One of the most important, if not the most important, theories in biology is the theory of evolution; so we begin by outlining this theory, with examples and with intent later to show a similarity with it to the ecosystem theories proposed earlier in the book. In biology, evolution is the process by which natural populations of organisms acquire and pass on novel characteristics from generation to generation (Darwin and Wallace, 1858; Darwin, 1859), and the theory of evolution by natural selection became decisively established within the scientific community. In the 1930s, work by a number of scientists combined Darwinian natural selection with the re-discovered theory of heredity (proposed by Gregor Mendel) to create the modern evolutionary synthesis. In the modern synthesis, "evolution" means a change in the frequency of an allele within a gene pool from one generation to the next. This change may be caused by a number of different mechanisms: natural selection, genetic drift, or changes in population structure (gene flow).
(a) Natural selection is survival and reproduction as a result of the environment. Differential mortality consists of the survival rate of individuals to their reproductive age. Differential fertility is the total genetic contribution to the next generation. The central role of natural selection in evolutionary theory has given rise to a strong connection between that field and the study of ecology.
Natural selection can be subdivided into two categories:
• Ecological selection occurs when organisms that survive and reproduce increase the frequency of their genes in the gene pool over those that do not survive.
• Sexual selection occurs when organisms that are more attractive to the opposite sex because of their features reproduce more and thus increase the frequency of those features in the gene pool.
Natural selection also operates on mutations in several different ways:
• Purifying or background selection eliminates deleterious mutations from a population.
• Positive selection increases the frequency of a beneficial mutation.
• Balancing selection maintains variation within a population through a number of mechanisms, including:
o Over-dominance or heterozygote advantage, where the heterozygote is more fit than either of the homozygous forms (exemplified by human sickle cell anemia conferring resistance to malaria). o Frequency-dependent selection, where the rare variants have a higher fitness.
• Stabilizing selection favors average characteristics in a population, thus reducing gene variation but retaining the mean.
• Directional selection favors one extreme of a characteristic; results in a shift in the mean in the direction of the extreme.
• Disruptive selection favors both extremes, and results in a bimodal distribution of gene frequency. The mean may or may not shift.
(b) Genetic drift describes changes in allele frequency from one generation to the next due to sampling variance. The frequency of an allele in the offspring generation will vary according to a probability distribution of the frequency of the allele in the parent generation.
Many aspects of genetic drift depend on the size of the population (generally abbreviated as N). This is especially important in small mating populations, where chance fluctuations from generation to generation can be large. Such fluctuations in allele frequency between successive generations may result in some alleles disappearing from the population. Two separate populations that begin with the same allele frequency might, therefore, "drift" by random fluctuation into two divergent populations with different allele sets (e.g. alleles that are present in one have been lost in the other).
The relative importance of natural selection and genetic drift in determining the fate of new mutations also depends on the population size and the strength of selection: when N-s (population size times strength of selection) is small, genetic drift predominates. When N-s is large, selection predominates. Thus, natural selection is 'more efficient' in large populations, or equivalently, genetic drift is stronger in small populations. Finally, the time for an allele to become fixed in the population by genetic drift (i.e. for all individuals in the population to carry that allele) depends on population size, with smaller populations requiring a shorter time to fixation.
The theory underlying the modern synthesis has three major aspects:
(1) The common descent of all organisms from a single ancestor.
(2) The manifestation of novel traits in a lineage.
(3) The mechanisms that cause some traits to persist while others perish.
Essentially, the modern synthesis (or neo-Darwinism) introduced the connection between two important discoveries: the evolutionary units (genes) with its mechanism (selection). It also represents a unification of several branches of biology that previously had little in common, particularly genetics, cytology, systematics, botany, and paleontology.
A critical link between experimental biology and evolution, as well as between Mendelian genetics, natural selection, and the chromosome theory of inheritance, arose from T.H. Morgan's work with the fruit fly Drosophila melanogaster (Allen, 1978). In 1910, Morgan discovered a mutant fly with solid white eyes—wild-type Drosophila have red eyes—and found that this condition though appearing only in males was inherited precisely as a Mendelian recessive trait. Morgan's student Theodosius Dobzhansky (1937) was the first to apply Morgan's chromosome theory and the mathematics of population genetics to natural populations of organisms, in particular Drosophila pseudoobscura. His 1937 work Genetics and the Origin of Species is usually considered the first mature work of neo-Darwinism, and works by E. Mayr (1942: systematics), G.G. Simpson (1944: paleontology), G. Ledyard Stebbins (1950: botany), C.D. Darlington (1943, 1953: cytology), and J. Huxley (1949, 1942) soon followed.
According to the modern synthesis as established in the 1930s and 1940s, genetic variation in populations arises by chance through mutation (this is now known to be due to mistakes in DNA replication) and recombination (crossing over of homologous chromosomes during meiosis). Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of genetic drift, gene flow, and natural selection. Speciation occurs gradually when geographic barriers isolate reproductive populations. The modern evolutionary synthesis continued to be developed and refined after the initial establishment in the 1930s and 1940s. The most notable paradigm shift was the so-called Williams revolution, after Williams (1966) presented a gene-centric view of evolution. The synthesis as it exists now has extended the scope of the Darwinian idea of natural selection, specifically to include subsequent scientific discoveries and concepts unknown to Darwin such as DNA and genetics that allow rigorous, in many cases mathematical, analyses of phenomena such as kin selection, altruism, and speciation.
Was this article helpful?