Disruptive selection
Encyclopedia
Disruptive selection, also called diversifying selection, describes changes in population genetics
in which extreme values for a trait
are favored over intermediate values. In this case, the variance
of the trait increases and the population is divided into two distinct groups. This evolution
ary process is believed to be the driving force behind sympatric
speciation
.
of “BB” would have a phenotype
of black fur, a genotype of “Bb” would have grey fur (a display of both black and white) and a genotype of “bb” would have a phenotype of white fur.
If this population of rabbits were put into an area that had very dark black rocks as well as very white colored stone, the rabbits with black fur would be able to hide from predators amongst the black rocks and the white furred rabbits would be able to hide in the white rocks, but the grey furred rabbits would stand out in both of the habitats and would suffer greater predation.
As a consequence of the selective pressures of their environment, our hypothetical rabbit population would be disruptively selected for extreme values of the fur color trait: white or black, but not grey.
in the Galápagos.
He observed that the species of finches were similar enough to ostensibly have been descended from a single species. However, they exhibited disruptive variation in beak size. This variation appeared to be adaptively related to the seed size available on the respective islands (big beaks for big seeds, small beaks for small seeds). Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive.
While it is true that disruptive selection can lead to speciation, this is not as quick or straightforward of a process as other types of speciation or evolutionary change. This is largely because the results of disruptive selection are less stable than the results of directional selection
(directional selection favours individuals at only one end of the spectrum).
For example, let us take the mathematically straightforward yet biologically improbable case of the rabbits: Suppose directional selection were taking place. The field only has dark rocks in it, so the darker the rabbit, the better. Eventually there will be a lot of black rabbits in the population (hence lots of “B” alleles) and a lesser amount of gray rabbits (who contribute 50% chromosomes with “B” allele and 50% chromosomes with “b” allele to the population). There will be few white rabbits (not very many contributors of chromosomes with “b” allele to the population). This could eventually lead to a situation in which chromosomes with “b” allele die out, making black the only possible colour for all subsequent rabbits. The reason for this is that there is nothing "boosting" the level of “b” chromosomes in the population. They can only go down, and eventually die out.
Consider now the case of disruptive selection. The result is equal numbers of black and white rabbits, and hence equal numbers of chromosomes with “B” or “b” allele, still floating around in that population. Every time a white rabbit mates with a black one, only gray rabbits results. So, in order for the results to "click", there needs to be a force causing white rabbits to choose other white rabbits, and black rabbits to choose other black ones. In the case of the finches, this "force" was geographic/niche isolation.
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
in which extreme values for a trait
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
are favored over intermediate values. In this case, the variance
Variance
In probability theory and statistics, the variance is a measure of how far a set of numbers is spread out. It is one of several descriptors of a probability distribution, describing how far the numbers lie from the mean . In particular, the variance is one of the moments of a distribution...
of the trait increases and the population is divided into two distinct groups. This evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary process is believed to be the driving force behind sympatric
Sympatric speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
.
Example
Suppose there is a population of rabbits. The color of the rabbits is governed by two incompletely dominant traits: black fur represented by “B” and white fur represented by “b”. A rabbit with the genotypeGenotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
of “BB” would have a phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
of black fur, a genotype of “Bb” would have grey fur (a display of both black and white) and a genotype of “bb” would have a phenotype of white fur.
If this population of rabbits were put into an area that had very dark black rocks as well as very white colored stone, the rabbits with black fur would be able to hide from predators amongst the black rocks and the white furred rabbits would be able to hide in the white rocks, but the grey furred rabbits would stand out in both of the habitats and would suffer greater predation.
As a consequence of the selective pressures of their environment, our hypothetical rabbit population would be disruptively selected for extreme values of the fur color trait: white or black, but not grey.
Significance
Disruptive selection is of particular significance in the history of evolutionary study, as it is involved in one of evolution's "cardinal cases", namely the finch populations observed by DarwinDarwin's finches
Darwin's finches are a group of 14 or 15 species of passerine birds. It is still not clear which bird family they belong to, but they are not related to the true finches. They were first collected by Charles Darwin on the Galápagos Islands during the second voyage of the Beagle...
in the Galápagos.
He observed that the species of finches were similar enough to ostensibly have been descended from a single species. However, they exhibited disruptive variation in beak size. This variation appeared to be adaptively related to the seed size available on the respective islands (big beaks for big seeds, small beaks for small seeds). Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive.
While it is true that disruptive selection can lead to speciation, this is not as quick or straightforward of a process as other types of speciation or evolutionary change. This is largely because the results of disruptive selection are less stable than the results of directional selection
Directional selection
In population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction...
(directional selection favours individuals at only one end of the spectrum).
For example, let us take the mathematically straightforward yet biologically improbable case of the rabbits: Suppose directional selection were taking place. The field only has dark rocks in it, so the darker the rabbit, the better. Eventually there will be a lot of black rabbits in the population (hence lots of “B” alleles) and a lesser amount of gray rabbits (who contribute 50% chromosomes with “B” allele and 50% chromosomes with “b” allele to the population). There will be few white rabbits (not very many contributors of chromosomes with “b” allele to the population). This could eventually lead to a situation in which chromosomes with “b” allele die out, making black the only possible colour for all subsequent rabbits. The reason for this is that there is nothing "boosting" the level of “b” chromosomes in the population. They can only go down, and eventually die out.
Consider now the case of disruptive selection. The result is equal numbers of black and white rabbits, and hence equal numbers of chromosomes with “B” or “b” allele, still floating around in that population. Every time a white rabbit mates with a black one, only gray rabbits results. So, in order for the results to "click", there needs to be a force causing white rabbits to choose other white rabbits, and black rabbits to choose other black ones. In the case of the finches, this "force" was geographic/niche isolation.
See also
- Character displacementCharacter displacementCharacter displacement refers to the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur but are minimized or lost where the species’ distributions do not overlap. This pattern results from evolutionary...
- Balancing selectionBalancing selectionBalancing selection refers to a number of selective processes by which multiple alleles are actively maintained in the gene pool of a population at frequencies above that of gene mutation. This usually happens when the heterozygotes for the alleles under consideration have a higher adaptive value...
- Directional selectionDirectional selectionIn population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction...
- Negative selection (natural selection)Negative selection (natural selection)In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilizing selection through the purging of deleterious variations that arise....
- Stabilizing selectionStabilizing selection-Description:Stabilizing or ambidirectional selection, , is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. This is probably the most common mechanism of action for natural selection...
- Sympatric speciationSympatric speciationSympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...