Biological evolution is not debated in the scientific community — organisms
become new species through modification over time. “No biologist today would
think of submitting a paper entitled ‘New evidence for evolution;’ it simply
has not been an issue for a century” (Futuyma, 1986). Precisely how and at what
rates descent with modification occurs are areas of intense research. For example,
much work is under way testing the significance of natural selection as the
main driving force of evolution. Non-Darwinian explanations such as genetic drift
have been explored as additional mechanisms that explain some evolutionary
changes. Darwin proposed that change occurs slowly over long periods of geologic
time. In contrast, a more recent hypothesis called punctuated equilibrium proposes
that much change occurs rapidly in small isolated populations over relatively
short periods of geologic time.
In Darwin’s time, the nature of inheritance and the cause of variation were
very poorly understood. The scientific understanding of heredity began with
the work of Gregor Mendel in the 1860s in Brno, Czech Republic. This understanding
accelerated throughout the 20th century and now includes knowledge of chromosomes,
genes, and DNA with its double helix.
Evolution could not occur without genetic variation. The ultimate source of
variation can now be understood as changes or mutations in the sequence of the
building blocks of the genetic material carried on the chromosomes in eggs and
sperm. Many of these changes occur spontaneously during the process of creating
copies of the genetic code for each egg or sperm. For example, the wrong molecule
may become attached to the newly formed strand of DNA, or the strand may break
and a portion can be turned around. Certain forms of radiation and chemical
toxins can also cause mutations in the DNA.
Because the sequence of building blocks in DNA is the genetic foundation for
the development of an individual’s features or characteristics, changes in the
sequence can lead to a change in the appearance or functioning of an individual
with that mutation. Although some changes may prove to be harmful or fatal,
other changes produce variations that convey a survival advantage to the organism.
It is these variations, when passed on, that give advantages to the next generation.