Principles of Evolution - Microevolution and Macroevolution
March 29 and April 3, 2000

Note: Chapter 16 and 18 material has been condensed and combined here!
Starr text: Ch 16 cover page,16.1 - 16.4, 16.6 Ch 18 cover page, 18.1-18.4, 18.6 - 18.7

"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]

Outline: I. Background II. Darwin and the voyage on the HMS Beagle III. Evolution and Natural Selection IV. Scientific evidence for Evolution V. Natural Selection in Action VI. What causes evolution to occur? VII. Other questions you may be asking...

Before we begin....

A. Please define (before class):

1. Evolution:Genetic change in a line of descent over time; brought about by microevolutionary processes (gene mutation, natural selection, genetic drift, and gene flow).

2. Natural Selection: A microevolutionary process; differential reproductive success among individuals that show variation in heritable traits.

 

3. Survival of the Fittest: individuals with the phenotype that is most adapted to the environment is the one that is preserved in nature.

4. Mutation: A heritable change in the molecular structure of DNA. Mutations are the sources of all alleles and, ultimately, of life's diversity.

5. Adaptation: A modification to a given set of environmental conditions.

 

6. Microevolution: Change in allele frequencies resulting from natural selection,

genetic drift, gene flow, and mutation.

 

7. Macroevolution: The large-scale patterns, trends, and rates of change among

families and other more inclusive groups of species.

 

8. Homologous structure: The same body parts, modified in different ways, in

different lines of descent from a common ancestor.

 

B. 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 believe this evidence to be true.

I. Background: Historical views on the origins of biological diversity: (Sect 16.1)

The idea of biological evolution came of age as a science when Charles Darwin published "On the Origin of Species." However, for centuries scientists, philosophers, and theologians have 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

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: (late 1700s) Catastrophism

5. Lamarck: (Early 1800s) Inheritance of Acquired Characteristics

6. Lyall: (Early 1820s) "Father of modern Geology" - Gradualism

7. Charles Darwin: (1859) Evolution and Natural Selection

If you are interested in more about the history of evolutionary thought, click here!

II. Darwin and the voyage on the HMS Beagle: (Sect 16.2)

Darwin was a 22-year-old enrolled in Medical School (against his wishes), until 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"!

Darwin's careful observations during the trip (1831 - 1836), plus the ideas concerning geology in Lyall's book, plus an essay on populations by Thomas Malthus, gave him ample evidence to formulate his hypothesis concerning evolution. Darwin was reluctant to publish his ideas, and it was only when Darwin received a letter from Alfred Russell Wallace outlining a theory almost identical to his own that he allowed his ideas to be published.

 

III. Evolution and Natural Selection: (Sect 16.3)

"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 friend and colleague T. H. Huxley had a different reaction: "How extremely stupid not to have thought of that'."

The Origin of Species had two major points:

1. Evolution: (Descent with Modification).

• Organisms are born with physical differences (we know now are crossing over or gene mutations) that make them unique from their parents and that also may (or may not) allow them to fit in with their environment.
• Similarities between species may reflect their descent from a common ancestor.
• Differences between species may be the result of natural selection.
  

2. Natural Selection: (differential reproductive success)

• Individuals in a population vary in many traits (hair color, eye color, enzymes of all sorts)
• Natural resources in an environment are limited (Thomas Malthus)
• Survival of the fittest: Individuals that are better able to capture natural resources because they happen to be more suited to the environment have larger numbers of offspring
• This differential reproductive success is called natural selection.
• The phenotype that is most adapted to the environment is the one that is preserved in nature.
  

In Darwin's theory of evolution by natural selection, any population can evolve (change over time) when its individuals vary in one or more traits that are responsible for differences in the ability to survive and reproduce (differential reproductive success). The effects of selection would lead to a species being modified over time.

Darwin's theory did not fit well with the "blending" theory of genetics. What Darwin didn't know was that Gregor Mendel had already shown that "heritable factors" (genes) remain distinct and can be passed on to subsequent generations. Mendel mailed his paper to Darwin, but Darwin never opened it...!

IV. Scientific Evidence for Evolution: (Chapter 18)

1. Fossil Record: (Sect 18.1)

• 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)
• Radiocarbon Dating

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2. Comparitive Anatomy: (Sect 18.2)

• Homologous structures - structures that are similar in function are similar in anatomy - the basic similarity indicates that the different structures are 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: (Sect 18.3)

• 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. (Figure 16.5)
• "Ontogeny recapitulates Phylogeny" - embryological development (ontogeny) indicates evolutionary relationships (phylogeny). Somewhat overstated theory by Ernest Haekel, 1866, but basic ideas are supported by embryology.
• 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: (Sect 18.4)

• 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 Darwin's boldest speculation - that all forms of life are related by branching descent from the earliest organisms.

5. Biogeography: (Sect 18.6)

• Islands usually have species very closely related to the species of the nearest mainland.
• Slight changes that occur over time in isolation = new species (more about this next lecture)

V. What causes evolution to occur?

1. Mutations: (Sect 16.4) - the raw material for evolution and the ultimate source of all variation found in populations

• New alleles arise only by mutation - rare and random accidents. Only when produced in gametes are they passed to offspring, but once there, they are inherited from that point on..
• Mutations cause evolution (change over time) by substituting one allele for another in the gene pool (example: sickle cell anemia, coat color, PKU)
• Recessive (unobserved) mutations are always occuring and could prove to be beneficial if the environment changes (ie: peppered moths)
• Each person bears 1-5 mutant alleles not present in either parent. May be beneficial, harmless, or fatal!
  

2. Gene Flow (Sect 16.10) causes evolution by the physical movement of alleles into and out of a population

• Occurs when individuals immigrate and emigrate between populations - exchange of alleles (and new variations) between populations due to migration
• Eventually, gene flow reduces differences between populations into a single population with a common genetic structure "melting pot"

 

 

 

 

 

3. Genetic Drift (Sect 16.11) causes evolution by the random change in allele frequencies observed in an unusually small population

• The Bottleneck Effect: gene pool randomly drifts when the population is drastically reduced by a disaster that kills unselectively
• The Founder Effect: a small population not necessarily representative of the parent population colonizes a new habitat. (Cheetahs, Northern Elephant seals, etc)
• Genetic drift usually reduces genetic variation over time due to small gene pool - some alleles become fixed within the population and some alleles are lost forever...

VI. Natural Selection in Action: (Sect 16.6-16.7)

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 and a few more general):

1. Industrial melanism: Before the industrial revolution, the peppered moth population was ~99% very light colored moths, ~1% dark-black colored moths. 50 years later, the black moths made up 99% of the peppered moth population in polluted areas only. The black moths in polluted areas were protected from predation by birds because they were not seen against the trees darkened by pollution

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:

Indiscriminate use of chemicals has caused organisms to develop resistance. Populations almost always have some organisms that are resistant to these chemicals; the new generation that survives the first chemical treatment will now be resistant.

• 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 - reproduce in 20 minutes - if 1 cell is resistant to antibiotic, in 10 hours, a billion resisant bacteria will be produced and the sensitive cells will be completely eliminated. Widespread (and often improper) use of antibiotics over the last 50 years has led to a growing number of bacterial diseases that cannot be controlled by the "first generation" (or even second, third...) antibiotics anymore.

3. Selection for or against extreme phenotypes (Sect 16.8)

• Stabilizing selection: A mode of natural selection by which intermediate phenotypes are favored, and extremes at both ends of the range of variation are eliminated. Example: Birth weight of human babies:

• Directional selection: A mode of natural selection by which a phenotype at one extreme is favored, with phenotypes at the other extreme eliminated.
• Disruptive selection: A mode of natural selection by which the two extreme phenotypes are favored, with phenotypes in the middle eliminated.

4. Heterozygote adventage (Sect 16.9) (we have discussed already, but also a form of natural selection)

VII. Questions you may be asking...

Does the environment create these genetic changes? No - The environment determines which organisms in a population will survive and differentially reproduce. 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, and the genome of an individual will always contain a few mutations, but adaptations must be inherited to persist within the population.

Does natural selection breed perfection??? NO!!! (don't we wish!) New alleles cannot be created on demand, and can disappear from a gene pool if conditions do not favor their continued existance.

Is evolution "just a controversal theory"?

Darwin attributed the diversity of life to natural causes rather that supernatural creation

Today (1998), in Tennessee, Texas, Kansas, and Alabama, biology textbooks carry a paste-in warning that evolution is a controversial theory, and require teachers to teach opposing theories 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, these theories, while sounding scientific, are untestable, as they are based on faith. 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 because it has been confirmed repeatedly by experimentation - predictions, based on hypotheses, are tested and confirmed by experimentation and observation.

If you are interested...Famous Trials in American History - the Scopes "Monkey" Trial