One of the most fascinating advances in evolutionary biology is in sexually antagonistic co-evolution (SAC). Some male animals have been observed to evolve persistence traits that increase their fitness to mate more successfully vis-à-vis male-male competition for potential mates. In response, their female counterparts have developed resistance mechanisms to reduce the direct costs of increased mating rates, by making it more difficult for males to mate successfully. SAC postulates that subsequently, these male animals evolve even more persistent traits, while females develop greater resistance characteristics, leading to an ever-escalating co-evolutionary “arms race” (Parker, 1979; West-Eberhard, 1983).
One example of SAC has been observed in seed beetles (Coleoptera bruchidae). Spines developed on the male genitalia helps enhance stability when mating and thereby greater success in copulation (Edvarsson & Tregenza, 2005). According to a study published in an online journal by Arnqvist et. al (2007), male seed beetles were recorded to evolve genital spines while female seed beetles developed tougher copulatory ducts to resist the spines. SAC was subsequently observed where male seed beetles developed more spines in response, while their female counterparts developed even stronger copulatory ducts by further reinforcing with thicker tissue. This evolution could be seen in the main pictures below, when comparing related species demonstrating the evolution of related male genital spines from C to B to A.
“Comparison between genital spines in C. analis (A), C. rhodesianus (B) and C. phaseoli (C).” By Arnqvist et. al (2007) in “Coevolution between Harmful Male Genitalia and Female Resistance in Seed Beetles.” Proceedings of the National Academy of Sciences of the United States of America, Vol. 104, No. 26 (Jun. 26, 2007), pp. 10921-10925. http://www.jstor.org/stable/25436040 (accessed on 04/04/2010).
Recent research has made SAC more rigorous. Some traits that were thought as primary sexual antagonisms, where the enhancement of such a trait would improve male copulation success, has been shown to be a result of other factors (Svensson & Gosden, 2007). For example, the increased forehead patch size in the collared flycatcher, Ficedula albicollis, has been primarily shown to be a result of climate change, with heightened sexual success a secondary result (Garant et al, 2004; Hegyi et al, 2006). This highlights the different primary causes of evolutionary changes that may have sexually enhancing and subsequently antagonistic consequences, making the study more careful and holistic.
Edvardsson M, Tregenza T (2005) Behav Ecol 16:788-793.
Garant, D., Sheldon, B.C. & Gustafsson, L., 2004. Climatic and temporal effects on the expression of secondary sexual characters: genetic and environmental components. Evolution Vol. 58, 634–644.
Hegyi, G., Torok, J., Toth, L., Garamszegi, L.Z. & Rosivall, B., 2006. Rapid temporal change in the expression and agerelated information content of a sexually selected trait. Journal of Evolutionary Biology Vol. 19, 228–238.
J. Rönn, M. Katvala, G. Arnqvist, 2007. Coevolution between Harmful Male Genitalia and Female Resistance in Seed Beetles. Proceedings of the National Academy of Sciences of the United States of America, Vol. 104, No. 26 (Jun. 26, 2007), pp. 10921-10925
E. I. Svensson & T. P. Gosden, 2007. Contemporary evolution of secondary sexual traits in the wild. Functional Ecology, Vol. 21, 422–433
West-Eberhard, M.J. ,1983. Sexual selection, social competition and speciation. Quarterly Review of Biology, Vol. 58, 155–183.
Parker, G. A. 1979. Sexual selection and sexual conflict. Pp. 123–166 in M. S. Blum and N. A. Blum, eds. Sexual selection and reproductive competition in insects. Academic Press, New York.