic drift is not the cause of this gradual change in gene frequency, but it serves first as an inhibitor of the process and then as a catalyst.
Gene flow is the evolutionary mechanism which describes the transfer of genes between populations (UCMP, n.d. b). Introduction of an organism which possesses one gene into a population which possesses another gene will produce genetic variation, as the organism may reproduce and pass its gene on to the next generation (UCMP, n.d. b). When there is transfer between two populations, however, genetic variance will increase, but after many generations, the genetic makeup of the two populations will become very similar, as the genes will reach an “equilibrium” (UCMP, n.d. b).
In the case of the noctuid moth, gene flow could cause the moths to become deafer, because if moths without the sensory organs are present in the area where the moths are lived, if they reproduced, the genes may not have gotten passed onto the next generation. Although this is not a certain cause of the moths’ deafness, it could be one, depending on the species indigenous to the areas in which the moths lived.