Darwin and Natural Selection

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Darwin and Natural Selection

Most educated people in Europe and the Americas during the 19th century had their first full exposure to the concept of evolution through the writings of Charles Darwin .  Clearly, he did not invent the idea.  That happened long before he was born.  However, he carried out the necessary research to conclusively document that evolution has occurred and then made the idea acceptable for scientists and the general public.  This was not easy since the idea of evolution had been strongly associated with radical scientific and political views coming out of post-revolutionary France.  These ideas were widely considered to be a threat to the established social and political order.

	Charles Darwin at age 7

Charles Darwin was born into a moderately wealthy family in Shrewsbury, England.  His father, Robert, had the largest medical practice outside of London at the time and his mother, Susannah Wedgwood, was from a family of wealthy pottery manufacturers.  She died when Charles was only 8 years old.  Thereafter, he was raised mostly by his father and doting older sisters.  Charles grew up in comparative luxury in a large house with servants.  However, this was a socially conservative time in England that set narrow limits on a young man's behavior and future possibilities.  The constraints on women in Darwin's social class were even greater.  Most were given only enough education to efficiently manage the homes of their future husbands and raise their children.  Young men were expected to go to university in order to prepare themselves to become medical doctors, military officers, or clerics in the Church of England.  Most other occupations were considered somewhat unsavory.

At his father's direction, Charles Darwin started university at 16 in Edinburgh, Scotland as a medical student.  He showed little academic interest in medicine and was revolted by the brutality of surgery being performed without pain relief.  Anesthesia was not used for operations until 1842.  Darwin dropped out of medical school after two years of study in 1827.  However, his knowledge of natural history was incidentally enriched in Edinburgh by the teaching of Robert Grant, a noted professor of anatomy and an avid marine biologist.  At Grant's suggestion, Darwin also became a member of Plinian Society for student naturalists at the University of Edinburgh.

Having given up on a medicine as a future career, Charles Darwin's father then sent him to Cambridge University in 1828 to pursue an ordinary degreeprogram with the goal of later becoming an Anglican parson.  In Cambridge his life's direction continued its radical change.  He became very interested in the scientific ideas of the geologist Adam Sedgwick and the naturalist John Henslow with whom he spent considerable time collecting specimens from the countryside around the university.  At this time in his life, Darwin apparently rejected the concept of biological evolution, just as his mentors Sedgwick and Henslow did.  However, Darwin had been exposed to the ideas of Lamarck about evolution earlier while he was a student in Edinburgh.

	Captain Robert Firzroy 1805-1865
Charles Darwin 1809-1882

Following graduation from Cambridge in 1831 with a Bachelor of Arts degree, Darwin was clearly more interested in biology and geology than he was in a clerical career.  Fortunately, John Henslow was able to help him secure a berth on a British Navy mapping expedition that was going around the world on what would ultimately become a nearly five year long voyage.  Initially, Darwin's father refused to allow him to go but was eventually persuaded by Charles and even agreed to pay for his passage and for that of his man servant on the journey.  They sailed two days after Christmas in 1831 aboard the survey ship H.M.S. Beagle with Darwin acting as an unpaid naturalist and gentleman companion for the aristocratic captain, Robert Fitzroy.  Darwin was 22 years old at the time, and Fitzroy was only 4 years older.  The Beagle was a compact 90 foot long ship with a crew of 74.  There was little space, even for the captain.  Darwin shared a cramped 10 X 11 foot cabin with two other men, a cabin boy, and their belongings.  Because of the Beagle's design and small size, it was generally thought by naval men that it was ill suited for the rough seas it would encounter, especially at the southern tip of South America.  Darwin frequently suffered from sea sickness on the voyage.  Fortunately, he was able to spend most of the time on land exploring.  In fact, he was at sea for only 18 months during the nearly 5 years of the expedition.

Captain Fitzroy was interested in advancing science and was especially drawn to geology.  He had a surprisingly good library of over 400 books onboard the Beagle that he made available to Darwin.  It was during the beginning of the voyage that Darwin read the first volumes of Charles Lyell's "Principles of Geology" and became convinced by his proof that uniformitarianism provided the correct understanding of the earth's geological history.  This intellectual preparation, along with his research on the voyage, was critical in leading Darwin to later accept evolution.  Especially important was his 5 weeks long visit to the Galápagos Islands  in the Eastern Pacific Ocean.  It was there that he made the observations that eventually led him to comprehend what causes plants and animals to evolve, but he apparently did not clearly formulate his views on this until 1837.  At the time he left the Galápagos Islands, he apparently still believed in a traditional Biblical creation of all life forms.

H.M.S. Beagle (90.3 ft long, 24.5 ft. wide)		Five year voyage of H.M.S. Beagle (1831-1836)

  Darwin's Beagle Diary
audio recording of excerpts from Darwin's popular account of his voyage of exploration.  This link takes you to an external website. To return here, you must click the "back" button on your   browser program.     (length = 68 mins, 19 secs)

The Galápagos Islands have species found in no other part of the world, though similar ones exist on the west coast of South America.  Darwin was struck by the fact that the birds were slightly different from one island to another.  He realized that the key to why this difference existed was connected with the fact that the various species live in different kinds of environments.

On returning to England, Darwin and an ornithologist associate identified 13 species of finches that he had collected on the Galápagos Islands.  This was puzzling since he knew of only one species of this bird on the mainland of South America, nearly 600 miles to the east, where they had all presumably originated.  He observed that the Galápagos species differed from each other in beak size and shape.  He also noted that the beak varieties were associated with diets based on different foods.  He concluded that when the original South American finches reached the islands, they dispersed to different environments where they had to adapt to different conditions.  Over many generations, they changed anatomically in ways that allowed them to get enough food and survive to reproduce.  This observation was verified by intensive field research in the last quarter of the 20th century.

		Galapágos Creatures--some of the unusual animals that Darwin         saw on the Galapágos islands.  Video clip from "Evolution:         Constant Change and Common Threads".  (Howard Hughes         Medical Institute, 2005)               (length = 24 secs.)
Finches from the Galápagos Islands

Today we use the term adaptive radiation to refer to this sort of branching evolution in which different populations of a species become reproductively isolated from each other by adapting to different ecological niches  and eventually become separate species.

              

Darwin came to understand that any population consists of individuals that are all slightly different from one another.  Those individuals having a variation that gives them an advantage in staying alive long enough to successfully reproduce are the ones that pass on their traits more frequently to the next generation.  Subsequently, their traits become more common and the population evolves.  Darwin called this "descent with modification."

The Galápagos finches provide an excellent example of this process.  Among the birds that ended up in arid environments, the ones with beaks better suited for eating cactus got more food.  As a result, they were in better condition to mate.  Similarly, those with beak shapes that were better suited to getting nectar from flowers or eating hard seeds in other environments were at an advantage there.  In a very real sense, nature selected the best adapted varieties to survive and to reproduce.  This process has come to be known as natural selection.

Thomas Malthus  (1766-1834)

Darwin did not believe that the environment was producing the variation within the finch populations.  He correctly thought that the variation already existed and that nature just selected for the most suitable beak shape and against less useful ones.  By the late 1860's, Darwin came to describe this process as the "survival of the fittest."   This is very different from Lamarck's incorrect idea that the environment altered the shape of individuals and that these acquired changes were then inherited.

Nineteenth century critics of Darwin thought that he had misinterpreted the Galápagos finch data.  They said that God had created the 13 different species as they are and that no evolution in beak shape has ever occurred.  It was difficult to conclusively refute such counter arguments at that time.  However, extensive field research since the early 1970's has proven Darwin to be correct.

In 1798, Thomas Malthus , an English clergyman and pioneer economist, published Essay on the Principles of Population.  In it he observed that human populations will double every 25 years unless they are kept in check by limits in food supply.  In 1838, Darwin read Malthus' essay and came to realize that all plant and animal populations have this same potential to rapidly increase their numbers unless they are constantly kept in check by predators, diseases, and limitations in food, water, and other resources that are essential for survival.  This fact was key to his understanding of the process of natural selection.  Darwin realized that the most fit individuals in a population are the ones that are least likely to die of starvation and, therefore, are most likely to pass on their traits to the next generation.

Who Was Charles Darwin?--video clip from PBS 2001 series Evolution         requires RealPlayer to view         (length = 6 mins, 26 secs)

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An example of evolution resulting from natural selection was discovered among "peppered" moths living near English industrial cities.  These insects have varieties that vary in wing and body coloration from light to dark.  During the 19th century, sooty smoke from coal burning furnaces killed the lichen on trees and darkened the bark.  When moths landed on these trees and other blackened surfaces, the dark colored ones were harder to spot by birds who ate them and, subsequently, they more often lived long enough to reproduce.  Over generations, the environment continued to favor darker moths.  As a result, they progressively became more common.  By 1895, 98% of the moths in the vicinity of English cities like Manchester were mostly black.  Since the 1950's, air pollution controls have significantly reduced the amount of heavy particulate air pollutants reaching the trees, buildings, and other objects in the environment.   As a result, lichen has grown back, making trees lighter in color.  In addition, once blackened buildings were cleaned making them lighter in color.  Now, natural selection favors lighter moth varieties so they have become the most common.  This trend has been well documented by field studies undertaken between 1959 and 1995 by Sir Cyril Clarke from the University of Liverpool.  The same pattern of moth wing color evolutionary change in response to increased and later decreased air pollution has been carefully documented by other researchers for the countryside around Detroit, Michigan.  While it is abundantly clear that there has been an evolution in peppered moth coloration due to the advantage of camouflage over the last two centuries, it is important to keep in mind that this story of natural selection in action is incomplete.  There may have been additional natural selection factors involved.

Dark moths on light colored bark are   easy targets for hungry birds but are   hidden on pollution darkened trees.

The Making of the Fittest: Natural Selection and Adaptation--an example of natural         selection in mice.  This link takes you to a video at an external website.  To return         here, you must select this page again.           (length = 10 mins, 25 secs)    Evolution in Action--an example of natural selection among salamanders in California         This link takes you to a video at an external website.  To return here, you must select         this page again.           (length = 3 mins, 20 secs)    Toxic Newts--the evolutionary arms race between predator and prey driving evolution         This link takes you to a video at an external website.  To return here, you must select         this page again.           (length = 5 mins, 28 secs)

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Darwin's use of the phrase "survival of the fittest" is frequently misunderstood.  Many people assume that "the fittest" refers to the strongest, biggest, or smartest and most cunning individuals.  This may or may not be the case.  From an evolutionary perspective, the fittest individuals are simply the ones who have the combination of traits that allow them to survive and produce more offspring that in turn survive to reproduce.  In fact, they may be relatively weak, small, and not particularly intelligent.  What makes an individual fit all depends on the environment at the time and the combination of traits that are most suited to flourishing in it.  In the case of Darwin's finches, specialized beaks provided the advantage.  However, in a changing environment, it is often the versatile generalist who has the greatest success.

Darwin did not believe that evolution follows a predetermined direction or that it has an inevitable goal.  His explanation that evolution occurs as a result of natural selection implied that chance plays a major role.  He understood that it is a matter of luck whether any individuals in a population have variations that will allow them to survive and reproduce.  If no such variations exist, the population rapidly goes extinct because it cannot adapt to a changing environment.  Unlike Lamarck, Darwin did not believe that evolution inevitably produces more complex life forms and that the ultimate result of this process is humans.  These were shocking, revolutionary ideas even for scientists who accepted evolution.

Darwin did not rush his ideas about evolution and natural selection into print.  He first concentrated his efforts on writing the account of his around the world voyage on the Beagle and analyzing the many preserved animal and plant specimens and extensive notes that he brought back with him.  This occupied him for more than 10 years.  An additional factor that may have held him back from publishing his ideas about evolution was the widespread Christian evangelical fervor in England during the 1830's and 1840's.  He could have been charged with sedition and blasphemy for widely publishing his unpopular theory. 

After returning from the voyage of the Beagle, Darwin settled down in England, married Emma Wedgwood (his wealthy first cousin), raised a large family, and quietly continued his research at his newly purchased country home 16 miles south of London.  In 1842 he wrote a 35 page summary of his theory about evolution.  This was expanded to a 230 page manuscript in 1844, but it was not published and apparently was only known to a few people in British scientific circles.  Darwin busied himself over the next two decades establishing his reputation as an important naturalist by growing and studying orchids, pigeons, earthworms, and other organisms at his home.  He spent 8 of these years studying and writing about barnacles that people had sent him from around the world.

Emma Darwin 1808-1896		Down House--Charles and Emma Darwin's country     home where he wrote his major publications and       their family lived contentedly for 40 years.		Charles Darwin 1809-1882

It was not until he was 50 years old, in 1859, that Darwin finally published his theory of evolution in full for his fellow scientists and for the public at large.  He did so in a 490 page book entitled On the Origin of Species.  It was very popular and controversial from the outset.  The first edition came out on November 24, 1859 and sold out on that day.  It went through six editions by 1872.  The ideas presented in this book were expanded with examples in fifteen additional scientific books that Darwin published over the next two decades.

Alfred Wallace 1823-1913

What finally convinced Darwin that he should publish his theory in a book for the general educated public was the draft of an essay that he received in the summer of 1858 from a younger British naturalist named Alfred Wallace , who was then hard at work collecting biological specimens in Southeast Asia for sale to museums and private collectors.  Darwin was surprised to read that Wallace had come upon essentially the same explanation for evolution.  Being a fair man, Darwin insisted that Wallace also get credit for the natural selection theory during debates over its validity that occurred at a meeting of the British Association for the Advancement of Science at Oxford University in 1860.  We now know that Darwin deserves most of the credit.  In 1837, one year after he returned from the voyage on the Beagle, he made detailed notes on the idea of evolution by means of natural selection.  At that time, Wallace was only 14 years old.  In addition, it was Darwin's book, rather than Wallace's essay, that had the most impact on the Victorian public.  Darwin not only described the process of natural selection in more detail, but he also gave numerous examples of it.  It was his On the Origin of Species that convinced most scientists and other educated people in the late 19th century that life forms do change through time.  This prepared the public for the acceptance of earlier human species and of a world much older than 6000 years. 

Darwin and Victorian Culture--interview with Darwin's biographer, James Moore        This link takes you to an audio file at an external website.  To return here, you must click        the "back" button on your browser program.           (length = 8 mins, 5 secs)

	Gregor Mendel 1822-1884

Both Darwin and Wallace failed to understand an important aspect of natural selection.  They realized that plant and animal populations are composed of individuals that vary from each other in physical form.  They also understood that nature selects from the existing varieties those traits that are most suited to their environment.  If natural selection were the only process occurring, each generation should have less variation until all members of a population are essentially identical, or clones of each other.  That does not happen.  Each new generation has new variations.  Darwin was aware of this fact, but he did not understand what caused the variation.  The first person to begin to grasp why this happens was an obscure Central European monk named Gregor Mendel.  Through plant breeding experiments carried out between 1856 and 1863, he discovered that there is a recombination of parental traits in offspring.  Sadly, Darwin and most other 19th century biologists never knew of Mendel and his research.  It was not until the beginning of the 20th century that Mendel's pioneer research into genetic inheritance was rediscovered.  This was long after his death.   He never received the public acclaim that was eventually showered on Darwin during his lifetime.

Charles Darwin's convincing evidence that evolution occurs was very threatening to many Christians who believed that people were created specially by God and that they have not changed biologically since that creation.  The idea that there could have been prehistoric humans who were anatomically different from us was rejected for similar reasons.  However, Charles Lyell's geological evidence that the earth must be much older than 6,000 years along with the rapidly accumulating fossil record of past evolution convinced educated lay people in the 1860's to think what had been unthinkable earlier.

Boucher de Perthes  1788-1868

Archaeological confirmation of the existence of prehistoric Europeans had been accumulating since the 1830's.  However, until the late 1850's, it had been widely rejected or misinterpreted.  Much of this evidence had been collected by Jacques Boucher Crèvecoeur de Perthes , a customs officer in northern France during the early 1800's.  His hobby was collecting ancient stone tools from deep down in the Somme River gravel deposits.  Since he found these artifacts in association with the bones of extinct animals, he concluded that they must have been made at the time that those animals lived.

	Prehistoric artifact incorrectly thought to be a "lightning bolt remnant"

Boucher de Perthes tried to publish his findings in 1838.  They were rejected by all important scientists and scientific journals.  The prehistoric stone tools usually were dismissed as being only "lightning stones" (i.e., the remnants of lightning bolts).  However, by 1858, his claims were beginning to be accepted by some enlightened Western European scientists.  Darwin's publication of On the Origin of Species the following year convinced even more educated people that Boucher de Perthes had been right.

Darwin's popularizing the idea of evolution also made it possible for scientists to begin to accept that some of the makers of Boucher de Perthes' prehistoric tools had already been discovered and that their bones were in museums.  These bones had been found in several Western European countries during the first half of the 19th century.  However, they had all been dismissed as being from odd looking modern people.  During the 1860's, some were correctly determined to be from an earlier species or variety of people who had lived during the last ice age--i.e., long before recorded history.  We now know that these ancient people were mostly Neandertals, who lived about 150,000-28,000 years ago.

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NOTE:   Charles Darwin was an active collector of plant and animal specimens and a prodigious note taker on the voyage of H.M.S. Beagle.  By the time the ship returned to England in 1836, he had accumulated 5,436 plant and animal specimens that had been dried or preserved in alcohol.  He had 368 pages of notes on plants and animals as well as 1,383 pages of geological observations.  In addition, he had a 770 page diary that was the basis for his later popular book of his narrative on the voyage ("Journal of Researches Into the Natural History and Geology of the Countries Visited During the Voyage of H.M.S. Beagle Round the World, Under the Command of Capt. Fitz Roy, R.N.").

NOTE:   From the time that Charles Darwin published "On the Origin of Species" in 1859 on up to the present, the presumptions of many people led them to misread the title.  They assumed that it was "On the Origin of the Species".  The implication of inadvertently adding "the" is that his book was about human evolution.  In fact, that was not the case, though it had implications for human evolution.  It focused on non-human animals and the mechanisms of evolution.  He did not pointedly address the question of human evolution until the publication of his 1871 book "Descent of Man and Selection in Relation to Sex".

NOTE:   The phrase "survival of the fittest" was apparently first used in 1851 by the influential British philosopher Herbert Spencer (1820-1903) as a central tenet of what later became known as "Social Darwinism."  He misapplied Darwin's idea of natural selection to justify European domination and colonization of much of the rest of the world.  Social Darwinism was also widely used to defend the unequal distribution of wealth and power in Europe and North America at the time.  Poor and politically powerless people were thought to have been failures in the natural competition for survival.  Subsequently, helping them was seen as a waste of time and counter to nature.  From this perspective, rich and powerful people did not need to feel ashamed of their advantages because their success was proof that they were the most fit in this competition.  Despite misgivings by Alfred Wallace and other naturalists, Charles Darwin began to use "survival of the fittest" as a synonym for "natural selection" in the 5th edition of Origin of Species, which was published in 1869.

NOTE:   H.M.S. Beagle, the famous ship that took Charles Darwin on his 1831-1836 voyage around the world, had a rather mundane history following her return to England.  She was transferred by the British Navy to the Customs and Excise Department and was used to catch smugglers along the southeast coast of England.  The Beagle was finally sold for scrap in 1870 after 50 years of service.