Flyway evolution is too fast to be explained by the modern synthesis: proposals for an ‘extended’ evolutionary research agenda
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In this paper, I argue that to fully grasp the generation and maintenance of variation in the migratory phenotypes of (shore-)birds we need to expand our scientific search image and include developmental processes and non-genetic pathways of inheritance in the explanatory frameworks. Traditionally, studies of micro-evolution of migratory phenotypes were restricted to comparative studies on migratory versus non-migratory taxa, and artificial selection and heritability experiments on quantitative behavioural traits related to migration. Such studies had a focus on the genetic axis of inheritance and were restricted to songbirds. In avian groups such as the shorebird families Scolopacidae and Charadriidae, all but a few island species are migrants, which precludes comparative studies at the species level. Like other taxa, shorebirds have geographically separate breeding populations (either or not recognized as subspecies on the basis of morphological differences) which differentiate with respect to the length, general direction and timing of migration, including the use of fuelling at staging sites and the timing of moult. However, their breeding systems preclude artificial selection and heritability experiments on quantitative traits. This would seem to limit the prospects of evolutionary analysis until one realizes that the speed of evolutionary innovation in shorebird migratory life-histories may be so fast as to necessitate other avenues of explanation and investigation. According to our best current estimates based on mitochondrial gene sequence variation, in Red Knots Calidris canutus considerable phenotypic variation has evolved since the Last Glacial Maximum ca. 20,000 years ago, to the extent that six subspecies are currently recognized. This would be too short a time for the origin of the qualitatively and quantitatively distinct and non-overlapping traits to be explained by random point mutations followed by natural selection, although we cannot dismiss the possibility of previously unexpressed (standing) genetic variation followed by selection. I argue that, to understand the flyway evolution of such shorebirds in the ‘extended’ evolutionary framework, we need to give due attention to developmental versatility and broad-sense epigenetic evolutionary mechanisms. This means that experimental studies at the phenotypic level are now necessary. This could involve a combination of observational studies in our rapidly changing world, common garden experiments, and even experiments involving global-scale displacements of particular migratory phenotypes at different phases of development. I provide suggestions on how such experiments could be carried out.
KeywordsCalidris canutus Common garden experiment Epigenetic Extended synthesis Migration Shorebirds
Most of what has been written about the evolution of bird migration (Alerstam et al. 2003) has had a strong focus on the (micro-)evolution of migration traits in songbirds (e.g. Berthold and Querner 1981; Berthold 1995; Able and Belthoff 1998) and on comparative studies between migrant and non-migrant taxa (e.g. Zink 2002; Helbig 2003; Outlaw et al. 2003; Rappole et al. 2003; Salewski and Bruderer 2007). The portfolio includes studies of artificial selection and heritability experiments on quantitative behavioural traits related to migration such as the duration and timing of migratory restlessness (e.g. van Noordwijk et al. 2006; Pulido 2007; Pulido and Berthold 2010). As impressive and as valuable all these studies are, I have found myself frustrated when applying the supposedly unifying but passerine-biased frameworks to the thinking about flyway evolution in the group of birds that I am most familiar with, the shorebirds, and especially the family of sandpipers Scolopacidae (Piersma et al. 1996) and the plover family Charadriidae (Piersma and Wiersma 1996).
Here is why. All but a few (island) species of shorebirds are migrants which precludes comparative studies on closely-related migratory and non-migratory species. Like other taxa, shorebirds do have geographically separate breeding populations (which may or may not be recognized as subspecies on the basis of morphological differences; Engelmoer and Roselaar 1998) which differentiate with respect to the length, general direction and timing of migration, including the use of fuelling at staging sites and the timing of moult. However, the peculiarities of their breeding systems preclude studies of inheritance of quantitative traits such as migratory restlessness. Importantly, the traits on the basis of which subspecies are distinguished are distinct and do not overlap, i.e. they seem to show evolutionary novelty rather than the gradual variation on which natural selection can work (van Noordwijk et al. 2006).
In this brief contribution, I aim to propose that to understand the flyway evolution of such shorebirds we need an ‘extended’ evolutionary framework with a mindset that is prepared to consider the many creative possibilities of individual development (Turner 2007; Badyaev 2011a, b) and broad-sense epigenetic evolutionary mechanisms (Jablonka and Lamb 2006). In addition to further genomic work to establish timelines of divergence in different clades of long-distance migrants, I will argue that we need intensive observational studies in our rapidly changing world and experimental studies at the phenotypic level.
Microevolution of contrasting seasonally changing phenotypes
Red Knots and other long-distance migrating shorebirds show amazing seasonal variation in phenotype. For example, body mass is doubled before take-off on flights of 5,000 km or more, and halved during the continuous flight which may take up to 10 days (Battley et al. 2000; Gill et al. 2009). These changes involve great modifications in fat stores, but also include modifications to most organs (other than the kidneys, the lungs and the brain; Piersma et al. 1999; Battley and Piersma 2005; Piersma and van Gils 2011). On top of all these seasonal variations, shorebirds also adjust phenotypes to unpredictable environmental changes in food type, climatic conditions and disease pressure (Vézina et al. 2006, 2010; Buehler et al. 2008a, b).
Beyond hard-coding: soft coding and the extended synthesis
Over the last 100 years, biology has strongly focussed on the two-stage evolutionary process of mutational changes to the genome with downstream consequences for the phenotype on which natural selection operates. This is a relatively slow process, and the finding that rapid phenotypic change commonly occurs in a wide variety of organisms has lead to explanations invoking selection for previously unexpressed (hidden or ‘standing’) genetic variation (Barrett and Schluter 2008; Le Rouzic and Carlborg 2008). Whilst I cannot dismiss this possibility, I also like to be open to explanations that go beyond the hard-coding of phenotypic traits.
There are various ‘soft-coded’ ways in which the actual translation into phenotypes of the genetic code in DNA may be blocked or facilitated (Fig. 4). In the cellular epigenetic scenario, the presence or absence of methyl, acetyl or phosphate groups to the DNA or histones can transiently or permanently modify gene expression by altering the local chromatin structure (Johannes et al. 2008; Law and Jacobsen 2009). These cellular epigenetic changes can be induced by environmental and/or genetic factors, and can remain relatively stable over several generations (Teixeira et al. 2009; Reinders et al. 2009). Phenotype can also be changed by mechanisms other than (epi)genome variation (Fig. 4). For example, the physiology of a bird embryo can also be affected by variable levels of maternal hormones, immune factors, etc.; developmental inheritance). Also, young birds can learn the migration route from more experienced birds (behavioural inheritance).
Research opportunities in the anthropocene
We live in the Anthropocene (Steffen et al. 2007; Kolbert 2011), an era of loss and of opportunity. Our time is one of strong declines in biodiversity (Pereira et al. 2010), yet it is also a time when we can learn from the rapid ongoing changes if we care to look. ‘Looking carefully’ at global scales is now made possible by the close connections (through internet and cheap airfares) of focussed observers worldwide (e.g. Rakhimberdiev et al. 2011), the availability of capture–recapture datasets spanning several generations of birds (van der Jeugd et al. 2009), and by the possibility to examine survival and dispersal using an amazing array of remote sensing and tracking tools (Robinson et al. 2010). Even for small organisms that travel the world, such as Red Knots, the tools are now available to track their survival and movements across the globe with considerable spatial and temporal precision.
Building on the ongoing demographic studies assembled within a worldwide collaborative research network with a particular emphasis on individually colour-ringing Red Knots and other shorebirds and resighting them (Global Flyway Network; see Piersma 2007), comparative analyses can be done of dispersal and survival based on sample sizes of several thousands individually marked Red Knots and multiple resightings of many of these individuals (e.g. Baker et al. 2004; Leyrer et al. 2011). Recent advances in multistate modelling (Lebreton et al. 2009) should enable the estimation of variation in survival rates over several generations within and between flyways and also obtain estimates of dispersal rates. In the case of the best studied Red Knot subspecies rufa, islandica and canutus, this would yield estimates of vital rates and dispersal during periods of habitat decline and restoration (Baker et al. 2004; Kraan et al. 2009) and thus enable us to examine the demographic drivers of micro-evolution.
Recent observations suggest that individual Red Knots (rufa as well as islandica) may sometimes switch flyways. This indicates that individual flexibility of flyway use is higher than we have assumed so far. An adult Red Knot marked in northern Norway (presumably islandica) was resighted a year later in the Mingan Archipelago, in the Gulf of St. Lawrence, Canada. Another bird marked at Mingan (presumably rufa) had moved to Iceland 2 years after marking (Wilson et al. 2010). Given the highly sociable behaviour of this species, individuals may be tuned by the behaviour of group members (Helm et al. 2006), enabling them to successfully switch between flyways.
I suggest that we could find out about the degrees of flexibility, and the epigenetic mechanisms involved (Fig. 4), by carrying out two types of phenotypic experiments: (1) bring birds together in common environments (common garden experiments), and (2) displacing birds to new conditions (translocation experiments). Building on preliminary results reported in Fig. 3, bringing birds from different flyways under identical environmental conditions would allow us to examine the degree that the seasonally changing phenotypes are (in-)flexible for genetic and non-genetic reasons, or whether the birds can adjust to alternative photoperiodic regimes. If the degree of adjustment is age-dependent, this would additionally tell us if social learning is involved.
Predictions to what will happen to birds translocated across the Atlantic from either the islandica or rufa subspecies of Red Knot, depending on how migration and annual routines are flexibly orchestrated, and whether learning from older birds takes place during early life
islandica moved to Canada
rufa moved to Wadden Sea
Fixed circannual programme 1
Stay in the north (as in Europe)
Stay in the north (as in Europe)
Fly south with canutus
Fly south with canutus
Fixed circannual programme 2
Return to Europe
Return to Europe
Return to the Americas
Returns to the Americas
Flexible during early ontogeny
Migrate to S. America
Stay north or return to Europe
Stay in Europe or fly south with canutus
Fly south with canutus or return to America
Migrate to S. America
Migrate to S. America
Stay in Europe or fly south with canutus
Stay in Europe or fly south with canutus
Thus, by including young and older birds in the experiment, we can address the role of age-dependent behavioural inheritance, i.e. learning, on the expectation that, in their first year, learning phenotypes would still be able to adjust phenotypically to new regimes, whereas older birds might not (Sutherland 1998). Fully flexible individuals will move with their new flocks, and even adjust the timing of fuelling and moult. If this were to occur, we have to explain an interesting contrast with the results from the preliminary aviary experiments (Fig. 3). Such flexibility would be at variance with the idea that the divergent traits have a rigid genetic background and would emphasize the creative force of two-way interactions between organisms and their environment (Laland et al. 2008).
The ideas presented here developed slowly over many years during discussions with Allan J. Baker, Ritsert Jansen, Deborah M. Buehler, Yvonne I. Verkuil, Franjo Weissing, David W. Winkler, Christiaan Both, Robert E. Gill, T. Lee Tibbitts, Bob Ricklefs, Hugh Boyd, Eva Jablonka, Barbara Helm, Andreas Helbig, Robert Zink, and many others. I am grateful to Franz Bairlein for providing the opportunity to present these ideas at the Centennial Symposium of the Vogelwarte Helgoland, and for his patient prompting to actually get a manuscript. Maarten Brugge was responsible for the husbandry of, and data collection on, the captive Red Knots, Dick Visser drew the figures, and Yvonne Verkuil and Eldar Rakhimberdiev provided constructive comments on the manuscript.
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