history and relatedness. Lions and tigers look like each other more than they look like bears.
But it’s not just appearance. The two cats are actually more closely related to each other
than to bears. How related organisms are is an important basis for classifying them.
Classifying Organisms
People have been concerned with classifying organisms back to the time of the Greeks and
Romans. The Greek philosopher Aristotle developed a classification system that divided
living things into several groups that we still use today, including mammals, insects, and
reptiles. Carl Linnaeus (1707-1778) (Figure2.20) built on Aristotle’s work to produce his
own extensive classification system and invented the way we name organisms by their genus
and species. For example, a coyote’s species name isCanis latrans. ”Latrans” is thespecies
and ”canis” is thegenus, a larger group that includes dogs, wolves, and other dog-like
animals. Linnaeus is considered the inventor of moderntaxonomy,the science of naming
and grouping organisms. He was especially interested in plants, and he used differences in
flowers to classify each plant into groups. Modern taxonomists have reordered many groups
of organisms since Linnaeus. The main categories biologists use are listed here from the most
specific to the broadest category (Figure2.21). In other words, there are many species in
each genus, many genera (plural for ”genus”) in each family, and so on. The broadest and
most inclusive category is the domain. It is currently believed that there are three domains
and six kingdoms. We will discuss these groups more later.
But how do taxonomists decide what domain or family an organism belongs to? Like Lin-
naeus, they still look at the physical features of the organisms and group organisms that
look similar together (Figure2.22). But taxonomists also try to piece together evolutionary
relationships when assigning organisms to a specific group. By looking atfossils,ancient
remains of living things, they can tell if organisms share a recent common ancestor–sort of
like a ”grandparent” species. A common ancestor is an ancestor shared by two groups of or-
ganisms. For example lions and tigers share a common ancestor; both species are descended
from an ancient cat. If two species share a recent common ancestor, it means they are closely
related and they will be placed in the same group.
Another way to determine evolutionary relationships is by looking for similarities or differ-
ences in organisms’DNA. The number of differences in two organisms’ DNA can show how
closely related the two organisms are. You might expect, for example, that human DNA
is more similar to chimpanzee DNA than to bacterial DNA. (And it is.) How biologists
determine evolutionary history will be discussed in more detail in theEvolutionchapter.
Naming Organisms
Carl Linnaeus recognized a need for a system of names for each species. If we just used
common names, we would have many different names in many different languages for the