An Online Introduction

to Advanced Biology

 
   

Terms and Concepts

 
 

SUBSITE TWO

CHAPTER 5 -

Explaining Life:  Evolution

 
     

 

  IDEAS ABOUT HOW ORGANISMS EVOLVE

 
 


Evolution is a change in type over time.  It connects back to that human compulsion to label and categorize things, combined with a knowledge of how the world of the past was different than todays world.  All sorts of things can evolve, so this may be the feature of Life found most often in things that are not alive.

Of course, for a long time it wasnt known that Nature had ever been any different than it is today - people accepted historical changes in cultures and societies, but not in the types of living things.  At the birth of modern western science during the Renaissance, it was still widely accepted that the Earth was only a few thousand years old, and Nature was a continuation of the Perfect Creation of Eden - only Man, with his ability to choose, was a flawed product of the Garden.  However, during this period a systematic approach was taken to studying fossils, and it was quickly found that the deeper (and older) a layer one looked at, the less like modern forms the fossilized animals were (animal hard parts fossilize much better than plants, and plant evolution is less dramatic than animal evolution).  In very old layers, there were types that seemed almost totally unlike anything existing today, such as the dinosaurs.  In newer layers, there seemed a progression of types that became more and more similar to things living today.  Attempts to explain fossils away as some sort of bizarre rock formations found some acceptance, and worked all right for separate bones, but it became impossible to explain toothy skulls and near-complete skeletons that way.

Most fossil beds contain the remnants of creatures from the sea floor, since most fossil layers are sedimentary rock that at one time was sediment in a body of water.  One of the first men to study such formations, Charles Lyell, became one of the most powerful influences ever on both geology and biology.  He decided that the production of fossil beds and the nature of the fossils themselves suggested a long, continuous process in a world not very different from what we see around us today (this idea, which didnt originate with Lyell but was not widely-accepted before his publications of the 1830s, is called uniformitarianism);  he rejected the idea of abrupt changes (and so rejected those who connected hundreds of meters of sedimentary rock to deposits of the Biblical flood) or that the ancient world was significantly different than today.  He was also one of the first widely-accepted voices to reject a Young Earth, as there was too much sediment accumulation shown in the rocks (even ignoring compression) to have been done in a few thousand years.  His calculations for the actual age of the Earth increased throughout his life as more information came in - by his death, he felt that the earth was several hundred million years old, and it is now thought that the Earth is 4 to 5 billion years old.

Two concepts arose that challenged the Word of the Bible and were considered blasphemous in those times.  The first we might hardly realize was controversial:  extinction.  Fundamentalist reading of Scripture declared the Bible to be totally incompatible with the idea that one of Gods creations could simply die out, although that idea has since largely been universally accepted.  Some Biblical scholars suggested  that Mans world is not the first Creation, and fossils are remnants of earlier versions, but this way of reading the text is not widely accepted anymore.

The second concept is evolution, the idea that types of living things change over time, that modern forms are significantly different from their distant ancestors, and that ongoing influences in Nature produce continuing change.  This idea is not totally incompatible with religious ideas, and in fact is accepted widely by people whose religious texts might seem to say otherwise, but there are many who choose to resist the idea.  Evolution, many have decided, represents a sort of "progressive Creation" which is all still part of Gods Plan for the world.  

In evolution, it seems to be that the underlying assumption that human beings have evolved from non-human ancestors that produces the biggest "sticking point," which is understandable.  Humans are strongly invested in the idea that they are somehow special and exist apart from the rest of the living world, and most evolutionary concepts challenge that assumption.  However, it should not be assumed that biologists are mostly atheists, as they are not.  As humans are expert at, they usually adjust their religious concepts to fit with the realities of their particular worlds.

Once the idea of evolution gained some momentum, the question of how such a process works became very important.  Many prominent naturalists came up with various explanations, but only a few are still widely remembered.

Lamarck.  Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck is known to history as a Man With a Silly Idea, which is unfair.  Strangely enough, during his life his ideas were largely ignored by his peers, but became influential long after his 1829 death.  Two concepts that are associated with Lamarck, neither true, are:

Evolution by Inheritance of Acquired Characteristics. Lamarck grasped that inherited changes, influenced by the environment, were important to evolution, and he wasnt wrong.  His mistake was in believing that changes in features developed during a lifetime (say, weightlifting to make yourself more muscular) could be passed to offspring (the children would themselves be born more muscular).  He thought that an organisms active adaptations to their environment could be passed on.  But would you be surprised if a world-class bodybuilders children were more muscular than most kids?  What we now accept, that something about that bodybuilders genes gave them the potential to train and to be more muscular than most, could be passed on, was unknown in Lamarcks time, and what seemed logical then no longer seems so now.  What should also be noted is that Lamarck seemed on the same track that Darwin wound up on in explaining evolution:  how traits that were well-adapted to the environment would be passed on and produce longterm changes.  He just was wrong about how traits got passed on.

Evolution = Improvement.  A typical philosophical approach for scientists of the 1800s was to hypothesize as a way to investigate and describe the Perfection of Gods Creation.  Part of the idea is that Modern Man, and the world he lives in, is the culmination of that plan, and so all evolution is a pathway, onward and upward, to that goal.  This idea, that evolution is a process of betterment working toward some goal of perfection, still has a strong influence on how people, even biologists, see evolution, although the widely-accepted idea is that evolution is a generation-to-generation adaptation to current conditions, which change randomly and over time produce random evolutionary pathways.

Thomas Malthus.  In the early 1800s, Malthus developed his ideas about how conditions affect populations.  He talked about the Natural world, where the rate at which a population could increase was exponential while resources remained about the same; populations would therefore be limited by disease, famine, and conflict, but there was a warning embedded about growing human populations as well (and maybe suggestions on how to use the principles to limit the numbers in the lower classes).  These ideas were huge influences on the evolutionary theories of both Lamarck and Darwin, who both talked about environmental influences on groups as a basis for evolution.

That idea, the current best explanation for how evolution works, is the Theory of Evolution by Natural Selection, developed and written down originally by Charles Darwin and Alfred Russel Wallace in 1858, with many slight adjustments and additions by many people since.  Generally, "disagreement" in scientific circles with this theory involves a dispute about how much Natural Selection influences evolution compared to other factors, not whether the basic ideas are accurate.

Charles Darwin.  Product of a fairly well-to-do but largely dysfunctional family, Darwin resisted his fathers ideas of what sort of career he should have, and in 1831 took a position on the merchant ship HMS Beagle, more or less to give the Captain someone of the same "station" for company (the social isolation of captains over long voyages had become quite a problem).  Darwin also took on the duties of the Ships Naturalist, and went on a voyage around the world, with notable stops along the coast and islands of South America.  Because of horrendous seasickness (and possibly personality clashes with the Captain), Darwin spent as much time as possible ashore, collecting samples and taking notes and making observations.  One thing he noticed was that the character of the animals and plants on islands resembled those of other islands nearby and the mainland, dependent upon how close together they were and how much the physical nature of the environments differed.  The strangest collections of creatures by far were found by Darwin on the Galapagos Islands, a collection of arid islands some 1000 kilometers (600 miles) from the equatorial jungles of South America.  The limited types of animals there had obvious relatives on the mainland, but were different species particularly suited to the different lives available on the islands.  Assuming that their ancestors were groups of mainland animals unlucky enough to wind up there, how did they evolve into these different species?  During his travels, Darwin began to develop his explanation for just such processes, which he came to call Evolution by Natural Selection.

What is Evolution by Natural Selection?  Sometimes nicknamed "Survival of the Fittest," it would be more appropriate to call it "Reproduction by the Fittest."  Simply put, in any given group of organisms, there will be some variety of features that directly affect how good a chance each individual has of living to reproductive age and then successfully reproducing - as a general trend, each generation of offspring will, more and more, reflect features that are advantageous to their environment.  Fitness actually means how well something "fits."  The important detail here is that environments change over time - and what was a good feature in one time and/or place may not be elsewhere - and these changes in environment (the "Nature" part of Natural Selection) influence (the "Selection" part) which individuals reproduce and what features preferentially wind up in the offspring.  Over time, depending on an organisms suitability to the new environment, new features and combinations of features (called adaptations, a confusing term that does not always mean the same thing even to biologists) may spread through the population as a whole until the basic "type," or species has changed significantly from the "type" of its ancestors that it needs to be relabeled.

Darwin was strongly influenced by some knowledge of animal husbandry - he knew that breeds of domestic animals could be changed over time by selecting which individuals breed, a process that has come to be called artificial selection.  In Natural Selection, the "picking" of the breeders is dependent on which individuals can survive and succeed under the natural conditions they were born into.

Evolution is not an "ever upward movement toward perfection," as Lamarck and many of his time believed;  species dont get better at anything other than fitting the environment of the day, which could change at any time.  There is no target, no progress, no ultimate peak at humans, and not everything evolves at the same rate, partly because the rate depends both on how fast the environment changes, which varies considerably from place to place (and even pieces within environments vary), and on the rate of reproductive "turnover" that can give short-lived organisms much faster potential rates than long-lived ones.

When Darwin returned to England, he settled into a role as a naturalist and went about writing up his theories, although he seemed in no hurry to actually publish them.  He made his reputation as an expert on smaller topics, such as earthworms, and certainly seemed comfortable discussing his ideas with other naturalists.  Perhaps he knew what sort of reaction a widespread distribution of his theory would get from the general public.

Then, along came Alfred Russel Wallace, whose travels through South America (where virtually his entire specimen collection was destroyed in a shipwreck) and Indonesia (perhaps the worlds largest collection of islands, presenting huge numbers of separate- but- somewhat- connected environments) led him to essentially the same ideas that Darwin had come up with.  In the late 1850s, Wallace wrote an article based upon his ideas and sent it off to England, where it was brought to Darwins attention (some account have him sending it through Darwin, but that seems unlikely).  Once published, the concepts would from then on have been linked only to Wallace;  Darwin arranged it so that when Wallaces paper was made public, it was accompanied by a similar one by Darwin.  They brought out the same idea "simultaneously."

In the years that followed, Charles Darwin became much more associated with Evolution by Natural Selection than Wallace.  There are many reasons for this:  Darwin was in England, where he was well-known in certain circles, while Wallace, who would not return from Indonesia for a few years, was less well-known or respected.  Also, Darwins ideas were soon presented in a book, On the Origin of Species, which was widely-read and became the particular reference source on the subject.

Darwin also developed a theory of Evolution by Sexual Selection to explain traits that had obvious advantages in the competition for mates but which might actually be disadvantages from a survival standpoint, traits such as a peacocks tail.  This was not difficult to integrate into his ideas, since it is actually mating success that ultimately decides which traits get passed on and affect the nature of descendants.  Evolution may be a balance between what helps you survive long enough to mate and what helps you actually accomplish the mating.  And, in an example of an established term broadening into a confusing area, "sexual selection" can be applied to any feature that aids reproduction success, even in asexual reproducers.

Not surprisingly, Darwins ideas were controversial and remain so to this day.  What also shouldnt be surprising is that, although the idea of natural selection has great explanatory power for evolution, there are parts that Darwin couldnt explain because no one knew how certain processes work.  Like any great, descriptive theory, the years since its release have seen a long procession of tinkerers that have adjusted it here and there.  As we continue through the history of biology, we will investigate the impact of later developments.

One side-note to the theory:  the French translation of On the Origin of Species tended to include "interpretations," including a subtitle, that suggested that the whole idea confirmed Lamarcks ideas.  Even the word "selection" was replaced with the word "election," implying more purpose and plan.  That version was used widely in Europe;  its difficult to assess how that affected later developments there.

 

 

FOSSILS AND EVOLUTION

 
 


Although fossils were some of the first aspects of nature whose explanation led to ideas of evolution, they have limited use in providing evidence for the theories.

As said before, fossils are usually found in layers of sedimentary rocks, from newer surface rocks to older deep layers.  The organisms died and sank to the bottom, where usually their soft parts were consumed by other organisms and only shells, bones, and teeth remain.  And for large organisms, even those that lived in water would be few and far between in the sediments (think about trying to find a shark carcass on the bottom), so its amazing that we have much record of big land animals (like dinosaurs) at all!

These ancient layers of sediment were laid down in bodies of water, but most known fossil beds are no longer under water (the major dinosaur beds of North America havent been under water for tens of millions of years).  So the record in any given place usually has time gaps of varied width, occurring at varied points in history when that land has risen above sea level.  Fossils can also be made when organisms are covered by sand, or tar (a rare happening), or pine sap (amber fossils), or volcanic ash (lava tends to be too destructive and produce too hard a rock to be good for fossils) - fossil footprints often were made in wet sand that was covered by ash that dried and protected them.  Objects in the buried layers lose water and accumulate minerals, eventually becoming petrified, or rock-like - becoming fossils.

So whats in the fossil record?  Mostly small ocean-living shelled animals, such as clams and snails.  The evolutionary record for these creatures is especially good, but of little interest to most people who want to see a progression of forms in the larger, sexier beasts, for which few fossils exist.  We may have a single individual for a wide span of time, and no way to know how typical it was or really how linked to an evolutionary sequence it may be.  Those who criticize evolution often use the "spotty" fossil record for their evidence, and its easy to see why.

 
     
 

ANOTHER WAY TO LOOK AT THINGS - COMPARATIVE EMBRYOLOGY

 
 


Through Darwins time, most of the time relationships between types of organisms were determined using comparative anatomy, as discussed earlier, relating homologous features between living forms and between fossils and living forms.   When homologous features become used for different purposes - are no longer analogous - the process is called divergent evolution, the splitting of a family tree in different directions.  When unrelated groups have analogous but nonhomologous features (wings in birds and butterflies, fins in squids and seals), the process is called convergent evolution (sometimes parallel evolution) - similar needs may produce similar structures, even if theyre based on different architecture.

With the development of the microscope, this could be extended to microscopic anatomy:  features could be compared on the level of the tiny, as well.  

One type of comparison was championed by Ernst Haeckel in the late 1800s:  looking for similarities between the embryo forms of animals, specifically backboned animals (vertebrates).  Relating the progression in form of developing mammal embryos to the progression of forms in the fossil record (fish to amphibian to reptile to mammal), Haeckel claimed that "Ontogeny recapitulates phylogeny," or that we "replay" our evolutionary history as embryos, going through a fishy, then an amphibianlike, then a reptilian form on our way to being mammals.  This theory was a product of real similarities, enhanced by Haeckels sincere belief that they existed (science truism - we often see just what we expect to see, even if it isnt really there), but it turned out to not really be true.  

But comparing embryos is very important, because evolutionary changes rarely impact early development.  For one, the "environment" of embryos (such as an inside an egg) may be much more similar than the broader environment around it.  Also, the earlier changes occur, the larger the impact can be, and evolution tends work more by small variations than huge leaps.  This means that organisms that dont look at all related as adults can be "connected" by finding embryo similarities "left over" from their shared distant ancestors.  It tells us that humans are more closely related to starfish than they are to clams, for instance, which would seem hard to determine looking at adults.

Some of the most widespread connections have come recently, as researchers have looked into the genes responsible for very early embryo development.  These homeogenes or hox genes code for proteins that determine front-and-back, right-and-left, inside-from-outside-layers, organ placement, and so one, and are turning out to be remarkably similar across wide ranges of organism.  For example, almost the same exact gene determines eye placement in both fruit flies and mice.  This makes sense, since basic layout genes could be used to produce a wide range of organisms.  And they do.

 
     
 

  EVOLUTION AT WORK ON THE GALAPAGOS ISLANDS - ISLAND IGUANAS 


Iguanas are large green lizards that live mostly in jungle trees, eating foliage and fruits.  You may have seen such iguanas as pets.  On the Galapagos islands, there are two species of iguanas:  marine iguanas, black lizards that live on the rocky shoreline and feed by diving into the cold surf and eating algae from the rocks, and upland iguanas, reddish lizards that eat the dry bristly plants of the hot deserty areas away from the ocean.

Assuming that both Galapagos species evolved from a group of mainland iguanas, how did they get there?  There have been recent observations of mats of trees and vegetation, with their resident iguanas, swept away by hurricanes and delivered across the ocean to islands hundreds of miles away.  One would think that the journey itself would select those individuals best able to deal with a harsh environment (and at least a few who could swim).  Once on the islands, there are no exact matches for the type of vegetation the green iguanas would have been used to - only those who could deal with what was available would survive, sending those abilities on to their offspring.  The splitting of descendant groups into those specialized to dive for algae and those specialized to live in the uplands makes sense, as very different features are needed for those two environmental lifestyles.  Before long, the two groups would become separate breeding groups and evolve along their own paths, becoming the two species they are today.

A similar process produced some dozen different types of finch species, known as Darwins finches, on the islands.  Some are very much like mainland finches, but others have moved into totally different niches.

According to modern theories, that is similar to what happened in our own, human, family tree.  A few million years ago, a change to a drier climate in parts of Africa gradually turned jungles into grasslands.  Groups of our chimpanzee-like ancestors that were able to feed in the new environment flourished and adapted, becoming upright, tightly-social, rather nasty human beings (monkey species adapted in similar ways, minus the upright walking, to produce baboons).  Some groups of chimpanzees stayed in the jungle areas, where they were already well-adapted and needed hardly any changes to remain that way.  Thats how humans could have evolved from chimp-like ancestors but chimps could still exist.

 
     
 

  EVOLUTIONARY RATES

 
 


How fast does evolution move along?  Darwin, with his background in uniformitarianism, thought it a long, slow, gradual process.  Evidence from the fossil record (such as it is) seems to indicate a punctuated process - long periods during which things barely change, and then quick transitions to new forms.  These ideas have produced two "warring camps" in evolutionary biology.  Probably, like most of the times such battles arise, the truth is a combination of both.

According to the theory of Evolution by  Natural Selection, evolution is tied to the environment, and we know that environmental change is sometimes slow and gradual and sometimes quite fast between relatively stable but different eras.  Why wouldnt evolutionary rates be the same?

In some ecosystems or parts of ecosystems, changes almost never happen.  For instance, there have been beach ecosystems and open, deep-water ecosystems since before there were animals to occupy niches in them;  they just havent always stayed in the same places.  Some species long ago adapted to such systems and have moved with them as the systems have migrated;  these species have also shown very little change over a very long timeframe.  Beach system inhabiting horseshoe crabs today look almost exactly like their 100-million-year-old fossil relatives.  Large open-water sharks have almost as long an unchanged history.  In fact, it has been found that such animals are particularly resistant to mutations, one of the bases for evolutionary change (thats why sharks rarely get cancer, commonly a product of mutation - its not the "magic" cartilage).  These organisms have actually evolved a resistance to evolution!

Another important part of evolution is that it is driven by reproduction over multiple generations, which means that a species that reproduces very rapidly can also evolve very rapidly (depending upon reproduction type and variability within each generation).  In the evolution of diseases, when small, quick-reproducing organisms invade larger, slower-reproducing ones, each evolves, but the "germs" tend to evolve / adapt faster.
 

 

 

 

Evolution, Religion, and Politics

The idea that all living things, including humans, evolve challenges many cultural and religious beliefs about the position and uniqueness of Man in the world.  This, of course, was among many such challenges that arose as Man learned more about how Nature works, and eventually, ideas about the flatness and centrality of the Earth, the extinction of created types, and many others have shifted from blasphemous to widely-accepted.  But evolution, which can be seen at work in many subtle ways, as an idea hasnt the blunt force support of something like a round or revolving Earth, and can still be denied.  And denial is fine, but this has become more like a battle between things that really cant fight.  In opposition to the concept of evolution are articles of pure faith, which are either factually unsupportable or scientifically untestable.  And shouldnt such faith-based things be untestable?

In the battle that does wage, however, the concepts of evolution are usually misrepresented.  One hears that evolution is "controversial," and filled with "dissenting opinions."  However, evolutionary theory is the supporting idea of biology the way that atomic theory supports chemistry or gravitational theory supports astronomy:  theres no real doubt that everything is driven by such forces, and although scientists may disagree on the details of the mechanisms, the concepts have as much weight as anything is allowed to have in science.

Somehow, since evolution seems to conflict with a particular interpretation of some religious treatises, this has led to a portrayal of science as being faithless, as if all scientists were atheists, which is of course untrue.  Religious scientists tend to have beliefs that are not literal fundamentalism, but that tends to be true with highly educated people in general;  that doesnt make them somehow less spiritual than literalists.  Evolution has also become a political issue, as if it was by definition anti-religion, when it surely isnt.  A persons appreciation of how the world works, with whatever driving force they believe, is enhanced by learning the rules - Darwin, Wallace, and Lamarck were trying to gain insight into Gods role in their world by working out the Rules of Existence.

 

 

 

INFORMATIONAL LINKS 

 
 


Everything You Might Want to Know about Charles Darwin -  The Darwin Page.

A discussion of a lab experiment showing natural selection in action.

A transcript from an interview with a Christian biologist.

A paper on the stepwise evolution of the bacterial flagellum, often presented by Intelligent Design advocates as a system that could not have evolved in useful stages, but must have appeared fully-formed.

Darwin rap-?

 
     
 

Terms and Concepts
Terms are in the order they appear.

 
 

Evolution
Fossils & Biblical beliefs
Fossil layers & age
Charles Lyell (1830s)
Uniformitarianism
Age of the Earth
Extinction & Biblical beliefs
Lamarck
Inheritance of Acquired Characteristics
Evolution as progress
Thomas Malthus (early 1800s)
Natural population controls
Charles Darwin (mid- to late-1800s)
Artificial Selection
Evolution by Natural Selection
Adaptations
Alfred Russel Wallace (mid- to late-1800s)
Evolution by Sexual Selection
How fossils form
The fossil record and evolution
Divergent vs Convergent Evolution
Ernst Haeckel (late 1800s)
"Ontogeny recapitulates phylogeny"
Comparative embryology 
Homeogenes
Gradual vs Punctuated Evolution  
Evolution Rates & Reproduction Rates  

 
     

Interdisciplinary Discussion Question -

The idea of uniformitarianism, as applied to the history of our planet, lost favor in geology and biology long ago, and yet it is still a very basic premise in physics -  the rules that govern the universe are assumed to have been the same since the Big Bang, and all of the observations (astronomical observations look back in time as well as far away) are explained with that uniformitarian foundation.  As a result, scientists are coming to assume that the universe is full of matter and energy that cannot be observed, because the universe doesnt seem to follow the rules and thats the only way to stretch the rules to fit the observations.  Are the physicists in the same mindframe that the early geologists were - using an assumption of convenience that will turn out to be incredibly naive?  One might expect them to be, as astronomy does allow direct observation of the past, although at great distances.  Will the next physics revolution come from the theoretician willing to look for changes in such things as the rules governing energy and matter over the lifetime of a universe?

     
 

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