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Natural Selection and
Variation
T
his chapter first establishes the conditions for natural
selection to operate, and distinguishes directional,
stabilizing, and disruptive forms of selection. We then
consider how widely in nature the conditions are met, and
review the evidence for variation within species. The review
begins at the level of gross morphology and works down to
molecular variation. Variation originates by recombination
and mutation, and we finish by looking at the argument to
show that when new variation arises it is not “directed”
toward improved adaptation.
4.1 In nature, there is a struggle for existence
The Atlantic cod ( Gadus callarias ) is a large marine fish, and an important source of human food. They also produce a lot of eggs. An average 10-year-old female cod lays about 2 million eggs in a breeding season, and large individuals may lay over 5 million (Figure 4.1a). Female cod ascend from deeper water to the surface to lay their eggs; but as soon as they are discharged, a slaughter begins. The plankton layer is a dangerous place for eggs. The billions of cod eggs released are devoured by innumerable plank- tonic invertebrates, by other fish, and by fish larvae. About 99% of cod eggs die in their first month of life, and another 90% or so of the survivors die before reaching an age of 1 year (Figure 4.1b). A negligible proportion of the 5 million or so eggs laid by a female cod in her lifetime will survive and reproduce a an average female cod will produce only two successful offspring. This figure, that on average two eggs per female survive to reproduce successfully, is not the result of an observation. It comes from a logical calculation. Only two can survive, because any other number would be unsustainable over the long term. It takes a pair of individuals to reproduce. If an average pair in a population produce less than two offspring, the population will soon go extinct; if they produce more than two, on average, the population will rapidly reach infinity a which is also unsustainable. Over a small number of generations, the average female in a population may produce more or less than two successful offspring, and the population will increase or decrease accord- ingly. Over the long term, the average must be two. We can infer that, of the 5 million or so eggs laid by a female cod in her life, 4,999,998 die before reproducing. A life table can be used to describe the mortality of a population (Table 4.1). A life table begins at the egg stage and traces what proportion of the original 100% of eggs die off at the successive stages of life. In some species, mortality is concentrated early in life, in others mortality has a more constant rate throughout life. But in all species there is mortality, which reduces the numbers of eggs produced to result in a lower number of adults. The condition of “excess” fecundity a where females produce more offspring than survive a is universal in nature. In every species, more eggs are produced than can sur- vive to the adult stage. The cod dramatizes the point in one way because its fecundity, and mortality, are so high; but Darwin dramatized the same point by considering the opposite kind of species a one that has an extremely low reproductive rate. The
72 PART 1 / Introduction
10
5
0
9-year-old cod 10-year-old cod
(a) (b)
Number of individuals 0 1 2 3 4 5 0 250 500 750 Millions of eggs Time (days)
Density (numbers/m
2 )
6
15
20
1,
Figure 4. (a) Fecundity of cod. Notice both the large numbers, and that they are variable between individuals. The more fecund cod lay perhaps five times as many eggs as the less fecund; much of the variation is associated with size, because larger individuals lay more eggs. (b) Mortality of cod in their first 2 years of life. Redrawn, by permission of the publisher, from May (1967) and Cushing (1975).
Cod produce far more eggs than are needed to propagate the population
As do all other life forms
factors limiting the sizes of real populations make up a major area of ecological study. Various factors have been shown to operate. What matters here, however, is the general point that the members of a population, and members of different species, compete in order to survive. This competition follows from the conditions of limited resources and excess fecundity. Darwin referred to this ecological competition as the “struggle for existence.” The expression is metaphorical: it does not imply a physical fight to survive, though fights do sometimes happen. The struggle for existence takes place within a web of ecological relations. Above an organism in the ecological food chain there will be predators and parasites, seeking to feed off it. Below it are the food resources it must in turn consume in order to stay alive. At the same level in the chain are competitors that may be competing for the same limited resources of food, or space. An organism competes most closely with other members of its own species, because they have the most similar ecological needs to its own. Other species, in decreasing order of ecological similarity, also compete and exert a negative influence on the organism’s chance of survival. In summary, organisms pro- duce more offspring than a given the limited amounts of resources a can ever survive, and organisms therefore compete for survival. Only the successful competitors will reproduce themselves.
4.2 Natural selection operates if some conditions are met
The excess fecundity, and consequent competition to survive in every species, provide the preconditions for the process Darwin called natural selection. Natural selection is easiest to understand, in the abstract, as a logical argument, leading from premises to conclusion. The argument, in its most general form, requires four conditions:
- Reproduction. Entities must reproduce to form a new generation.
- Heredity. The offspring must tend to resemble their parents: roughly speaking, “like must produce like.”
- Variation in individual characters among the members of the population. If we are studying natural selection on body size, then different individuals in the population must have different body sizes. (See Section 1.3.1, p. 7, on the way biologists use the word “character.”)
- Variation in the fitness of organisms according to the state they have for a heritable character. In evolutionary theory, fitness is a technical term, meaning the average number of offspring left by an individual relative to the number of offspring left by an average member of the population. This condition therefore means that indi- viduals in the population with some characters must be more likely to reproduce (i.e., have higher fitness) than others. (The evolutionary meaning of the term fitness differs from its athletic meaning.) If these conditions are met for any property of a species, natural selection automatic- ally results. And if any are not, it does not. Thus entities, like planets, that do not reproduce, cannot evolve by natural selection. Entities that reproduce but in which parental characters are not inherited by their offspring also cannot evolve by natural selection. But when the four conditions apply, the entities with the property conferring
74 PART 1 / Introduction
.
The struggle for existence refers to ecological competition
The theory of natural selection can be understood as a logical argument
higher fitness will leave more offspring, and the frequency of that type of entity will increase in the population. The evolution of drug resistance in HIV illustrates the process (we looked at this example in Section 3.2, p. 45). The usual form of HIV has a reverse transcriptase that binds to drugs called nucleoside inhibitors as well as the proper constituents of DNA (A, C, G, and T). In particular, one nucleoside inhibitor called 3TC is a molecular analog of C. When reverse transcriptase places a 3TC molecule, instead of a C, in a replicating DNA chain, chain elongation is stopped and the reproduction of HIV is also stopped. In the presence of the drug 3TC, the HIV population in a human body evolves a discriminating form of reverse transcriptase a a form that does not bind 3TC but does bind C. The HIV has then evolved drug resistance. The frequency of the drug-resistant HIV increases from an undetectably low frequency at the time the drug is first given to the patient up to 100% about 3 weeks later. The increase in the frequency of drug-resistant HIV is almost certainly driven by nat- ural selection. The virus satisfies all four conditions for natural selection to operate. The virus reproduces; the ability to resist drugs is inherited (because the ability is due to a genetic change in the virus); the viral population within one human body shows genetic variation in drug-resistance ability; and the different forms of HIV have differ- ent fitnesses. In a human AIDS patient who is being treated with a drug such as 3TC, the HIV with the right change of amino acid in their reverse transcriptase will reproduce better, produce more offspring virus like themselves, and increase in frequency. Natural selection favors them.
4.3 Natural selection explains both evolution
and adaptation
When the environment of HIV changes, such that the host cell contains nucleoside inhibitors such as 3TC as well as valuable resources such as C, the population of HIV changes over time. In other words, the HIV population evolves. Natural selection pro- duces evolution when the environment changes; it will also produce evolutionary change in a constant environment if a new form arises that survives better than the current form of the species. The process that operates in any AIDS patient on drug treatment has been operating in all life for 4,000 million years since life originated, and has driven much larger evolutionary changes over those long periods of time. Natural selection can not only produce evolutionary change, it can also cause a popu- lation to stay constant. If the environment is constant and no superior form arises in the population, natural selection will keep the population the way it is. Natural selection can explain both evolutionary change and the absence of change. Natural selection also explains adaptation. The drug resistance of HIV is an example of an adaptation (Section 1.2, p. 6). The discriminatory reverse transcriptase enzyme enables HIV to reproduce in an environment containing nucleoside inhibitors. The new adaptation was needed because of the change in the environment. In the drug- treated AIDS patient, a fast but undiscriminating reverse transcriptase was no longer adaptive. The action of natural selection to increase the frequency of the gene coding
CHAPTER 4 / Natural Selection and Variation 75
HIV illustrates the logical argument
Natural selection drives evolutionary change...
... and generates adaptation
lighter than average did not survive as well as babies of average weight. Stabilizing selec- tion has probably operated on birth weight in human populations from the time of the evolutionary expansion of our brains about 1–2 million years ago until the twentieth century. In most of the world it still does. However, in the 50 years since Karn and Penrose’s (1951) study, the force of stabilizing selection on birth weight has relaxed in wealthy countries (Figure 4.4b), and by the late 1980s it had almost disappeared. The pattern has approached that of Figure 4.2d: percent survival has become almost the same for all birth weights. Selection has relaxed because of improved care for premature
CHAPTER 4 / Natural Selection and Variation 77
(a) Directional selection (b) Stabilizing selection (c) Disruptive selection (d) No selection
Frequency
Fitness
(number of offspring produced)
Average size (in population)
Body size Body size Body size Body size
Time Time Time Time
Body size Body size Body size Body size
Figure 4. Three kinds of selection. The top line shows the frequency distribution of the character (body size). For many characters in nature, this distribution has a peak in the middle, near the average, and is lower at the extremes. (The normal distribution, or “bell curve,” is a particular example of this kind of distribution.) The second line shows the relation between body size and fitness, within one generation, and the third the expected change in the average for the character over many generations (if body size is inherited).
(a) Directional selection. Smaller individuals have higher fitness, and the species will decrease in average body size through time. Figure 4.3 is an example. (b) Stabilizing selection. Intermediate-sized individuals have higher fitness. Figure 4.4a is an example. (c) Disruptive selection. Both extremes are favored and if selection is strong enough, the population splits into two. Figure 4.5 is an example. (d) No selection. If there is no relation between the character and fitness, natural selection is not operating on it.
78 PART 1 / Introduction
Bella Coola
Upper Johnston Strait
Mean weight (lb)
’51 ’52 ’53 ’54 ’55 ’56 ’57 ’58 ’59 ’60 ’61 ’62 ’63 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 ’ Year
Odd years
Even years
Odd years
Even years
Mean weight (lb)
Figure 4. Directional selection by fishing on pink salmon, Onchorhynchus gorbuscha. The graph shows the decrease in size of pink salmon caught in two rivers in British Columbia since 1950. The decrease has been driven by selective fishing for the large individuals. Two lines are drawn for each river: one for the salmon caught in odd- numbered years, the other for even years. Salmon caught in odd years are consistently heavier, which is presumably related to the 2-year life cycle of the pink salmon. (5 lb ≈ 2.2 kg.) From Ricker (1981). Redrawn with permission of the Minister of Supply and Services Canada,
99 98 96 90 80 70 60 50 40 30 20 10 0
97 95
(a)
Percentage survival to age 4 weeks
Mean survival rate (females) Mean survival rate (males)
Females Males
(b)
2 3 4 5 6 7 8 9 10 Birth weight (lb)
0 5 10 15 20 25 30 35 Average mortality × 1,
25
20
15
10
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Minimal mortality
×^
1,
Italy (1954–74)
Japan (1969–84) USA, whites (1950–76)
USA, non-whites (1950–76)
Figure 4. (a) The classic pattern of stabilizing selection on human birth weight. Infants weighing 8 lb (3.6 kg) at birth have a higher survival rate than heavier or lighter infants. The graph is based on 13,700 infants born in a hospital in London, UK, from 1935 to
- (b) Relaxation of stabilizing selection in wealthy countries in the second half of the twentieth century. The x -axis is the average mortality in a population; the y -axis is the mortality of infants that have the optimal birth weight in the population (and so the minimum mortality achieved in that population). In (a), for example, females have a minimum mortality of about 1.5% and an average mortality of about 4%. When the average equals the minimum, selection has ceased: this corresponds to the 45° line (the “no selection” case in Figure 4.2d would give a point on the 45° line.) Note the way in Italy, Japan, and the USA, the data approach the 45° line through time. By the late 1980s the Italian population had reached a point not significantly different from the absence of selection. From Karn & Penrose (1951) and Ulizzi & Manzotti (1988). Redrawn with permission of Cambridge University Press.
deliveries (the main cause of lighter babies) and increased frequencies of Cesarian deliveries for babies that are large relative to the mother (the lower survival of heavier babies was mainly due to injury to the baby or the mother during birth). By the 1990s in wealthy countries, the stabilizing selection that had been operating on human birth weight for over a million years had all but disappeared. The third type of natural selection occurs when both extremes are favored relative to the intermediate types. This is called disruptive selection (Figure 4.2c). T.B. Smith has described an example in the African finch Pyrenestes ostrinus , informally called the black-bellied seedcracker (Smith & Girman 2000) (see Plate 2, between pp. 68 and 69). The birds are found through much of Central Africa, and specialize on eating sedge seeds. Most populations contain large and small forms that are found in both males and females; this is not an example of sexual dimorphism. As Figure 4.5a illustrates, this is a case in which the character is not clearly either discretely or continuously dis- tributed. The categories of discrete and continuous variation blur into each other, and the beaks of these finches are in the blurry zone. We shall look more at the mean- ing of continuous variation in Chapter 9, but here we are using the example only to illustrate disruptive selection and it does not much matter whether Figure 4.5a is called discrete or continuous variation. Several species of sedge occupy the finch’s environment, and the sedge seeds vary in how hard they are to crack open. Smith measured how long it took a finch to crack open a seed, depending on the finch’s beak size. He also measured fitness, depending on beak size, over a 7-year period. Figure 4.5c summarizes the results and shows two fitness peaks. The twin peaks primarily exist because there are two main species of sedge. One sedge species produces hard seeds, and large finches specialize on it; the other sedge species produces soft seeds and the smaller finches specialize on it. In an evironment with a bimodal resource distribution, natural selection drives the finch population to have a bimodal distribution of beak sizes. Natural selection is then dis- ruptive. Disruptive selection is of particular theoretical interest, both because it can
80 PART 1 / Introduction
.
20 15 10 5 0
15
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12 14 16 18
45 50 55 60 Tail (mm)
Lower bill width (mm)
Percentage
Percentage
(a)
(b)
(high) 0. Seedcracking performance
(low) 0.
Fitness
10 11
12 13
14 15
16 17
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Lower bill width (mm)
Figure 4.5 (c) Disruptive selection in the seedcracking finch Pyrenestes ostrinus. (a) Beak size is not distributed in the form of a bell curve; it has large and small forms, but with some blurring between them. The bimodal distribution is only found for beak size. (b) General body size, such as measured by tail size, shows a classic normal distribution. The distributions shown are for males. (c) Fitness shows twin peaks. Notice that the peaks and valleys correspond to the peaks and valleys in the frequency distribution in (a). Fitness was measured by the survival of marked juveniles over the 1983–90 period. Performance was measured as the inverse of the time to crack seeds. (1 in ≈ 25 mm.) Modified from Smith & Girman (2000).
... or disruptive
increase the genetic diversity of a population (by frequency-dependent selection a Section 5.13, p. 127) and because it can promote speciation (Chapter 14). A final theoretical possibility is for there to be no relation between fitness and the character in question: then there is no natural selection (Figure 4.2d; Figure 4.4b provides an example, or a near example).
4.5 Variation in natural populations is widespread
Natural selection will operate whenever the four conditions in Section 4.2 are satisfied. The first two conditions need little more to be said about them. It is well known that organisms reproduce themselves: this is often given as one of the defining properties of living things. It is also well known that organisms show inheritance. Inheritance is produced by the Mendelian process, which is understood down to a molecular level. Not all the characters of organisms are inherited; and natural selection will not adjust the frequencies of non-inherited characters. But many are inherited, and natural selec- tion can potentially work on them. The third and fourth conditions do need further comment. How much, and with respect to what characters, do natural populations show varia- tion and, in particular, variation in fitness? Let us consider biological variation through a series of levels of organization, beginning with the organism’s morphology, and working down to more microscopic levels. The purpose of this section is to give ex- amples of variation, to show how variation can be seen in almost all the properties of living things, and to introduce some of the methods (particularly molecular methods) that we shall meet again and that are used to study variation.
Morphological level At the morphological level, the individuals of a natural population will be found to vary for almost any character we may measure. In some characters, like body size, every individual differs from every other individual; this is called continuous variation. Other morphological characters show discrete variation a they fall into a limited number of categories. Sex, or gender, is an obvious example, with some individuals of a population being female, others male. This kind of categorical variation is found in other characters too. A population that contains more than one recognizable form is polymorphic (the condition is called polymorphism). There can be any number of forms in real cases, and they can have any set of relative frequencies. With sex, there are usually two forms. In the peppered moth ( Biston betularia ), two main color forms are often distinguished, though real populations may contain three or more (Section 5.7, p. 108). As the number of forms in the population increases, the polymorphic, categorical kind of variation blurs into the continuous kind of variation (as we saw in the seedcracker finch, Figure 4.5).
Cellular level Variation is not confined to morphological characters. If we descend to a cellular char- acter, such as the number and structure of the chromosomes, we again find variation.
CHAPTER 4 / Natural Selection and Variation 81
.
The extent of variation, particularly in fitness, matters for understanding evolution
Variation exists in morphological,
...
extra chromosomes, in addition to the normal number for the species. These “super- numerary” chromosomes, which are often called B chromosomes, have been particu- larly studied in maize and in grasshoppers. In the grasshopper Atractomorpha australis , normal individuals have 18 autosomes, but individuals have been found with from one to six supernumary chromosomes. The population is polymorphic with respect to chromosome number. Inversions and B chromosomes are just two kinds of chromoso- mal variation. There are other kinds too; but these are enough to make the point that individuals vary at the subcellular, as well as the morphological level.
Biochemical level The story is the same at the biochemical level, such as for proteins. Proteins are molecules made up of sequences of amino acid units. A particular protein, like human hemoglobin, has a particular characteristic sequence, which in turn determines the molecule’s shape and properties. But do all humans have exactly the same sequence for hemoglobin, or any other protein? In theory, we could find out by taking the protein from several individuals and then working out the sequence in each of them; but it would be excessively laborious to do so. Gel electrophoresis is a much faster method. Gel electrophoresis works because different amino acids carry different electric charges. Different proteins a and different variants of the same protein a have different net electric charges, because they have different amino acid compositions. If we place a sample of proteins (with the same molecular weight) in an electric field, those with the largest electric charges will move fastest. For the student of biological variation, the importance of the method is that it can reveal different variants of a particular type of protein. A good example is provided by a less well known protein than hemoglobin a the enzyme called alcohol dehydrogenase, in the fruitfly. Fruitflies, as their name suggests, lay their eggs in, and feed on, decaying fruit. They are attracted to rotting fruit because of the yeast it contains. Fruitflies can be collected almost anywhere in the world by leaving out rotting fruit as a lure; and drowned fruitflies are usually found in a glass of wine left out overnight after a garden party in the late summer. As fruit rots, it forms a number of chemicals, including alcohol, which is both a poison and a potential energy source. Fruitflies cope with alcohol by means of an enzyme called alcohol dehydrogenase. The enzyme is crucial. If the alcohol dehydro- genase gene is deleted from fruitflies, and those flies are then fed on mere 5% alcohol, “they have difficulty flying and walking, and finally, cannot stay on their feet” (quoted in Ashburner 1998). Gel electrophoresis reveals that, in most populations of the fruitfly Drosophila melanogaster , alcohol dehydrogenase comes in two main forms. The two forms show up as different bands on the gel after the sample has been put on it, an electric current put across it for a few hours, and the position of the enzyme has been exposed by a specific stain. The two variants are called slow ( Adh-s ) or fast ( Adh-f ) according to how far they have moved in the time. The multiple bands show that the protein is poly- morphic. The enzyme called alcohol dehydrogenase is actually a class of two polypep- tides with slightly different amino acid sequences. Gel electrophoresis has been applied to a large number of proteins in a large number of species and different proteins show different degrees of variability (Chapter 7). But the point for now is that many of these
CHAPTER 4 / Natural Selection and Variation 83
... biochemical, such as in enzymes,...
proteins have been found to be variable a extensive variation exists in proteins in nat- ural populations.
DNA level If variation is found in every organ, at every level, among the individuals of a popula- tion, variation will almost inevitably also be found at the DNA level too. The inversion polymorphisms of chromosomes that we met above, for example, are due to inversions of the DNA sequence. However, the most direct method of studying DNA variation is to sequence the DNA itself. Let us stay with alcohol dehydrogenase in the fruitfly. Kreitman (1983) isolated the DNA encoding alcohol dehydrogenase from 11 inde- pendent lines of D. melanogaster and individually sequenced them all. Some of the 11 had Adh-f , others Adh-s , and the difference between Adh-f and Adh-s was always due to a single amino acid difference (Thr or Lys at codon 192). The amino acid difference appears as a base difference in the DNA, but this was not the only source of variation at the DNA level. The DNA is even more variable than the protein study suggests. At the protein level, only the two main variants were found in the sample of 11 genes, but at the DNA level there were 11 different sequences with 43 different variable sites. The amount of variation that we find is therefore highest at the DNA level. At the level of gross morphology, a Drosophila with two Adh-f genes is indis- tinguishable from one with two Adh-s genes; gel electrophoresis resolves two classes of fly; but at the DNA level, the two classes decompose into innumerable individual variants. Restriction enzymes provide another method of studying DNA variation. Restric- tion enzymes exist naturally in bacteria, and a large number a over 2,300 a of restric- tion enzymes are known. Any one restriction enzyme cuts a DNA strand wherever it has a particular sequence, usually of about 4–8 base pairs. The restriction enzyme called EcoR1 , for instance, which is found in the bacterium Escherichia coli , recognizes the base sequence ...GAATTC... and cuts it between the initial G and the first A. In the bacterium, the enzymes help to protect against viral invasion by cleaving foreign DNA, but the enzymes can be isolated in the laboratory and used to investigate DNA sequences. Suppose the DNA of two individuals differs, and that one has the sequence GAATTC at a certain site whereas the other individual has another sequence such as GTATT. If the DNA of each individual is put with EcoR1 , only that of the first individual will be cleaved. The difference can be detected in the length of the DNA frag- ments: the pattern of fragment lengths will differ for the two individuals. The variation is called restriction fragment length polymorphism and has been found in all populations that have been studied.
Conclusion In summary, natural populations show variation at all levels, from gross morphology to DNA sequences. When we move on to look at natural selection in more detail, we can assume that in natural populations the requirement of variation, as well as of reproduction and heredity, is met.
84 PART 1 / Introduction
... and genetic characters
failure in orchids with these deceptive flowers can be remarkably high a even higher than the 80% in Encyclia cordigera. More extreme examples exist. Gill (1989) measured reproduction in a population of almost 900 individuals of the pink lady’s slipper orchid Cypripedium acaule in Rockingham County, Virginia, from 1977 to 1986. In that 10-year period only 2% of the individuals managed to produce fruit: the rest had been avoided by pollinators and failed to breed. In four of the years none of the orchids bred at all. Thus the ecological factor determining variation in reproductive success in orchids is the availability of, and need for, pollinating insects. If pollinating insects are unnecessary, all the orchids in a population produce a similar number of fruit. But if pollinating insects are necessary and scarce, because of the way the orchid “cheats” the pollinators, only a small minority of individuals may succeed in reproducing. Pollinators happen to be a key factor in orchids; but in other species other factors will operate and eco- logical study can reveal why some individuals are more reproductively successful than others. The results in Figure 4.8 show the amount of reproductive variation among the adults that exist in a population, but this variation is only for the final component of the life cycle. Before it, individuals differ in survival, and a life table like Table 4.1 at the beginning of the chapter quantifies that variation. A full description of the variation in lifetime success of a population would combine variation in survival from concep- tion to adulthood and variation in adult reproductive success. Examples such as HIV, or the pink salmon, show that natural selection can operate; but that leaves open the question of how often natural selection operates in natural populations, and in what proportion of species. We could theoretically find out how widespread natural selection is by counting how frequently all four conditions apply in nature. That, however, would at the least be hard work. The evidence of variation in phenotypic characters and of ecological competition suggests that the preconditions required for natural selection to operate are widespread, indeed probably universal. Whenever anyone has looked they have found variation in the phenotypic characters of populations, and ecological competition within them. Indeed, you do not need to be a professional biologist to know about variation and the struggle for existence. They are almost obvious facts of nature. It is logically possible that individual reproductive success varies in all populations in the manner of Fig- ure 4.8, but that natural selection does not operate in any of them, because the variation in reproductive success is not associated with any inherited characters. However, though it is logically possible, it is not ecologically probable. In almost every species, a high proportion of individuals are doomed to die. Any attribute that increases the chance of survival, in a way that might appear trivial to us, is likely to result in a higher than average fitness. Any tendency of individuals to make mistakes, slightly increasing their risk of death, will result in lowered fitness. Likewise, once an individual has sur- vived to adulthood, there will be many ways in which its phenotypic attributes can influence its chance of reproductive success. The struggle for existence, and phenotypic variation, are both universal conditions in nature. Variation in fitness associated with some of those phenotypic characters is therefore also likely to be very common. The argument is one of plausibility, rather than certainty: it is not logically inevitable that in a population showing (inherited) variation in a phenotypic character there will also
86 PART 1 / Introduction
.
The conditions for natural selection to operate are often met
Natural selection is likely at work in natural populations all the time
be an association between the varying character and fitness. But if there is, natural selection will operate.
4.7 New variation is generated by mutation and
recombination
The variation that exists in a population is the resource on which natural selection works. Imagine a population evolving increased body size. To begin with there is varia- tion and average size can increase. However, the population could only evolve a limited amount if the initial variation were all there was to work with; it would soon reach the edge of available variation (Figure 4.9a). In existing human populations, for instance, height does not range much beyond about 8 feet (2.4 m). The evolution of humans more than 8 feet high would be impossible if natural selection only had the currently existing variation to work on. Evolution from the origin of life to the level of modern diversity must have required more variation than existed in the original population. Where did the extra variation come from? Recombination (in sexual populations) and mutation are the two main answers. As a population evolves toward individuals of larger body size, the genotypes encoding larger body size increase in frequency. At the initial stage, large body size was rare and there might have been only one or two individuals possessing genotypes for large body size. The chances are that they would interbreed with other individuals closer to the average size for the population and produce offspring of less extreme size. But as the
CHAPTER 4 / Natural Selection and Variation 87
Frequency
Body size
Frequency
Body size
Frequency
Body size
Frequency
Body size
Frequency
Body size
Frequency
Body size
Time
(a) No new genetic variation
(b) New genetic variation introduced
Figure 4. Natural selection produces evolution by working on the variation in a population. (a) In the absence of new variation, evolution soon reaches the limit of existing variation and comes to a stop. (b) However, recombination generates new variation as the frequencies of the genotypes change during evolution. Evolution can then proceed further than the initial range of variation.
Long-term evolutionary change requires an input of new variation
It comes from recombination...
change was needed for the virus to continue to reproduce. Could it arise by directed mutation? At the genetic level, the mutation consisted of a set of particular changes in the base sequence of a gene. No mechanism has been discovered that could direct the right base changes to happen. If we reflect on the kind of mechanism that would be needed, it becomes clear that an adaptively directed mutation would be practically impossible. The virus would have to recognize that the environment had changed, work out what change was needed to adapt to the new conditions, and then cause the correct base changes. It would have to do so for an environment it had never previously experienced. As an analogy, this ability would be like humans describing subject matter they had never encountered before in a language they did not understand; like a seventeenth century American using Egyptian hieroglyphics to describe how to change a computer program. (Hieroglyphics were not deciphered until the discovery of the Rosetta Stone in 1799.) Even if it is just possible to imagine, as an extreme theoretical possibility, directed mutations in the case of viral drug resistance, the changes in the evolution of a more complex organ (like the brain, or circulatory system, or eye) would require a near miracle. Mutations are there- fore thought not to be directed toward adaptation. Although mutation is random and undirected with respect to the direction of improved adaptation, that does not exclude the possibility that mutations are non- random at the molecular level. For example, the two-nucleotide sequence CG tends to mutate, when it has been methylated, to TG. (The DNA in a cell is sometimes methy- lated, for reasons that do not matter here.) After replication a complementary pair of CG on the one strand and GC on the other will then have produced TG and AC. Species with high amounts of DNA methylation have (perhaps for this reason) low amounts of CG in their DNA. Molecular mutational biases are not the same as changes toward improved adapta- tion, however. You cannot change a drug-susceptible HIV into a drug-resistant HIV just by converting some of its CG dinucleotides into TG. Some critics of Darwinism have read that Darwinian theory describes mutation as “random,” and have then trotted out these sorts of molecular mutational biases as if they contradicted it. But mutation can be non-random at the molecular level without contradicting Darwinian theory. What Darwinism rules out is mutation directed toward new adaptation. Because of this confusion about the word random, it is often better to describe mutation not as random, but as “undirected” or “accidental” (which was the word Darwin used).
CHAPTER 4 / Natural Selection and VariationCHAPTER 4 / Natural Selection and Variation 8989
Adaptively directed mutation is unlikely, for theoretical reasons
But some non-adaptive mutational processes are directed
Further reading
An ecology text, such as Ricklefs & Miller (2000), will introduce life tables. For the the- ory of natural selection, see Darwin’s original account (1859, chapters 3 and 4), Endler (1986), and Bell (1997a, 1997b). Law (1991) describes the selective effects of fishing. Travis (1989) reviews stabilizing selection. Ulizzi et al. (1998) update the human birth- weight story. Greene et al. (2000) describe another possible example of disruptive selec- tion. Chapter 3 in this text gave references for HIV. Genetic variation is described in all the larger population genetics texts, such as Hartl (2000), Hartl & Clark (1997), and Hedrick (2000). White (1973) and Dobzhansky (1970) describe chromosomal variation. Variation in proteins and DNA will be dis- cussed further in Chapter 7, which gives references. The authors in Clutton-Brock (1988) discuss natural variation in reproductive sucess. I have concentrated on the theoretical argument against directed mutation, but experiments have also been done. The classic one was by Luria & Delbruck (1943). It was challenged by Cairns et al. (1988) but modern interpretations of results such as Cairns et al. rule out directed mutation: see Andersson et al. (1998) and Foster (2000). Two other themes are the evolution of mutation rates (see Sniegowski et al. 2000), and the possibility that the high mutation rates of HIV could be used against them by trig- gering a mutational meltdown. The underlying theory is discussed in Chapter 12 later
90 PART 1 / Introduction
Summary
chromosomes, the amino acid sequences of their proteins, and in their DNA sequences. 6 The members of natural populations vary in their reproductive success: some individuals leave no off- spring, others leave many more than average. 7 In Darwin’s theory, the direction of evolution, particularly of adaptive evolution, is uncoupled from the direction of variation. The new variation that is created by recombination and mutation is accidental, and adaptively random in direction. 8 Two reasons suggest that neither recombination nor mutation can alone change a population in the direction of improved adaptation: there is no evidence that mutations occur particularly in the direction of novel adaptive requirements, and it is theoretically difficult to see how any genetic mechanism could have the foresight to direct mutations in this way.
1 Organisms produce many more offspring than can survive, which results in a “struggle for existence,” or competition to survive. 2 Natural selection will operate among any entities that reproduce, show inheritance of their character- istics from one generation to the next, and vary in “fitness” (i.e., the relative number of offspring they produce) according to the characteristic they possess. 3 The increase in the frequency of drug-resistant, relative to drug-susceptible, HIV illustrates how natural selection causes both evolutionary change and the evolution of adaptation. 4 Selection may be directional, stabilizing, or disruptive. 5 The members of natural populations vary with respect to characteristics at all levels. They differ in their morphology, their microscopic structure, their
