Tracking the Course of Evolution


by Kevin Padian

Major goals
1. Evolution is the central unifying theory of biology
2. Evolution is integrally related to other sciences
3. About half of science is neither experimental nor quantitative
Goals are seen here as broad concepts that form an individual's literacy and worldview.
1A. Everything on Earth, including the Earth, has "evolved" (changed through time), and carries the marks of its history. Deciphering these marks is the reconstruction of that history.
B. The similarities of organisms, from adaptations to molecular structures, cannot be explained fully either by assumptions of optimal design or by a transcendental plan.
C. Not only organisms and their structures, but ecological levels of organization, such as guilds, faunas, floras, communities, and biotic provinces, have evolved through time. Their features can be traced readily, and their histories can be similarly reconstructed.

2A. Evolution cannot be roped off from other sciences, because everything has evolved.
B. Historically, the understanding of evolution comes from discoveries made in and dependent on other sciences, including their basic principles and methods.
C. Evolution is an explanation of patterns and phenomena seen in other sciences.

3A. In purely experimental sciences, time, age, and other historical factors do not influence their phenomena and hence they are universally predictive ("lawlike"). Newtonian physics and simple aspects of chemistry apply here.
B. Many sciences are not largely amenable to experimentation either because most of their phenomena cannot be manipulated (astronomy, meteorology, oceanography) or because their principal determining factors have happened in the past (geology, cosmology, biology). Aspects of all sciences are tractable to experimentation, but it does not follow that experimentation is the foundation of their major discoveries.
C. Most sciences are inductive, not deductive. That is, they do not rely on classic experimentation but on observation, description, and the elucidation of patterns and the inference of processes that control and influence them.

A.Explain what evolution is and isn't. It is a unifying theory that explains phenomena from cosmology to immunology. It is not speculation poorly grounded in imperfect fossil records, assumptions about leaps between major body plans, and extrapolations from poorly controlled experiments on populations. It is a theory subject to naturalistic testing. It isn't anti-religious and it doesn't claim that everything is random nor that there is necessarily no direction or purpose to life.
B.Separate the various levels of evolution to avoid misconceptions. Biological evolution is descent with modification. The modification is heritable, which separates biological evolution from the evolution of non-living things. Microevolution is what happens in populations, between successive generations of organisms. Speciation is the separation of one lineage into two. Macroevolution describes the processes and patterns that take place between species and larger lineages of organisms and communities in their environments, which also change through time. The definition of evolution as "a change in gene frequency" is like a definition of the Internet as "where you get your e-mail." The word "evolution" has different meanings in biology and in other sciences (the fact, the theory, the process), but so do many other words such as "species," "adaptation," and so on. The more we learn, the more concepts expand. But this is equally true in common parlance with no loss of clarity in discourse among reasonable people (think of the various meanings of the word "business").
C.Take an historical approach so that students can see how the idea gained acceptance. One advantage to this is that it temporarily shifts the focus from scientific knowledge to historical knowledge, and postpones the role of the teacher as advocate of a scientific theory that makes some students (at least initially) uncomfortable. Another advantage is that it actually shows how the pieces of the theory were assembled. Because they all made sense individually, they made better sense as a unified theory. This introduces students to the inductive method as well as to the ways in which theories in the historical sciences (or historical theories in the sciences) are formed and tested.

Alignment of rock layers throughout England by civil engineers (William Smith, late 18th C.) showed common and deep geological history, far beyond the few thousand years sometimes proposed.
Succession of fossil-bearing rocks in the Paris Basin (Cuvier and Brongniart, early 19th C.), representing sequences of ancient marine and terrestrial environments, gave evidence of many great changes in Earth's surface through time, not evidence of a single deluge.
Discovery of fossil animals such as the mosasaur, found nowhere on Earth today, demonstrated the reality of extinction (Cuvier, early 19th C.).
Fossil-bearing sequences through the geologic column showed that faunas become quantitatively more similar to living forms as the present is approached (Lyell, early 19th C.).
The sequences of fossils in rocks leads to general scientific acceptance that evolution on a grand scale has occurred (early 19th C., Britain and Europe).
Darwin and Wallace propose mechanism of natural selection, analogous to artificial selection, to explain differential survival and long-term change of organisms, providing a mechanism for the pattern of evolution (mid-19th C.).
Comparative embryology demonstrates that many organisms, such as chordates and echinoderms, that are very different as adults, pass through uniquely similar larval stages, providing further evidence of common ancestry (mid-19th C.)
The concept of homology (comparable structures among organisms), based originally on the position, composition, and development of organs and tissues, acquires an evolutionary explanation for its similarities (Owen, mid-19th C.).
Basic principles of what will be called genetics (dominance, recessiveness, blending, linkage of traits) are elucidated (Mendel, late 19th C.) but his work is overlooked until the beginning of the 20th C.
Radioactivity is discovered (early 20th C.). Different proportions of radioactive isotopes of the same elements in rocks are seen to provide a basis for calibrating their age, resulting in the geologic time scale. Radioactivity, not combustion, is identified as the source of the Sun's energy, indicating that it is much older than the 100,000 years previously calculated by physicists.
The rise of population genetics models (early 20th C.) provides illustrations of how forces of selection, genetic drift, and population size can change the genetic composition of natural populations. These models are consistent with observations of population changes in the wild and in laboratories.
Continental drift is proposed as an explanation for similarities among landforms, sequences of rock formations, and fossil floras and faunas spread across widely separated continents (early 20th C.). Geophysicists object that there is no known mechanism to move solid rock through solid rock. Later, such mechanisms are discovered in seafloor spreading, subduction, and mountain-building, leading to the acceptance of plate tectonics as the central organizing theory of geology and the basis for changes in the distributions of plants and animals through time (mid-20th C.).
The Modern Synthesis of Evolution fuses the growing understanding of genetics, the observations and models of population biology, and the vast patterns of the fossil record into a comprehensive theory of continuous change at all levels of biological evolution (mid-20th C.).
The genetic material, DNA and RNA, is described as a double helix of parallel codon triplets of amino acids. The recombination, mutation, and language of its genetic code form the basis for understanding the particulate mechanism of heritable evolutionary change (mid-20th C.).
The Neutral Theory of Evolution shows that much of the genetic material is not critical to the reproduction of offspring but is redundant (mid-20th C.). Observations and calibrations of relative changes in this redundant genetic material allow the reconstruction of relationships of organisms on the basis of degree of divergence of their neutral genetic material.

Everything evolves!

To move the discussion a little more broadly, it might be worth summarizing the following points:
Our basic understanding of cosmology is essentially a story of universal evolution. We form and test these hypotheses just as we test hypotheses about biological evolution.
The Solar System, the Earth, and all its initial conditions (distance from the Sun, size, rotational parameters) had a great influence on the evolution of life.
Tectonics, the unifying theory of earth sciences, is responsible for the oceans and atmosphere (as well as earthquakes, mountain building, seafloor spreading, subduction, etc.) and without it life could not exist (and does not exist on other planets).
Radiometric dating provides "absolute dates" (in numbers of years), compared to the "relative dates" given by typical geologic methods of matching strata. Hence these methods are independent, and do not rely on preconceptions of evolutionary history read from fossils. Radiometric dating doesn't depend on the geologic record at all, but on the principles of physics and chemistry.

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