IGCSE Biology 0610: Species, Binomial Names and Constructing a Dichotomous Key

A wildlife club records the great pond snail, which has the binomial name Lymnaea stagnalis, during a stream survey.

(a) State what is meant by the term "species". [1]

(b) State the genus given in the name Lymnaea stagnalis, and describe why the binomial system is useful to scientists working in different countries. [2]

(c) Later in the survey, the club finds three unidentified freshwater snails, labelled snail 1, snail 2 and snail 3, and records the following features before returning each snail to the stream.

Snail 1: shell coils in a flat spiral; shell has no dark bands
Snail 2: shell coils to a pointed spire; shell has no dark bands
Snail 3: shell coils to a pointed spire; shell has dark bands

Using only the features given, construct a dichotomous key that could be used to identify snail 1, snail 2 and snail 3. [3]

Part (a): Defining a species

A species is a group of organisms that share enough features to be able to reproduce (breed) together and produce offspring that are themselves fertile, meaning those offspring can go on to reproduce as adults.

Part (b): The binomial name and system

In “Lymnaea stagnalis”, the first word, Lymnaea, is the genus, and the second word, “stagnalis”, is the species.

The binomial system is an internationally agreed naming system, so every scientist, wherever they are in the world and whatever language they speak, uses and recognises the same two-part scientific name for an organism. This avoids the confusion that would arise from relying on local common names, since the same snail might be called something different in every country, or even in different regions of the same country.

Part (c): Constructing the dichotomous key

A dichotomous key works through a series of numbered statements, each offering two contrasting (opposite) choices based on an observable feature, until only one named organism remains.

Comparing the three snails, shell shape separates snail 1 from the other two, and the presence of dark bands then separates snail 2 from snail 3:

(a) Shell coils in a flat spiral → snail 1 (b) Shell coils to a pointed spire → go to statement 2
(a) Shell has no dark bands → snail 2 (b) Shell has dark bands → snail 3

Checking the key against the data confirms it works: snail 1 is picked out immediately at statement 1 by its flat spiral shell. Snails 2 and 3 both have pointed spire shells, so they both move on to statement 2, where the presence or absence of dark bands correctly separates them.

Final answers

(a) A species is a group of organisms that can reproduce to produce fertile offspring.
(b) Genus = Lymnaea; the binomial system lets scientists everywhere use and understand the same scientific name for an organism.
(c) See the two-statement dichotomous key above, which correctly identifies snail 1, snail 2 and snail 3.