Directional selection : Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.
Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. Similarly, the hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed.
The Evolution of the Peppered Moth : Typica and carbonaria morphs resting on the same tree. Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection, this is seen in many populations of animals that have multiple male mating strategies, such as lobsters.
Diversifying or disruptive selection : Diversifying selection occurs when extreme values for a trait are favored over the intermediate values. This type of selection often drives speciation.
Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass.
Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.
Types of natural selection : Different types of natural selection can impact the distribution of phenotypes within a population. In a stabilizing selection, an average phenotype is favored. In b directional selection, a change in the environment shifts the spectrum of phenotypes observed. In c diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against. In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection.
Another type of selection, called frequency-dependent selection, favors phenotypes that are either common positive frequency-dependent selection or rare negative frequency-dependent selection.
An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females; blue males are medium-sized and form strong pair bonds with their mates; and yellow males are the smallest and look a bit like female, allowing them to sneak copulations.
Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females.
Frequency-dependent selection in side-blotched lizards : A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. Frequency-dependent selection allows for both common and rare phenotypes of the population to appear in a frequency-aided cycle.
In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes. In one generation, orange might be predominant and then yellow males will begin to rise in frequency.
Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored. An example of negative frequency-dependent selection can also be seen in the interaction between the human immune system and various infectious microbes such as pathogenic bacteria or viruses.
As a particular human population is infected by a common strain of microbe, the majority of individuals in the population become immune to it. This then selects for rarer strains of the microbe which can still infect the population because of genome mutations; these strains have greater evolutionary fitness because they are less common.
An example of positive frequency-dependent selection is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless. The scarlet kingsnake, a harmless species, mimics the coloration of the eastern coral snake, a venomous species typically found in the same geographical region. Predators learn to avoid both species of snake due to the similar coloration, and as a result the scarlet kingsnake becomes more common, and its coloration phenotype becomes more variable due to relaxed selection.
Lampropeltis elapsoides, the scarlet kingsnake : The scarlet kingsnake mimics the coloration of the poisonous eastern coral snake. Avian Egg Coloration and Visual Ecology.
The Ecology of Avian Brood Parasitism. The Maintenance of Species Diversity. Neutral Theory of Species Diversity. Population Genomics. Semelparity and Iteroparity.
Geographic Mosaics of Coevolution. Comparative Genomics. Cybertaxonomy and Ecology. Ecological Opportunity: Trigger of Adaptive Radiation. Evidence for Meat-Eating by Early Humans. Resource Partitioning and Why It Matters. The Evolution of Aging. Citation: Safran, R. Nature Education Knowledge 3 10 How do new species form?
Like most areas of Evolutionary Biology, research related to the formation of new species - 'speciation ' - is rich in historical and current debate.
Here, we review both early and modern views on speciation, starting with Darwin and finishing with current genomics-era insights. Aa Aa Aa. Darwin's "Mystery of Mysteries". The Modern Synthesis. Barriers to reproduction. The role of geography in speciation. Biologists have long been fascinated with — and sought to explain — the origin and maintenance of biological diversity within and among species. Natural selection is generally recognized as a central mechanism of evolutionary change within species.
Thus, natural selection plays a major role in generating the array of phenotypic and genetic diversity observed in nature. But to what extent is selection also responsible for the formation of new species i. To what extent do phenotypic and species diversity arise via the same processes, as proposed by Darwin? Figure 4. Ecological speciation in host-plant associated populations of Timema cristinae walking-stick insects individual populations feed on either the Ceanothus spinosus host plant or on Adenostoma fasciculatum.
The role of sexual selection in speciation. A view that is becoming increasingly popular is that sexual selection, or selection related to variation in reproductive success, plays a role in speciation Panhuis et al. This model suggests that differential patterns of trait variation related to reproductive success within populations contribute to the reproductive isolation among populations.
A compelling example is related to the explosive radiation of cichlid fishes in the African Rift Lakes, where populations with overlapping distributions are diverging as a function of the differential preference of male color in mate selection Seehausen et al. Some models of speciation do not include a role for selection of any sort, but rather invoke a key role for chance events.
Current views: Mutation-order vs. A lack of strong examples for speciation by genetic drift, yet evidence for ecologically-similar species pairs Price , has led to the development of a powerful alternative mechanism to ecological speciation. In essence, different populations find different genetic solutions to the same selective problem.
In turn, the different genetic solutions i. Whereas different alleles are favored between two populations under ecological speciation, the same alleles would be favored in both populations under mutation-order speciation i. Divergence occurs anyway because, by chance, the populations do not acquire the same mutations or fix them in the same order. Divergence is therefore stochastic but the process involves selection, and thus is distinct from genetic drift. Selection can be ecologically based under mutation-order speciation, but ecology does not favor divergence as such, and an association between ecological divergence and reproductive isolation is not expected.
How might mutation-order speciation arise? Sexual selection might cause mutation-order speciation if reproductive isolation evolves by the fixation of alternative advantageous mutations — for example those which increase individual attractiveness — in different populations living in similar ecological environments. For a summary of these models, see Table 1. References and Recommended Reading Butlin, R. Sympatric, parapatric or allopatric: The most important way to classify speciation?
Coyne, J. Sunderland, MA: Sinauer Associates, Animal Species and Evolution. Price, T. Speciation in Birds. Woodbury, NY: Roberts and Company, Article History Close. Share Cancel. Revoke Cancel. London: John Murray. Without doubt the most famous book about natural selection, and in the whole of evolutionary biology.
Still a very useful and entertaining read that is astonishing for the care with which its enormous evidence base was garnered and organized. Darwin understood the operation of directional selection better than most biologists that followed him for a century afterwards. Endler, J. Natural selection in the wild. Princeton, NJ: Princeton Univ. The first review and synthesis of studies and concepts relating to natural selection in wild populations.
It contains detailed consideration of the philosophy and methods for the study of natural selection. A seminal work. Futuyma, D. Sunderland, MA: Sinauer Associates. The latest edition of the standard undergraduate textbook for evolution.
Contains introductory material on directional selection. Mitton, J. Selection in natural populations. A review and synthesis of studies of selection on genetic variation and protein polymorphisms in natural populations, and why this comes about.
The approaches described have been outdated by the modern march to use genomic methods, but the book documents many classic and easily understood examples. Williams, G. Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population. Key Terms directional selection : a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction disruptive selection : or diversifying selection a mode of natural selection in which extreme values for a trait are favored over intermediate values stabilizing selection : a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value.
Stabilizing Selection If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. Directional Selection When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.
Diversifying or Disruptive Selection Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In a stabilizing selection, an average phenotype is favored. In b directional selection, a change in the environment shifts the spectrum of phenotypes observed.
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