CXC CSEC Biology

Grouping organisms by what you can see

Living things in a habitat such as a school garden, a wall or a small pond can be sorted into groups by their visible features. Plants are compared by leaf shape, the pattern of veins (venation), the colour and number of flower parts, and whether they are hairy or smooth. Animals are compared by the number of legs and wings, by body colour, and by whether the body is divided into segments. Common names are enough for this kind of grouping, so a student can sort ants, beetles, spiders and millipedes without knowing their scientific names.

The steps are the same each time.

1. Observe each organism and record its features in a table.
2. Pick one clear difference (for example, six legs or eight) and split the organisms into two sets on it.
3. Split each set again on another difference, and continue until every organism is on its own.

Written out as a flowchart of yes or no questions, this becomes a key. A key only works if every question has a clear answer that depends on a feature that can be seen, so "is it large?" is a poor question and "does it have wings?" is a good one. Drawings of each organism, with the features labelled, go beside the table.

The five kingdoms

Classification is the sorting of organisms into groups based on their shared features. Every living organism is placed in one of five kingdoms.

The Protoctista are the group that is left when plants, animals and fungi are taken out, so it holds a mixture of organisms that are only loosely related. Amoeba takes in food like an animal but is a single cell.

Dividing the animal kingdom

The animal kingdom is divided into phyla (singular phylum). Two are met most often in the Caribbean. Arthropoda have a jointed exoskeleton and jointed legs, and include insects, spiders, crabs and millipedes. Chordata have a nerve cord running along the back, and their best-known members are the vertebrates.

The vertebrates, the animals with a backbone, are divided into classes: fish, amphibians, reptiles, birds and mammals. Insects are a class within the Arthropoda, with a body in three parts, six legs and usually wings. Each class is then divided into smaller and smaller groups, so a full classification runs from the largest group to the smallest:

The last of these, the species, is the group of organisms that are so similar that they can breed together and produce fertile offspring. The scientific name of an organism has two parts, its genus followed by its species, and is written in italics, for example Herpestes auropunctatus for the small Indian mongoose.

How modern classification works

Older classification depended on how organisms look, and that can mislead, because unrelated organisms can come to look alike when they live in the same conditions. Modern classification compares the sequence of bases in the DNA of different organisms. The more alike two sequences are, the more recently the two organisms shared a common ancestor, so organisms with very similar DNA are placed close together and those with very different DNA are placed far apart. This method sometimes moves an organism into a different group from the one its appearance suggested.