What is evolution?
Evolution is change over generations in the inherited characteristics found in a population. Evolution is a slow and gradual process, but in the 3,500,000,000 years since life began on Earth, there has been plenty of time for it to produce the great diversity of life we see today from the ancient common ancestor shared by all living species.
What is an adaptation?
An adaptation is an inherited characteristic that improves the chance that an organism will survive and reproduce in particular circumstances or in a particular environment. For example, there is usually a higher frequency of darker shells in Cepaea populations living in shaded habitats such as woodlands than in open habitats such as grasslands. This is thought to be because shell colours that match the background (darker in shaded habitats) provide better camouflage against birds that eat the snails. Dark shell colour in shaded habitats is therefore an adaptation to these habitats and light shell colour is adaptive in open habitats.
What is natural selection?
Natural selection is the mechanism by which evolution produces adaptation. It is the process in which inherited characteristics that improve the chances of an individual leaving offspring increase over the generations at the expense of other, less advantageous characteristics.
What is the founder effect?
Some populations are founded (started) by a small number of individuals whose descendants then all share the inherited characteristics of these few ancestors. Because Cepaea are hermaphrodites, even one snail can found a new population if it has mated before arriving in a new location. Although many Cepaea populations are polymorphic, a few may contain only one or two morphs due to the founder effect. This is particularly seen outside the native range of Cepaea, such as in populations that have been introduced in N. America.
Why are Cepaea used for evolutionary study?
Cepaea are very good model organisms for the study of evolution in the wild because they 'carry their genes upon their backs'. More exactly, you can record the genetic composition of a population by looking at shell characteristics. These characteristics are adaptive and variable in Cepaea, so it is possible to look for the effects of natural selection. Because of these advantages, there is a large dataset of historical information on Cepaea populations that we are now able to use as a baseline for the Evolution MegaLab investigation.
Why are populations of Cepaea polymorphic?
This is an excellent question! (Which is a scientist's way of saying 'we don't exactly know'). The likeliest answer is that shell polymorphisms are subject to a variety of different sources of natural selection (e.g. birds and climate) and that these vary on quite a local scale, probably even within habitats.
What is the genetic basis of Cepaea shell polymorphisms?
It's a bit complicated! The simplified version is as follows: The colour of the shell is controlled by a genetic locus called C, which has alleles for brown, pink or yellow shells in that order of dominance (brown > pink > yellow). Another locus called B has two alleles that control the presence/absence of bands (absence dominant to presence). The C and B loci are linked (on the same chromosome).
A third locus called U, which is not linked to C or B, has two alleles: mid banded and 5-banded. The former is dominant to the latter.
More details of the genetics are given in a review specially written for the Evolution MegaLab by Prof. Laurence Cook (link below).