Agriculture Reference
In-Depth Information
process of natural selection, explained brilliantly in Darwin
s On the
Origin of Species (Darwin 1859). However, importantly, in natural
selection, the environment plays the role in selecting form and function
(phenotype) by a passive
'
filtering mechanism and not by an active or
directing process, as Lamarck had proposed. However, Darwin still did
not understand the mechanism driving either the appearance of life or
the phenotype changes upon which natural selection worked. He so
clearly stated at the end on the On the Origin of Species that the
beginning of life and the origin of variation remained as mysterious in
the context of evolution as it did before. Perhaps this was best said by
Hugo de-Vries when he remarked that although natural selection could
explain the survival of the
fittest, it could not explain the arrival of the
fittest through the development of organisms in their life cycles (De
Vries 1904). As we shall see, his participation in revealing the work of
Mendel started with an explanation that is now undergoing a major
adjustment.
B. Genotypes and Phenotypes
The basis of phenotype change in the evolutionary context of life took a
giant mechanistic leap forward with the work of Mendel, which
described the patterns of polymorphic phenotype inheritance, eventu-
ally called genetics by Bateson, that acted through units eventually
called genes by Johannsen (Johannsen 1911). As Mendel
s work was
rediscovered and expanded, new observations of inheritance revealed
that polymorphic phenotypes could be inherited no matter to what
environment their parents were exposed because the genotype (the basis
of heredity) could not be in
'
uenced by the environment over the course
of development and the reproductive life of an organism. Johannsen then
became the primary proponent of the theory of genotype rather than
phenotype inheritance (Johannsen 1911). However, even after these
observations and conclusions fully discredited the Lamarckian view
of the direct role of the environment in evolution, active environmental
effects could not be ruled out entirely because the source of genotype
variation was still obscure. Famously, the environment by genotype
interaction made this painfully evident to investigators of heritability
(Finnegan 2001; Sherman and Talbert 2002; Steward et al. 2002;
Tsaftaris et al. 2008). These inconsistent roles of the environment
between the theory of evolution (a passive role) and the often observed
environmental effects on the heritability of traits (an apparently active
role) originally resulted from an incomplete understanding of how
genetic information could be altered, then manifested in a phenotype,
Search WWH ::




Custom Search