HUMAN BIOLOGY

prinCiples of evolution 443

hoW does a neW species come about?

• New species can arise as the result of the accumulation of
  genetic changes due to natural selection and chance events.


• Members of a species form a genetic unit consisting of one
  or more populations. Unlike individuals of different species,
  members of the same species can interbreed and produce
  fertile offspring under natural conditions.

taKe-Home message

speciation Accumulation

of so many genetic differ-

ences that members of two

populations can’t interbreed

successfully.

species A group of popu-

lations in which the indi-

viduals share enough traits

that they can successfully

interbreed.

Figure  23.3B is an artist’s rendition

of an extinct human species, Homo

habilis. Clearly, there are many dif-

ferences between H. habilis and our

own species, Homo sapiens.

Biologists disagree about the pace

and timing of microevolution. Some

hold to a model of gradual specia-

tion, in which new species emerge

through many small changes in

form over long spans of time. Other scholars favor a model

in which most evolutionary change occurs in bursts. That

is, each species undergoes a spurt of changes in form when

it first branches from the parental lineage, then changes

little from then on.

The driving force behind rapid changes in species may

be dramatic changes in climate or some other aspect of

the physical environment. This type of change alters the

physical conditions to which populations of organisms

are adapted. On the other hand, bursts of evolutionary

change could help explain why the fossil record has scanty

evidence of a continuum of micro evolution—the “missing

links” between closely related species. Both models prob-

ably have a place in explaining the history of life.

The ability to interbreed defines a species

For humans and other sexually reproducing organisms,
a species is a genetic unit consisting of one or more
populations of organisms that usually closely resemble
each other physically and physiologically. Members of the
same species can interbreed and produce fertile offspring
under natural conditions. It doesn’t matter how diverse
their traits are, so long as they can interbreed successfully
and so share a common gene pool. From this perspective, a
female lawyer in India may never meet up with an Icelan-
dic fisherman, but there’s no biological reason why the two
couldn’t mate and produce children. But neither of them
could mate successfully with a chimpanzee, even though
chimps and humans are closely related species and have
more than 90 percent of their genes in common. Humans
and chimps are “reproductively isolated.” This means that
their genetic differences ensure that they can’t mate and
produce fertile offspring.
Reproductive isolation develops when gene flow between
two populations stops. This often occurs when two popu-
lations are separated geographically. When populations
are in different environments, mutation, natural selection,
and genetic drift begin to operate independently in each
one. These processes can change the gene pools of each
in different ways. Eventually, the differences can result
in changes in body structure, function, or behavior that
reduce the chances of successful interbreeding. For exam-
ple, the two popu lations may breed in different seasons, or
there may be bodily changes that physically interfere with
mating. Other changes may prevent zygotes or hybrid off-
spring from developing properly.
The buildup of genetic differences between isolated
populations is called divergence (Figure  23.3A). When
the genetic differences are so great that members of
the two populations can’t interbreed, speciation has
occurred: The populations have become separate species.

Figure 23.3 Divergence is the key first step toward the formation
of new species. A Horizontal lines in this diagram represent different
populations. Because evolution is gradual, we can’t say at any one
point in time that there are now two species rather than one. At time
A there is only one species. At D there are two. At B and C the split
has begun but isn’t complete. B Artist’s view of Homo habilis.
(© Cengage Learning)

time A time B

time

time C time D

parent species

daughter

species

daughter

species

A

B

Mauricio Anton/Science Source

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