The Ecology Book

89

See also: Evolution by natural selection 24–31 ■ The role of DNA 34–37 ■ The selfish gene 38–39

■ Competitive exclusion principle 52–53

ORDERING THE NATURAL WORLD

organisms may appear identical

and live in the same place, but this

does not mean that they are the

same species. For example, the

Western Meadowlark (Sturnella

neglecta) and Eastern Meadowlark

(Sturnella magna) look similar and

have overlapping ranges, but they

have evolved to produce different

songs. This prevents them from

mating with each other, making

them two distinct species.

Another scenario is when

members of the same species look

very different, but because they can

mate and reproduce they are still

considered to be the same species.

The most obvious instance of this is

the domestic dog (Canis familiaris),

a species in which there are great

differences between individuals.

However, as is also evident, different

breeds are capable of reproduction

with each other, and therefore

belong to the same species.

Complex permutations

According to the biological species

concept, the potential for inter-

breeding is key to the definition

of a species. Geographical

separation alone does not prevent

species from reproducing if they

are brought together. Evolutionary

divergences—such as the different

mating songs of the Western

and Eastern Meadowlarks—are

what prevents interbreeding.

The biological species concept

is not applicable to asexual

organisms, such as bacteria,

or asexual creatures—for example,

species of whiptail lizard.

Sometimes, too, different animal

species are able to mate and

produce offspring, as is the case

of a female horse (Equus ferus

caballus) and a male donkey

(Equus africanus asinus), which

together can produce a hybrid—

the mule. However, mules

themselves are generally incapable

of reproduction, and therefore the

horse and donkey remain different

species. Another example is the

liger, a zoo-bred hybrid of a female

tiger and a male lion.

Such anomalies highlight the

complexities of defining a species.

The biological species concept

remains the most popular, but

scientists are now looking at the

idea of shared genes, and using

DNA sequence analysis. To date,

no one has come up with a single

definition that covers every known

species, and it seems unlikely

that anyone ever will. In the

absence of better models, Ernst

Mayr’s biological species concept

provides an extremely useful

way of thinking about species

and evolution. ■

Male fireflies are an example of

a typological species. They emit a

pattern of flashes to attract females,

who recognize their species’ code

and flash back—if they wish to mate.

Alternative species concepts

Although Mayr’s idea about

biological speciation is perhaps

the most common way to define

species and explain how they

evolve, it is far from the only

one. In fact, there are more than

20 recognized species concepts,

ranging across two broad groups:

typological and evolutionary

concepts. Typological species

concepts are based on the idea

that a population of individuals of

the same type—or sharing the

same set of traits—are what

makes up a species. The traits can

be based on genetics, such as

DNA or RNA base sequences, or

on phenotypes, such as the size

of certain body parts or

particular markings, such as the

arrangments of spots on insects’

wings. The evolutionary species

concept is based on species

lineages. A species is defined as

the organisms that share a

lineage from the time when the

species initially split off until

extinction, or until an additional

splitting off and creation of a

new species.

Endless forms

most beautiful and

most wonderful have

been, and are

being, evolved.

Charles Darwin

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