HOMOLOGOUS STRUCTURES
http://itc.gsw.edu/faculty/bcarter/histgeol/paleo2/homol1.htm
There are two reasons why a structure might be shaped similarly in two different objects. One is that the structure is
analogous, meaning that it performs the same function. The wings of a dragonfly and of a bird are analogous, and, in
fact, are also analogous to the wings of a 747. In order to fly an object needs to generate lift. This is easily done (in
various ways) by having large flat surfaces project out from both sides of the object. Therefore flying things have such
structures.
But knowing the function of a structure doesn't necessarily tell us all we want to know about why the structure
exists. Part of why a wing exists is to perform a certain function, but part of it lies in the construction of the
wing. Perhaps this question is better worded as HOW the wing came to be rather than WHY it came to be. The picture
below shows the basic structure of the limbs of several vertebrates. Notice that all the limbs, whether wings, legs, arms,
or flippers are built upon the same basic structure. (Diagram from Monroe and Wicander, The Changing Earth, 3rd
Edition.)
ALL vertebrate limbs are put together this way, regardless of their use. Clearly
there is no analogous similarity between a bat's wing and a horse's leg, and the
extreme difference in uses of the two makes the underlying similarity seem
unusual. These types of similarity are called homologous and are very interesting
indeed.
Biologists generally find depictions of angels absurdly funny because they have two sets of forelimbs. Bird wings are
homologous to human arms, not completely different structures. Angels would necessarily have two tibia, two radii,
two ulnae, and more than five sets of digits if they really looked like that, not to mention all the attachment bones that
would have to be duplicated in the torso, like shoulder blades and collar bones. Then there would have to be two sets of
muscles, a huge sternum and keel, ... Even wimpy angels would be barrel-chested individuals indeed! Pegasus, the
flying horse of Greek myth, represents the same basic misunderstanding of anatomy.
Organisms have genes that perform two separate functions. One set of
genes controls the existence (or lack thereof) of a particular feature. These
are called structural genes. The other set controls if, when, and for how
long the structural genes are allowed to function. These are called
regulatory genes. the diagram below shows the effects of different
regulatory gene control on the development of two vertebrate limbs: a
human arm and a bat wing. Examine each corresponding bone and
compare their relative sizes after growth.
There are many examples of homologies among animals and plants. The
limbs of vertebrates are not the only obvious homologies in the skeleton -they are the rule rather than the exception.
The number of neck vertebrae is constant among mammals, for example. The neck of a giraffe is made of seven bones,
just as yours is, and just as a whale's is. In the whale's neck the bones are exceedingly short, in the giraffe they are
exceedingly long. One has to wonder why a structure that basically won't bend is made of seven ridiculously short
bones rather than one big one, or why a giraffe doesn't have more bones in a ten-foot neck than a person with a tencentimeter neck. Perhaps if they did they wouldn't have to splay their front legs to get a drink of water. There appears
to be some constraint upon the way mammal necks are built.
3. Evolution Makes Sense of Homologies
http://www.zoology.ubc.ca/~bio336/Bio336/Lectures/Lecture5/Overheads.html
Richard Owen (1848) introduced the term homology to refer to structural similarities among organisms. To Owen, these
similarities indicated that organisms were created following a common plan or archetype.
That is, although each species is unique, the plans for each might share many features, just as the design plans for a Honda
Civic and a Honda Prelude might be similar. Nevertheless, if every organism were created independently, it is unclear
why there would be so many homologies among certain organisms, while so few among others. It is also hard to make
sense of the fact that homologous structures can be inefficient or even useless. Why would certain cave-dwelling fish have
degenerate eyes that cannot see?
Darwin made sense of homologous structures by supplying an evolutionary explanation for them:
• A structure is similar among related organisms because those organisms have all descended from a common ancestor
that had an equivalent trait.
Ridley uses a specific definition of homology: "A similarity between species that is not functionally necessary. "I interpret
this as: "A similarity between species that exists despite several plausible alternative traits that would function equally
well."
The plasma cell membranes of all organisms, eukaryotic and prokaryotic, are structurally similar, consisting of a
phospholipid bi-layer.
Many other possible membrane structures exist.
The hydrophobic fatty acid tails could be joined. There could be three
hydrophobic fatty acid chains. Other hydrophilic groups could be involved
besides glycerol phosphoric acid. The similarity of the plasma membrane (as well
as other cell structures) suggests that all living cells have descended from an
ancestor with a similar membrane structure.
One of the classic examples of a homologous structure is the pentadactyl (= five
digit) limb. All tetrapods (= four legged) have limbs with five digits, at least at
some stage in development. Certain tetrapods lose some of these digits during
development, as in the bird wing shown here. But if the bird wing does not need
five digits, why do five initially develop in the growing embryo? The most
plausible explanation is that while the five digits are not functionally necessary,
they represent a genetic artefact inherited from the ancestors of birds.
Homologous structures teach us an important lesson about evolution: Evolution works primarily by
modifying pre-existing structures. That is, even when two species function in completely different
ways, they often use homologous structures to carry out those functions. For example, birds and
bats fly rather than run on all fours, yet their wings are modified fore-limbs rather than completely
novel structures. Similarly, the stinger of wasps and bees is a modified ovipositor, rather than an
entirely new structure. (Explaining why only female bees sting!!)
Homologies
http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_05
Evolutionary theory predicts that related organisms will share similarities that are derived from common ancestors.
Similar characteristics due to relatedness are known as homologies. Homologies can be revealed by comparing the
anatomies of different living things, looking at cellular similarities and differences, studying embryological development,
and studying vestigial structures within individual organisms.
In the following photos of plants, the leaves are quite different from the
"normal" leaves we envision. Each leaf has a very different shape and
function, yet all are homologous structures, derived from a common
ancestral form. The pitcher plant and Venus' flytrap use leaves to trap and
digest insects. The bright red leaves of the poinsettia look like flower petals.
The cactus leaves are modified into small spines which reduce water loss
and can protect the cactus from herbivory.
Another example of homology is the forelimb of tetrapods (vertebrates
with legs). Frogs, birds, rabbits and lizards all have different forelimbs, re
flecting their different lifestyles. But those different forelimbs all share the
same set of bones - the humerus, the radius, and the ulna. These are the
same bones seen in fossils of the extinct transitional animal,
Eusthenopteron, which demonstrates their common ancestry.
Individual organisms contain, within their bodies, abundant evidence of
their histories. The existence of these features is best explained by
evolution.
• Several animals, including pigs, cattle, deer, and dogs have reduced, nonfunctional digits, referred to as
dewclaws. The foot of the pig has lost digit 1 completely, digits 2 and 5 have been greatly reduced,
and only digits 3 and 4 support the body. Evolution best explains such vestigial features. They are
the remnants of ancestors with a larger number of functional digits.
• People (and apes) have chests that are broader than they are deep, with the shoulder blades flat i n
back. This is because we, like apes, are descended from an ancestor who was able to suspend
itself using the upper limbs. On the other hand, monkeys and other quadrupeds have a different
form of locomotion. Quadrupeds have narrow, deep chests with shoulder blades on the sides.
Organisms that are closely related to one another share many anatomical similarities. Sometimes the similarities are
conspicuous, as between crocodiles and alligators, but in other cases considerable study is needed for a full appreciation
of relationships.
Modification of the tetrapod skeleton
Whales and hummingbirds have tetrapod skeletons inherited from
a common ancestor. Their bodies have been modified and parts have
been lost through natural selection, resulting in adaptation to their
respective lifestyles over millions of years. On the surface, these
animals look very different, but the relationship between them is easy
to demonstrate. Except for those bones that have been lost over time,
nearly every bone in each corresponds to an equivalent bone in the
other.
Studying the embryological development of living things provides clues to the evolution of present-day organisms.
During some stages of development, organisms exhibit ancestral features in whole or incomplete form.
Snakes have legged ancestors.
Some species of living snakes have hind limb-buds as early embryos but rapidly lose the
buds and develop into legless adults. The study of developmental stages of snakes, combined with fossil evidence of
snakes with hind limbs, supports the hypothesis that snakes evolved from a limbed ancestor.
Baleen whales have toothed ancestors.
Toothed whales have full sets of teeth throughout their lives. Baleen whales, however, only possess teeth in the early
fetal stage and lose them before birth. The possession of teeth in fetal baleen whales provides evidence of common
ancestry with toothed whales and other mammals. In addition, fossil evidence indicates that the late Oligocene whale
Aetiocetus (below), from Oregon, which is considered to be the earliest example of baleen whales, also bore a full set of
teeth.
Homologies: cellular/molecular evidence
All living things are fundamentally alike. At the cellular and molecular level living things are remarkably similar to each
other. These fundamental similarities are most easily explained by evolutionary theory: life shares a common ancestor.
The cellular level
All organisms are made of cells, which consist of
membranes filled with water containing genetic material, proteins, lipids,
carbohydrates, salts and other substances. The cells of most living things use
sugar for fuel while producing proteins as building blocks and messengers.
Notice the simil arity between the typical animal and plant cells pictured
below — only three structures are unique to one or the other.
The molecular level
Different species share genetic homologies as well as anatomical ones. Roundworms, for example,
share 25% of their genes with humans. These genes are slightly different in each species, but their striking similarites
nevertheless reveal their common ancestry. In fact, the DNA code itself is a homology that links all life on Earth to a
common ancestor. DNA and RNA possess a simple four-base code that provides the recipe for all living things. In some
cases, if we were to transfer genetic material from the cell of one living thing to the cell of another, the recipient would
follow the new instructions as if they were its own.
These characteristics of life demonstrate the fundamental sameness of all living things on Earth and serve as the basis of
today's efforts at genetic engineering.
Examples of Analogies
Analogy: Squirrels and Sugar Gliders
Beyond being cute and cuddly, flying squirrels and sugar gliders have
many striking similarities: big eyes, a white belly, and a thin piece of skin
stretched between their arms and legs, a trait which helps them "glide"
and remain stable when leaping from high places. But sugar gliders and
flying squirrels also have some key differences. Most importantly, they
reproduce and bear their babies in fundamentally different ways:
• Flying squirrels are placental mammals. Placental mammals spend a long time
developing inside the mother's body being nourished by a placenta before
they are b orn.
• Sugar gliders are marsupial mammals, like kangaroos. Marsupial mammals may only
spend a short time developing inside the mother's body and are very tiny
when born. After birth, a baby marsupial crawls into its mother's pouch and is
nourished by her milk as it continues to grow and develop.
Flying squirrels and sugar gliders are only distantly related. So why do they look so
similar then? Their gliding "wings" and big eyes are analogous structures. Natural
selection independently adapted both lineages for similar lifestyles: leaping from
treetops (hence, the gliding "wings") and foraging at night (hence, the big eyes)
Analogy: Desert-dwellers
Both of these two plants have thick, water-filled branches and sharp spines. You might
guess that they are closely relat ed — but they are not. In fact, one "cactus" is more
closely related to a common weed, and the other is more closely related to a carnation.
Their similarities are analogies — independently evolved adaptations that aid survival in
the desert: succulent, waxy stems help store water, and spines provide shade and
protect the plant from herbivores.
Analogy: Jaws versus Flipper
Although their differences are certainly substantial (e.g., sharks are cold-blooded and dolphins are warm-blooded),
sharks and dolphins have some undeniable similarities: side fins, a dorsal fin, and a torpedo-shaped body. These
similarities are analogies — traits the two lineages evolved independently as adaptations for moving swiftly in the water.
Sharks were gliding through the oceans long before dolphins descended from land-dwelling mammals. Dolphins evolved
flippers, a dorsal fin, and a torpedo-shaped body as natural selection shaped them for the life of an ocean predator.
Analogy: Kings of the Anthill
Anteaters live in Latin America and South America, grow to two meters (about six feet) long,
and give birth to live young. Echidnas live in Australia, grow to half a meter (about one and a
half feet) long, and, as the only close, living relative of the duck-billed platypus, lay eggs. But
if you get up close and personal with them, you will find that anteaters and echidnas have
some striking similarities: both are t oothless, with a pointy snout, a long, sticky tongue, and
sharp, curved claws. All of these traits are analogies — testaments to the selective power of
the anthill. These two rather distantly related lineages independently evolved snout, tongue,
and claw traits that allow them to more efficiently pillage and plunder an anthill.
Analogy: When is a thumb a thumb?
Through careful study, biologist can also identify analogies. For example, panda bears have a "thumb" on their hands.
They use this "thumb" to hold onto bamboo as they eat. Is the panda's "thumb" homologous or analogous to the thumb
on your own hand? Studying the anatomy of panda hands and human hands shows that these "thumbs" must be
analogous. When you look at the bones of each, you see that the thumbs are not very similar at all! The human thumb
has joints and is made of many bones. The panda thumb is just
one bone sticking out of the side of the hand. Furthermore, the
panda thumb is the sixth "finger" on its hand! If you watched the
hand of a baby panda grow, you would see that the "thumb"
develops from a wrist bone. The panda thumb and the human
thumb don't grow from the same bones. This is more evidence
that they are analogous structures. In fact, the panda thumb is
homologous to a wrist bone in humans, and the human thumb is
homologous to the first finger in pandas!