Natural selection operates among individual organisms which differ in their genetic constitution. The degree of hereditary variability within a species is greatly enhanced by cross-fertilization. Indeed, the mechanism of sexual reproduction occurred very early in evolution, for it is seen today even in bacteria. In Escherichia coli, fertilization occurs by passage of the single chromosome from the male into the female bacterium (Lederberg, 1959).
KeywordsPlacental Mammal Mendelian Gene Female Heterogamety Genome Lineage Hemizygous State
Unable to display preview. Download preview PDF.