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| Selection |
Selection refers to any process by which some individuals are allowed to reproduce at the expense of others, leading to a shift in the composition of a population over generational time.
In the context of animal and plant breeding by humans, the process of selection is referred to as artificial selection. In the wild, it is referred to as natural selection and is viewed by scientists as the principal means by which adaptations (traits that promote the survival and reproduction of organisms) arise and new species evolve over geological time frames.
Natural Selection
In On the Origin of Species by Means of Natural Selection, published in 1859, Charles Darwin presented arguments for two related theories. The first argument documents evidence for evolution on the basis of an extensive study of biogeographical, embryological, and fossil data.
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Within ten years of its publication, nearly all individuals who today would be considered scientists were convinced that evolution had occurred.
Darwin’s second argument develops the case for what he identified as the chief mechanism of evolution: his theory of natural selection. This argument elegantly draws attention to the probabilistic consequences of three conditions in nature.
First, organisms vary from one another in ways that affect their ability to survive and reproduce. Second, at least some of this variation is heritable. Third, there is competition, owing to the fact that organisms produce more offspring than can possibly survive.
Darwin’s genius was to recognize that as a consequence of these conditions, members of a population possessing favored variations would be more likely to leave offspring than those that did not, and as such, the composition of the population as a whole would change over generational time in the direction of the favored form.
Evidence
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| Evidence of natural selection |
In addition to evidence for the existence of the above conditions in nature, Darwin also gave indirect evidence in support of the long-term consequences of these conditions he had deduced. In addition to thought experiments, he developed an analogy between artificial and natural selection.
He pointed out that during the relatively short time that pigeon and dog species had been domesticated, breeders had been able, by picking which individuals were allowed to reproduce, to create new varieties as distinct in appearance as true species in nature.
Darwin then drew attention to the fact that conditions in nature (coupled with the occasional introduction of new variants by mutation) operating over geological time periods could have a similarly dramatic effect on populations in the wild.
Although the vast time and spatial frames involved make it difficult to observe directly the origin of new adaptations and species, many field experiments in the years since Darwin first wrote have confirmed both the power and ubiquity of natural selection in nature.
A particularly important example in the context of botany was provided in the late 1960’s by Janis Antonovics and others, who demonstrated that the evolution of heavy metal tolerance in many plant species was the result of natural selection.
Other contemporary areas of research on selection include the study of developmental plasticity, or the ability of an organism to respond to environmental conditions during its development, as occurs when a growing sapling forms leaves to maximize its light exposure in the presence of partial shading by other trees in a forest. This ability is itself the object of natural selection.
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Genotypes vs. Phenotypes
Although evolution is often discussed in terms of a change in the frequency of genes in a population, in fact, natural selection acts directly on the phenotypes (observable characteristics of organisms) and only indirectly on the genotypes (the specific forms of the gene, or alleles, an individual has inherited from its parents) responsible for them.
While often portrayed as a process that removes genetic variability from populations, natural selection can promote variability, as occurs when the population exists in an environment where one form of a trait is advantageous in some areas but another form is advantageous in other areas. Such processes may lead to clines, or gradations, in the frequencies of genes over the range of the population.
Ecosystems
The foregoing has discussed selectionwith reference to isolated populations. In ecosystems composed of multiple populations of distinct species, natural selection can promote the evolution of ecotypes, or populations having a distinct set of characteristics unique to the region they inhabit and themode of life they pursue.
Natural selection can favor the coevolution of populations of distinct species with one another. This occurs in the evolution of predator and prey species, in which, for example, the origin of an adaptation that allows a predator to consume a grass species that was previously toxic to it changes the selective environment of the prey species, leading to the evolution of entirely new plant defenses.
Convergent vs. Divergent Evolution
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| Convergent vs. Divergent Evolution |
A good example is provided by cacti in North and South American deserts and euphorbs in African deserts: Both have thornless leaves with a similar structure that has evolved to maximize water retention.
Natural selection can also lead to a partitioning of resource space, a phenomenon known as divergent evolution, in which distinct populations of a species evolve divergent modes of life.
This may reflect the imposition of a barrier that prevents interbreeding or competition among cohabiting populations leading to a partitioning of niche space. A good example is the common mistletoe, Viscum album, a parasitic higher plant having three distinct races that specialize on deciduous trees, firs, and pines.