Evolution in action: the polymorphism paradox

05 February 2016

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Darwin’s theory of evolution underpins all of biology from the intricate workings of a cell to the fascinating behaviour of the honeybee’s dance. Everywhere you look in nature, specialist traits brought about by evolution via natural selection can be seen ranging from the bizarre to the beautiful. All have been selected for their success in aiding the organism’s survival or reproductive efforts and all have been passed on to the next generations. The concept of evolution can be summed up by a phrase first coined by Herbert Spencer and used by Darwin in later editions of ‘On the Origin of Species’ namely ‘survival of the fittest’.

Survival of the fittest means that those traits that are a hindrance to survival or reproduction are less likely to be passed on to the next generation, as the organism in question is less likely to survive or reproduce. Conversely, those characters that increase the chances of survival or reproduction are more likely to be passed on, as the organism is more likely to survive and reproduce. Over time the more successful traits that convey the better fitness benefits prevail and evolution occurs. For example, in a population of rats those that are resistant to rat poison will be more likely to survive and pass on their genes to the next generation than those that are susceptible.

Evolution itself is dependent on chance alone, because it is chance that leads to mutations, which are the basis of variation of fitness within a population. However the process of natural selection is far from random as it is only the most successful traits that will persist within a particular environment. Whilst the concept of ‘survival of the fittest’ is accepted by biologists, a thoughtful consideration of some populations of organisms could lead you to think otherwise.

One such organism challenging the foundations of evolution comes in the form of a leisurely, slightly slimy and unassuming character that you might find eating the plants at the bottom of your garden: the banded land snail, Cepaea nemoralis. It lives throughout Europe in habitats ranging from hedgerows and woodland to meadows and garden shrubs. Banded snails come in three different colours, pink, yellow and brown. They also vary in the distinctive banding patterns on their shells.

The different combinations of colours and patterns (also called morphs) are found together in many different areas of the continent, although their relative frequencies vary. After a quick snail hunt in your garden, you’d be sure to find at least two of the colours and banding patterns if not all three. However, the presence of these different morphs (polymorphisms) presents a problem to the concept of survival of the fittest.

Why does the banded snail have so many different colours and forms? Shouldn’t one colour be more advantageous than another and therefore be selected for over time? Or another colour which doesn’t confer as successful an advantage for survival and reproduction be selected against and disappear over time? Why do we still have three different colours and three further different banding patterns rather than just one morph that is the most successful in aiding survival and reproduction?

These questions are not limited to snails. In fact, there are many other instances of polymorphisms in nature, including the presence of different blood types in humans, and differences in flower structures in primroses. We also know that the banded snail polymorphism has been around for some time, because different banding morphs have been found dating back to 12,000 years ago, so this particular polymorphism is not a short-term transitional phase.

One theory to explain why there are so many different snail morphs is that the different morphs occupy different habitats. Scientists have found that dark forms of the snail (the pink and brown) are more commonly found in woodland, whereas the yellow five banded form is more common in grassland. So the different colours may act as camouflage and persist due to their selective advantage in different habitats. This is an especially important defense against the banded snail’s number one enemy, the song thrush. Better camouflaged snails would be less likely to be spotted by the thrush and more likely to survive and pass on their genes.

Camouflage is probably part of the reason for the polymorphism but not the whole answer because different morphs are found within the same habitats and other areas have very different habitat types but very similar morph types. So others have suggested advantages of different morphs with regards to temperature regulation. It was found that there were more dark forms in northern regions whereas locations further south boasted more of the lighter yellow form. Perhaps being darker allows the snails to gain more heat faster in the colder more northern regions providing a physiological advantage.

Indeed scientists have found a correlation between temperature and colour in an Icelandic population of C. hortensis (a close relative of C. nemoralis) with no song thrush predators, indicating temperature may well be a factor contributing to the persistence of the many morphs. It is unlikely that temperature alone explains the polymorphism as there is such compelling evidence for some kind of camouflage selection pressure.

Some scientists claim that the polymorphism has been maintained by a process called apostatic selection. Apostatic selection is best explained using an example: the more common morphs of the banded snail are more easily spotted simply because they are the most common. After initial catching success with the most common morph the song thrushes form a ‘search image’ of that particular banding pattern. This morph then decreases in frequency as it is more likely to get caught by the song thrush and different morphs survive to reproduce. Morphs other than that which was most common increase in proportion and another morph takes the place of most common. The song thrush ‘search image’ now forms with the different more common morph which in turn declines leaving room for a different morph to increase in number and so on.

Apostatic selection has been shown in the lab but evidence is much harder to collect in the field. The relative importance of each of these selection pressures is debated by scientists and it is likely that each factor has a role to play in maintaining the polymorphism. A further consideration of the complexities of the selection pressures acting on these snails comes from the fact that the distribution of morphs within C. nemoralis and C. hortensis populations in the same area differ significantly despite these two species of snail looking almost identical.

The answer as to which of the selection pressures is the strongest is far from clear however a team of scientists are trying to find out and they need your help. A Europe-wide project run by Evolution Megalab hopes that by tracking the proportion of morphs of Cepaea nemoralis and its close relative Cepaea hortensis over the last 30 years or so they can look at any changes that have occurred and perhaps get one step closer. Differences in morph frequency may have occurred due to changes in climate or could be affected by the decline in song thrushes that has been seen in some parts of Europe. It is a huge undertaking and the organisers are calling on members of the general public like you and me to help them out. You can take part in the survey of these snails yourself and help investigate evolution first hand. Go to the website cited below to find out more. It’s a chance to take part in a real scientific enquiry and watch evolution in action from your own back yard. What are you waiting for?

Bengtson SA, Nilsson A, Nordstrom S, Rundgren S. 1979. Distribution patterns of morph frequencies in the snail Cepaea hortensis in Iceland. Holarctic Ecology 3: 144‐149
Cain AJ, Sheppard PM. 1952. The effects of natural selection on body colour in the land snail Cepaea nemoralis. Heredity 217‐231
Jones JS, Leith BH, Rawlilngs P. 1977. Polymorphism in Cepaea: a problem with too many solutions? Annual Review of Ecology and Systematics. 8: 109‐143

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