Download Body parts are considered homologous if they have

Body parts are considered homologous if they have
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the same basic structure
the same relationship to other body parts, and, as it turns out,
develop in a similar manner in the embryo.
It seems unlikely that a single pattern of bones represents the best possible structure to
accomplish the functions to which these forelimbs are put. However, if we interpret the
persistence of the basic pattern as evidence of inheritance from a common ancestor, we
see that the various modifications are adaptations of the plan to the special needs of the
organism. It tells us that evolution is opportunistic, working with materials that have
been handed down by inheritance.
Color the homologous bones according to the color scheme below:
 Green = humerus
 Red = radius
 Blue = ulna
 Yellow = carpels & metacarpals
 Purple = phlanges
Many groups of organisms show similarities in appearance that cannot be
explained by similarities in their functional requirements. For example, the
forelimbs of a diverse group of mammals, including humans, cats, whales, and
bats are all constructed from the same bones even though they perform vastly
different functions. This puzzling similarity among mammals and a variety of
other animals prompted Darwin to exclaim, "What could be more curious than
that the hand of a man, formed for grasping, that of a mole for digging, the leg
of a horse, the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include the same bones, in the
same relative positions?"
Morphologically similar structures that perform different functions are called
homologous structures. Perhaps the oddest kinds of homologous structures are
vestigial organs, rudimentary structures that are of marginal or no use to an
organism, or structures in which the shared form appears to be inefficient.
Examples of vestigial structures include:
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Skeletons of some snakes retain the vestiges of pelvic girdle and leg
bones of walking ancestors. Also, modern whales use hind limbs with
bony supports. However, if we dissect a whale we find, at the
appropriate place down the spine, a set of bones that are clearly
homologous with the pelvis of any four-limbed vertebrate.
Some blind, cave-dwelling fish have eye-sockets but no eyes.
Humans have muscles that make our body hair stand on end when we
get cold or excited. Did you ever wonder why we have goose bumps?
Erectile fur that is important during aggressive encounters is found
among mammals .
Comparative Embryology
Developmental biologists have discovered that closely related organisms go
through similar stages during their embryonic development. For example, all
vertebrate embryos go through a stage in which they have gill pouches on the
sides of their throats, a post-anal tail, and segmented muscles. During the early
stages of development the embryos of vertebrates are strikingly similar.
However, as development continues, the various groups of vertebrates diverge
considerably taking on the distinct characteristics of their classes. Among
fishes, the gill pouches become gills. In humans, they develop into eustachian
tubes that connect the throat and the middle ear.
The embryonic similarities among vertebrates can be explained in terms of
relative stasis in the environment for developing embryos, resulting in similar
selection regimes. In the process of adapting to different environments, adult
vertebrates have experienced different selective pressure and have undergone
considerable degrees of divergence.
5. Molecular Biology
Molecular biological studies have indicated that the structure of DNA and the
nature of its hereditary materials are universal to all life. One way in molecular
biology has confirmed this is by isolating the messenger RNA (a type of RNA
that specifies a particular protein) for hemoglobin from a rabbit and injecting it
into a bacterium. The bacterium does not normally make hemoglobin, but when
injected with the messenger RNA for this product, it will produce rabbit
hemoglobin. The bacterium is capable of manufacturing the rabbit protein
because the machinery for decoding the message in the messenger RNA must
be common to rabbits and the bacterium. Also, an examination of the structure
of proteins and DNA for a vast diversity of organisms has revealed a high
degree of similarity between organisms that are taxonomically closely related
to one another.
6. Artificial selection
Artificial selection refers to the selective breeding of domesticated plants (a
variety of grains and vegetables that you buy at the local super market) and
animals (e.g., dogs, pigeons, cattle, etc.) to encourage the occurrence of
desirable traits. In artificial selection, a breeder selects the parents possessing
traits that are considered desirable for each generation and culls, or destroys the
undesirable varieties. If the breeder continues to select in a particular direction
for an extended period of time the initial trait(s) will appear considerably
different from their initial appearance. Broccoli and cauliflower have a
common ancestor (wild mustard plant) and by accentuating different parts of
this plant from generation to generation, plant breeders were able to generate
the divergent features that are characteristic for each of these vegetables.
2. Biogeography
When naturalists began to travel and study organisms from different parts of
the globe, they observed interesting patterns in their geographical distributions.
A number of patterns that emerged from examining the geographical
distribution of species.
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Isolated islands harbor unique species of plants and animals. However,
these organisms do bare some resemblance to species of plants and
animals found on the nearest mainland.
Islands with similar environments in different parts of the world do not
appear to be populated with the same kinds of species. Instead, they are
inhabited by species that are taxonomically closely related to the plants
and the animals of the nearest mainland, where the environment is often
quite different.
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Australia is comprised of a large number of species of marsupial
mammals, but has relatively few species of placental mammals.
1. The Fossil Record
The fossil record, documenting the history of life on Earth, is rather
incomplete. The main reason that fossils are not too plentiful is that the
conditions necessary for fossilization are very specific, depending on a linking
of a series of improbable events: organisms must be buried under conditions
that prevent the decay of their hard parts; organisms must be buried in
sediments that subsequently become solidified into rock; rocks containing
fossils must avoid erosion and metamorphosis for long periods of time;
subsequently, rocks containing fossils must be eroded to expose them; the
location of rocks containing fossils needs to be in an area where humans are
likely to discover them. Given these precise circumstances, it may be a wonder
that we find any evidence of past life forms on this planet!
Despite these limitations, certain groups of organisms have left a reasonably
rich fossil record, allowing scientists to characterize the patterns observed. A
close examination of the fossilized organisms has uncovered an orderly history
of life, with different groups originating at different times rather than all at
once. For example, there is a geological succession of the different vertebrate
classes, with fishes appearing first followed by the appearance of amphibians,
reptiles, and mammals. The fossil record also has revealed the smooth sequence
of intermediate fossil forms through geological time. Furthermore, it indicates
that extinction is a common occurrence in the history of life and whether or not
a group of organisms becomes extinct is independent of their emergence in the
fossils record. Species that appear suddenly in younger rocks are just as likely
to go extinct as fast or as slow
as species that have been
recovered from much older
layers of rock. In addition, the
fossil record has revealed the
occurrence of organisms that
possess characteristics that are
intermediate between specific
groups of organisms. These
fossil organisms have
sometimes been referred to as
"missing links." For example,
in 1861 several fossils were
discovered exhibiting characteristics that were intermediate between reptiles
and birds. The fossil, Archaeopteryx (pictured at left), had a number of reptilian
features including teeth and a long tail, but it also had a number of birdlike
features such as feathers and a wishbone. Perhaps even more revealing is the
evolutionary transition to mammals from mammal-like reptilian ancestors.
A classic example of the stepwise
occurrence of organisms in the
fossil record is provided by
horses. The earliest fossils were
very small and possessed four
toes on its pad-footed front legs.
The teeth were simple and were
specialized for browsing on soft
vegetation. During the
approximately 60 million years
since the occurrence of these
early forms, horses have
undergone substantial changes
They became progressively larger
and there was a gradual change in
the configuration of the toes to
the point where a single toe on
the arch of the foot is used for
locomotion (pictured at right). In
addition, the teeth developed
unique characteristics adapted for
chewing hard abrasive grasses. The relationship between the first horses to
appear in the fossil record and modern day horses is complete with a showcase
of intermediate stages . However, it is important to understand that changes
among the various forms of horses did not progress in a strictly linear fashion.
There is evidence of a substantial amount of splitting or branching of groups
with some individual branches maintaining fairly distinct features until they
become extinct.
The Nature of Science
Science is a fallible enterprise. What does this mean? In part, it means that
scientists, being human beings, will make mistakes. Crucial data will be
overlooked, calculations will be in error, or worse, scientists will fudge their
data to get a desired result. Science, in other words, is a human enterprise with
all that entails.
But science is fallible in a deeper sense. Science always only gives us tentative
results. Science is a discipline that yields an understanding of reality given all
available evidence. But of course the available evidence is not all the evidence.
There could be another experiment, a new discovery, refined instruments, etc.,
any of which might prompt a revision of the current scientific understanding of
reality. So, even when scientists avoid the errors mentioned above, scientific
claims are always subject to revision in light of new evidence. (Once again,
being human, scientists will sometimes claim their views are certain. To say
this is to go beyond the limits of science.) Put another way, science does not
deliver absolute, certain, indubitable truth.
Neither, however, does science deliver mere conjecture, nor a view of reality
with nothing to support it but blind faith. Certain truth and blind faith represent
two ends of a continuum. Most beliefs we have lie somewhere between these
two poles. One great value of science is that it provides us with methods by
which we can arrive at justified beliefs (i.e., beliefs nearer the certain end of the
continuum). The methods available are as rich and varied as the sciences
themselves--there really is no such thing as the scientific method--and the
results these methods yield at any moment may be of greater or lesser
significance. Consequently, it is perhaps better to speak of scientific theories as
being confirmed or disconfirmed, and these may each be to a greater or lesser
degree. Let us briefly explore the nature of confirmation.
One important way in which theories are confirmed is by making successful
predictions. The classic example of this is the triumphant discovery of Neptune.
Newton's celestial mechanics were at odds with the observed orbit of Uranus.
That is, given what was known at the time, Newton's laws predicted a different
orbit for Uranus than was observed. At this point, one might suspect a
disconfirming observation: Newton's theory got it wrong. But in the face of a
failed prediction, one need not reject a theory! Instead, one might seek an
explanation of the failure. This is just what John Adams and Urbain Leverrier
did. These scientists hypothesized (independently) an unseen planet beyond
Uranus. The existence of such a celestial body would then bring the observed
orbit of Neptune into accord with Newton's laws. Using Newton's laws, the
position of the planet was calculated, and Neptune was discovered right where
Newton's laws predicted it would be. A stunning confirmation!
Remarkably, there was a second opportunity for Newton's theory to shine as it
had with Neptune. Like Uranus, the planet Mercury was discovered to deviate
from the expected path given Newton's laws. Scientists again hypothesized an
as yet undiscovered planet the existence of which would explain Mercury's
observed deviation. This time, however, no such planet was discovered.
Newton's theory failed in its prediction. Predictions, however, are not the sole
test of a theory. Indeed, perhaps the most important test is the explanatory
scope of a theory. That is, scientists are not simply in the business of collecting
facts and charting predictive successes and failures. Science is a much more
active enterprise seeking to unite these facts by explaining how the world is
such that these facts might be expected. Newton's theory was remarkable for its
capacity to unify what had heretofore been a disparate set of phenomena; the
motion of celestial bodies was to be explained by the very same laws as the
falling of an apple! In the end, what undermines a theory is not a false
prediction, but a better successor theory (i.e., one that does a better job of
explaining the facts). Thus, the mere failure to find a hidden planet in the case
of Mercury did not lead to a rejection of Newton's theory. Mercury's motion
was considered an unexplained anomaly until Einstein's theory came along and
explained it. Newton's theory turned out to be false. Nonetheless, it was highly
confirmed, and he and the rest of humanity were well justified in believing it
(even in the face of failed predictions) until Einstein came along.
Consider an analogy to a court of law. In a trial, the jury is presented with a
complex set of facts, or evidence. Their task is to make sense of the evidence,
to seek an understanding of how these facts can best be explained. Similarly,
scientists seek to understand how the world works such that the observed facts
are to be expected. Unlike jurors, however, scientists are not limited in their
capacity to test their explanations as jurors are. Moreover, scientists must
defend their explanations to a public community of scientists who have access
to the very same resources. That is, while jurors can (and often do) justify their
verdict based on gut instinct or feelings, scientists must appeal to publicly
available evidence. Consequently, good scientific explanations are testable. (It
is of singular importance that the testing of theories must bear a degree of
independence from the theories themselves. The hypothesis that there was a
hidden planet whose existence explained the orbit of Uranus needed testing
beyond the observation of the orbit of Uranus, and it got it--by telescopic
observation of the planet Neptune.) Such explanations generate predictions and
raise questions that lead to new investigations. In the end, these investigations
may lead to new and better theories (as was ultimately the case with Newton's
celestial mechanics), but along the way the investigations may also offer
confirming evidence of the theory (as was also true with Newton's theory in the
case of Uranus).
There is a further analogy with law. Sometimes in a court of law, there is
nobody alive who witnessed the crime for which the jury sits in judgement;
they are forced to arrive at a reasonable explanation of an event for which there
is no eye-witness. Similarly, scientists often seek to explain that which no
living person can observe. For example, geologists indicate that the Grand
Canyon was formed by the erosive action of the flowing water over millions
and millions of years. In one sense, we cannot test this hypothesis because we
cannot directly observe the effects of water erosion over the period of time that
is supposedly needed to produce the end result. However, we can measure
erosion of sedimentary rocks over a much shorter period of time (months or
years), and infer that if this process were extended over millions of years it
could produce the patterns of erosion consistent with those that we find in the
Grand Canyon. It is a circumstantial case, and scientists make use of this
pattern of reasoning regularly.
As you read through the evidence section of this site, you will see just how
extraordinarily well Darwin's theory of evolution by natural selection fares. It is
certainly one of the most highly confirmed theories science has ever enjoyed. It
is unifying, predictively successful and certainly has and continues to raise
challenging questions for investigation. Does its success in these respects show
it to be certain? No. While we have every reason to believe it is true, and so are
justified in believing it, the same thing once was true of Newton's theory.
Perhaps, then, there is an Einstein to Darwin's Newton awaiting us in the
future. In the meantime, let us explore the extent to which evolutionary theory
is confirmed by the evidence currently available.
Introduction http://www2.evansville.edu/evolutionweb/evidence.html
Modern biologists recognize that evolution is the process that has transformed
life on Earth from its early beginnings to the diversity of forms that
characterizes it today. Evolution is regarded as the most pervasive concept in
biology, providing deeper meaning and understanding to all aspects of the
discipline. The celebrated geneticist, Theodosius Dobzhansky, vividly captured
the essence of this perspective when he said "nothing in biology makes sense
except in the light of evolution."
Our modern understanding of the evolution of life began in 1859, with the
publication of Charles Darwin's On the Origin of Species. In this book, Darwin
presented a convincing case for evolution. He argued that species were not
specially created in their present forms, but had evolved from ancestral species.
Darwin coined the phrase "descent with modification" to encapsulate the idea
that all organisms on this planet are related to one another through descent from
an unknown ancestor that lived in the remote past. A critical component to
Darwin's concept of evolution was his proposal of a mechanism to explain how
species change through time and how they become better adapted to their local
environments. Many of Darwin's predecessors suggested that life on Earth had
evolved, but their theories were speculative and failed to adequately explain
how the evolutionary process operates. The mechanism for evolution proposed
by Darwin is called natural selection. According to Darwin's concept of natural
selection, populations change over time because individuals with certain
heritable traits leave more offspring than others. Interestingly, while some form
of descent with modification was readily accepted by most of the scientific
community, Darwin’s explanation for this process (natural selection) was being
rejected. Some of the main objections to natural selection were: it lacked an
explanatory theory of heredity; it attributed evolution to chance; and it could
not explain all the patterns (e.g., gaps between forms) that were evident in the
fossil record. Darwin also indicated that evolution proceeded as a branching
tree; new species arise from small populations that branch from an ancestral
species..
Although Darwin's theory of evolution by means of natural selection was a
profound interpretation of the natural world, it was also a rather simple one.
Thomas Huxley, who became Darwin's main public defender, is reported to
have said, "How extremely stupid not to have thought of that!" The essence of
Darwin's theory can be summarized as follows: There is a high degree of
heritable variation in most natural populations. That is, populations exhibit a
substantial amount of variation in form and function and this variation is passed
on through the generations. Populations typically produce more offspring than
can survive and reproduce successfully. The survival and reproductive success
of individuals in a population are not equal. If some offspring have traits that
give them an advantage under a particular set of conditions those organisms
will be more likely to survive and pass on those traits to future generations. As
the differences accumulate from generation to generation, this will lead to
gradual change in the population such that populations of organisms will
diverge from their ancestors, becoming reproductively isolated and eventually
forming new species.
Behind Darwin's rather simple explanation for evolutionary change and
organismal adaptations was a substantial amount of evidence supporting his
claims. In the 140 year period since the publication of On the Origin of Species,
evidence in support of Darwin's theory of evolution continues to accrue. This
essay is intended to present evidence for an evolutionary way of life. Before the
evidence of evolution is presented, it will be useful to introduce a short
discussion on science and the scientific method. There are certain criteria and
avenues of inquiry that scientists must adhere to when attempting to interpret
the natural world objectively. We certainly want to be sure that the evidence
used to support evolution is in compliance with them. Indeed, many of the
objections to modern evolutionary theory, result from misunderstandings about
the nature of science and the scientific method.
NEXT
The coelacanth is a "living fossil" with much the same limb bone structure as the fish which
gave rise to terrestrial vertebrates. The bones of this fish are homologous to the limb bones of all
terrestrial vertebrates. Through reduction, duplication and/or modification, evolutionary pressures
have given rise to organisms which have diverged to occupy different ecological niches. The fin of
the coelacanth, the leg of the frog and the wing of the bird indicate evolutionary relationships
between these organisms.