Evolutionary biology is concerned with explaining and understanding the diversity of living things and their characteristics: what has been the history that produced this diversity, and what have been the causes of this history?
Biological evolution is inherited change in the properties of groups of organisms over the course of generations. Evolution is descent with modification from common ancestors. Evolutionary theory constitutes a body of ideas about the causes of evolution, including mutation, recombination, gene flow, isolation, random genetic drift, the several forms of natural selection, and other factors. Evolutionary theory is the foundation of the science of life.
Charles Darwin published On the Origin of Species in 1859. It contains two major theories. The first is Darwin’s idea of descent with modification. It holds that all species, living and extinct, have descended, without interruption, from one or a few original forms of life. The second is natural selection, which Darwin proposed is the chief cause of evolutionary change. Darwin’s theory of evolution includes five distinct components:
- Evolution as such is the simple proposition that the characteristics of organisms change over time.
- Common descent. Species diverged from common ancestors.
- Gradualism is the proposition that differences between organisms evolved by small steps through intermediate forms.
- Populational change is the hypothesis that evolution occurs by changes in the proportions (frequencies) of different variant kinds of individuals within a population.
- Natural selection is the hypothesis that accounts for adaptations, features that appear “designed” to fit organisms to their environment.
Darwin proposed that the various species that descend from a common ancestor evolve different features because those features are adaptive under different habitats or habits. The pressure of competition favors the use of different foods or habitats by different species. New hereditary variations continue to arise; there is no evident limit to the amount of divergence that can occur.
The modern synthesis reconciled Darwin’s theory with Mendelian genetics; its chief principle, often referred to as neo-Darwinism, is that adaptive evolution is caused by natural selection acting on particulate (Mendelian) genetic variation. Mutation and natural selection together cause adaptive evolution; mutation is the raw material on which natural selection acts. Mutation, gene flow or migration, natural selection, and genetic drift are the major causes of evolution within species (microevolution); these same causes, over long periods of time, account for the origin of new species and for the evolution of the major alterations that distinguish higher taxa (macroevolution). Since the evolutionary synthesis, progress in evolutionary biology has modified some of these ideas and many extensions of these ideas.
The following are the fundamental principles of biological evolution that emerged from the modern synthesis:
- An individual’s phenotype (its observed traits) is distinct from its genotype (its DNA).
- Acquired characteristics are not inherited. *
- Hereditary variations are based on particles – the genes. *
- Genetic variation arises by random mutation. Mutations do not arise in response to need.
- Evolution is a change of a population, not of an individual.
- Changes in allele frequencies may be random or nonrandom.
- Natural selection can account for both slight and great differences among species.
- Natural selection can alter populations beyond the original range of variation.
- Populations usually have considerable genetic variation.
- The differences between species evolve by rather small steps. *
- Species are groups of interbreeding or potentially interbreeding individuals that do not exchange genes with other such groups. *
- Speciation (the origin of two species from a single ancestor species) usually occurs by the genetic differentiation of geographically isolated populations. *
- Higher taxa arise by the sequential accumulation of small differences.
- All organisms form a great Tree of Life (or phylogeny).
Some of the above statements, marked by an asterisk (*), have to be qualified to some degree, in light of later research.
The implications of Darwin’s theory, which revolutionized Western thought, include the ideas that change, rather than stasis, is the natural order; that biological phenomena, including those seemingly designed, can be explained by purely material causes rather than by divine creation; and that no evidence for purpose or goals can be found in the living world, other than in human actions.
—August 2020
—September 2021
—January 2022
Lance Jones 