Principles of Evolution:
Why are there so many living things?
March 26, 2003

Readings: Chapter 18, pages 272 - 288

Before we begin...

• (1) If your personal views on evolution are different from the scientific theory of evolution, you are, of course, perfectly entitled to hold (and express) such views! You are, however, asked to understand what scientists have found to be the scientific evidence for evolution, even if you do not personally accept the evidence in support of Evolutionary Theory.
• (2) What is Evolution, anyway? In its simplest form, biological evolution can be defined as a heritable genetic change over time in a population of organisms.

"When the views entertained in this volume ... are generally admitted, we can dimly foresee that there will be a considerable revolution in natural history."

- Charles Darwin, The Origin of Species [1859]

I. Background: How did Evolutionary Thought Evolve?

The idea of biological evolution came of age as a science when Charles Darwin published "The Origin of Species" in 1859. However, for centuries scientists, philosophers, and theologians had grappled with ways to explain the vast amount biological diversity on Earth:

• the great diversity of organisms
• the origins and relationships of organisms
• the similarities and differences between organisms
• the adaptation of organisms to their surroundings

1. Old Testament account of Creation: God created all life in its present form
2. Aristotle: (~350 BC) The"scala naturae"
3. Linnaeus: (mid 1700s) "Father of Taxonomy" Binomial system of classification
4. Cuvier: (1800) Catastrophism. Studies of fossils indicated periodic extinctions of species
5. Lamarck: (1809) Inheritance of Acquired Characteristics
6. Lyall: (1830) Gradualism / Uniformitarianism. Estimated that the Earth is very old.
7. Charles Darwin: (1859) Evolution and Natural Selection

If you are interested in more (and there is a lot more) about the history of evolutionary thought, click here!

II. Darwin and the voyage on the HMS Beagle: How did Darwin account for species?

Darwin was a 22-year-old enrolled in Medical School (against his wishes), when he was offered a place on the HMS Beagle for a 5-year journey to chart the coast of South America. It so happened that he received as a going-away gift a copy of Lyall's Principles of Geology . "Timing and logic converge"!

On the Beagle: (1831 - 1836) Many opportunities for

Observation:
Questioning:
Hypotheses:

Return to England: (1836) While Darwin assessed his notes, he read an essay by Thomas Malthus, a clergyman and economist concerned about human population growth. Malthus argued that people tended to reproduce faster than their food supply, and so were forced to compete for their existence.

Darwin realized that over-reproduction and a struggle for existance occur in virtually all species, and the limited resources of the environment keeps the number of reproducing individuals in check. But which individuals are the 'winners' (who reproduce to pass on their genes) and which are the 'losers'?

Darwin suspected that natural variations in size, coloration, and other traits all living organisms might give some individuals a competitive edge to secure resources and reproduce in a particular environment.

Darwin's careful observations during the trip (1831 - 1836), plus the ideas concerning geology in Lyall's book, plus the ideas concerning population growth by Malthus, gave him ample material to think about concerning the mechanisms of biological diversity on Earth. His reasoning, and his conclusions, are as follows:

Darwin was reluctant to publish his work, anticipating the stir these ideas might produce, and it was only when he received a letter from Alfred Russell Wallace outlining a theory almost identical to his own (in 1858!) that he allowed his ideas to be published.

And, he certainly got noticed!

"In 1859, when Charles Darwin published The Origin of Species, it not only challenged the accepted scientific views of the times, it also challenged the religious views of Western culture that had been taught for centuries. The first edition of The Origin of Species sold out on the day it was published in 1859. Theologians quickly labeled Charles Darwin 'the most dangerous man in England'. Yet, after reading it, Darwin's colleague T. H. Huxley had a different reaction: "How extremely stupid not to have thought of that'." [Quote]

III. The Theory of Evolution, and Natural Selection:

In Darwin's theory of Evolution by Natural Selection, any population can evolve, or change over time. This phenomenon had been noticed for decades- even centuries - before Darwin. Darwin's theory went a step further than most by proposing a mechanism to explain how evolution might occur:

"This preservation of favorable variations and the rejection of injurious variation, I call Natural Selection, or Survival of the fittest".

The theory states that:

1. Evolution: Genetic change or 'descent with modification' occurs over time.
2. Natural Selection: is the mechanism of evolution. When individuals vary in one or more traits (as they always do), there may be slight differences in the ability to survive and reproduce (or a differential reproductive success). Keep in mind that in the 1850s, almost nothing was known about DNA, genes, mutations, even cells! However, we now know that mutations happen, with great regularity in organisms, and whether a mutations is neutral, harmful or beneficial depends on circumstances and the environment.
3. Survival of the Fittest: is the result of evolution by natural selection. Individuals that happen to be most suited to the environment are the ones that reproduce are whose traits are preserved in nature.

 

Darwin postulated that if populations change over time, this could lead to the formation of new species over long periods of time, especially when two populations become separated and spent long periods of time in isolation from each other. Darwin's boldest speculation was that all forms of life are related by descent from the earliest organisms, also known as the Central Proposition of Evolution.

IV. Natural Selection in Action:

Artificial selection: or, variation under domestication: Usually, natural selection takes hundreds or thousands of years to produce a noticeable change in the phenotype (appearance of an organism); however, humans have long practiced artificial selection = breeding.

Artificial selection refers to the selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits. Humans have practiced artificial selection of plants and animals for over 5,500 years, with the result that all of our common vegetables, fruit, and livestock have been long modified by selective breeding.

• In plants, desirable features include disease resistance, high yield, or attractive appearance.
• In animal, selection has led to the development of breeds of cattle specialized for meat production (such as the Aberdeen Angus) or milk production (such as Jerseys).

Darwin saw a similarity between this phenomenon and the processes of natural selection (If interested, see Breeds of the Domestic Pigeon, their Differences and Origin). Darwin looked to artificial selection to find clues for how natural selection might occur.

Natural Selection: Usually, natural selection takes hundreds or thousands of years to produce a noticeable change in the phenotype; however, there are over 100 examples of ongoing natural selection that scientists are currently observing (here are two specifics):

1. Industrial melanism: Before and after the industrial revolution

In polluted areas, the environment favored the darker moths: they were protected from predation, and therefore lived to transmit their genes to the next generation, a larger percentage of which would now be dark.

2. Resistance of bacteria to antibiotics, insects to pesticides, and weeds to herbicides:

• Tuberculosis: A human disease that showed promise of eradication in the1960s - but before we could eradicate, resistent strains survived survived and multiplied, making the problem more difficult than ever. Now working on vaccines.
• Bacteria - Bacteria replicate at incredible rates - a new generation every 20 minutes. Antibiotics easily wipe out most bacteria. If, however, 1 cell is resistant to antibiotic due to natural variation in DNA between bacteria, and survives antibiotic treatment, in 10 hours, a billion resisant bacteria will be produced while the sensitive cells will be eliminated.

V. Scientific Evidence of Evolution:

1. The Fossil Record provides Evidence of Change over Time:

• Fossils form when an organism becomes buried in mud, volcanic ash, or sediments. Over time, the organic remains (bones, teeth, skin, shells) become replaced with inorganic minerals, and pressure from more sediments preserves and hardens the original structure
• Chronological appearance of organisms - example: bacteria - invertebrates - fish - amphibians - reptiles - birds - mammals, with the occasional evidence of intermediate forms (evolutionary transitions) between groups of organisms (Archaeopteryx)

.

2. Comparitive Anatomy provides Structural Evidence of Evolution:

• Homologous structures - structures that are similar in function are similar in anatomy - the basic similarity indicates that the different structures may be a consequence of descent from a common ancestor.
• Morphological diversity: (p 280 in book) Forelimbs of mammals are constructed from the same skeletal elements - the ancestral forelimb became specialized for many different functions
• Vestigal structures (appendix and canine teeth in humans, molar teeth in vampire bats, pelvic bones in whales and snakes) that serve no apparent purpose in one organism, are hologous to structures in related organisms.

3. Comparitive Embryology provides Evidence of Common Ancestry:

• Closely related organisms - animals or plants - go through a shared program of embryonic development. Early embryonic stages of all vertebrates are similar - a 4 week old human embryo has gill pouches and a tail, two trademarks of all vertebrate embryos, even ones that do not have gills or tails at birth.
• Can often establish homology (similarity) between organisms that would otherwise not be apparent - example - gill pouches develop into eustachian tubes in humans.

4. Comparitive Biochemistry / Molecular Biology reveal Relatedness among Diverse Organisms:

• Biochemical similarities (Protein and DNA) are greatest among the species most closely related by appearance, anatomy, patterns of embryonic development.
• All living organisms use the same biochemical molecules - ATP, enzymes, metabolic pathways. In addition, the genetic "triplet" code is universal
• Evolutionary trees (or "phylogenetic trees") based on biochemical data are very similar to these based on anatomical data.
  The result of this work provides an entirely new reason to believe that evolution has shaped life since the beginning, says Washington University biologist Goodenough. "In the past 10 or 20 years, we have developed this completely independent record. Studies of the genome [an organism's genetic code] have in most cases completely confirmed...the relationships deduced from the fossil record."
• Provides evidence for the Central Proposition of Evolution.

How closely related are humans to apes and other animals? How do scientists measure that?

"If life is the result of 'descent with modification,' as Charles Darwin put it, we can try to represent its history as a kind of family tree derived from these morphological and genetic characteristics.Humans, chimpanzees, gorillas, orangutans and other extinct primates form a family of organisms known as the Hominidae. Researchers have evidence that that among the living animals in this group, humans are most closely related to chimpanzees, judging from comparisons of anatomy (comparitive anatomy, fossil record) and genetics (protein sequence data and DNA sequence data). Using such reasoning, it has been estimated that the last common ancestor of humans and chimpanzees (with whom we share 99 percent of our genes) lived five million years ago". Source: Bernhard Haubold [Image: Caracture of Darwin from Newspaper, ca. 1858]

From the PBS Evolution Series: Hominid Evolution

VI. Questions you may be asking...

(1) Does the environment create these genetic changes? No - The environment merely makes possible the growth, reproduction and survival of some individuals over others. IF the environment changes, particularly in a short time, organisms that once 'fit in' with their environment and escaped predation may now stand out like a sore thumb!

• Antibiotics do not 'create' antibiotic-resistant strains of bacteria, but they do create an environment in which only those organisms with a slight genetic resistance to the antibiotic survive and reproduce.
• Sooty tree bark does not 'cause' more dark moths to appear, but does create an environment in which only the black moths survive predation in dark environment.

(2) Does Survival of the Fittest mean that only the fastest, smartest, and strongest survive? NO! Just the organisms whose phenotype (physical makeup / appearance) is the most suited to that particular envronment. Sometimes it is the quietest and the least noticible that survive....

(3) Do individuals evolve? No - a population is the smallest unit that can evolve. Individuals interact with their environment, and accumulate or inherit mutations, but the changes must be passed on to persist within the population.

(3) Does natural selection breed perfection??? NO!!! (don't we wish!) The differential reproductive success (survival of the individual and its genes) of one organism over another does not lead to "perfection" but to populations of organisms being well-suited (adapted) to their environment.

(4) Could "blind chance" have led to the complexity of life of Earth? No - Natural selection is anything BUT a "blind", random process. All individuals in a population show some sort of normal genetic variation, and natural selection describes the mechanism whereby these slight genetic variations can influence the survival of an organism in its environment. Natural Selection (or non-random survival - survival based on the 'fittest' in the environment) leads to the cumulative process of Evolution (change over time). We can show this through artificial selection (breeding), test hypotheses about Natural Selection, and even use "directed evolution" or "molecular evolution" to design needed drugs, enzymes, computer systems, etc....Labs around the world and many Biotech companies concerned with directed evolution:

Applied Molecular Evolution - San Diego, CA; directed evolution protein engineering
Diversa- San Diego, CA; molecular evolution/biodiversity
Genopsys - Santa Cruz, CA; molecular optimization of nucleic acid sequences
Nautilus Biotech - Evry, France; directed evolution of natural proteins

(5) Is Evolution (and Natural Selection) "just a theory"? Evolution happens. In principle, evolution could someday be better explained by something other than Natural Selection, but there is NO indication that the last 150 years of evidence supporting natural selection could be replaced by another theory anytime soon!

- Scientists accept evolution (and natural selection as its mechanism) because it is the best supported scientific explanation for the diversity of life that we have! Predictions, based on hypotheses, have been repeatedly tested and confirmed by experimentation and observation.

- Remember that the popular use of the word "theory" (a guess, an opinion), is NOT the same as the scientific use of the word "theory" - which is an explanation built on MANY confirmed facts combined with the absence of facts that contradict the explanation. When scientists propose a theory to explain a phenomenon, they do NOT use this word to mean that they doubt that is it true - they mean just the opposite - that it is supported by a WIDE body of evidence!

- Unfortunately, many educated (one would think) people - OK, lets pick on Ronald Reagan and George W. Bush, for instance - use the phrase "just a theory" to discount the evidence in support of evolutionary theory, not understanding the distinction between the the scientific and popular use of the word theory. (I do not mean to pick on their beliefs, only their understanding of the word THEORY)

• From our late Pres Ronald Reagan: "…Evolution is a theory, a scientific theory only, and it has in recent years been challenged in the world of science…”
• From Pres George W. Bush: ".... I believe children ought to be exposed to different theories about how the world started. The theory of evolution is an open question and the verdict is still out on how God created the Earth."

- Fortunately, many other world leaders have a better grasp on the use of the word Theory: From Pope John Paul's 1996 Statement on Evolution to the Pontifical Academy of Sciences "...It is indeed remarkable that this theory has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge. The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favour of this theory." (Point #4)

If you are so inclined: READ THIS STATEMENT from the Pope!! It is an excellent paper on a difficult subject!

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A classic paper in Evolutionary thought: Nothing in Biology Makes Sense Except in the Light of Evolution by Theodosius Dobzhansky. Again, if you are so inclined: READ IT - another excellent article!

Science teachers: click here for LOTS more Evolution resources

Darwin attributed the diversity of life to natural causes rather than creation by a supreme being. Today, some states require teachers to teach opposing views about the origins of species:

1. "Abrupt appearance theory": organisms are created fully formed and do not evolve
2. "Intelligent design theory": organisms are so perfectly formed that they must be the products of conscious design

However, while sounding scientific, these ideas are untestable. This is not to say that they are not true, just that they lie outside of the realm of science.

Famous Trials in American History - the Scopes "Monkey" Trial  An interesting bit of history from the recent past: Gym teacher (and science teacher) John Scopes was brought to trial by a Tennasee court in 1925 for teaching evolution in his classroom.    Lawyer William Jennings Bryan had in the years prior to the trial led a crusade to banish the theory of evolution from American classrooms (in Tennessee ca. 1925, he was instrumental in making it illegal "to teach any theory that denies the story of divine creation as taught by the Bible and to teach instead that man was descended from a lower order of animals."); Clarence Darrow represented Scopes, free speech and the constitutionality of Tennessee's anti-evolution laws....

Final Thought: Evolution by Natural selection is THE unifying theme of biology, the study of life, precisely because of its power to explain the unity, diversity, and complexity of life.

Objectives:

1. Explain the contributions of Lyall, Lamarck, Cuvier, Wallace, and Malthius to evolutionary theory.
2. What is evolution? What is natural selection? How do they differ?
3. State the 3 observations and the 3 deductions that Dawrin used to formulate the Theory of Evolution .
4. Distinguish between artificial selection and natural selection. Give examples.
5. What is the Central Proposition of Evolution?
6. Explain the scientific evidence for evolution.