| Classification | Plant
Evolution | Animal Evolution
| Homology |
Homology forms the basis of organisation for
comparative
biology. In 1843 Richard
Owen defined homology as "the same organ in different
animals under every variety of form and function." Scientists
had noticed that, within a group of related species, some
structures shared similarities in form. For example, organs
as different as a bat's wing, a seal's flipper, a cat's paw
and a human's hand have a common underlying structure, with
identical or very similar arrangements of bones and muscles.
Owen reasoned that there must be a common structural plan
for all vertebrates, as well as for each class of vertebrates.
He called this plan the
archetype.
Richard Owen also distinguished homology from analogy (also
known as
homoplasy), which
he defined as a 'part or organ in one animal which has the
same function as another part or organ in a different animal'
(Owen, 1843: 374).
Homologous structures are structures that are derived from a common ancestor i.e. they have a common evolutionary ancestry. This is not to say that homologous structures have the same function e.g. a whale's flipper is homologous to a human arm. These two limbs are superficially different, but their internal skeletal structure is essentially the same. Similarly, the wings of a bird and the wings of a bat are homologous structures.
Homologous structures in modern organisms may show even less similarity in form, but it's still possible to trace their development and use them as a measure of evolutionary relatedness.
The ear
ossicles
of tetrapods are homologous to bones in a fish jaw
The
stapes,
incus, and
malleus
of the mammalian
ear are homologues of parts of the fish jaw and gill
arches.
Early fish, such as the ancestors of modern hagfish and lampreys,
didn't have a jaw. Their gills acted to filter food particles
from the water. These gills were supported by a series of
gill arches, formed of cartilage or bone. As early fish evolved
jaws, the gill arches closest to the mouth were co-opted to
act as jaw bones. The enlarged first arch, called the mandibular
arch, became the basis of the upper and lower jaws. The second,
or hyoid, arch, extended from the
quadrate
bone (at the rear the cranium) to the angle of the
jaw, and acted to support the jaw. The hyoid arch later became
the hyomandibular bone,
which braces the quadrate bone in bony fish.
As tetrapods evolved from one group of lobe-finned fish,
the quadrate bone fused with the skull, giving a stronger
bite. This meant that the hyomandibular bone lost its function
of supporting the jaw. However, its location near the ear
seems to have allowed the hyomandibular to transmit vibrations
to the inner ear. The hyomandibular bone of fish is homologous
to the stapes, or
columella,
of a reptile's ear. (Reptiles have only a single bone, the
stapes, to transmit vibrations to the inner ear.)
One group of reptiles, the
synapsids,
went on to evolve into mammals. Among the multitude of changes
this entailed was the development of the other two inner-ear
bones of mammals, the incus ("anvil") and malleus ("hammer").
Like the stapes (or "stirrup"), these two bones are also derived
from bones of the jaw.
In early synapsids two bones formed the joint between the
upper and lower jaws. The quadrate was part of the skull while
the
articular bone was
part of the lower jaw. Like the hyomandibular bone (the stapes)
to which they were connected, these two bones became progressively
smaller and eventually completely lost their connection with
the jaw. This evolutionary sequence can be traced through
an excellent series of transitional
fossils.
Once separated from the jaw these three bones became the
ossicles of the middle ear. The incus is homologous to the
quadrate bone, the malleus to the articular bone, and the
stapes to the hyomandibular bone. A mammal's skull has a swollen
area just below and behind the jaw joint. This is the
auditory
bulla, which contains the ossicles.
The evolution of these structures supports the hypothesis that early mammals were active at night (nocturnal), when they would have depended heavily on their senses of hearing, smell, and touch to find food and avoid being preyed upon.
Homology in birds and
bats
Birds and bats have independently evolved wings from their
forelimbs (an example of
convergent
evolution). However, while their wings look superficially
quite different, examination of the underlying bones reveals
them to be homologous. The forelimb of the embryonic bird
begins its development with much the same structure as that
of a mammalian embryo. As the bird develops the forelimb becomes
ever more wing-like and less leg-like. Many of the hand bones
fuse with each other and some are lost. In contrast, the bat
retains all the bones of its hand, but these are greatly elongated.
Another group of flying vertebrates, the
pterosaurs,
had similar modifications of the basic tetrapod forelimb.
All three organisms, reptile, mammal, and bird, have the same
bones of the upper and lower arm, although their proportions
differ. However, the hand
bones supporting the wing surface are quite different.
In a pterosaur the wing membrane is supported by the 5th finger
only. A bird's primary flight feathers are mounted on the
2nd finger, while in a bat the 2nd, 3rd, 4th and 5th fingers
all support the wing membrane.
Vestigial organs
Some organisms have structures or organs with no apparent or predictable function. For example some snakes have rudiments of a pelvis and hind limbs, many flightless birds have remnants, humans have a tail bone that is completely internal, and whales still have the remains of a pelvis and thigh bones. Those seemingly functionless parts are called vestigial organs or vestigial structures. Vestigial organs are often homologous to organs that are fully functional in other species e.g. the vestigial human tail bone (or coccyx) is homologous to the functional tail of other primates.
Quite often rudimentary organs can be detected in the embryo,
but are lost later during development, e.g. the teeth in the
upper jaws of embryos of whales, or the
pharyngeal
slits seen in the embryos of all chordates but gone in all
adult forms apart from the fish. Vestigial structures are
evidence for evolution: a species with a vestigial form of
an organ is related to other species where the homologous
organ is fully functional.
Molecular homology
A species' evolutionary history leaves signs in its DNA and the proteins that DNA codes for. Two species that share a DNA base sequence (and also the specific protein coded for) probably have a common ancestor. Usually the DNA base sequence will be slightly different between the two, as each species will have accumulated different mutations once they separated. The number of mutations can be used to indicate how closely related the species are. It can also be used as an indication of how long ago they became separate species. In fact a common genetic code is shared by all species. This shows that natural selection reuses genes and structures that have worked well in the past.
All living organisms have their instructions for reproducing and operating encoded in a chemical language using four bases, adenine (A), cytosine (C), guanine (G) and thymine (T). Combinations of the bases specify which amino acids the cell uses in making proteins for use in cell functions. The fact that every living species carries the same genetic code indicates a common single ancestor at some point in the distant past.
 Reference
Websites
http://www.virtuallaboratory.net/Biofundamentals/lectureNotes/Topic1-5_Evo.htm
 Reference
Books
Campell N.A. and Reece J.B. (2001): Essential Biology. San Francisco: Benjamin Cummings
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