Studies of local adaptation provide important insights into the power of natural selection relative to gene flow and other evolutionary forces. They are a paradigm for testing evolutionary hypotheses about traits favoured by particular environmental factors. This paper is an attempt to summarize the conceptual framework for local adaptation studies. We first review theoretical work relevant for local adaptation. Then we discuss reciprocal transplant and common garden experiments designed to detect local adaptation in the pattern of deme · habitat interaction for fitness. Finally, we review research questions and approaches to studying the processes of local adaptation -divergent natural selection, dispersal and gene flow, and other processes affecting adaptive differentiation of local demes. We advocate multifaceted approaches to the study of local adaptation, and stress the need for experiments explicitly addressing hypotheses about the role of particular ecological and genetic factors that promote or hinder local adaptation. Experimental evolution of replicated populations in controlled spatially heterogeneous environments allow direct tests of such hypotheses, and thus would be a valuable way to complement research on natural populations.

■ Abstract The ability to adapt to marginal habitats, in which survival and reproduction are initially poor, plays a crucial role in the evolution of ecological niches and species ranges. Adaptation to marginal habitats may be limited by genetic, developmental, and functional constraints, but also by consequences of demographic characteristics of marginal populations. Marginal populations are often sparse, fragmented, prone to local extinctions, or are demographic sinks subject to high immigration from high quality core habitats. This makes them demographically and genetically dependent on core habitats and prone to gene flow counteracting local selection. Theoretical and empirical research in the past decade has advanced our understanding of conditions that favor adaptation to marginal habitats despite those limitations. This review is an attempt at synthesis of those developments and of the emerging conceptual framework.

Experimental evolution is the study of evolutionary processes occurring in experimental populations in response to conditions imposed by the experimenter. This research approach is increasingly used to study adaptation, estimate evolutionary parameters and test diverse evolutionary hypotheses. Long applied in vaccine development, experimental evolution also finds new applications in biotechnology. Recent technological developments provide a path towards detailed understanding of the genomic and molecular basis of experimental evolutionary change, while new findings raise new questions that can be addressed with this approach. However, experimental evolution has important limitations, and the interpretation of results is subject to caveats resulting from small population sizes, limited timescale, the simplified nature of laboratory environments and, in some cases, the potential to misinterpret the selective forces and other processes at work.

Many animal species face periods of chronic nutritional stress during which the individuals must continue to develop, grow, and/or reproduce despite low quantity or quality of food. Here, we use experimental evolution to study adaptation to such chronic nutritional stress in six replicate Drosophila melanogaster populations selected for the ability to survive and develop within a limited time on a very poor larval food. In unselected control populations, this poor food resulted in 20% lower egg-to-adult viability, 70% longer egg-to-adult development, and 50% lower adult body weight (compared to the standard food on which the flies were normally maintained). The evolutionary changes associated with adaptation to the poor food were assayed by comparing the selected and control lines in a common environment for different traits after 29-64 generations of selection. The selected populations evolved improved egg-to-adult viability and faster development on poor food. Even though the adult dry weight of selected flies when raised on the poor food was lower than that of controls, their average larval growth rate was higher. No differences in proportional pupal lipid content were observed. When raised on the standard food, the selected flies showed the same egg-to-adult viability and the same resistance to larval heat and cold shock as the controls and a slightly shorter developmental time. However, despite only 4% shorter development time, the adults of selected populations raised on the standard food were 13% smaller and showed 20% lower early-life fecundity than the controls, with no differences in life span.The selected flies also turned out less tolerant to adult malnutrition. Thus, fruit flies have the genetic potential to adapt to poor larval food, with no detectable loss of larval performance on the standard food. However, adaptation to larval nutritional stress is associated with trade-offs with adult fitness components, including adult tolerance to nutritional stress.

Materials and Methods Conditioning procedureConditioning and memory tests were performed on samples of 50 adult flies (sexes mixed), raised in standard conditions and aged 3-5 days. The conditioning procedure consisted of 5 training sessions separated by 20 min intervals (spaced protocol) or immediately following one another (massed protocol).(A) Associative conditioning: In each training session flies were first exposed for 30 s to one odorant simultaneously with mechanical shock (2000 rpm vibration pulses of 1 s duration, delivered every 5 s by a test tube shaker). This period was followed by a 60 s rest period (no odor and no shock). Then, for 30 s another odorant was delivered, withoutshock. The training session ended with a second rest period of 60 s. 3-octanol and 4-methylcyclohexanol (both 0.6ml/l of paraffin) were used as odorants.(B) Non-associative conditioning was similar except that the conditioned stimulus (odor) and the unconditioned stimulus (shock) were not presented simultaneously. Instead, in each training session the flies first received for 30 s shock only (on odors). Then one