1.1Diversity and Universality of Organisms
Organisms have long been classified by differences in morphology and lifestyle. Higher animals provide a good example; variations in skeletal fossils have shown that rabbits and mice, as well as elephants and dugongs, have evolved from common ancestors, and that horses have evolved in a unilinear manner (in accordance with orthogenetic theory). With this methodology, however, variations that cannot be gleaned from skeletal morphology, such as skin color and intelligence, have been overlooked.
All humans have different faces, skin colors and personalities, and these characteristics are strongly influenced by genetic base-sequence differences. Just one base-sequence difference results in significant variations in morphology and phenotype (i.e., traits), as demonstrated by the occurrence of hereditary diseases. In humans, polydactylism, achondroplasia and color blindness are examples of this. In mice, a mutation in the hairless gene is known to produce hairless mice.
However, as genome analysis advances into the 21st century, it is becoming increasingly clear that the diversity of organisms is determined by the organization of genes and their expression patterns. It is interesting that genes are not the only determinant. As an example, identical human twins have the same gene composition but not the same personality; this stems from differences in the expression pattern of genes among individuals. It is known that if one twin is infected with a disease, the expression pattern of antibody genes in that twin changes, resulting in a difference in resistance to the illness between the two siblings. It is also clear that one twin mastering the piano at a certain stage of life results in differences in the range of the motor and auditory areas of the brain. A simpler example is that identical twins are born with different weights; this is believed to be caused by a difference in nutritional balance between the two in the mother’s body.
Humans, despite the varying appearance of different races, constitute a single species as far as the reproductive pattern is concerned. This universality in humans is obvious when compared with other primates; differences in intra-species variation and inter-species gaps in terms of traits are clear between humans and other primates, indicating the importance of understanding the extent to which genes influence traits in defining species.