Abstract
The potential for selective harvests to induce rapid evolutionary change is an important question for conservation and evolutionary biology, with numerous biological, social and economic implications. We analyze 39 years of phenotypic data on horn size in bighorn sheep (Ovis canadensis) subject to intense trophy hunting for 23 years, after which harvests nearly ceased. Our analyses revealed a significant decline in genetic value for horn length of rams, consistent with an evolutionary response to artificial selection on this trait. The probability that the observed change in male horn length was due solely to drift is 9.9%. Female horn length and male horn base, traits genetically correlated to the trait under selection, showed weak declining trends. There was no temporal trend in genetic value for female horn base circumference, a trait not directly targeted by selective hunting and not genetically correlated with male horn length. The decline in genetic value for male horn length stopped, but was not reversed, when hunting pressure was drastically reduced. Our analysis provides support for the contention that selective hunting led to a reduction in horn length through evolutionary change. It also confirms that after artificial selection stops, recovery through natural selection is slow.
Highlights
Human activities such as habitat modifications, expanding road networks, overexploitation and climate change affect animal populations
Intense exploitation by humans may outpace (Darimont et al 2009) or oppose (Carlson et al 2007) the selective effects of natural predators, potentially leading to evolutionary changes in behaviour, phenotype or life history (Hard et al 2008; Devine et al 2012). van Wijk et al (2013) showed that selective harvesting of guppies (Poecilia reticulata) led to changes in size and in the frequency of alleles associated with size in just two generation
Coltman et al (2003) used a pedigree up to six generation deep to report a decline in estimated breeding values (EBV) of horn length and body mass in bighorn rams over 30 years, suggesting an evolutionary response to size-selective harvests
Summary
Human activities such as habitat modifications, expanding road networks, overexploitation and climate change affect animal populations. One approach to study evolution in nature, often referred to as the animal model, involves mixed models combining a pedigree with data on phenotype and environmental conditions to estimate genetic parameters (Kruuk 2004) Using this approach, Coltman et al (2003) used a pedigree up to six generation deep to report a decline in estimated breeding values (EBV) of horn length and body mass in bighorn rams over 30 years, suggesting an evolutionary response to size-selective harvests. Coltman et al (2003) drew that conclusion after analysing data for the only sport-hunted population of ungulates for which a pedigree and horn measurements are available (Pelletier et al 2012) By extension, these criticisms question phenotype-based studies that reported long-term trends consistent with an evolutionary impact of selective hunting (Garel et al 2007; Hengeveld and Festa-Bianchet 2011). Univariate models are less prone to problems when fitted with limited data given their simpler structure (Wilson et al 2010)
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