Voyagers residents and sleepers

There are two lasting bequests we can give our children. One is roots.

The other is wings.

Hodding Carter Jr

One of the most important developments in evolutionary biology came in 1962 with the publication of'Animal dispersion in relation to social behaviour' by Vero Wynne-Edwards. This was probably the most controversial biology book of its generation. In it, Wynne-Edwards proposed that animal populations were regulated by dispersal of surplus, less fit, individuals, which prevented overpopulation. Wynne-Edwards believed this behaviour to have evolved through differential survival and extinction of populations; those groups of individuals without this benevolent behaviour went extinct, leaving the remaining habitat to become populated by those that had (Figure 6.1). This process, group selection, ran counter to the Darwinian notion of selection between individuals, and was robustly rejected by the academic community. The principal legacy of the book was to focus attention on the mechanisms of natural selection and the evolution of social behaviour. The ideas that followed laid the foundations of modern evolutionary ecology.

Though Wynne-Edwards's ideas on group selection rightly never gained acceptance, the questions on which his work was based remain as some of the most challenging in evolutionary biology. Many animal and plant species display risky and costly dispersal behaviour. Having been born in a habitat that was obviously suitable for their parents, they undertake movements to new breeding habitats, exposing themselves to predators and unsuitable environments, often at great energetic cost, and with no guarantee of success. How could such behaviour evolve? In particular, can we explain it in the standard Darwinian context of selection on individuals, rather than invoking group selection? The issue is, as Wynne-Edwards appreciated, a very important one. Dispersal is the process that binds the populations of a species together. It can not only regulate the dynamics of populations, but also their genetic differentiation, and coexistence and evolution with other species

No dispersal

High dispersal

High dispersal, one non-dispersing mutant

No dispersal

High dispersal

High dispersal, one non-dispersing mutant

Fig. 6.1 Wynne-Edwards's view of the evolution of dispersal by group selection, and why it does not work. In the left column is a population with no dispersers (a), which becomes overpopulated (b) and goes extinct (c). A population with high dispersal propensity (middle column) would thus survive, even though the dispersers have very low survival chances. This was Wynne-Edwards's argument. However (right column), such populations are susceptible to cheats (open circle), with a low dispersal propensity. Cheats do not pay the cost of dispersal but gain the benefits from the rest of the population, and hence increase in frequency.

Fig. 6.1 Wynne-Edwards's view of the evolution of dispersal by group selection, and why it does not work. In the left column is a population with no dispersers (a), which becomes overpopulated (b) and goes extinct (c). A population with high dispersal propensity (middle column) would thus survive, even though the dispersers have very low survival chances. This was Wynne-Edwards's argument. However (right column), such populations are susceptible to cheats (open circle), with a low dispersal propensity. Cheats do not pay the cost of dispersal but gain the benefits from the rest of the population, and hence increase in frequency.

(see Chapter 11). Understanding why it evolves might unlock our understanding of a whole gamut of evolutionary and ecological phenomena.

In addition to dispersal via their seeds, plants display an equally puzzling but related phenomenon: seed dormancy (delayed germination). There are two obvious costs to dormancy. First, there is some year-to-year mortality of dormant seeds, so the total number of germinating seeds is reduced. Second, in increasing populations there is selection for early reproduction, giving more descendents by virtue of more generation cycles. Dormancy delays reproduction. Hence, it is another costly and curious phenomenon. And seed dormancy, like dispersal, is a population parameter that can affect dynamics and coexistence. Rather nicely, but with hindsight not surprisingly, the evolutionary solutions to dormancy and dispersal are intricately related at a theoretical level, and, in plants at least, can no longer be considered in isolation.

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