Principles of Evolution:
Why are there so many living things?
January 17, 2001
"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 
Before we begin... If your personal views on evolution are different from the current scientific theories, 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.
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) "Father of modern Geology" - Gradualism / Uniformitarianism
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
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.
Example: Sea stars (starfish)
Thus, 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:
Observation 1 & 2: All organisms are capable of producing more offspring than the environment can support.
Observation 3: Natural resources are limited.
Observation 4: Individuals within a species vary in size, color, behavior, and other traits, and this variation is heritable.
Therefore, (Darwin deduced)
- Deduction 1: Individuals must compete for resources, and only some will survive.
- Deduction 2: Individuals with variations more suited to the environment will be more likely to survive and reproduce.
- Deduction 3: Over many generations and long periods of time, these variations accumulate in the population, resulting in evolution, or change over time.
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. 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:
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 that Evolution Occurs:
1. Fossil Record:
- 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:
- 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.
- Forelimbs of mammals are constructed from the same skeletal elements - the ancestral forelimb became specialized for many different functions
3. Comparitive Embryology:
- Closely related organisms go through similar stages in their 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:
- 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
- Biochemical similarities are greatest among the most closely related species.
- Provides evidence for the Central Proposition of Evolution.
Worksheet: Comparitive Biochemistry
- Islands (which arise from volcanic action) usually have species very closely related to the species of the nearest mainland.
- Slight changes that occur that occur over time in isolation may result in the formation of a new species (more about this next lecture)
VI. Questions you may be asking...
Does the environment create these genetic changes? No - The environment merely favors the growth, reproduction and survival of some individuals over others.
Do individuals evolve? No - a population is the smallest unit that can evolve. Individuals interact with their environment, but adaptations must be inherited to persist within the population.
Does natural selection breed perfection??? NO!!! (don't we wish!)
Is evolution "just a controversal theory"?
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.
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. What is sometimes controversial are the details surrounding the mechanism of evolution (more on this next time).
Famous Trials in American History - the Scopes "Monkey" Trial
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.
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