Alternative Phenotypes

 

Fig. 1.To demonstrate alternative phenotypic variation within a single species, a selection of  photographs of the European adder (Vipera berus) is provided. The examples are  taken mainly from the internet At bottom left is an example from my own garden.

 As shown by Mary Jane West-Eberhard in her game changing book, the role of alternative phenotypes in adaptive evolution appears underappreciated. To quote her  - ‘Alternative phenotypes are a necessary phase in the evolution of every novel discreet trait‘(West-Eberhard, 2003), p30. Indeed, alternative phenotypes or polymorphisms as they are commonly called provide the variability without which natural selection is blind. A beautifully example is provided by the European adder (Fig. 1). There are many other examples, of course, including the famous case of the land snail Cepea (Ford, 1971). Often the adaptive benefit of alternative forms is thought to provide crypsis by environmental matching against visual predators such as birds. Here individual with the best background match gain in fitness, variation being maintained by diversity of backgrounds. The melanic form shown at bottom right seems to be an exception. There is evidence of adaptive temperature regulation, rather that background matching in that case (Forsman, 1995).

 

      In my personal research I have studied a case quite different from the background matching considered above. That study involved morphological and behavioural alternative phenotypes in both larval and adult parts of the life cycle of the common chironomid midge. Size polymorphism in the larvae, expressed as condition sensitive alternative phenotypes within an Evolutionarily Stable Strategy (ESS), has been considered earlier (McLachlan, 1989), and will not be pursue further here. I consider the adult stage of the life cycle in two papers (McLachlan, 2014, 2018), and will expand on that here. The story starts with the observation that male sizes in mating swarms show a discontinuous distribution. One mode contains large male called α males, and the other composed of small males called γ males. The important point is that the two phenotypes adopt quite different mating behaviours. Only the α males create the swarms which attracts females. The γ males do not take part in swarm formation but, on arrival, rest in the grass under the swarm. Arriving females rest there too before entering the swarm and this provides γ males with the opportunity to acquire a mate. The tactic of γ males is rather like the ‘sneak tactic’ of many animals considered by Krebs and Davies (Krebs & Davies, 1981). The mating behaviour of these midges has been elaborated by my friend and colleague John Lazarus in an evolutionary game (McLachlan, 2014). His reasoning depends on the male phenotypes being condition dependent (size dependent), within an ESS (Maynard Smith, 1982). This approach provides an idea of the level of understanding that can be achieved by the application of ESS theory.

 References

 

Ford, E. B. (1971). Ecological Genetics. London: Chapman & Hall.

Forsman, A. (1995). Heating rates and body temperature variation in melanistic and zigzag Vipra bersus: does colour make a difference? Annales Zoologici Fennice., 32, 365-374.

Krebs, J. R., & Davies, N. B. (1981). An Introduction to Behavioural Ecology (3 ed.). London: Blackwell Scientific Publications.

Maynard Smith, J. (1982). Evolution and the Theory of Games. Cambridge, UK: Cambridge University Press.

McLachlan, A. J. (1989). Animal populations at extreme densities: size dimorphism by frequency dependent selection in ephemeral habitats. Functional Ecology, 3, 633-643.

McLachlan, A. J. (2014). Phenotype limited male mating tactics among some non-biting midges. (pp. http://www.co.uk/atholmclachlan.blogspot.co.uk): Google.

McLachlan, A. J. (2018). http://www.google.co.uk/atholmclachlan.blogspot.co.uk.

West-Eberhard, M. J. (2003). Developmental Plasticity and Evolution. Oxford: Oxford University  Press.