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The domestication of plants and animals is historically one of the most important topics in evolutionary biology. The evolutionary genetic changes arising from human cultivation are complex because of the effects of such varied processes as continuing natural selection, artificial selection, deliberate inbreeding, genetic drift and hybridization of different lineages. Despite the interest of domestication as an evolutionary process, few studies of multicellular sexual species have approached this topic using well-replicated experiments. Here we present a comprehensive study in which replicated evolutionary trajectories from several Drosophila subobscura populations provide a detailed view of the evolutionary dynamics of domestication in an outbreeding animal species. Our results show a clear evolutionary response in fecundity traits, but no clear pattern for adult starvation resistance and juvenile traits such as development time and viability. These results supply new perspectives on the confounding of adaptation with other evolutionary mechanisms in the process of domestication.
- Top of page
- Materials and methods
The domestication of plants and animals is historically one of the most important topics in evolutionary biology, figuring prominently in Darwin's Origin of Species. Traditionally, the term ‘domestication’ refers to the genetic changes undergone by our commensal species, from dogs to agricultural animals to grains to legumes, sometimes with an additional connotation related to behavioural change, especially reduction in ‘wildness’ (Soanes, 2003). A more useful definition, however, for scientific purposes is that domestication is the evolutionary genetic change arising from the transition of a population from nature to deliberate human cultivation. In some laboratory populations, such as those of Drosophila or Escherichia coli, the ‘state of nature’ may be laboratory culture under a sequence of ill-defined, arbitrarily or haphazardly changing conditions (cf. Matos et al., 2002; Lenski, 2004).
Domestication, as defined here, is of both practical and theoretical scientific interest. One of the enduring problems in the breeding of both plants and animals is the interpretation of the evolutionary conditions that they have been subjected to, a topic that was a particular favourite of Darwin (1859, 1883). The development of modern animal and plant breeding has depended, in part, on the spread of this type of evolutionary understanding from theorists, like Darwin, to practical breeding. Understanding the impact of captivity is also becoming prominent in conservation genetics, as more and more species are being maintained in ex situ conservation programmes (Frankham et al., 2002).
For evolutionary biology itself, domestication provides one of the more important contexts for experimental evolution. It is both a background to evolutionary studies of diversification under selection (e.g. Rose et al., 2004) and an important topic in itself. In studying domestication in well-defined laboratory experiments, we can measure in detail the evolutionary process with replication and specific environmental controls. In this context, such key evolutionary processes as adaptation and inbreeding occur transparently and reproducibly, a fruitful setting for testing biological hypotheses (see Mueller & Joshi, 2000; Houle & Rowe, 2003; Prasad & Joshi, 2003).
Domestication of Drosophila populations that have been founded from wild samples has been studied using two different approaches. First, comparison of populations that have and have not been subject to particular domestication regimes (e.g. Sgrò & Partridge, 2000; Hoffmann et al., 2001; Krebs et al., 2001; Gilligan & Frankham, 2003; Griffiths et al., 2005). Secondly, temporal analysis of the evolutionary trajectories of domesticated populations since their foundation from the wild (e.g. Matos et al., 2000b, 2002), our approach for the last 15 years.
In particular, we have been studying evolutionary convergence between the recent and the long-established populations, suggesting laboratory adaptation, particularly in fecundity traits (Matos et al., 2000b, 2002). In the present study, we extend these previous studies to include more generations, more fitness-related traits, and two new, independent, synchronous foundations. Here we offer the most detailed view yet of domestication in an outbreeding animal species, with new information on the confounding of adaptation, founder effects and inbreeding in the process of domestication.
In this study these specific questions were addressed:
Are there directional patterns of adaptation across traits?
Are there plateaus in long-term domestication?
Do long-maintained populations show progressive inbreeding depression?
Is there a temporal increase in divergence between replicate populations?
How important are effects of foundation for evolutionary patterns and processes during local adaptation?
Finally, we address the relevance of these laboratory studies to practical domestication and conservation issues, such as ex situ breeding programmes.