Evolution And History

Macroevolution and the Process of Speciation 127

bilateral symmetry, a body plan in which the right and left

sides of the body are mirror images of each other, is an an-

cestral trait in humans. Because it is a characteristic of all

vertebrates including fish, reptiles, birds, and mammals, bi-

lateral symmetry does not contribute to the reconstruction

of evolutionary relationships among fossil primates. Instead,

paleoanthropologists pay particular attention to recently

evolved derived features in order to construct evolutionary

relationships among fossil groups. For example, because

changes in bones associated with bipedalism are present

only in the human line, these derived features can be used to

separate humans and their ancestors from other hominoids.

Sorting out evolutionary relationships among fossil spe-

cies may be complicated by a phenomenon called conver-

gent evolution, in which two more distant forms develop

greater similarities. The classic examples of convergence

involve analogies discussed in Chapter 2, such as the wings

of birds and butterflies, which resemble each other because

these structures serve similar functions. Convergent evolu-

tion occurs when an environment exerts similar pressures on

distantly related organisms causing these species to resemble

each other. Distinguishing the physical similarities produced

by convergent evolution from those resulting from shared

ancestry may be difficult, complicating the reconstruction of

the evolutionary history of any given species.

Among more closely related groups, convergence of ho-

mologous structures can occur, as when an identical struc-

ture present within several distinct species takes on a similar

form in distantly related groups. Among the primates, an

example is hind-leg dominance in both lemurs and humans.

In most primates, the hind limbs are either shorter or of the

same length as the forelimbs. Lemurs and humans are not

as closely related to each other as are humans and chimps,

for example, but both have longer hind limbs related to their

patterns of locomotion. Humans are bipedal while lemurs

use their long legs to push off and propel them from tree to

tree. Hind-leg dominance appeared separately in these two

groups and is not indicative of a close evolutionary relation-

ship. Only shared derived features can be used to establish

relationships among groups of species.

The Nondirectedness of Macroevolution

In the nonscientific community, evolution is often seen as

leading in a predictable and determined way from one-

celled organisms, through various multicelled forms, to

derived Characteristics that define a group of organisms and

that did not exist in ancestral populations.

ancestral Characteristics that define a group of organisms

that are due to shared ancestry.

convergent evolution In biological evolution, a process by

which unrelated populations develop similarities to one another

due to similar function rather than shared ancestry.

Geospiza magnirostris Geospiza fortis

Geospiza parvula Certhidea olivacea

Figure 6.2 Scientists have begun to unravel the genetic
mechanisms controlling beak shape and size of finches studied
by Darwin on the Galapagos Islands. Darwin noted how beak
size and shape were related to each species’ diet and used the
birds to illustrate adaptation to a particular ecological niche.
Finches with blunt crushing beaks are seed eaters while others
with long probing beaks pick between cactus thorns for food or
use the beaks to get insects.

It may be difficult to determine whether variation pre-
served in the fossil record presents evidence of separate
species. How can we tell whether two sets of fossilized
bones represent organisms capable of interbreeding and
producing viable fertile offspring? Paleoanthropologists
use as many data sources as possible, checking the pro-
posed evolutionary relationships in order to approximate
an answer to this question. Today, paleoanthropologists
use genetic data as well as observations about the biology
and behavior of living groups to support theories about
speciation in the past. Thus reconstructing evolutionary
relationships draws on much more than bones alone. Fos-
sil finds are always interpreted against the backdrop of sci-
entific discoveries as well as prevailing beliefs and biases.
Fortunately, the self-correcting nature of scientific inves-
tigation allows evolutionary lines to be redrawn in light
of all new discoveries and more compelling explanations.

Constructing Evolutionary Relationships

In addition to designating species in the fossil record, pa-
leoanthropologists and paleontologists construct evolu-
tionary relationships among fossil groups. Scientists pay
particular attention to features appearing more recently in
evolutionary history that are unique to a line, calling these
features derived. The counterparts to derived traits are
ancestral characteristics, which occur not only in the pres-
ent-day species but in ancestral forms as well. For example,