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Biology 14.2 How Biologists Classify Organisms
What is a species?
Have you ever wondered how scientists
tell one species from another. Many
species look identical but have invisible
differences that set them apart.
In 1942, biologist Ernst Mayr proposed
the biological species concept.
Mayr defined a biological species as a
group of natural populations that are
interbreeding or that could
interbreed, and that are
reproductively isolated from other
similar groups.
The biological species concept
defines a species as members of
populations that actually or potentially
interbreed in nature, not according to
similarity of appearance. Although
appearance is helpful in identifying
species, it does not define species.
Appearance isn’t everything
Organisms may appear to be alike
and be different species. For example,
Western meadowlarks (Sturnella
neglecta) and Eastern meadowlarks
(Sturnella magna) look almost
identical to one another, yet do not
interbreed with each other—thus, they
are separate species according to this
What is a species?
Reproductive isolation occurs when a
barrier separates two or more groups
of organisms and prevents them from
In nature, however, reproductive
barriers between sexually reproducing
species are not always complete.
Sometimes individuals of different
species interbreed and produce
offspring called hybrids.
For example; wolves and dogs are
separate species in the genus Canis,
but interbreeding happens and
produces fertile offspring that are
Evaluating the Biological Species Concept
The biological species concept works
well for most members of the kingdom
Animalia, in which strong barriers to
interbreeding exist.
For example, Asian elephants and
African elephants do not interbreed in
But the biological species concept fails
to describe species that reproduce
asexually; such as all species of
bacteria and some species of protists,
fungi, plants, and even some animals.
Evaluating the Biological Species Concept
With many groups of organisms, there
are no barriers to interbreeding
between the species.
Many species of plants, some mammals,
and many fishes are able to form
fertile hybrids with one or a few
closely related species.
In practice, modern biologists
recognize species by studying an
organisms features.
Liger (lion-tiger hybrid)
Evaluating the Biological Species Concept
The number of actual species in
the world is much larger than what
is now known. Large number of
species are still being discovered.
Only about 1.5 million species have
been described to date. Scientists
estimate that 5 million to 10 million
more species may live in the
tropics alone.
Since than, no more than 500,000
tropical species have been named;
it is clear that our knowledge of
Earth’s diversity of species is
Linnaeus’s classification system
was based on his observations that
organisms have different degrees
of similarity. A tiger resembles a
gorilla more closely than it
resembles a fish.
According to Darwin’s view,
organisms that are more similar to
one another than they are to other
organisms have descended from a
more recent common ancestor.
Therefore, classification based on
similarities should reflect an
organism’s phylogeny; it’s
evolutionary history.
Phylogeny of certain dinosaurs
Concurrent Evolution
Inferring evolutionary connections
from similarities, however, can be
Not all features that are similar
are from a common ancestor.
Consider the wings of a bird and
the wings of an insect. Both enable
flight but the structures of the
two kids of wings are radically
Fossil evidence indicates that they
evolved independently from one
Concurrent Evolution
Through a process called
concurrent evolution, similarities
evolve in organisms not closely
related to one another, often
because the organisms live in
similar environments.
Similarities that arise through
concurrent evolution are called
analogous characters.
Most biologists today analyze
evolutionary relationships using
Cladistics is a method of analysis
that reconstructs phylogenies
(evolutionary histories) by
inferring relationships based on
shared characteristics.
Inferring means to make an
educated guess based on examining
Cladistics can be used to
hypothesize the sequence in which
different groups of organisms
For example, cladistics can be used
to hypothesize the evolutionary
development of winged birds.
To do this, cladistics focuses on
the nature of the characteristics
in different groups of organisms.
It looks at the traits that a group
has in common and what traits are
Ancestral Characters
With regard to two or more
groups, a character is defined as
an ancestral character if it evolved
in a common ancestor of two or
more later groups in an
evolutionary chain.
For example, four limbs may be an
ancestral characteristic that
developed long ago when the first
organisms came from the sea to
walk on land.
When looking at humans and apes,
a hand with a thumb is an ancestral
character that developed long ago
when both shared a common
Derived Characters
When considering the relationship
between a bird and a mammal, a
backbone is an ancestral
Having feathers, however, is a
derived character.
A derived character evolved in an
ancestor of one group but not the
Feathers evolved in an ancestor of
birds that was not also ancestral
to mammals.
Derived characters
Cladistics is based on the principal
that shared derived characters
provide evidence that two groups
are closely related.
Shared ancestral characters
however, are not always closely
How can this be?
Ancestral Characters
For an example; let us compare lizards,
dogs, and whales.
Lizards and dogs have limbs but a whale
has no limbs.
Having limbs does NOT provide evidence
that dogs and lizards are more closely
related. Recall that whales are
descended from ancestors that did have
 Therefore, the presence of limbs is a
shared ancestral character of all three.
However, dogs and whales have
mammary glands; a shared derived
character not found in lizards. This
provides evidence that dogs and whales
share a more recent common ancestor
than either shares with a lizard.
A biologist using cladistics constructs
a branching diagram called a
A cladogram shows the evolutionary
relationships among groups of
Organisms that share derived
characters are grouped together on
the cladogram.
Cladogram of dinosaur development
As groups evolve, new derived
characters appear on the cladogram
that were not present before.
Cladogram of ape-human development
Cladograms have strengths and
The strength of a cladogram is it’s
objectivity. If a computer is fed the
data, it will make the same cladogram
each time.
Cladogram of dinosaur development
The disadvantage of cladistics is that
the degree of difference (how
different something is) is not
considered. Cladistic analysis simply
indicates whether a character is
present or not
Cladogram of ape-human development
Cladistic analysis does not take into
account variations in the “strength” of
a character; such as the size or
location of a fin or the effectiveness
of a lung.
Each character is treated equally.
Thus, in a cladogram of vertebrate
evolution seen at right, birds are
grouped with dinosaurs and reptiles.
This reflects their ancestry; birds
evolved from dinosaurs.
The immense evolutionary impact of
the development of feathers is
ignored by the cladogram.
Evolutionary success often depends on
the high-impact developments; such as
the development of feathers and
Some modern cladistic studies
therefore attempt to weigh the
evolutionary importance of the
characters being studied.
In evolutionary systematics,
taxonomists give varying degrees of
importance to characters and thus
produce a more subjective analysis of
evolutionary relationships.
In this type of analysis, evolutionary
relationships are displayed in a
branching diagram called a phylogenic
In a phylogenic tree, evolutionary
systematics places birds in an entirely
separate class from reptiles., giving
more importance to characteristics
like feathers that made flight
A phylogenic tree and a cladogram are
similar in that each represents a
hypothesis of evolutionary history,
which must be inferred because it can
not be observed.
Phylogenic tree