Darwin’s Evolutionary Theory: Contributions & Shortcomings

Darwin's  1837 "Tree of Life" sketch from his notebooks (Stanford University)

Darwin’s 1837 “Tree of Life” sketch from his notebooks (Stanford University)

Biological evolution occurs when a population evolves physical changes that allow it to better survive and reproduce in its environment. It’s discovery was first published by Charles Darwin’s in 1859 in his book On the Origin of Species by Means of Natural Selection in 1859 (later re-published as The Origin of Species). Darwin’s called his theory “descent with modification” through the process of “natural selection.”  He believed that environmental pressures–nature–selected for the most advantageous traits in living things much in the same way as domestic animals were selectively bred for desirable traits using “artificial selection.”

Darwin’s contributions to Evolutionary Theory

As the original title implies for Darwin’s book, On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life, Darwin introduced his scientific theory that populations evolve the physical differences that favor their survival through a process of natural selection. Darwin believed that all life forms originated from common ancestors. These lifeforms competed for scarce resources in a competitive environment. They evolved physical differences and the one’s with physical traits that gave them a survival advantage, survived and through the process of natural selection, the population as a whole and over many generations, evolved into a separate species. This process of natural selection for survival of the fittest created a tree-like branching pattern of evolutionary speciation (Editor 2009). Darwin supported his scientific theory of “descent with modification due to natural selection” with evidence of exotic species of flora and fauna that he had collected from the Galapagos Islands on his long voyage as a naturalist on the HMS Beagle in the 1830s and with his subsequent research back home in England.

Definition of natural selection is the factor that causes a “directional change in allele frequency” as a result of environmental pressures (Jurmain 2010: 103).  While variation is produced by mutation, recombination, gene flow and genetic drift, only natural selection and its variants of sexual selection (also known as “assortive mating”), artificial selection (selective breeding of domesticated animals and plants), and group selection) act on phenotypic variations to produce evolution in a population (Godoy 2008; Jurmain 2010:104).

Examples of natural selection for polygenetic traits include stature, body shape, eye color, hair color and skin color. Skin color variations are responses in populations to different levels of ultra-violet light (Jablonski 2004). A contemporary example of natural selection would be the lowered birthrates in post-industrial Basque populations. These lower birthrates are due to “sociocultural fitness” pressures that give a reproductive advantage to those with more resources and reduced family size (Alfonso-Sanchez 2004). Biological evolution explains modern human variation as well as the evolution of our species Homo sapiens from earlier hominins. According to evolutionary theory, all living things are the end-result of successful environmental adaptations from earlier forms of species.

Darwin’s Shortcomings:

Darwin did not  know how trait variations were inherited from one generation to the next (Mendal’s laws of inheritance) nor could he have known what produced the physical differences in the first place (genetic mutation, recombination and gene flow). These were discovered with the science of genetics

1. How physical traits were inherited: Mendelian Inheritance
Darwin did not know how physical differences where inherited. The mechanisms of biological inheritance were not known until the discoveries of Gregor Johann Mendel, a German-Czech monk/scientist who did experiments cross-breeding pea plants. His findings were published in 1866 but were not applied to evolution until around 1900 when they were re-discovered. Mendel figured out how physical traits or genes were inherited. He discovered that physical traits were not inherited as a blend from both parents (as Darwin thought) but were inherited from both parents as particulates.  He also discovered that some genes were dominant and others recessive and their inheritance could be described mathematically in ratios. For example, recessive genes were passed on in a ratio in a second generation (F2) of 1:2:1. This means that one individual would be positive homozygous for the trait, two individuals would be positive heterzygous for the trait and one individual would be negative homozygous for the trait. This is known as “Mendalian inheritance”. However, Mendel did not know what genes were exactly, only that they were heritable factors. The discovery of molecular structure of genes came much later.

2. What produced trait differences: genetic mutation and recombination: Darwin also did not  know how physical differences were produced. These differences are produced by genetic mutation, genetic recombination, genetic drift and gene flow. These molecular mechanisms were not understood until after Mendel’s work was rediscovered and the structure of DNA was discovered.

Mendel’s work fell into obscurity until around 1900 when it was re-discovered and scientists tried to find the particulate molecules in cells responsible for coding for physical traits. In 1910 Thomas Hunt Morgan called these genetic molecules “chromosomes”. It wasn’t until 1944 that it was discovered that these chromosomes were linked together in DNA and could transform cells. The double-helix molecular structure of DNA was finally discovered by James Watson and Francis Crick in 1953 in bacteria cells. It wasn’t until much later that it was discovered how DNA worked to influence the form of protein cells with messenger RNA.  DNA-sequencing technology was discovered in 1977 and provided the means to map the nucleotide sequence of a DNA molecule. In 2003 the entire human genome was finally sequenced.

The basic creative force in evolution is genetic mutation. All evolution is ultimately the result of genetic mutation in the DNA of sex cells that are inherited through meiosis and passed on from one generation to the next (Jurmain 2010:99). 

Genetic recombination produces variation by the splitting of a sex cell during its first meiotic division after being produced by either a male testes or female ovary.  Each of the two daughter sex cells produced by meiosis, called gametes, have only 23 chromosomes and are different from each other and their parent cell. A gamete united with another viable gamete from the opposite sex becomes a fertilized zygote with a new combination of 46 chromosomes, half from the mother and half from the father.

From a genetic perspective, evolution is a “change in allele frequency from one generation to the next” and can occur randomly by gene flow and genetic drift. Gene flow occurs when members of one population migrate and interbreed with another population. An example of gene flow would be the native Inuit populations of Greenland and Canada. The incoming Thule culture from Canada interbred with the existing Dorset groups in Greenland in ancient times to produce the native Inuit peoples (Helgason 2005). Genetic drift is produced by small populations. It is the change of allele frequencies produced by random factors that results in an increased proportion of certain traits in a small interbreading population (Jurmain 2010:100). Example: “founder effect” is a type of genetic drift that is caused by a genetic mutation in the founders of a small population. The mutation becomes more common in each succeeding generation of the founder’s descendants as long as they are isolated from other populations. An example of genetic drift occurred in the population of native Polynesians of Niue Island. Native Niue islanders suffer from an increased incidence of complex genetic diseases (Abbott 2006). Another example of genetic drift is “bottleneck”, which describes a pattern where a population is reduced so that the genes of it’s few remaining members’ are represented in a larger ration once the population increases.

Darwin theory of natural selection was combined with Mendel’s discovery of the mechanism of trait inheritance and the discovery of DNA’s role in producing trait variation. Together these created the modern theory of evolution called “The Modern Synthesis” (Jurmain 2010:97). According to the Modern Synthesis, the definition of evolution is a genetic “change in allele frequency from one generation to the next” (Jurmain 2010:98).

RESOURCES

Abbott, W.G. H et al

2006 “Genetic Diversity and Linkage Disequilibrium in the Polynesian Population of Niue Island,” Human Biology, 78(2), Pp. 131-145.

Alfonso-Sånchez, M.A., R. Calderon and J.A. Peña 2004  “Opportunity for Natural Selection in a Basque population and Its Secular Trend: Evolutionary Implications of Epidemic Mortality,” Human Biology, 76(3), Pp. 361-381.

Editor 2009 “Interaction: Darwin’s Legacy at Stanford,” Stanford University. Electronic document: http://multi.stanford.edu/features/darwin/, Accessed March 8, 2012.

Godoy, R. et al 2008 “Assortive mating and offspring well-being: theory and empirical finding from a native Amazonian society in Bolivia,” Evolution and Human Behavior, 29: 201-210.

Helgason, A. et al

2005            “MtDNA Variation in Inuit Populations of Greenland and Canada: Migration History and Population Structure,” American Journal of Physical Anthropology, 130, Pp. 123-134.

Jablonski, N.G.

2004 “The evolution of human skin and skin color,” Annual Review of Anthropology, 33: 585-623.

Jurmain, Robert et al

2010 Introduction to Physical Anthropology, 2009-2010 Ed., Belmont, California: Wadsworth Cengage Learning, Pp. 585.

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