Have you ever wondered why there is a new flu shot each year? Or, why people who are traditionally from the same region have similar physical features? Or, why it’s probably a bad idea to marry your cousin? And, why are Darwin’s finches so important? 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) where he tracked the variation in finch beaks and other kinds of similarities and differences in other species as well.
This paper will explain the fundamental cause of the variation of all life: evolution. In particular I will explain evolution talking about the five forces or mechanisms of evolutionary change in a population: genetic mutation, genetic recombination, gene flow, genetic drift and natural selection.
1. Genetic mutation is the production and distribution of trait variations via genetic mutation. 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).A mutation that increases reproductive fitness in a large population thereby becomes more frequent as more offspring with it survive and reproduce. A famous example of mutations increasing the reproductive fitness of a species and then becoming an endemic trait for that species are the various types of finches Darwin discovered in the Galapogos Islands. A differently shaped beak, for example a stout strait beak, gave stout strait beaked finches an adaptive advantage on the island with abundant hard seeds and nuts that required such a beak to exploit. Finches with long curved beaks survived and reproduced on another island that had an abundant food supply which required long curved beaks to consume, such as fruit.
Another example of natural selection acting on a mutation is the incidence of sickle-cell anemia in human populations who are native to tropical areas affected by malaria (Juramain 2010:99). In these regions being heterozygous for the recessive sickle-cell allele (a Mendalian “point mutation” in one allele of DNA) gives one protection from the disease by producing some abnormal blood cells hemoglobin S (Hbs). A person only gets the fatal sickle-cell anemia disease when he or she inherits the sickle-cell trait from both parents (Jurmain 2010:104-107).
2. 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 flowand genetic drift.
3. 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).
4. 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). The “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 drive is a population “bottleneck” effect that occurs when a population is decreased significantly and then regains size. The gene frequencies of the relatively few survivors will be disproportionately inherited by their successive decedents.
5. 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.
The Modern Synthesis
Mendel’s discovery of the mechanism of trait inheritance and the discovery of DNA’s role in producing trait variation was combined with Darwin’s natural selection theory to create 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).
The process of evolution occurs in two stages. The first stage is when traits are produced due to mutation, recombination and gene flow/genetic drift. The second stage is when natural selection acts on these traits to select the most advantageous trait variations for survival in a population (Jurmain 2010:96).
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.