Philosophy Of Biology

Elliott Sober

• Publisher: Routledge
• Year: 2018
• ISBN:
  9780429974502
• Edition: 2
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

Perhaps because of it implications for our understanding of human nature, recent philosophy of biology has seen what might be the most dramatic work in the philosophies of the?special? sciences. This drama has centered on evolutionary theory, and in the second edition of this textbook, Elliott Sober introduces the reader to the most important issues of these developments. With a rare combination of technical sophistication and clarity of expression, Sober engages both the higher level of theory and the direct implications for such controversial issues as creationism, teleology, nature versus nurture, and sociobiology. Above all, the reader will gain from this book a firm grasp of the structure of evolutionary theory, the evidence for it, and the scope of its explanatory significance.

1
What Is Evolutionary Theory?

1.1 What Is Evolution?

We talk of stars evolving from red giants to white dwarfs. We speak of political systems evolving toward or away from democracy In ordinary parlance, "evolution" means change.

If evolution is understood in this way, then the theory of evolution should provide a global account of cosmic change. Laws must be stated in which the trajectories of stars, of societies, and of everything else are encapsulated within a single framework. Indeed, this is what Herbert Spencer (1820-1903) attempted to do. Whereas Charles Darwin (1809—1882) proposed a theory about how life evolves, Spencer thought he could generalize Darwin's insights and state principles that govern how everything evolves.

Although the allure of a unified theory of everything is undeniable, it is important to realize that evolutionary biology has much more modest pretensions. Evolutionary biologists use the term "evolution" with a narrower meaning. One standard definition says that evolution occurs precisely when there is change in the gene frequencies found in a population. When a new gene is introduced or an old one disappears or when the mix of genes changes, the population is said to have evolved. According to this usage, stars do not evolve. And if political institutions change because people change their minds, not their genes, then political evolution is not evolution in the biologist's proprietary sense.

Biologists usually compute gene frequencies by head counting. Suppose two lizards are sitting on a rock; they are genetically different because one possesses gene A and the other possesses gene B. If one grows fat while the other grows thin, the number of cells containing A increases and the number of cells containing Β declines. However, the gene frequencies, computed per capita, remain the same. The growth of organisms (their ontogeny) is not the same thing as the evolution of a population (Lewontin 1978).

The idea that change in gene frequency is the touchstone of evolution does not mean that evolutionists are interested only in genes. Evolutionary biologists try to figure out, for example, why the several species in the horse lineage increased in height. They also seek to explain why cockroaches have become more resistant to DDT. These are changes in the phenotypes of organisms—in their morphology, physiology, and behavior.

When a population increases its average height, this may or may not be due to a genetic change. Children may be taller than their parents simply because the quality of nutrition has improved, not because the two generations are genetically different. However, in the case of the horse lineage, biologists believe that the increasing height of successive species does reflect a change in their genetic endowment. The definition of evolution as change in gene frequency will count some cases of height in crease—but not others—as instances of evolution. This definition does not deny that phenotypic change can count as evolution. What it rejects is change that is "merely" phenotypic.

Another worry is that the definition of evolution as a change of gene frequencies ignores the fact that evolution involves the origin of new species and the disappearance of old ones. Evolutionists use the term microevolution to describe the changes that take place within a persisting species. Macroevolution is reserved for the births and deaths of species and higher taxa. Does the definition of evolution as change in gene frequency mean that macroevolution is not evolution? This is not a consequence of the definition, as long as daughter species differ genetically from their parents. If speciation—the process by which new species come into being—entails change in gene frequency, then speciation counts as evolution as far as this definition is concerned.

To further explore this definition of evolution, we need to review some elementary biology Genes are found in chromosomes, which, in turn, are found in the nuclei of cells. It is a simplification, though a useful one for getting started, to think of the genes in a chromosome as arranged like beads on a string. Some species—including human beings—have chromosomes in pairs. Such species are said to be diploid. Others have their chromosomes as singletons (haploid) or in threes (triploid) or fours (tetraploid). A species also may be characterized by how many chromosomes the organisms in it possess.

If we consider a pair of chromosomes in a diploid organism, we can ask what gene occurs on each of the two chromosomes at a given location (a locus). If there is only one form that the gene can take, then all members of the species are identical at that locus. However, if more than one form (allele) of the gene occurs, then the organisms will differ from each other at that locus.

Suppose there are two alleles that a diploid organism may have at a given location, which I'll call the A-locus. These alleles I'll call A ("big A") and a ("little a"). Each organism will either have two copies of A or two copies of a or one copy of each. The genotype of the organism at that locus is the pair of genes it possesses there. AA and aa organisms are termed homozygotes; Aa individuals are called heterozygotes.

Now I come to sex. This is a common but by no means universal mode of reproduction. A diploid organism forms gametes, which contain just one of the two chromosomes that occur in each chromosomal pair: The gametes are haploid. The process by which diploid parents produce haploid gametes is called meiosis. An individual who is heterozygous at the v4-locus typically will have 50 percent A gametes and 50 percent a gametes (though not always—see Section 4.5). The nonsex cells (somatic cells) in an individual are genetically identical with each other (ignoring for the moment the infrequent occurrence of mutations), but the gametes that an individual produces may be immensely different because the individual is heterozygous at various loci. Diploid parents produce haploid gametes, which come together in reproduction to form diploid offspring.

If I describe the genotypes of all the males and females in a population, can you figure out what the genotypes will be of the offspring they produce? The answer is no. You need to know who mates with whom. If a mother and father are both AA (or both aa), their offspring will all be AA (or aa). But when heterozygotes mate with heterozygotes (or with homozygotes), the offspring may differ from each other.

Mating is said to be random within a population if each female is as likely to mate with one male as with any other (and vice versa). Mating is assortative, on the other hand, if similar organisms tend to choose each other as mates. I now want to describe how assortative mating provides a counterexample to the claim that evolution occurs precisely when there is change in gene frequencies.

Suppose that each organism mates only with organisms that have the same genotype at the A-locus. This means that there are only three kinds of crosses in the population, not six. These are AA X AA, aa X aa, and Aa X Aa. What are the evolutionary consequences of this pattern of mating?

Consider a concrete example. Suppose the process begins with 400 individuals, of which 100 are AA, 200 are Aa, and 100 are aa. Notice that there are 800 alleles in the population at the locus in question (2 per individual times 400 individuals). Notice further that there are 400 copies of A (200 in the homozygotes and 200 in the heterozygotes) and 400 copies of a. So, initially, the gene frequencies are 50 percent A and 50 percent a.

Suppose these 400 individuals pair up, mate, and die, with each mating pair producing 2 offspring. In the next generation, there will be 400 individuals. The following table describes the productivities of the mating pairs:

Parental pairs		Offipring
50 AA×AA	produce	100 AA
100 Aa×Aa	produce	50 AA 100 Aa 50 aa
50 aa×aa	produce	100 aa

If you don't understand how I calculated the numbers of different offspring in the heterozygote mating, don't worry. The present point is simply that not all the offspring of such matings are heterozygotes.

Let's compare the frequencies of the three genotypes before and after reproduction. Before, the ratios are 1/4, 1/2, 1/4. After, they are 3/8, 1/4, 3/8. The frequency of heterozygosity has declined.

What has happened to the gene frequencies in this process? Before reproduction, A and a were each 50 percent. Afterwards, the same is true. There are 800 alleles present in the 400 offspring—400 copies of A (300 in homozygotes and 100 in heterozygotes) and 400 copies of a. The frequencies of genotypes have changed, but the gene frequencies have not.

In this example, the population begins at precisely 50 percent A and 50 percent a, and the assortative pattern is perfect—like always mates with like. However, neither of these details is crucial to the pattern that emerges. No matter where the gene frequencies begin and no matter how biased the pattern of positive association, assortative mating causes the frequency of heterozygosity to decline though gene frequencies remain unchanged.

Is the process generated by assortative mating an evolutionary one? It is standard fare in evolution texts and journals. To exclude it from the subject matter of evolutionary theory would be a groundless stipulation. I conclude that evolution does not require change in gene frequency.

Genes are important in the evolutionary process. But the gent frequency in a population is only one mathematical description of that population. For example, it fails to describe the frequencies of gene combinations (e.g., genotypes). The mistake in the definition of evolution as change in gene frequency comes from thinking that this single mathematical description always reflects whether an evolutionary change has taken place.

Genes are related to genotypes as parts are related to wholes: Genotypes are pairs of genes. This may lead one to expect that by saying what is true of the genes, one thereby settles what is true of the genotypes. After all, if I tell you what is going on in each cell of your body, doesn't that settle the question of what is going on in your body as a whole? This expectation is radically untrue when the properties in question are frequencies. Describing the frequencies of genes does not determine what the genotype frequencies are. For this reason, genotype frequencies can change whereas gene frequencies remain constant.

A second question about the definition of evolution as change in gene frequency is worth considering. I said earlier that genes are found in chromosomes, which are located in the nuclei of cells. However, it has been known for some time that there are bodies outside the nuclei (in the cytoplasm) that can provide a mechanism of inheritance (Whitehouse 1973). Mitochondria influence various phenotypic traits, and the DNA they contain is inherited. If a population changes its mitochondrial characters while its chromosomal features remain the same, is this an instance of evolution? Perhaps we should stretch the concept of the gene to include extrachromosomal factors. This would allow us to retain the definition of evolution as change in gene frequency, though, of course, it raises interesting questions about what we mean by a "gene" (Kitcher 1982b).

Another feature of the definition of evolution as change in gene frequency is that it does not count as evolution a mere change in the numbers of organisms a species contains. If a species expands or contracts its range, this is of great ecological significance, and a historian of that species will want to describe such changes in habitat. But if this change leaves gene frequencies unchanged, should it be excluded from the category of evolution? I won't try to answer this question. The point, again, is that change in gene frequency covers one type of change but fails to include others.

A final limitation in the definition of evolution as change in gene frequency is noteworthy. The genetic system itself is a product of evolution. Hence, an evolutionary process was underway before genes even existed. This objection to the standard definition is perhaps the most serious one, because it is difficult to see what better definition could be constructed in response.

The term "evolution" denotes the subject matter of an extremely variegated discipline, whose subfields differ in their aims, methods, and results. In addition, evoluti...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Dedication
• Contents
• List of Boxes and Figures
• Acknowledgments
• Introduction
• 1 What Is Evolutionary Theory?
• 2 Creationism
• 3 Fitness
• 4 The Units of Selection Problem
• 5 Adaptationism
• 6 Systematics
• 7 Sociobiology and the Extension of Evolutionary Theory
• References
• Index