The process of natural selection is one of the most cited reasons for the evolution of a species, and it was made famous by Charles Darwin’s famous observations of finches in the Galapagos Islands. It is believed to work by way of random mutations; random mutations occur, as it indicates, randomly and spontaneously in a population for a multitude of genetic reasons. Mutations help to create variation of different traits within a species, and they can be expressed with different phenotypes.
In times where a mutation has no negative effect on the individual organism, this trait could be advantageous in a new habitat or predatory situation, etc. With an advantage in survival, and thus reproduction, these organisms are often selected for over other members of the species, creating a change in variation of a species over generations. Molluscs, although a very large and diverse group of animals, often share certain features: a mantle, radula, shell, and foot. The mantle is a thin and fleshy layer which secretes the hard shell of a mollusc. The radula is a grate-like structure in the mouth used to scrape surfaces and drill holes.
The foot is a muscle which assists in locomotion and movement of the molluscs. It is hypothesized that all current-day molluscs share a common ancestor, called the hypothetical ancestral mollusc or HAM, because of the similar characteristics and body plans that have been modified in diverse ways over time to adapt to different environments. Two members of the mollusc family include the clam, of the class bivalvia, and the squid, of the class cephalopoda, and these are an example of variation from the HAM that helped them adapt for their particular environment, feeding behavior, and movement (Sigwart, 2007).
The squid species has undergone many adaptations. The squid’s radula resembles a beak-like structure that it uses to devour its food, very different from the chiton-like HAM which probably had a small radula. Over time, the radula’s shape has been mutated and then selected for as the squid changed its eating habits – if it provided quicker eating time, then it would be selected for. The squid lacks an outer shell (it’s internalized), so the mantle of the squid has also been modified into muscular “flaps” that it uses to propel itself quickly through the water, nd thus capture prey or avoid predators, a necessary function due to the squid’s lack of a hard protective barrier (Sigwart, 2007) A large shell would be unnecessary for the squid since it can propel quickly away from predators, and may even hinder its ability to swim; this would lead one to believe that the squid developed a muscular mantle from HAM and then a mutation for loss of the shell was selected for following the muscular body.
Also, the foot of the squid has adapted from the single foot of the HAM to become tentacles and arms, used to quickly capture and bring prey closer to its beak-like radula to eat, which is buried behind its many arms. This modified version of its foot is an extremely effective feeding method that, coupled with the squid’s speed, enables it to catch and hold prey that otherwise might be able to escape (Sutton, 2007). The clam is a sedentary mollusc that uses its foot to burrow into the sand.
The clam is a filter feeder, straining food particles from the water. The clam doesn’t have a radula, probably the result of a random mutation that caused some clam ancestors to lack the structure, or it may have been modified into another structure that aided in the filter feeding process. The filter feeding process is environmentally selected for because it enables the animal to take in food particles while buried under the sand. The mantle of the clam is found inside of its hard, hinged shell.
The strong shell of the clam was environmentally selected for since it prevents predators from easily eating the clam, allows it to bury safely into the sand without affecting its internal organs, and improves the clams ability to survive and reproduce. The foot of the clam has adapted so that the species can dig into the sand with this muscle which can be moved out of the shell, a very different type of foot than the HAM’s muscular, positioned foot which allows for scavenging (Sutton, 2007). The HAM, very similar to the chiton, served as a good base model for this very diverse phylum of animals. Lots of spontaneous and random mutations must have occurred for such diversity – to have sedentary, burrowing animals and predatory, jet-propelling animals in the same phylum of animals is quite a feat.