NATURAL SELECTION
Natural selection is the centerpiece of Darwin's theory of evolution. It gives us a natural explanation for the origins of adaptation, including all developmental, behavioral, anatomical, and physiological attributes that enhance the organism's ability to use environmental resources to survive and reproduce. Darwin developed his theory of natural selection as a result of a series of five observations and three inferences drawn from them.
Observation 1
Organisms have great potential fertility. All populations produce large number of gametes and potentially large numbers of offspring each generation. Population size would increase exponentially at an enormous rate if all individuals that were produced each generation survived and reproduced. Darwin calculated that, even in a slowbreeding species such as the elephant, a single pair breeding from age 30 to 90 and having only six young could produce 19 million descendants in 750 years.
Observation 2
Natural populations normally remain constant in size, except for minor fluctuations. Natural populations fluctuate in size across generations and sometimes go extinct, but no natural populations show the continued exponential growth that their reproductive biology theoretically could sustain.
Observation 3
Natural resources are limited. Exponential groqth of a natural population would require unlimited natural resources to provide food and habitat for the expanding population, but natutal resources are finite.
Inference 1
There exists a continuing struggle for existence among members of a population. Survivors represent only a part, often a very small part, of the individuals produced each generation. Darwin wrote in the origin of species that it is the doctrine applied with manifold force to the whole animal and vegetable kingdoms. The struggle for food, shelter, and space becomes increasingly severe as overpopulation develops.
Observation 4
All organisms show variation. No two individuals are exactly alike. They differ in size, colour. physiology, behavior, and many other ways.
Observation 5
Variation is heritable. Darwin noted that offspring tend to resemble their parents, although he did not understand how, the hereditary mechanism discovered by Gregor Mendel would be applied to Darwin's theory many years later.
Inference 2
There is differential survival and reproduction among varying organisms in a population. Survival in the struggle for existence is not random with respect to hereditary variation present in the population. Some traits give their possessors an advantage in using the environment for effective survival and reproduction.
Inference 3
Over many generations, differential survival and reproduction generates new adaptations and new species. The differential reproduction of varying organisms individually transforms species and results in the long-term "improvement" of types. Darwin knew that people often use hereditary variation to produce useful new breeds of livestock and plants. Natural selection acting over millions of years should be even more effective in producing new types than the artificial selection imposed during a human lifetime. Natural selection acting independently on geographically separated populations would cause them to diverge from each other, thereby generating the reproductive barriers that lead to speciation.
Natural selection is often viewed as a two-step process with a random component and a nonrandom component. The production of variation among organisms is the random component. The mutational process does not preferentially generate traits that are favorable to the organism; if anything, the reverse is probably true. The nonrandom component is the survival of different traits. This is determined by the effectiveness of different traits in permitting their possessors to survive and to reproduce.
Differential survival and reproduction among varying organisms is now called sorting and should not be equated with natural selection. We now know that even random processes (genetic drift), can produce sorting among varying organisms. Selection states that sorting occurs because certain traits give their possessors advantages in survival and reproduction relative to others that lack those traits. Selection is therefore a specific cause of sorting.
Darwin's theory of natural selection has been challenged repeatedly. One challenge claims that directed (nonrandom) variation governs evolutionary change. In the decades around 1900, several evolutionary theories collectively called orthogenesis proposed that variation has momentum that forces a lineage to evolve in a particular direction, which is not always adaptive.
The extinct Irish elk was a popular example of orthogenesis; variation preferentially increased the size of antlers and natural selection was unable to stop the antlers eventually from becoming so large and cumbersome that they forced the Irish elk into extinction. Orthogenesis explained apparently nonadaptive trends that led a species into decline. Subsequent genetic research on the nature of variation however, has rejected the genetic predictions of orthogenesis.
Another recurring criticism of natural selection is that it cannot generate new structure or species but can only modify old ones. Most structures in their early evolutionary stages could not have performed the functions that the fully formed structures perform, and it is therefore unclear how natural selection occurred.
MENDELIAN HEREDITY AND THE CHROMOSOMAL THEORY OF INHERITANCE
The chromosomal theory of inheritance is the foundation for current studies of genetics and evolution in animals. This theory comes from the consolidation of research done in the fields of genetics, which was founded by the experimental work of Gregor Mendel, and cell biology.
GENETIC APPROACH
The genetic approach consists of mating or "crossing" populations of organisms that are true-breeding for contrasting traits, and then following the hereditary transmission of those traits through subsequent generations. "True-breeding" means that a population maintains across generations only one of the contrasting states of a particular feature when propagated in isolation from other populations.
Mendel studied the transmission of seven variable features in garden peas, crossing populations that were true-breeding for alternative traits (for example, tall versus short plants). In the first generation (called the F1) there was no indication of blending of the parental traits. In the example, the offspring (called F1 hybrids) formed by crossing the tall and short plants were tall, regardless of whether the tall trait was transmitted through the male or the female parent. These F1 hybrids were allowed to self-pollinate and both parental traits were found among their offspring (called the F2 generation), although the trait observed in the F1 hybrids (tall plants in this example) was three times more common than the other trait. Again, there was no indication of blending of the parental traits.
_ Mendel's experiments showed that the effects of a genetic factor can be masked in a hybrid individual, but that these factors were not physically altered during the transmission process. He postulated that variable traits are specified by paired hereditary factors, which we now call "genes", When germ cells (eggs or sperm) are produced, the two genes controlling a particular feature are segregated from each other and each germ cell receives only one of them. Fertilization restores the paired genes for a feature. If an organism possesses ditferent forms of the paired genes for a feature, only one of them is expressed in its appearance but both genes nonetheless will be transmitted unaltered in equal numbers to the gametes produced. Transmission of these genes is particulate not blending. Mendel observed that the inheritance or one pair of traits is independent of the inheritance of other paired traits. We now know, however, that not all pairs oftraits are inherited independently of each other.
PRINCIPLES OF lNHERITANCE
Heredity establishes the continuity oflife forms. Although offspring and parents in a particular generation may look different, there is nonetheless a basic sameness that runs from generation to generation for any species of plant or animal. In other words, "like begets like". Yet children are not precise replicas of their parents. Some of their charactetistics show resemblances to one or both parents, but they also demonstrate many traits not found in either parent. What is actually inherited by an offspring from its parents is a certain type of germinal organization (genes) that, under the influence of environmental factors, guides the orderly sequence of differentiation of the fertilized egg into a human being, bearing the unique physical characteristics as we see them. Each generation hands on to the next the instructions required for maintaining continuity of life.
The gene is the unit entity of inheritance, the germinal basis for every characteristic that appears in an organism. The study of what genes are and how they work is the science of genetics. It is a science that deals with the underlying causes of resemblance, as seen in the remarkable fidelity of reproduction, and of variation, which is the working material for organic evolution. Genetics has shown that all living forms use the same information storage, transfer, and translation system, and thus it has provided an explanation for both the stability of all life and its probable descent from a common ancestral form. This is one of the most important unifying concepts of biology.
MENDEL'S INVESTIGATIONS
The first man to formulate the cardinal principles of heredity was Gregor Johann Mendel (1822-1884), who lived with the Augustinian monks at Brunn (Brno), Moravia. At that time Brunn was a part of Austria, but now it is in the central part of Czechoslovakia. While conducting breeding experiments in a small monastery garden from 1856 to 1864, Medel examined with great care the progeny of many thousands of plants. He worked out in elegant simplicity the laws governing the transmission of characters from parent to offspring. His discoveries, published in 1866, were of great significance, comingjust after Darwin's publication of the Origin of Species. Yet these discoveries remained unappreciated and forgotten until 1900 some 35 years after the completion of the work and 16 years after Mendel's death.
Mendel's classic observations were based on the garden pea because it had been produced in pure strains by gardeners over a long period of time by careful selection. For example, some varieties were definitely dwarf and others were tall. A second reason for selecting peas was that they were self-fertilizing but also capable of crossfertilization. To simplify his problem mendel chose single characteristics and characteristics that were sharply contrasted. He carefully avoided mere quantitative and intermediate characteristics. He selected pairs of contrasting characteristics, such as tall plants. dwarfplants. smooth seeds. and wrinkled seeds.
Mendel crossed plants having one of these characteristics with others havin g the contrasting characteristic. He did this by removing the stamens from a flower to prevent self-fertilization and then placing, on the stigma of this flower, pollen from the flower of the plant that had the contrasting characteristic. He also prevented the experimental flowers from being pollinated from other sources such as wind and insects. When the cross-fertilized flower bore seeds, he noted the kind of plants (hybrids) that were produced from the planted seeds. Subsequently he crossed these hybrids among themselves to see what would happen.
CHROMOSOMAL BASIS OF INHERITANCE
In sexually reproducing animals, gametes (eggs and sperm) are responsible for providing the genetic information to the offspring. Scientific explanation for genetic principles required a study of germ cells and their behavior, which meant working backward from certain visible results of inheritance to the mechanism responsible for such results. The nuclei of sex cells were early suspected of furnishin g the real answer to the mechanism. This applied especially to the chromosomes, since they appeared to be the only entities passed on in equal quantities from both parents to offspring.
When Mendel's laws were rediscovered in 1990, their parallelism with the cytological behavior of the chromosomes was obvious. Later experiments showed that the mechanism of heredity could be definitely assigned to the chromosomes. The next problem was to find out how chromosomes affected the hereditary pattern.
Mendel knew nothing of the cytological basis of heredity, since chromosomes and genes were unknown to him. Although we can admire Mendel's power of intellect in his discovery of the principles of inheritance without knowledge of chromosomes, the principles are certainly easier to understand if we first examine chromosomal behavior, especially in meiosis.
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