Homology and Analogy in Evolution: Vertebrate Forelimbs
Integrated Science 4 Honors
Name:
Per.
Background
The absence or presence of a backbone is a characteristic used to classify animals. Those
animals that have flexible backbones or vertebral columns are called vertebrates. Fishes, frogs,
turtles, pigeons, cats, and humans are among the diverse organisms that belong to this group.
Examination of other similar skeletal features in vertebrates indicates important links
among members of this widely varied group. Biologists consider these skeletal similarities to be
evidence that similar bones in different animals evolved form the same bone structures of a
common ancestor. Similar structures that are the result of descent from common ancestors are
called homologies and are considered important evidence in reconstructing evolutionary
phylogenies. However, not all similar structures are the result of common ancestry. These
structures, called analogies, are the result of adaptation to similar environments. One of the most
important (and difficult) tasks for an evolutionary biologist is to separate structural homologies
from structural analogies.
Consider the structure and function of your hand and arm. Can you detect any similarities
between the structures and functions of your hand and arm and those of a pigeon's wing or a
cat's paw and leg? Not only do the skeletal systems of most vertebrates perform the same kinds
of functions, but they also contain basically the same bones. The patterns of change in these
bones provide important information about evolutionary relationships amongst the vertebrates.
The vertebrate forelimb provides a good example of both homologous and analogous
structures. All vertebrate forelimbs contain the same bones: humerus, radius, ulna, carpals,
metacarpals, and phalanges. Despite this fact, these forelimbs have been modified for very
different uses in different vertebrate groups. The fact that these different limbs contain the same
bones is a strong indicator of evolutionary relationship.
The forelimbs of bats and birds also provide an interesting example of analogous
structures. Although both contain the same bones and both are adapted for flight, the modified
bones that support the wing are actually different. This is an example of different structures
adapted for the same purpose. This is considered by evolutionary biologists to indicate a
different evolutionary history for these two groups of organisms.
In this activity you will study the forelimb bones of various vertebrates to observe how
organisms that are evolutionarily related have similar structures but very different functions.
Procedures
1. Compare the vertebrate forelimbs on the attached sheet.
2. Shade in the bones that are similar using the same color. Note, the carpals and phalanges
are groups of bones.
3. Examine the available vertebrate skeletons: salamander; frog; turtle; snake; pigeon; cat; bat
and human. Pay close attention to the skeletons represented by diagrams in this lab
handout.
Analysis and Conclusions
1. Compare and contrast the forelimbs of the lizard, bird and whale. Why do individual bones
differ among these organisms?
2. Consider all of the forelimbs shown. Identify the class to which each belongs and describe
how each is suited for the environment and habitat of the organism.
3. How are human forelimbs different from those of the cat? What are human forelimbs used
for and how are they adapted for this purpose?
4. Why are homologous structures useful in constructing evolutionary histories, while
analogous structures are not? Be sure to include specific examples of each type of structure
in your answer