Order, Class, Phylum, and Kingdom — Climbing to the Top

You have already explored the three ranks closest to the base of the classification ladder: species, genus, and family. Each step up widened the group and reduced the number of features all members share. Now it is time to keep climbing. The four remaining ranks — order, class, phylum (or division in plants), and kingdom — take you all the way to the broadest possible grouping of life on Earth. As you will see, the pattern of widening groups and shrinking common ground continues at every step.

Order — Families That Still Share Something in Common

At the species, genus, and family levels, groupings lean heavily on a detailed comparison of characters. Once you reach the level of order, the approach shifts slightly. Because an order sits higher in the hierarchy and includes a wider range of organisms, it is identified based on aggregates of characters rather than a single defining trait. An order is a collection of families that still exhibit a few similar characters, though the similarities are fewer compared to what you would find among genera within the same family.

Here are two concrete examples:

• Plants: The families Convolvulaceae (the morning glory family) and Solanaceae (the potato and nightshade family) look quite different from each other in many respects. Yet when botanists examined their flowers, they found enough shared floral characters to place both families in the same order, Polymoniales.
• Animals: The animal order Carnivora brings together families of meat-eating mammals. It includes Felidae (cats, lions, tigers) and Canidae (dogs, wolves, foxes). These two families differ in many ways, but they share a set of carnivore-specific features that justify grouping them under one order.

The key takeaway here is that orders work with broader strokes than families do. The families within an order are related, but the connection is based on fewer shared features than what ties genera together within a family.

Class — Orders That Belong Together

Move one more rung up the ladder and you reach class. A class groups together related orders. The organisms within a class share certain overarching features, but the diversity is now much wider than at the order level.

Consider the class Mammalia (mammals). It contains several orders, including:

• Primata — monkeys, gorillas, gibbons, and humans
• Carnivora — tigers, cats, dogs, bears

Primates and carnivores are clearly very different animals. A gorilla and a dog do not look or behave alike in most ways. Yet they share the defining features of mammals: they are warm-blooded, have body hair, and females nourish their young with milk. Those shared mammalian characters are what hold the class together, even though the individual orders within it are quite distinct from one another.

Mammalia includes many other orders beyond Primata and Carnivora. Each order brings its own set of unique characteristics, but all of them fit within the broader umbrella of mammalian features.

Phylum (Animals) and Division (Plants) — Uniting Entire Classes

One level above class sits phylum in the animal kingdom, or division in the plant kingdom. Both terms describe the same rank; the only difference is that zoologists say “phylum” while botanists say “division.”

A phylum brings together several related classes. Take the phylum Chordata as an example. It includes the classes of:

• Fishes
• Amphibians
• Reptiles
• Birds
• Mammals

These classes are strikingly different from one another. A fish lives underwater and breathes through gills, while a bird flies through the air and breathes with lungs. A reptile is cold-blooded and covered in scales, while a mammal is warm-blooded and covered in hair. Despite all these differences, every one of these animals possesses two shared features at some stage of its development:

1. A notochord (a flexible, rod-shaped support structure that runs along the body)
2. A dorsal hollow neural system (a hollow nerve cord running along the back)

These two characters are the thread that ties all chordates together. They are broad, fundamental features, which is exactly what you would expect at such a high level of the hierarchy, where the grouping is very wide and the organisms are extremely diverse.

On the plant side, the same logic applies. Classes of plants that share a few broad characters are grouped into a division.

Kingdom — The Broadest Category of All

At the very top of the taxonomic ladder sits the kingdom. This is the most inclusive category. Every organism that exists belongs to a kingdom.

• Kingdom Animalia contains all animals, from every phylum. Whether it is a jellyfish, an insect, a fish, or a human, if it is an animal, it falls under this kingdom.
• Kingdom Plantae contains all plants, from every division. Whether it is a moss, a fern, a pine tree, or a flowering shrub, it belongs here.

The kingdom is the endpoint of the classification journey when you are moving upward. It holds the widest possible collection of organisms, united by only the most fundamental characteristics.

The Big Pattern — Fewer Shared Features as You Climb

Now step back and look at the complete hierarchy from species at the bottom to kingdom at the top. A clear pattern emerges:

Direction	What happens to shared characteristics	What happens to group size
Moving upward (species to kingdom)	The number of common characteristics decreases	The group becomes larger and more diverse
Moving downward (kingdom to species)	The number of common characteristics increases	The group becomes smaller and more specific

At the species level, organisms share the greatest number of features. By the time you reach kingdom, the only things all members have in common are a handful of very broad characteristics. This is why classification becomes progressively more difficult as you move to higher ranks: the organisms are more diverse, the shared features are fewer, and figuring out how different groups at the same level relate to one another becomes a bigger challenge.

Sub-Categories — Filling the Gaps

The seven main categories (kingdom, phylum, class, order, family, genus, species) provide a solid framework, but they are broad steps. To allow more precise and scientifically sound placement of organisms, taxonomists have developed sub-categories within this hierarchy. Examples include sub-order, sub-class, super-family, and so on. These sit between the main ranks and let biologists make finer distinctions without breaking the overall structure.