The Open Door Web Site
Part XIX : Colonizing and Populating Habitats :
Shells of the banded snail are very variable. They have bands of different thickness and some are completely brown or completely yellow.
COLONIZING AND POPULATING HABITATS
Sexual Reproduction in Populations of Wild Animals and Plants
Amongst the populations of wild animals and plants sexual reproduction also produces offspring which are varied. This is important because it can help them to survive in an environment that is always changing. This will become clearer if we study an example.
A banded snail
The banded snail is a common snail which is found all over Europe. The shell of the banded snail may be of different colours: yellow with brown bands around it, all yellow, all pink or all brown. This snail lives in woodland, open grassland and in hedges. Biologists know that the colours of the snail's shell are inherited from its parents. Also amongst a population of snails one may find all the different types of shell.
In different habitats some types of shell colour are more common than others. In open grassland you will find more yellow-shelled snails. In woodland there are more brown-shelled and pink-shelled snails. Amongst the hedgerows the snails often have banded shells.
Biologists think that one of the reasons for this difference in the numbers of shell colours is camouflage.
Stone used by thrush to break shells
These snails are eaten by birds, rats and mice. The yellow-shelled snails are more difficult to see in grassland but they are very easy to see amongst the dead leaves of a forest. The brown-shelled snails are less easy to see in woodland but they will stand out in grassland. The predators of these snails will eat those snails which are easy to find.
The snails that can hide are the survivors. Because of their good camouflage, these snails are being selected to breed the next generation of snails. After a few years there will be mainly one colour.
What would happen if the environment changed? For example, if a forest started to grow in grassland? The snail population would survive because there are always a few brown-shelled snails amongst the yellow-shelled snails. The numbers of yellow-shelled snails would decrease because they would be more easily found by the predators. The number of brown-shelled snails would increase because they could hide more easily.
You can see from this example that it is very important that a population of a species shows some variation. This is possible with sexual reproduction. Asexual reproduction produces offspring which are identical. If in this example all the snails had been yellow when their environment changed they would all have been eaten. A species of living organism which shows no variation may eventually become extinct.
The Open Door Web Site is non-profit making. Your donations help towards the cost of maintaining this free service on-line.
Donate to the Open Door Web Site using PayPal