We address the evolutionary consequences of large-scale and rapid environmental perturbations in marine ecosystems. This includes fundamental research questions such as:
Scales of global environmental impacts are typically thousands of kilometres or more spatially, and hundreds or thousands of years in time. A major problem in environmental research is that it is not possible to perform experiments at such large scales. One solution to this dilemma is to study adaptation and evolution across existing natural and anthropogenic gradients.
The Baltic Sea is an ideal natural test-bed for studying adaptation of populations and species over a temporal and spatial scale similar to those of current environmental changes. During recent geological time the Baltic Sea salinity has changed dramatically: First time 8500 years ago when the sea broke through the barriers of the Ancylus Lake, causing salinity to increase from 0‰ to 15‰ over a few hundred years, and the second time 4500 years ago, when salinity rapidly dropped to the current 7‰.
As a consequence two permanent and stable salinity gradients were formed along the Swedish coast; one large-scale from the inner Baltic to the North Sea and one small-scale from the surface to deep water. Both shifts and gradients constitute excellent natural test-beds for studies of evolutionary processes.
A Darwinian laboratory
The formation of the Baltic Sea provides a geologically recent and unusually simple environmental change linked to variation in one easily measurable variable, salinity, which is a crucial factor for almost all marine species. In addition, the Baltic is also a species-poor and trophically simple ecosystem. These characteristics make the Baltic Sea salinity shifts and gradients exceptionally tractable natural experiments for studies of rapid evolution.
Fig. The Baltic Sea salinity gradient is paralleled by genetic gradients (in microsats, allozymes, etc.) in several Baltic Sea species. (Data from Johannesson & André 2006)
The fact that the Baltic Sea may not be representative of the rest of the world has no relevance here. Indeed, the Galapagos became a useful model for Darwin not because these islands were representative of the rest of the world, but because they provided a system simple enough to show effects of evolutionary processes that were easy to observe.
Most of our understanding of how organisms adapt to changes in their environment comes from experimental work with laboratory organisms, typically Drosophila, or from plant and animal breeding. While those experiments often show that individual traits have considerable potential for evolutionary changes, rapid change in one trait is often accompaigned by negative responses in other traits. This could be one reason for the evolutionary stasis observed in some species.
Perhaps the best-known example of successful adaptation to a rapid and indeed anthropogenic environmental impact is the peppered moth Biston betularia. In just av few decades air-pollution changed the colour of this moth from white to dark. Other well-known examples include the evolution of elevated heavy metal tolerance in grass populations growing in the vicinity of mines, and changes in time of parturition in arctic squirrels following a decade of temperature increase.
Notwithstanding these well-known cases, both geological and recent history are crowded with examples of species that failed to adapt and went extinct during periods of rapid environmental change. Hence, it is logical to conclude that natural populations are under serious constraints and trade-offs that will subdue the evolution of rapid adaptation. Why this is so, or when such constraints are important, are pressing issues to which answers are urgently needed.
The Baltic Sea was formed from a postglacial freshwater lake about 8000 years ago. Today it is one of the world’s largest brackish-water seas with a surface salinity of 2-10‰.