13 April 2005

Lecture 37

Reading, Chapter 11, Chapter 12

F. The Evidence

Biological evolution is a theory, not a fact. This phrase has been used to bolster the argument that alternative, faith-based explanations for the diversity of life on Earth should be presented as science in the public schools. The phrase ignores the fact that a theory is the highest form of knowledge that science offers. It is a synthesis of many facts into a large concept that attempts to explain them all. A scientific theory is always based on physical evidence and always subject to revision in the light of new evidence. Biological evolution is no exception.

Biological evolution is based on a vast diversity of individual facts but many of these can be organized into three classes of evidence:

1) Direct observations of micro-evolution. Within the human time scale, it is possible to see small amounts of biological evolution happening by direct observation. These are used to test hypotheses for how evolution occurs. For example, recent studies of closely related species and sub-species of hummingbirds in South America suggest that geographic isolation may not be required for speciation to occur in animals (See this link for more information, if you are interested).

2) Fossils. Fossils of living things that are no longer around and the absence of modern organisms from the fossil record has always been the most solid evidence of macro-evolution. Fossils are the most direct way that we can see the past history of life on Earth. Traditional study of evolution using fossils consists mostly of comparing the skeletons of fossil organisms to the skeletons of extant organisms. Similarity of key skeletal features are taken as support of relationship by descent. Hypotheses for how modern organisms evolved are developed by lining up a series of fossils that have some key skeletal features in common and others that show a gradual transition from the ancestral form in the oldest fossils to the modern form in the most recent. An example of this is the hypothesis for whale evolution that is illustrated on page 255 of your text. More information about the use of fossils to understand whale evolution can be found via this link.

3) Molecular evidence. Fossils and phenotypes of extant organisms were all that early evolutionary biologist had to work with. In the last 30 years, a new type of evidence based on gene sequences has become increasingly important.

Changes in the nucleotide sequences of genes, or in the amino acid sequences of their proteins, accumulate over time by mutation and other processes. Some of these cause changes in phenotype, which we observe as visual evidence of evolution. Many others do not and must be examined directly by study of the DNA. The degree of sequence difference present in the same gene from two different organisms is now used as a way to measure how much time has passed since the two diverged from a common ancestor. Your text presents examples of this on page 256.

As DNA evidence has been utilized to test hypotheses of evolution, it has been learned that different genes accumulate changes at different rates. Sequences of non-coding DNA change very rapidly. Since this DNA codes for nothing, sequence changes have no consequences for the survival of the organism. They are not filtered out by selection and they accumulate quickly, some with each generation. Changes in non-coding DNA are those used to identify individual humans from DNA samples, e.g. in criminal cases. Other genes change very slowly because the proteins they code for are essential to survival and most changes are filtered out by selection. The cytochrome c protein that carries electrons in mitochondrial electron transport and the histone proteins that bind DNA in chromosomes are two examples of genes that change slowly. Figure 11.8 on page 257 of your text shows approximate rates of change for several genes. Those that change most slowly are used for comparing distantly related organisms that descended from a common ancestor a very long time ago. Those that change quickly are better for looking at closely related organisms that diverged from a common ancestor recently.

Molecular evidence is now used in combination with fossil evidence to test hypotheses of evolution. It has revolutionized the study of biological evolution to an even greater degree than DNA evidence has revolutionized the criminal justice process.

1. Whale evolution as an example.

An excellent example of how molecular evidence has been used to verify or refute hypotheses of evolution based on fossil evidence comes from studies of whale evolution. As shown on page 255 of your text, the terrestrial ancestor of whales was proposed to be a carnivorous mammal that lived 60 million years ago. This was based on similarity of ear bones in the skull fossils of the carnivore to ear bones of modern whales. However, molecular evidence showed that modern whales were more closely related to modern hippos than to modern carnivores, casting doubt on the fossil-based hypothesis. These two hypotheses were argued for some time with no progress until the originator of the carnivore hypothesis, Dr. Philip Gingerich of the University of Michigan, discovered more fossils of an alternative whale ancestor that showed key skeletal similarities to both modern hippos and modern whales. In the light of this new information, Dr. Gingerich regretfully discarded his celebrated carnivore hypothesis and began testing the hippo hypothesis, which was supported by better evidence.

G. Alternative views

There has been much written recently about teaching biological evolution in public schools so we must take a few moments to address the issue. Some political and religious groups have proposed teaching faith-based alternatives for understanding biodiversity and the history of life. The leading alternatives in the US are:

1. Special creation

This view of biodiversity and the history of life is based on a strict interpretation of the book of Genesis in the Christian Bible. Proponents of special creation believe that all living things were made simultaneously by a supernatural being less than ten thousand years ago. There is no credible physical evidence to support this view. It rests solely on faith in the belief that Genesis must be interpreted narrowly and literally. Honest proponents of special creation will admit that physical evidence is secondary to their faith in this belief.

As we discussed early in this class, faith is one way to understand the world around us, in conjunction with reason and instinct. Science rejects faith as a way to understand the natural world, however, because it has been so misleading in the past. Special creation is certainly a tolerable religious belief held by many good people but it is not science and should not be presented as such.

Nonetheless, "creation science" has been proposed as the scientific study of special creation. It consists largely of verbal arguments against widely accepted principles of chemistry, geology, and biology. More information about creation science is available via this link.

2. Intelligent design

Intelligent design is a recent reformulation of special creation. Intelligent design is the idea that living things are too complex to have evolved without the guidance of a deity. Proponents of intelligent design do not explicitly espouse any one religion. This makes intelligent design a faith-based hypothesis that can be taught in the public schools without being unconstitutional. Intelligent design seems to be based largely on the fact that it feels better to a lot of people than a strictly scientific theory of biological evolution. Thus we can say it is based on a combination of faith and instinct. Intelligent design is a legitimate religious belief and some of its proponents are scientists but it does not qualify as a scientific hypothesis because it is not based on physical evidence and cannot be tested by experiment or observation.

For more information on intelligent design, visit the website for the Discovery Institute, which promotes teaching intelligent design in the public schools.

Life manifests itself in many forms. There are lots of different ways to be alive and it is instructive to consider their variety.

Systematics is the branch of biology that attempts to organize living things so that we can best understand their diversity. Modern systematics attempts to organize living things based on their evolutionary history. It is proposed that the diversity of life on earth is related by descent from a single or very few original life forms. Organisms that are closely related (descended from a recent common ancestor) are grouped together while organisms that are distantly related (descended from a very ancient common ancestor) are grouped separately.

In the past, systematists relied on the phenotype of organisms as evidence of evolutionary relationships. More recently, systematists examine the DNA sequences of organisms to understand how they are related. The accumulation of mutations in groups of genes and even the whole genotype of different organisms is taken as an indicator of how much time has passed since they diverged from a common ancestor.

Very similar organisms are grouped into a "genus". Organisms that are very closely related and actively interbreeding are put into a "species". Species are given names that have both their genus and species combined (sort of a first and last name). For example, Homo (genus) sapiens (species) is the human species. Eschericia coli is the species of bacterium that lives in your intestine.

Species are the smallest systematic groupings. The largest are "kingdoms", which represent groups of organisms descended from a very ancient common ancestor. The six kingdoms presently accepted in modern biology are:

1. Animalia

animals, including you

many cells

eukaryotic

2. Plantae

plants, mostly land plants

many cells

eukaryotic

3. Fungi

mushrooms, molds, yeasts

single cells or many cells

eukaryotic

4. Protista

algae, amoebas

single cells or many cells

eukaryotic

5. Eubacteria

bacteria

single cells but some colonies of multiple cells

prokaryotic

6. Archebacteria

bacteria

single cells but some colonies of multiple cells

prokaryotic

  

Within these six groups, the Eubacteria and Archebacteria consist of cells that are "prokaryotic" while the others consist of cells that are "eukaryotic". Eukaryotic cells are like those of plants and animals (your cells are eukaryotic). Prokaryotic cells are fundamentally different from eukaryotic cells. If you could only have two kingdoms, they would be prokaryotes and eukaryotes.

Not long ago, Archbacteria and Eubacteria were grouped together into the same kingdom, called Monera. It is now clearer that the genetic and other differences between Archebacteria and Eubacteria are as great as those between prokaryotes and eukaryotes. Therefore, they have each been given their own kingdoms.