The two major paradigms that guide zoological research are Darwin’s theory of evolution and the chromosomal theory of inheritance. Darwin’s theory of evolution is based on Charles Darwin’s 1859 book, “On the Origin of Species by Means of Natural Selection.” The chromosomal theory of inheritance is based on work done independently by Walter Sutton and Theodor Boveri between 1902 and 1904.
The Theory of Evolution
The theory of evolution states that variation normally exists among members of a species. All members of the same species need certain resources to survive and reproduce successfully. Because resources are limited, members of the same species will inevitably compete with each other for the necessary resources. The species members which have variations which give them an advantage in the existing environment are more likely to survive and reproduce than the species members which don’t have advantageous variations.
If an advantageous variation is inheritable, it will be passed to the offspring, which will also have a competitive advantage in that environment. An advantageous variation which is inheritable is called an adaptation.
After several generations where the environment is constant, a particularly favorable variation can spread through the entire species. At the same time, members of species which are less well adapted to the environment are less likely to reproduce. This process is known as natural selection.
The theory of evolution explains ecologically how species change over time to become better adapted to their environments. It does not attempt to explain where life initially came from. This is not its purpose.
The Chromosomal Theory of Inheritance
The chromosomal theory of inheritance states that chromosomes are linear sequences of genes, which carry inheritance patterns. In general, the location of the gene on the chromosome determines the pattern of inheritance.
Darwin recognized that advantageous traits must be inheritable for a species to adapt to an environment. However, the mechanism for heredity was unknown in Darwin’s time. While Gregor Mendel identified inheritance patterns for traits which were clearly dominant or clearly recessive, it was impossible to identify the biological mechanism which carried those traits with the technology available at the time.
This changed when August Kohler developed Kohler illumination for microscopes in 1893. Earlier scientific observation had already identified the processes of mitosis and meiosis in cell reproduction. This innovation in microscope technology improved sample illumination to the point where chromosomes could be accurately observed in samples taken at all points during reproduction.
While the chromosomal theory of Inheritance was able to explain the observations, it was frustratingly difficult to prove at first. However, by 1927, Nikolai Koltsov brought the strands of research together by proposing that inherited traits were inherited through a giant hereditary molecule. That molecule turned out to be DNA.