1.3 Mechanisms of Evolutionary Changes: Lamarckism vs. Mendelism

In order for a population to survive under natural selection it has to adapt to changing environmental conditions and also pass on to its offspring its capacity to survive, so-called directional selection (see I. 1.4.5). In Darwin's time little was publicly known about the science of genetics even though Mendel's work was published in 1865, a mere six years after Darwin's Origin of Species. The dominant view of inheritance at that time was a blending type. The hereditary information from the two parents was believed to be blended in the offspring, just as a container of white paint mixed with a container of red paint blends into a pink liquid. This theory [33] would predict that half of the variations carried by the parents were lost in each generation. Eventually the source of diversities would be eliminated, and natural selection would run out of raw material for operation.

Chevalier de Lamarck was the first person to propose a comprehensive theory known as "the inheritance of acquired characteristics" to explain evolutionary change. Lamarck believed that the evolutionary process that started out with the simplest organism became progressively more complex in succeeding generations. The process was triggered by the environmental effect on an organism's vital fluid. This fluid was a source of energy that differentiated an organism from the nonliving world. Organizational gains in an organism were conserved and passed on to the offspring.

Lamarck argued that temperature around the world affected the life processes of plants differently, and variations of plants were distributed according to the temperature zones of the world. He maintained that animals also could adapt to new environments through the use of organs and features best suited to a new surrounding. The vital fluid associated with these organs and features would be stimulated, assuring their further development, but if some parts did not suit the new environment, they would deteriorate, and only the well-adapted parts of the organism would be inherited by the offspring. In this way, the diverse forms of life evolved.

Darwin did not publicly agree with Lamarck but was more or less influenced by his theory in his formulation of a hypothesis to account for the transmission of parental variation. He believed that different parts of the body sent particles into the blood as messengers to the gonads, the sexual reproductive structures. As the body organs change under environmental influences, so would the messenger particles. Therefore, the new variations transmitted by these particles would be constantly replenished. However, Darwin's hypothesis was not really satisfactory because it could not account for the patterns of inheritance of parental characteristics.

Francis Galton (1822-1911) tested Darwin's hypothesis by attempting to produce hybrid rabbits with intermediate color by injecting blood from male rabbits of one color into the female rabbits of another. Darwin's theory would predict that the offspring would have coat colors intermediate between those of the parents and the blood donors because they had received a mixture of messenger particles carried by the blood of the donors and the parents. However, the coat colors of the offspring were consistently the same as those of the parents with no sign of being influenced by the blood donors (1). [34]

Gregor Mendel (1822-84) proposed another theory. He described the hereditary units as particulate traits or genes instead of vital fluid. Mendel tried to test the validity of the dominant blending view of heredity by mating different parental stocks and analyzing the characteristics of the offspring to see if there were any specific patterns of inheritance. Mendel was successful in discovering two basic laws of inheritance while most of his colleagues who were involved in more or less the same kind of experimentation failed to come up with any significant conclusions. This was no accident, for he had set up his experiments carefully.

Mendel chose the garden pea Piston sativum for experimentation. The pea had several advantages that allowed Mendel to have direct answers to his questions. First, the fertilization process could easily be controlled since the plant normally undergoes self-fertilization. This permitted Mendel to develop a technique to allow easy cross fertilization of different pea plants by removing the stamens (pollen organs) from one plant before self-fertilization and transferring them to another unfertilized destamenized plant. Second, because the generation tune of the pea plant was brief, Mendel could trace the distribution of parental characteristics in many generations in a relatively short time. Third, peas had many sharply defined characteristics each represented in two alternative forms that Mendel called traits.

Mendel did two sets of experiments with the garden peas. In the first set of experiments he mated pure breeding plants showing alternative traits for a specified characteristics. He then observed the distribution of a given pair of traits in the offspring. From the results of these experiments, he formulated the law of segregation. In the second set of experiments Mendel traced the patterns of inheritance of two pairs of traits. These experiments led him to formulate the law of independent assortment. The parental generation was designated as P, and the first generation and second generation offspring were designated F1 (first filial generation) and F2 (second filial generation), respectively.

The first set of experiments Mendel performed were monohybrid crosses involving one pair of traits. The second set of experiments were dihybrid crosses in which the distribution of two pairs of traits was followed. Mendel observed that all the traits of pure breeding parents did not always appear in the F1 generation. Apparently one form of each paired trait took precedence over the other. He termed those traits that appear in the F1 generation dominant characters and the traits that are not seen (latent) in the F1 generation recessive characters.

Figure 1.1 summarizes the traits that Mendel examined, and Table 1.1 lists all the combinations of the monohybrid crosses. The uniformity in [35]

Figure 1.1. Seven characteristics in peas that were observed and scored by Mendel in his published experiments. As in other legumes such as various species of beans (Phaseolus), each flower produces a seed pod, containing up to 10 seeds (7 ovules). Reprinted, with permission, from Strickberger, M. W. Genetics. New York: Macmillan Publishing Co.; 1976. © 1976. [36]

Table 1.1. Mendel's results from crosses involving single character differences. See Figure 1.1 for further description of these characters.
* The F2 generation results from self-fertilization of F organisms.

the F1 generation expressing the dominant characters and the apparent 3:1 distribution of the dominant versus the recessive characters in the F2 generation could best be explained by assuming that the appearance (phenotype) of the dominant character is controlled by an A gene, and the phenotype of the recessive trait is controlled by ana gene. The parental [37] genetic compositions (genotype) would be AA (homozygous dominant) and aa (homozygous recessive).

During the process of sexual reproduction, the genes of each parent segregate into gametes. In this case only one type of gamete will be obtained from each of the parents as a result of segregation, namely, A and a. When these gametes are brought together again in the fertilization process, then the genotype of the hybrid will be Aa (heterozygous). Since A gene is dominant over a gene, the only phenotype that is expressed in the F1 generation will be the dominant character. This accounts for the observation of the uniformity in the appearance of the dominant traits in Mendel's F1 offspring. During the subsequent mating among the F1 generation segregation occurs again, but now two different gametes can be obtained from each parent. Thus the possible combinations of genotype will be AA:Aa:aa with a 1:2:1 ratio. Since A is the dominant gene, AA and Aa will both have the phenotype of the dominant characters. This gives rise to the 3:1 phenotypic ratio in the F2 generation.

In Mendel's dihybrid crosses where he mated pure breeding plants characterized by round yellow seeds with pure breeding plants characterized by wrinkled green seeds, he got the following results:

Again the concept of segregation explains the data. If the genotypes of the plants in the parental generation with round yellow seeds and those with wrinkled green seeds are represented by RRYY (homozygous dominant) and rryy (homozygous recessive), respectively, during the F1 generation, only one genotype is possible, namely, RrYy since each parent can produce only one type of gamete by segregation, i.e., RY and ry, respectively. However, in the crosses among the F1 offspring, segregation can give rise to several possible combinations of gametes. Since each gamete must contain one type of inherited factor from each gene pair, a condition that Mendel had established from the monohybrid crosses, the gametes produced by a heterozygous F1 generation (RrYy) will have the composition of RY, Ry, rY, or ry. If each of these compositions are produced at equal frequency, i.e., the segregation of the two pairs of traits is independent of each other, then sixteen combinations with nine genotypes are possible, namely, RRYY, RRYy, RRyy, RrYY, RrYy, Rryy, rryy, rrYy, and rryy with a ratio of 1:2:1:2:4:2:1:2:1, respectively. These [38] genotypes will give a phenotypic ratio of 9:3:3:1. This interpretation is shown in Figure 1.2.

Figure 1.2. Schematic drawing showing the outcome of Mendel's second law of segregation. See text for explanation.

Although Mendel's work was first published in 1865, it went unnoticed until 1900, when Correns, de Vries, and Tschermak rediscovered and confirmed his findings. Furthermore, Hugo de Vries (1848-1935) proposed a new concept of mutation (2) that he used to describe changes in Oenothera, a plant commonly known as evening primrose. Oenothera brought about new types apparently at a single step. Mutations have since been shown to involve changes in single genes. Although de Vries coined the term mutation to describe sudden and abrupt changes, it has been found subsequently that a single mutation usually results in only a slight or barely perceptible modification of a phenotypic characteristic. This concept will be discussed further in the next section. With the introduction of Mendelian principles and de Vries's concept of mutation, the mechanism of evolutionary change was redefined with a more precise genetic framework.

The source of variation Darwin failed to explain by natural selection is essentially the mutations that occur in genes and that can be rearranged [39] by genetic recombination. However, through the process of natural selection new varieties can evolve by the adaptation of the preexisting mutants to the new environment.

Figure 1.3. Comparison of the ideas of Lamarckian and Mendelian theories of evolutionary change. Adapted, with permission, from Savage, J. M. Evolution. 2nd ed. New York: Holt, Rinehart, and Winston; 1969. © 1969.
[40] Therefore, the Lamarckian theory and the Mendelian theory of inheritance differ basically in the explanation of the source of diversification. Lamarckism interpreted it as a result of the inheritance of environmentally induced characteristics driven by the inner need of the vital fluid that stimulates development. Mendelism perceived it as the intrinsic characteristics of individuals controlled by particulate genes that can undergo mutation independently of environmental changes. The two interpretations of evolutionary change can best be illustrated by the hypothetical evolutionary scheme of the giraffes depicted in Figure 1.3 (3).

After additional research by August Weismann (1834-1914), Lamarckism fell into disfavor. Weismann believed that alterations of body characteristics by adaptation to the environment cannot be transmitted through the gametes. The failure to demonstrate the inheritance of acquired characteristics in experimental animals was consistent with his thesis. Characteristics in human populations also seem to conform to Weismann's interpretation. For ages Chinese women have had their feet tightly bundled shortly after birth, yet today's Chinese women have feet of regular size.

Lamarckian followers argue that since germ cells and body cells can be differentiated only in sexually reproducing organisms, Weismann's theory cannot be applied to asexual organisms, such as bacteria. Therefore, the Lamarckian view continued to be popular among bacteriologists in the early part of this century.

The Nobel laureate Joshua Lederberg (b. 1925) and his associates devised an ingenious experiment shown in Figure 1.4 to refute the Lamarckian view and establish the Mendelian view (4). The idea was to test the source of streptomycin-resistant mutant in a bacterial culture that was sensitive to the killing effects of streptomycin, a drug that inhibits the protein synthesis process in the bacteria. If the streptomycin-resistance trait can be shown to be inherent in the bacterial culture and not the result of the exposure to streptomycin, the Lamarckian theory that predicts the drug-resistant characteristic is produced only by the adaptation of the bacteria to the drug is refuted.

Lederberg developed a simple technique he called replica plate for his experiment. A wooden block was covered with a piece of sterile velveteen. The block was slightly smaller than the petri dish containing the solid agar medium that would support the growth of the bacteria. A few bacteria taken from a culture that was sensitive to streptomycin were spread on the surface of the agar plate containing no streptomycin. After an appropriate incubation time, each bacterium gave rise to a colony (an [41] area of bacterial growth on an agar plate) containing millions of bacterial cells with the same genetic make-up. This is represented by the dark dots on the agar plate in Figure 1.4. The cover of the plate was removed, and the velveteen was used as a stamp on which an imprint of each colony was made by gently tapping the plate onto the velveteen. The imprint was correlated with the actual position of the master plate, i.e., the plate containing the original colonies. The imprint on the velveteen was transferred to an agar plate that was supplemented with streptomycin by the same procedure as the original stamp making. The principle behind the procedure was that the velveteen's sticky hairlike texture picked up bacteria from each colony on the master plate and introduced them to the streptomycin plate at the same location.

Figure 1.4. Diagrams illustrate the replica plating technique used by Lederberg to demonstrate that streptomycin resistance results from mutations that can occur quite independently of exposure of the culture to the drug. Explanation in the text. Adapted, with permission, © 1961, from Sager, R.; Ryan, F. Cell heredity. New York: John Wiley & Sons; 1961.

[41] After incubation, the colony that survived on the streptomycin plate was scored, and by comparing it with the position of the imprint, the corresponding colony of the master plate was identified as the streptomycin-resistant clone. This was demonstrated by taking a few bacteria from the clone and transferring them to a liquid medium containing streptomycin. They multiplied and made the liquid medium in the tube cloudy, whereas the control colony as identified by the replica plating technique did not grow in the tube with the same medium. Thus, bacterial cells that had never been exposed to the drug streptomycin were shown to be resistant to the drug. The inheritance of acquired characteristics was ruled out by this experiment.

References 1.3

1. Lerner, LM.; Libby, W. J. Heredity, evolution and society. 2nd ed. San Francisco: Freeman; 1976: 5.
2. de Vries, H. Die mutations theorie. Leipzig: Veit; 1901.
3. Savage, J. M. Evolution. 2nd ed. New York: Holt, Rinehart, and Winston; 1969.
4. Lederberg, J.; Lederberg, E. M. J. Bacteriol. 63:399; 1952.