This article appeared in The
Textbook Letter, November-December 2000.
It accompanied a review of the 1998 version of Biology: Living Systems,
which Glencoe/McGraw-Hill markets as a high-school biology textbook.
According to the doctrine of the ladder, living things can be neatly arranged in a continuous series that starts with lowly and defective creatures, progresses through "higher" forms that are increasingly "complex" and competent, culminates in the mammals, and attains its grand climax in man.
Schoolbook-writers use an array of deceits to teach that the living world conforms to that simple-minded scheme, and some of their most prominent deceits concern modes of reproduction in animals. In particular: Schoolbook-writers obsessively preach that external fertilization, which they associate with animals that occupy the ladder's lower rungs, is contemptibly crude and ineffective, but internal fertilization, which they associate with animals residing on higher rungs, is refined and efficient and worthy of esteem. To boost this notion, the writers tell students that animals which employ external fertilization discharge vast quantities of gametes, but most of the gametes fail to achieve syngamy (i.e., the union of a sperm cell with an egg). Animals which exhibit internal fertilization, the writers say or imply, are properly frugal: They shed fewer gametes at a time, and most or all of the gametes accomplish syngamy and form zygotes.
Such claims contravene what we see in the real world.
First let's briefly consider sperm cells. These are released by the millions or billions in external-fertilizing species and internal-fertilizing species alike -- and in external-fertilizing species and internal-fertilizing species alike, huge numbers of sperm cells die without participating in syngamy. Among internal-fertilizing species, Homo sapiens serves as a good example. A healthy male human typically discharges tens of millions of sperm cells during each ejaculation -- and typically, none of those sperm cells or only one of them succeeds in uniting with an egg. Less often, two or three succeed. The rest perish. Enough said.
Now, what about eggs? We must give much more attention to eggs, for these are the gametes on which ladder-peddlers like to focus. The peddlers cherish the dogma that external fertilization is so clumsy and haphazard that many eggs are left untouched by sperm cells; and schoolbooks have been conveying this fantasy to students for many years. As an example: The 1991 version of D.C. Heath's Life Science: The Challenge of Discovery displayed a picture of two fish spawning, and the caption accompanying the picture said: "There is a good chance that many eggs will not be fertilized." The caption didn't suggest what "a good chance" might mean, nor did it identify the fish. When I reviewed Heath's book, I described the caption as pseudoscientific drivel, and I stated that fertilization rates in fishes typically attain or closely approach 100%. I then supported my statement by reporting what I had learned during a survey of ichthyological publications, and I cited various examples to show that external fertilization in fishes proceeds with perfect or near-perfect effectiveness. Teachers may find my report helpful for disabusing students of the notion that "There is a good chance that many eggs will not be fertilized" when fishes spawn [see note 1, below].
I shall not say anything more about fertilization rates here, but I want to expose another deceit by which the ladder-peddlers depreciate external fertilization and mislead students. I want to rebut what the peddlers say about the matter of clutch size.
The word clutch denotes the complement of eggs that a female animal produces each time she breeds, and the phrase clutch size means the number of eggs that the clutch contains. From the dogma that external fertilization is inefficient and leaves many eggs barren, the ladder-peddlers have derived a corollary: They preach that females of external-fertilizing species must produce clutches that are much larger than those produced by females of internal-fertilizing species. This corollary, like its parental dogma, is false and can be refuted by counterexamples:
Among the cichlids, a large and diverse family of fishes that are renowned for their elaborate mating rituals and for the care that they confer on their eggs and young, most species use ordinary external fertilization. In some of these species, a typical clutch comprises about 2,000 eggs; in some others, a clutch has only 100 or so. The clutches of these latter species, then, are comparable to the clutches seen in some of the poeciliids -- small fishes in which fertilization is internal, the female retains the fertilized eggs in her genital tract, and the offspring develop there till she releases them as free-swimming, competent individuals that resemble tiny adults. In certain poeciliids, clutches containing about 100 eggs are common, and some species generate clutches comprising 200 eggs or more -- clutches substantially larger than those of some cichlids and substantially larger than the clutches of various fishes belonging to other families (e.g., the characids and the doradid catfishes) that employ external fertilization.
In Sebastes marinus, a fish that is widely distributed in the North Atlantic, fertilization is internal and a clutch has 50,000 eggs or more! This is another internal-fertilizing fish in which the female keeps the fertilized eggs in her genital tract for a while -- so reproduction in S. marinus bears some resemblance to reproduction in the poeciliids. But in S. marinus, the offspring complete only a minor part of their early development within the female's body, and she releases them as finless, helpless larvae that still carry their yolk sacs. The larvae then live as plankters while they develop further and, eventually, undergo metamorphosis.
Now let's look at salamanders. Some families of salamanders use external fertilization, but in most salamanders fertilization is internal. The species that produce the smallest clutches (fewer than 10 eggs) use internal fertilization, and so do those that produce the largest clutches (about 5,000 eggs).
The marine gastropods, too, deserve our attention here. All of these animals employ internal fertilization, yet the number of eggs constituting a clutch ranges from fewer than 20 to more than 10,000, depending on the species.
As these facts have demonstrated, the ladder-peddlers' notion of a simple, invariable relation between clutch size and mode of fertilization is nonsense. The matter of clutch size is complicated -- all the more so because the concept of clutch size sometimes breaks down. Look at the killifishes, a family that comprises more than 200 species, all of which use external fertilization. In many species of killifish, including those that are popular among aquarists, spawning is conducted more or less continually, rather than as a single event: A female expels one egg or a few eggs every day (or every few days) for the entire breeding season. When spawning occurs in that way, the idea of clutch size becomes elusive.
And in some instances, the very distinction between external fertilization and internal fertilization becomes blurred, as we can learn by viewing the African cichlid fishes Tilapia macrochir and Haplochromis burtoni. In each of these species, the female broods her fertilized eggs in her mouth. What interests us here is how she comes to have fertilized eggs in her mouth to begin with:
A question: In these two animals, is fertilization external or internal? In each case, to be sure, the gametes of both sexes are exposed, however briefly, to the outside world; and in each case, syngamy takes place in the female's alimentary tract, not in her genital tract. I suggest, though, that what T. macrochir and H. burtoni do cannot be called external fertilization in any ordinary sense of that term [note 2].
The Glencoe writers are zealous ladder-peddlers, and in the 1994 version of Living Systems -- in a section titled "External Fertilization" -- they used this malarkey to convey the lesson that external fertilization is haphazard and unreliable:
Animals with external fertilization release huge numbers of gametes at one time. A female salmon, for example, lays from 2000 to 10 000 eggs during spawning. There is safety in numbers in that at least some of the eggs are fertilized and have a greater chance of survival.
When I reviewed the 1994 version, I said that the claim about "huge numbers of gametes" was a false generalization and the stuff about "at least some of the eggs" being fertilized was rubbish. (It was, in fact, just another way of saying "There is a good chance that many eggs will not be fertilized.")
In the 1998 version of Living Systems, Glencoe's writers have altered the "External Fertilization" section, but they have succeeded only in making it worse:
Animals with external fertilization release huge numbers of gametes at one time. In most cases, the majority of eggs released is wasted. However, there is safety in numbers in that at least some of the eggs are fertilized and have a greater chance of survival.
The writers still are preaching their old hokum, but they have replaced their specific statement about a salmon with the claim that "In most cases, the majority of eggs released is wasted." Do you know how eggs can be "wasted"? Neither do I. The writers want students to view a reproductive process as if it were a manufacturing operation, to be judged competent or incompetent according to a simple-minded technological criterion that has nothing to do with the natural world or natural selection. In nature, any reproductive process is competent if it enables an organism to leave descendants. Nothing else counts.
William J. Bennetta is a professional editor, a fellow of the California Academy of Sciences, the president of The Textbook League, and the editor of The Textbook Letter. He writes often about the propagation of quackery, false "science" and false "history" in schoolbooks.
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