Guide for this unit:
Darwin on Struggle for Existence
Darwin on Natural Selection
Questions for this Unit
Click here for: On-line
Resources about and by Darwin
Click here for: Ch. 3 of Origin of Species on Struggle for Existence
Click here for: Ch. 4 of Origin of Species on Natural Selection Click here for: Malthus on the Principle of Population
Darwin's Origin of Species first appeared in 1859. Its full title, in the first edition was "The Origin of Species By Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life." The subtitle is significant, as it brings up the two key concepts we will be looking at this week and next: "struggle for life" (or struggle for existence), and "natural selection".
We'll begin by looking at Darwin's own words, in chapters 3 and 4 of the Origin (chapter 3 on "struggle for existence", and chapter 4 on "natural selection".)
In chapters 1 and 2 of Origin, Darwin had discussed the concept of "variation", both in domestic species (ch. 1, especially pigeons) and in the wild (ch. 2). This forms the background for the more substantial discussion of "struggle for life" in ch. 3. Darwin's point about variation can be summarized as follows: Although individuals making up a species share common characteristics -- with progeny resembling their parents in this regard -- there are individual variations within any co-specific population. For example, among a population of humans there is variation as to height, eye color, hair texture, and many other characteristics, even though humans share common characteristics that define us all as members of the species "homo sapiens", such as a large brain, two arms and legs, mainly hairless body, etc.
Darwin now enters into his discussion of the conditions that obtain within a population of individuals, characterized by shared species characteristics and individual differences. In section two of this chapter, "Geometrical Ratio of Increase", Darwin borrows from the work of the political economist Robert Malthus, who argued as follows in his Principles of Population (first edition 1798, second edition 1803):
(i) Population increases "geometrically", where a geometrical progression is defined as one where you get each successive term of a series through multiplication, eg: 1, 2 ,4, 8, 16, 32, 64, etc. Here each later number is gotten by multiplying the previous by 2 (x2).
(ii) Food increases only "arithmetically", where an arithmetical progression is defined as one where each successive term of a series is produced through adding a constant to the previous one, eg: 1, 3, 5, 7, 9, 11, 13, etc. In this example, each term is gotten by adding 2 to the previous (+2)
When you compare a geometrical to an arithmetic progression, you can readily see that the first rapidly outstrips the second:
1, 2, 4, 8, 16, 32, 64, etc -- this is a geometric progression (x2) 1, 3, 5, 7, 9, 11, 13, etc -- this is an arithemetic progression (+2)
You can see that the first term exceeds the second as of the fourth term. As the first (geometric) series represents population, and the second (arithmetic) series represent food, Malthus argued that a growing population cannot feed all of its members. In fact, Malthus used an arithmetic progression which increased by only 1, so that population outstrips food resources even faster. Click here to read Malthus on population and food. Malthus used this theory -- which he applied only to humans -- against proponents of egalitarianism, who were in 1798, the supporters of the French revolution. His argument was as follows: even if food and other essential resources were redistributed by a revolution from the rich to the poor, the population would soon again increase more rapidly than the available supplies of the necessaries of life, and poverty would again result. Darwin borrowed the core of Malthus' argument and applied the theory of geometric vs arithmetic progression to animals. This led him to conclude that under ordinary circumstances, too many individuals can be produced within a species for the available supply of food and other necessaries of life. In other words, there is over-population.
In a situation of overpopulation relative to food supplies, at least two possibilities exist: (i) individuals might compete each against the other to get as much as possible for themselves, (ii) individuals might cooperate to find new food resources or divide available resources more equitably amongst themselves, or (iii) some combination of competition and cooperation might occur.
For Darwin it appears axiomatic that this overpopulation can be resolved only through competition. Here he shares a cultural (or ideological) viewpoint with Malthus. Both were imbued with the culture of the industrial revolution in England, characterized by competition among producers for markets, among countries for colonies, and among workers for employment.
Though Darwin opts reflexively for competition as the basic form of interaction within a species, he does note some degree of association (a kind of unconscious cooperation) between individuals of different species in special circumstances. The section of this chapter entitled "Complex Relations of all Animals and Plants" heralds many ideas of ecology (a term coined only later by the German evolutionist Ernst Haeckel). Darwin briefly describes the complex interactions of members of different species in what we would today call an "ecosystem", using the example of a woodland and its various animal, plant, and insect inhabitants. But as Darwin's main focus is on "struggle for existence", he accords only a passing comment in this section to association rather than competition, and does not return to the matter in the rest of the volume. We will, however, in our later considerations of the problem of the relation between competition and cooperation in evolutionary theory. For the moment, it is an interesting, but off-hand discussion that seems almost out of place in the somewhat dismal picture Darwin paints of populations engaged in continual struggle for existence.
In ch. 3 of Origin we saw that Darwin, following and extending the argument of Malthus, argued that a struggle for life occurs among individuals making up each species. The question now arises: how is this struggle to be resolved? In order to consider Darwin's reasoning, let's look at an argument be analogy which he uses. Most of us are no doubt familiar with the actions of an animal breeder or plant fancier who is trying to get a particular characteristic in its animals or plants. We'll consider the case of animals, and for the sake of specificity, dogs.
Suppose that a dog breeder wants to breed animals with longer necks. From among a pack of six animals, which should he or she choose? The obvious answer, and common practice is to choose the dogs which have longer necks relative to the others, in the hope that they will pass on that characteristic to their progeny. Breeding a male and a female with longer necks would seem to be the best option. Darwin calls this "artificial selecton", for a reason that will be clear in a moment.
Now, let's try and follow Darwin as he considers the situation of animals in the wild, such as wolves. He makes the following argument by analogy:
An argument from analogy takes the following form: X is to A as Y is to B. Since we know three of the four terms (A, Y and B), we can get an idea of what X might be. In this case, the term for "X", the unknown process in nature which resolves the struggle for life, is understood on analogy to "Artificial Selection", it is "Natural Selection". Nature selects (metaphorically speaking, Darwin admits) those animals that by chance have the variations necessary to survive in their circumstances. The process is a two-fold one:
It is crucial to note that the first and second parts of the process are, according to Darwin, completely independent of each other. The process of variation cannot look ahead to see what variations will be needed later, in the next and succeeding generations. Variations, though following from laws of heredity, appear as random distributions in the population. Natural Selection then acts to choose those individuals with the "right" variations. These are the individuals that tend to reproduce, passing on their traits to the next generation. After this process if repeated over and over again, the population has modified characteristics; characteristics so modified that a biologist, looking at this group many generations later would classify them as a different species from the original. That, in a nutshell, is how Darwin sees the evolution of species, at least insofar as its chief factor is concerned: natural selection.
Consider a made-up, simplified example: Suppose we have a population of small mammals that are living in a cave. These animals vary in the amount of fur each carries, in their weight and height, etc. While circumstances are unchanged, these variations are without import. But suppose either of two circumstances occurs: (i) The temperature in the area in which they live falls significantly; or (ii) The cave exit is covered over by a rock fall, leaving only a small space open. In the first place, bigger animals with more fur will tend to be chosen for reproduction, as they can best withstand the temperature drop. In the second case, smaller animals with less fur will be selected, for they can best crawl through the tiny opening. Now, the change in the environment (temperature drop or rock fall) cannot be predicted in advance by the animals. So which variety (big or small) will be selected depends on chance. Of course, the decline in temperature or the rock fall are caused by physical processes governed by laws of nature. But the animals don't and can't know this, any more than dinosaurs knew that a comet (on the current hyothesis) would fall on earth, bringing their domination to an end in its fiery aftermath.
Darwin hypotheses that in general, the extremes tend to be selected, as each finds a different means of surviving in changed environments, though its important to note that sometimes none or too few of the animals have the needed characteristics. In this case, the population or species goes extinct. As a result of some varieties being selected and some going extinct, "gaps" appear between populations over time. This is known as "divergence of character". Darwin iterates (repeats) this process over and over again, producing the following hypothetical chart representing tens of thousands of generations:
Consider the species represented by "A". Some of the varieties (organisms sharing a distinct variation) go extinct -- their "tree line" ends; others (at the two extremes) tend to get selected as the environment varies. Each horizontal line with a Roman numeral to its right represents 1,000 generations. As you follow the paths of diverging branches up the tree, you get to level X, where the survivors, labeled a10, f10, and m10 have diverged so far, that a taxonomist would classify them as different species. They themselves would very inhabit different ecological niches and not recognize each other as relatives; indeed, one might be a predator of another. By generation XIV, each of these distinct species has diverged into a number of further species, forming new genera. The spreading species lines in the tree represent "descent with modification" from common ancestors. Put from the point of view of the biologist, each separate cluster of like individuals represents the origin of a species.
Note that the base line with the letters through has dotted lines under it, which if extended beyond the diagram might meet. If they did, this would represent the species from which all other species are evolved. Darwin was not sure that all species had a common ancestor, though he speculated that they did, with this ancestor of ancestors arising, as he put it in a letter: in some warm little pond.
(1) What does Darwin mean by "struggle for existence"?
(2) How are population and food related in Darwin's view of the matter, derived from Malthus?
(3) How does Darwin differ in his treatment of associations between organisms of different species and competition between organisms of the same species?
(1) Distinguish between the random nature of variations, and the
causal nature of selection.
(2) Define the following concepts: artificial selection, natural selection.. How are these similar and how are they different?
(3) Define the following concepts: divergence of character, descent with modification, origin of species