|Plants & Human Affairs (BIOL106) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; email@example.com; http://www.employees.csbsju.edu/ssaupe|
A Primer on Evolution
"Nothing in biology makes sense except in the light of
American Biology Teacher 35:125 (1973)
Recall from our science unit that a paradigm is a world view, philosophy, perspective, way of looking at things and I used the example of my stupidity at Tim's & Kelly's wedding. We concluded that paradigms affect the way we think and that it is difficult to think beyond the prevailing paradigm. When we do - which is called a paradigm shift - it is usually an exciting time of intellectual upheaval.
Some examples of scientific paradigms that are old (outdated) and new
(modern) are listed in Table 1:
1: Examples of old
(outdated) and new paradigms
|Cause of disease||
(about 10,000 years)
(about 5 billion years)
(the earth's features were shaped by major cataclysmic events,
like earthquakes and floods)
gradualism (the earth's features were shaped by slow processes like
(Great Chain of Being, Special Creation
|not fixed (evolution)|
creation (Bible) and spontaneous generation (outdated theory that living
organisms can "spontaneously develop" from non-living
material. One example, eels
could spontaneously generate from the muck at the bottom of a pond)
|chemical evolution/biological evolution|
In our science unit we also mentioned that paradigms die hard. People are often unwilling to abandon well ensconced paradigms. As an example, we discussed "The Tomato Effect." Some other examples include:
In 1966 Sheik Abd el Aziz bin Baz, King of Saudi Arabia presumably said,
"The Holy Koran, The Prophet's teachings, the majority of Islamic scientists, and the actual facts all prove that the sun is running in its orbit...and that the earth is fixed and stable, spread out by God for his making...Anyone who professes otherwise would utter a charge of falsehood toward God, the Koran and the Prophet".
Buffon (1707‑1788), a French naturalist, was one of the first to
publicly state that the earth is old and that species are not fixed, but that
they changed. He said,
"Each family, among the animals as well as among the plants, comes from the same origin, and even that all animals are come from but one animal, which, in the succession of eras...has produced all the races of animals that now exist".
Buffon was attacked for his heretical ideas and later recanted,
declare that I had no intention of contradicting Scriptures, that I most firmly
believe in everything they say about Creation, both as to order of time and
matter of fact. I abandon
everything in my book respecting the formation of the earth, and in general, all
that may contradict the account of Moses."
(a) paradigms become deeply entrenched in our psyche and are hard to
remove; (b) it's a bad idea to base science on religious dogma.
II. The Evolutionary Paradigm
Evolution is the process by which: (a) one species gives rise to another; and (b) a species becomes adapted to its environment.
Evolution suggests all species share a common ancestry, some more recent than others. For example, we share very recent common ancestors with our parents and immediate family, and going back even further, with our grandparents, etc. If you go back far enough, we share a primate ancestor with chimps, and if you go back far enough, we share a cellular origin with all life. Closely related species simply share more recent common ancestors.
Special Creation is the notion that a Creator created the earth and its occupants, pretty much in the form that we now see.
Let's use an analogy. Imagine all of the earth's species, both extinct and extant, are like the leaves of a tree. When a creationist looks at the leaves, s/he sees only leaves - no twigs, branches or trunks. A creationist assumes (paradigm) that these parts were never present; God made the leaves just as they appear. On the other hand, an evolutionary biologist sees the leaves and assumes that they are part of a tree with invisible twigs, branches and trunk. The twigs, etc. represent the course of evolution; species on the same branch would be more similar that species on separate branches.
III. Importance of
As the opening quote implies, evolution is the single most important conceptual idea in biology. Why?
A. Evolution explains the unity and diversity of life.
We learned during our life unit that one of the features of life is that it is diverse; that is, we share the planet with as many as 30 million other species (further, 90% of the species that ever lived on the planet are probably now extinct). Conclusion: there are many different kinds of leaves on our "tree."
However, even though living things show considerable diversity, they also share many features in common. For example the genetic code (i.e., codons), molecular building blocks (i.e., proteins, polysaccharides), biochemical pathways (i.e., glycolysis, respiration) are similar in diverse species. Chimp and human hemoglobin are identical. Conclusion: even though our "tree" is made up of many different kinds of "leaves," these leaves share many features.
How can we explain why such diverse species as, say thermophilic bacteria and elephants, share similar characteristics? Let's use both paradigms:
Special Creation paradigm - the Creator made them that way;
Evolution paradigm - since species are related to one another by descent you expect them to share similar characteristics. Since these species adapt to different habitats, they also have different/unique features. Thus, in the evolutionary paradigm, the similarities between species illuminates the "invisible" connections between species and the differences explain why the tree branches as much as it does (each species adapts to slightly different sets of conditions).
Evolution ties together assorted biological "facts" and unifies
them into a coherent whole.
C. Evolution highlights the relationship between structure and function (since structures are the products of evolutionary change).
III. The Evidence for Evolution � Taxonomy
Biologists/naturalists, especially during the Age of Expansion (1450 � 1750), combed the globe returning to Western centers of learning with many new species and observations. Among the insights were:
there is a bewildering diversity of species;
there were many species in distant parts of the world that were similar to those in Europe;
no two individuals of a species are alike.
Many important naturalists contributed to our knowledge during this time, including Buffon and Linnaeus. Why should there be so many different kinds of leaves on the tree? Or, why should species from very different places have similar features?
Creation paradigm: just because
that's the way the Creator wanted to do it.
Evolution paradigm: different habitats = different species. Species share similarities because they are related; i.e., have have a recent common ancestor (one basic stock, manipulated by the environment).
IV. The Evidence for Evolution: Biogeography
These are studies relating to the distribution of plants and animals. Among the observations:
species often intergrade;
there are different species on east vs. west side of the Andes (mountains);
flightless birds tend to exist on islands.
finches in the Galapagos - unique but similar to mainland species
How can we explain these observations?
Creation paradigm - just because
paradigm - different habitats = different species.
V. The Evidence for
Evolution: Comparative Anatomy
These are studies in which the structures of different organisms are compared. Anatomists learned about:
A. Homologous vs. analogous structures.
Homologous structures are those with the same embryological origin, but may differ in function/structure. i.e., forelimbs of mammals. In contrast, analogous structures have a similar function/structure, but different origin. i.e., wings of birds, bats, butterflies. Homologous structures provide evidence for evolution. Why should a horse, dolphin and human forelimb all be constructed from the same basic parts? Creationist - just because. Evolutionist - because they all share a common ancestor, but adapted and modified for survival in different environments.
B. Vestigial organs.
These are structures with no apparent function. Why should humans have an appendix or a snake hips? Creationist model - just because. Evolutionist model - these structures are remnants of past evolutionary history. Thus, they clearly help make visible the branches and twigs of the evolutionary tree.
Embryonic similarities of animals.
Why should various vertebrate embryos look so similar during early stages of development?
Special Creation model - just because
model - these structures are remnants of past evolutionary history, making
visible the branches and twigs of the evolutionary tree.
The Evidence for Evolution: Geology
Catastrophism - the earth's surface was shaped by major, cataclysmic events, like Noah's Flood, major earthquakes and volcanos. This was the prevailing paradigm of early geologists. A logical corollary ‑ the earth was relatively young, around 10,000 years old.
Uniformitarianism - By the nineteenth century, Lyell and Hutton had
argued convincingly that the earth's surface is changed by slow, gradual
progressive changes and that these processes (like weathering, erosion) are
happening even today. Conclusion:
the earth must be really old.
The biological impact:
helped biologists to see beyond the paradigm that species are "fixed". If the earth can change, why can't something obviously more dynamic like a species, change? and
uniformitarianism provided the time needed for evolution to occur (evolution takes time and 10,000 was not nearly enough time).
VII. The Evidence for Evolution: Paleontology
Paleontology refers to fossil evidence. Initially, folks didn't know quite what to make of fossils, but eventually fossils were recognized as life forms form the past. Studies of the fossils showed that:
A. Fossils vary from strata to strata; simple fossils in old rock and more complex fossils in newer rock
Creationist paradigm - just because
Evolution paradigm - our ancestors were much less complex, again showing the invisible connections;
B. Extinct forms existed, many with no living relatives. Why would a Creator make fossils, especially of species with no descendants?
Creationist paradigm - just because.
Evolution paradigm - predicts that extinct forms are those species that failed to adapt to environment;
C. Intermediates existed (missing links). In other words, some fossil forms showed characteristics intermediate between two groups. For example, Archaeopteryx has teeth, tail, feathers. Transition between reptiles and birds.
Creation explanation - just because;
Evolution - share similar common ancestor.
D. Living Fossils
Interestingly, many of our cultivated plants no longer exist in the wild (e.g.,
maize, poppy, tobacco).
These are species that were first known from the fossil record and presumed to be extinct, only to be "discovered" later. Examples: Coelocanths (Old four legs) is a fish discovered on Dec 22, 1938 Marjorie Courtney Latimer hauled in by a trawler from near the the mouth of the Chalumna River (South Africa). Ginkgo (Ginkgo biloba) was discovered in Chinese monastic gardens and Wollemi pine has recently been discovered in Australia.
IX. The Evidence for Evolution � Breeding Studies
Artificial selection. This is a common practice in crops and livestock. Thus, farmers and other realized that (i) organisms show slight individual differences; (2) some variations are inherited; (iii) selection for a trait can "intensify" its expression; that is, show a directional change (i.e., faster race horse, more milk production, higher crop yields).
Interestingly, many of our cultivated plants no longer exist in the wild (e.g.,
maize, poppy, tobacco).
X. Jean Baptiste Lamarck
Lamarck, a student of Buffon and a distinguished botanist/natural scientist. He held the position of Professor of Insects, Worms and Microscopic Animals at the Natural History Museum in Paris. He was one of the first to use dichotomous keys, and made many contributions to anatomy and taxonomy. He coined the terms biology and invertebrate.
Lamarck was one of the first to suggest that species are not fixed, but that they can change. He believed that species are inter‑related and that higher forms are derived from simpler ones. According to the prevalent idea of the time, the Great Chain of Being (from Aristotle), each species was considered to have a place on a ladder where it was relatively fixed.
Lamarck suggested that the ladder may be more like an escalator.
He believed that: (i)
species have a built in desire for perfection (to be like a human).
Species "desire" to climb the ladder.
This "desire" ultimately powers the escalator; (ii) spontaneous
generation replaces species at the bottom of the ladder; (iii) new structures
can appear, be acquired, in response to a need (use and disuse).
Acquired traits are those obtained during an individual's lifetime, or in
other words, the individual was not born with them (e.g., suntan, large
muscles); and (iv) acquired traits are inherited.
This idea is
obviously bogus. Experimental proof
‑ chopped rat tails; circumcision. Even
though Lamarck was wrong about HOW evolution occurred, he was one of the first
to clearly state and support the notion of evolution ‑ that species DO
change over time. For this, he
deserves much credit.
As an aside, Lamarck was chastised during his time for his very vitalistic
outlook on species. And, even Darwin looked down on Lamarck: In a
letter to Hooker in 1844
he wrote that
didn't know of any books on a particular topic with the exception Lamarck's, which
he called "rubbish." Uff da.
As an aside, Lamarck was chastised during his time for his very vitalistic outlook on species. And, even Darwin looked down on Lamarck: In a letter to Hooker in 1844 he wrote that didn't know of any books on a particular topic with the exception Lamarck's, which he called "rubbish." Uff da.
Classic examples: giraffe, ducks, snakes. Check out the
reading in the public folder.
Classic examples: giraffe, ducks, snakes. Check out the
reading in the public folder.
XI. Chuck Darwin. Evolution by way of natural selection
personal issues. Came from a
wealthy family (gave him leisure time); liked nature, especially bugs and
rocks; trained as a physician, but couldn't stomach it; trained in theology,
bored him; Henslow, a botanist at Cambridge, was a good mentor.
The Boat Ride: Darwin served as naturalist on the HMS Beagle. Darwin was selected to serve as a companion to Captain Fitzroy who was a strong‑willed, hot tempered creationist. Note: Darwin as essentially a creationist at the beginning of the voyage, but changed his mind as he gathered more and more observations. Some of Darwin's insights on board ship include:
he became aware of the unity and diversity of life (e.g., biogeography, fossils. Darwin wondered why glyptodont fossils occurred where armadillos did, but not elsewhere?);
he read Principles of Geology by Lyell (uniformitarianism);
visited the Galapagos Islands off the coast of Ecuador. He noted that each island in this archipelago had its own unique fauna and flora that was reminiscent to the mainland but more similar to each other. He also knew the islands were volcanic, which meant that they were younger than the mainland. Why were the species different than those on the mainland? But, why did the species resemble those on the mainland? Why did each island differ?
Creationist paradigm - just because.
Evolutionary paradigm - some ancestral species made it to the islands, and evolved into others.
Rev. Thomas Malthus. 18th century
theologian/economist. Wrote Essay
on Populations (1798). In it he
said that populations increase geometrically
(exponentially, which means doubles or triples, etc. at each time increment; 2,
4, 8, 16, 32, 64, etc) and resources increase arithmetically (increase by a
constant amount at each increment; 2, 4, 6, 8, 10, 12 etc).
Thus, populations grow faster and will eventually use up resources.
This will lead to a competition for resources (struggle for survival).
Natural Selection. Simply stated - those individuals with traits (features or
phenotypes) that are most successful (better adapted) will increase in frequency
in a population over time (because they have more offspring).
The environment "selects" for the most "fit."
Peppered moths (Biston betularia)
Widespread in England. Polymorphic - some individuals dark colored, others gray speckled. They fly at night and sit on trees during day. Lichens, common on trees, are light colored - thus the light variant is well camouflaged. Predators (birds) easily spot dark moths and eat them. Thus, the population was mostly light colored moths until mid 1800's. From 1848 -1898 the dark colored variant increased in frequency 94%. Why? industrial revolution made soot/pollution that killed the lichens and darkened trees. Now the black variant is more fit. Note: (i) adaptive value of trait ‑ light "better" in some circumstances, black in others. (ii) a predator was the selective agent; (iii) dark forms due to a change in one gene locus; (iv) the appearance of the dark form is not linked to the environment, presumably it was a mutation; (v) selection resulted in a change in gene frequency (alleles for black increased); (vi) the environment influenced the population. As an aside, note we also learned that certain lichens are good monitors of air quality, especially for sulfur dioxide.
Antibiotic resistance in microbes.
Recent example, resurgence of antibiotic resistant strains of TB.
Crabs and Periwinkles - worksheet
XII. Alfred Russel Wallace
He was English, but unlike Darwin didn't come from a wealthy family. He spent lots of time in the field. Lived in South America and experienced hardships (yellow fever, lost his collections on the way home in a fire on board). He was an excellent illustrator. He spent 8 years in the Malay archipelago (Singapore/Borneo). He had published several works on origin of species. Sent a manuscript outlining natural selection to Darwin to critique. Darwin immediately recognized his ideas. Both papers were presented at meeting.
Let's use my "favorite" animal, the cat, to develop a model for evolution. Assume that we want to open a restaurant in St. Joe serving our specialty, roast cat, to throngs of hungry students and others. Wow! Business is booming and so we franchise (reproduce) the operation. Soon there are Cats Delight Restaurants all over the Midwest (and very few stray cats). Our restaurant is certainly "fit." Our recipe for cooked cat is very successful.
Now, let's imagine that in one of the restaurants, there is a slight error in the recipe book - when it was duplicated (replicated) the amount of salt was changed from 1 teaspoon to 1 tablespoon. This would represent a mutation in the instructions. Any franchise that arises from this one would also have increased levels of salt in the recipe. There is now some variation, regular and extra-salty, in our population of restaurants. Note that the recipe did not change in response to the environment - the recipe change was accidental and was then 'tested' by the environment.
If the extra salty-roast cat tastes better than regular, these restaurants will do a booming business. The other restaurants may fold. In the meantime, the extra-salty will thrive, spawning franchises here and there. Thus, the frequency of the extra-salty recipe would be increase and the regular decrease - viola, evolutionary change via natural selection. The consumer is the selective agent that is acting upon the individual (restaurant). Those restaurants that are most fit (salty recipe) will do better than those without, and hence, lead to more franchises. Over time we'll see the loss of the regular version restaurant and all salty ones. Yummy.
This is the end product of evolution; individuals become adapted to their environment. Thus, most features of individuals have some survival value that increases the reproductive fitness of the individual. And most do. If not, it would have been selected against during the course of evolution, just like the unsalted roasted cat. This is why biologists are able to ask questions about structures and functions. If we observe a structure we can ask scientific questions about it's utility for the organism.
However, there are some exceptions in which a structure isn't a
particularly useful adaptation. For
example, have you ever wondered why males have nipples?
They have no function. So
why hasn't evolution "eliminated" them?
Simply because they DO have a function (suckle offspring) for females -
and since males and females are constructed from the same basic plan, males have
nipples because females need them, they are homologous structures.
Or, have you ever wondered why humans have orgasms? One answer is that orgasm is a "payoff" for sexual reproduction. Thus, we would expect that the site of intercourse should be the site of orgasm; which it is in males but not females. Why should females have clitoral orgasms when the vagina is the site of intercourse? Why didn't evolution "get it right?" For the same reason that males have nipples - because males and females are constructed from the same basic plan. During embryo development tissues that become the penis make the homologous structure in females, the clitoris. Check out the article "Freudian Slip" by Stephen J. Gould for more details.
Does evolution ever fashion a perfect, perfectly adapted, organism? NO. Individuals are: (i) locked into historical constraints; (ii) adaptations are a compromise; (iii) not all evolution is adaptive (as described above); (iv) selection can only edit variations that exist.
Evidence for Evolution
Above we described many examples of evidence that support evolution. Stephen J. Gould suggests that there are three major lines of evidence:
Observation - such as antibiotic resistance in microbes and peppered moths;
we have been able to "watch" evolution happen.
Evolution is revealed by the "imperfections of nature", or to use Darwin's
By imperfection, Gould means any structure that is modified from
other parts. In other words,
imperfect solutions to particular problems.
e.g., Panda's Thumb, used
for stripping leaves from bamboo, is fashioned from a wrist bone; forelimbs
of mammals (homologous structures); floral parts; vestigial structures.
Record. numerous transitional
fossils are known, and fossils vary by strata.
XV. Evolution as a
Evolution adheres to the rigors of the scientific method. Creationism does not (must be taken on faith). For example:
is testable � meaning that hypotheses and predictions about evolution can be developed and
tested. Let's give some examples:
IF evolution by means of natural selection is true, THEN...
simplest fossils must be found in oldest strata. We can test this by collecting fossils. And, all of the fossils collected to date support this notion.
there must be variation among individuals. There is.
more offspring must be born than will survive. It's true.
species from the distant past must be different that those today. They are.
the earth must be old. It is.
related species should show similarities in structures, cells, molecules, etc. They do.
and so on....
is falsifiable - it is possible to imagine evidence that exists that could
prove it false. If complex
fossils are found in old rock strata, it would disprove our hypothesis.
However, this has never occurred.
This is one area that makes creationists salivate.
For example, in Texas there is a riverbed that supposedly shows
dinosaur footprint side-by-side with human footprints.
If true, this would be a falsifying case.
Unfortunately for the creationists, this fossil bed turned out to be
is predictive. For example,
most biologists could have predicted that the heart transplant received by
Baby Fae on October 26, 1984 would not be successful (she died 21 days
later). This experiment was
doomed because baboons are not as closely related to humans as other
primates (like chimps). For
example, humans quickly reject transplanted baboon kidneys, but chimp
kidneys are no more quickly rejected than one from a human.
So, why did the surgeons pick a baboon? The surgeon: (a) hoped that
immunosuppressant drugs would prevent rejection; (b) hoped that Baby Fae's
immature immune system wouldn't reject the tissue; and (c) didn't believe in
Another example - Bolley (1904) studied flax fields that were severely affected by flax wilt disease. Bolley predicted that if seeds from flax plants that survived wilting disease were crossed, disease resistant plants could be developed. He was right. Note the variation in individuals (some susceptible, others resistant), selection agent (wilt), change in gene frequency.
Evolution as Fact & Theory
Darwin On-Line - full text versions of Darwin's books
Issues in Evolution - from ActionBioscience.org. Great links and reference materials
Understanding Evolution - Univ of California-Berkeley
Tree of Life - consortium hosted by Univ of Arizona Library
EvolveIT - evolution simulation
Last updated: 11/04/2008 / � Copyright by SG Saupe