Darwin had the advantage of data from previous experimental findings, which he studied and compiled into his theories. It should also be noted that Darwin had access to these studies even though they were often in foreign countries and many years before his birth. Darwin also was aware of the extreme criticism that previous theories of evolution brought from the politicians and religious leaders of the time. To prevent a personal attack on his controversial ideas, he withheld his manuscript for 14 years.
Darwin's first theory on evolution, called descent with modification, was based on a combination of Lamarckian ideas and recent fossil discoveries. He theorized that organisms living today have been changed over time and probably stemmed from a single, or perhaps a few, starting ancestral organisms. This also explained why the members of a species living in one area may appear different from a similar species living in a different environment. While in South America, he examined fossilized armadillos from a local archaeological dig and noted that although similar to modern armadillos, they had minute differences. Darwin saw descent with modification as evolution.
Darwin's second theory, called modification by natural selection, expands on Malthus's work by stating that although populations have the ability to grow uncontrollably, they do not do so for very long because of natural limiting factors such as increasing death rate or decreasing birth rate. He further referenced the farmers' understanding by stating that within a population, individual variability exists and may provide an advantage for one individual over another. Individuals who possess more desirable traits are more adapted to their environment, so they are able to reproduce more successfully and populate succeeding generations with a higher percentage of descendants than those individuals less adapted to who environment. Likewise, whether artificial or natural, the possession of certain traits that are favored provides the individual with an adaptive advantage.
The process of selection, both artificial and natural, favors those traits that are considered desirable as well as those traits most adapted to their environment at the expense of those who are not. In so doing, selection tends to reduce the genetic variability of the population as the unequal reproductive rates favor the descendants of one individual over another, leading to decreased genetic variability. In certain haploid organisms, the lack of genetic variability is often a reason for extinction or greatly reduced numbers.
Fortunately, in diploid organisms, populations are less likely to become genetically similar because recessive genes are hidden in a heterozygous individual. Because of this heterozygous advantage, a recessive allele can remain viable and present in a population, because its phenotype is never expressed and therefore never selected for or against. Albinism is a condition characterized by organisms with a lack of pigment. It is a recessive gene that is carried in both plants and animals, including the human population, but is not expressed unless the offspring inherits the recessive gene from both parents.
Likewise, populations on the endangered-species list may have populations so small that their genetic variability is subsequently low. Adding to this problem is the unlikely chance that new reproductive members will be found to increase the genetic diversity or spontaneous random mutations creating new alleles to increase genetic variability. In keeping with “survival of the fittest,” the most fit individual is the one most capable of passing on its genes, so the chance for the survival of the genes is critical in small populations, such as endangered species.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.