Every cell protects the integrity of the genome. DNA is chemically stable, and the cell defends it with various protective and repair devices. When genes are altered, the consequences for the cell might not be disastrous: the system as a whole is robust, and the redundancy of the genetic code provides a buffer. So, in eukaryotes, does the abundance of "junk" DNA. Nevertheless, DNA can change in a number of ways.
These changes can alter organisms subtly or markedly. If a developmental "master gene" is modified, the effect can be particularly dramatic. These "master genes" encode transcription factors and control the expressions of many other genes, so significant changes in them can alter the entire course of development, perhaps creating a novel sort of organism.
Over thousands of millions of years, cumulative changes in DNA have resulted in the diversification of countless species. One result of the revolutionary progress in molecular biology during the last quarter of the twentieth century was a revision of the "tree of life". Before the 1970s, the historical connections among species were reconstructed mainly from comparative anatomical and embryological evidence and the fossil record. Now there is ever-increasing reliance on comparative gene sequencing. This modern, molecular biological, approach gives results that are reassuringly consistent with those of the traditional approach. However, comparative gene sequencing allows greater refinement of detail and has provided insights into early life on Earth, even into times before the fossil record began.
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