HUMAN BIOLOGY

446 Chapter 23

n Comparing stages of development in major groups of

organisms can shed light on evolutionary history.

Comparing body forms is one way to

uncover evolutionary connections

Comparative morphology uses information contained

in patterns of body form to recon struct evolutionary his-

tory. When populations of a species diverge, they diverge

in their appearance, or in the functions of certain body

parts, or both. Yet the related species also remain alike

in many ways, because their evolution modi fies a shared

body plan. In such species we very often see homologous

structures. These are the same body parts that have been

modified in different ways in different lines of descent

from a common ancestor (homo- means “same”).

For example, most land vertebrates have homologous

structures and probably share a common ancestor that

had four five-toed limbs. The limbs diverged in form and

became wings in pterosaurs, birds,

and bats (Figure  23.7). All these

wings are homologous—they have

the same parts. The five-toed limb

also evolved into the flippers of por-

poises and the anatomy of your own

forearms and fingers.

Body parts in organisms that

don’t have a recent common ances-

tor may also come to resemble one

another in form and function. These

analogous structures (from anala-

gos, meaning “similar”) arise when

different lineages evolve in the same

or similar environments. Different

body parts, which were put to similar uses, were modified

through natural selection and ended up resembling one

another. For example, a dolphin, a fast-swimming marine

mammal, has a sleek, torpedo-shaped torso—and so does

a tuna, a fast-swimming fish.

Development patterns also provide clues

Vertebrates include fishes, amphibians, reptiles, birds, and

mammals. Yet despite how different these groups are, com-

paring the ways in which their embryos develop provides

strong evidence of their evolutionary links.

Early in development, the embryos of all the different

vertebrate lineages go through strikingly similar stages

(Figure  23.8). During vertebrate evolution, mutations that

disrupted an early stage of development would have had

devastating effects on the organized interactions required

for later stages. Evidently, embryos of different groups

remained similar because mutations that altered early

steps in development were selected against.

Figure 23.7 Animated! The form of forelimbs of humans and

other vertebrates diverged as different groups evolved. This

diagram starts with a generalized form of ancestral early reptiles.

Diverse forms evolved even as similarities in the number and

position of bones were preserved. The drawings are not to the

same scale. (© Cengage Learning)

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pterosaur
chicken
penguin
porpoise
bat
human
early reptile
elephant
23.5 Comparing the form and Development of Body parts
analogous structures
Body parts that are similar
in different lineages because
they are adaptations to a
similar environment.
comparative morphology
Comparing the body form
of different groups to help
reconstruct evolutionary
history.
homologous structures
Body parts that are similar
in different lineages because
the lineages have a com-
mon ancestor.
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