Natural populations are heterogeneous mixtures of individuals differing in physiology, morphology, and behavior. Despite the ubiquity of phenotypic variation within natural populations, its effects on the dynamics of ecological communities are not well understood. Here, we use a quantitative genetics framework to examine how phenotypic variation in a predator affects the outcome of apparent competition between its two prey species. Classical apparent competition theory predicts that prey have reciprocally negative effects on each other. The addition of phenotypic trait variation in predation can marginalize these negative effects, mediate coexistence, or generate positive indirect effects between the prey species. Long-term coexistence or facilitation, however, can be preceded by long transients of extinction risk whenever the heritability of phenotypic variation is low. Greater heritability can circumvent these ecological transients but also can generate oscillatory and chaotic dynamics. These dramatic changes in ecological outcomes, in the sign of indirect effects, and in stability suggest that studies which ignore intraspecific trait variation may reach fundamentally incorrect conclusions regarding ecological dynamics. Show
Charmantier, A., & Garant, D. Environmental quality and evolutionary potential: Lessons from wild populations. Proceedings of the Royal Society, Biological Sciences 272, 1415–1425 (2005) Falconer, D. S., & Mackay, T. F. C. Introduction to Quantitative Genetics (Harlow, UK, Longman, 1996) Hill, W. G., et al. Data and theory point to mainly additive genetic variance for complex traits. PLoS Genetics 4, e1000008 (2008) Macgregor, S., et al. Bias, precision and heritability of self-reported and clinically measured height in Australian twins. Human Genetics 120, 571–580 (2006) Visscher, P. M., et al. Assumption-free estimation of heritability from genome-wide identity-by-descent sharing between full siblings. Public Library of Science Genetics 2, e41 (2006) ———. Heritability in the genomics era—Concepts and misconceptions. Nature Reviews Genetics 9, 255–266 (2008) (link to article) All Subject Expand Expand
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Humans belong to the same species but look and behave differently. Differences in observable traits are called phenotypic variations. In this article we will discuss the definition, causes, and provide some examples of phenotypic variations. We will also discuss why phenotypic variation is important in natural selection. Definition of phenotypic variationThe phenotype of an organism refers to its observable traits- its physical appearance, behavior, learning ability, mode of reproduction, and so on. Phenotypic variations are differences among organisms in these observable traits. Figure 1 below shows four phenotypes of foxglove, each with petals of different colors. Coloration is an example of a phenotypic trait. Causes of phenotypic variation in a populationPhenotypic variation in a population is caused by two main factors: genotypic variation and environmental influence (Fig. 2). Genotypic variationGenotype refers to the genetic information of an organism. Specifically, it is the combination of alleles (gene variants) that an organism has. The genotype contributes to the phenotype of an organism. Organisms can either be homozygous or heterozygous for a specific genotype (Fig. 3):
It is important to distinguish between homozygous and heterozygous genotypes because these affect how phenotypes are passed on from parent to offspring. This will be further discussed in the examples later. Differences in genotype (or genotypic variations) contribute to phenotypic variation. Genotypic variations are caused by mutations, gene flow, and sexual reproduction:
Environmental influenceEnvironmental conditions such as climate, availability of food, and interactions with other organisms can influence the development of inherited traits, thus contributing to phenotypic variation. The ability of a genotype to produce different phenotypes in response to different environmental conditions is called phenotypic plasticity. For example, differences in the availability of food can result in differences in size and weight among organisms of the same genotype. Similarly, differences in climate (dry season vs. wet season) can result in differences in crop yield. In both cases, stunted growth could result from a lack of nutrition. It is important to note that phenotypic variations due to environmental influence are not passed on from parent to offspring. The thermosensitivity of the embryo of some species of reptiles is another interesting case showing the impact of environmental influence on the phenotype of an organism. The exposure of embryos of freshwater turtles, for example, to different temperature ranges during a specific 2-week period (called the "thermosensitive period") in their development can influence the sex of the resulting baby turtle. When exposed to 30°C only female turtles hatch, while at 25°C only male turtles hatch. When exposed to around 28.5°C a mixture of males and female turtles hatch. Examples of phenotypic variationsPhenotypic variations can be classified as either discrete or continuous. Examples of each type will be discussed in the following section. Discrete variations are traits with qualitative differences. These are distinct and separate categories with nothing ‘in between’. Think blood type: you can have only one of four possible types: A, B, AB, or O. In discrete variations, the combination of alleles at a single gene locus (the position of a gene on a chromosome) have a significant impact on the phenotype. Some examples include the inheritance of sickle cell anemia in humans and the stem color of tomato plants. Let's briefly discuss these examples. Suppose a gene determines whether or not a human has sickle cell anemia. The alleles of this gene could either lead to normal or sickle-cell hemoglobin. As mentioned earlier, a genotype could either be homozygous or heterozygous, resulting in the following genotypes (Fig. 4):
There are also instances where the phenotype of a heterozygous organism is affected by only one allele. For example, the stem color of a tomato plant is determined by two alleles: one produces green stems and the other produces purple stems. A tomato plant that has one allele for purple stems and one allele for green stems will have purple stems identical to a tomato plant that has two alleles for purple stems. In this example, the allele that produces purple stems is dominant, while the allele that produces green stems is recessive.
Other examples of discrete variations in humans are the ability to roll the tongue (whether one can or cannot) and the hand used for writing (left- or right-dominant). Continuous variationsContinuous variations are traits expressed as quantitative differences with a wide range of values. This is because, for continuous variations, the combination of alleles at a single gene locus has small or additive effects on the phenotype. Examples of continuous variations in humans are height, weight, and skin color. Let’s say the height of an organism is determined by two genes: Aa and Bb, where the dominant alleles (A and B) add 2x cm each and the recessive alleles add 1x cm each. This means:
In reality, more genes are involved in determining the height of the organism, each having an additive effect. Traits determined by multiple genes are called polygenic traits. Polygenes: multiple genes that have an additive effect on one trait. Polygenic traits: traits determined by multiple genes. Why is phenotypic variation necessary for evolution by natural selection?Phenotypic variations lead to different survival and reproduction rates among organisms. Evolution by natural selection--where individuals with traits that are more adapted to the environment have more chances of survival and reproduction--can only take place when these differences are present. Without phenotypic variation, a population cannot evolve. Some factors can lead to reduced phenotypic variation. An example of this is inbreeding, or when closely-related individuals reproduce. This could take place in nature when a population becomes so small that the individuals capable of reproducing are closely related. This could also take place when humans practice artificial selection, the process of breeding organisms with desirable traits. Inbreeding will cause a reduction in the number of alleles in the gene pool of the population, leading to loss in genetic variation. Another example is genetic drift. Genetic drift is when chance events cause allele frequencies to change at random. This means that the alleles that are passed onto succeeding generations are not the most well-adapted to the environment, potentially causing individuals with these inherited traits to die out hence reducing genetic variation. With less genetic variation, it is less likely that there are individuals with the traits needed to survive changes in environmental conditions (for instance, climate change or the emergence of infectious diseases), which can lead to the extinction of the population or even the entire species. Phenotypic Variations - Key takeaways
The phenotype of an organism refers to its observable traits- its physical appearance, behavior, learning ability, mode of reproduction, and so on. Phenotypic variations are differences among organisms in these observable traits.
Phenotypic variation in a population is caused by two main factors: genotypic variation and environmental influence.
Differences in genotype (or genotypic variations) produce phenotypic variation. Genotypic variations are caused by mutations, gene flow, and sexual reproduction. Environmental conditions such as climate, availability of food, and interactions with other organisms can contribute to phenotypic variation.
Phenotypic variations lead to different survival and reproduction rates among organisms. Natural selection--where individuals with traits that are more adapted to the environment have more chances of survival and reproduction--can only take place when these differences are present.
Phenotypic variation in a population is caused by two main factors: genotypic variation and environmental influence.
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What is "phenotypic variation"?
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Phenotypic variations are differences among organisms in terms of observable traits (phenotypes).
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What factors cause phenotypic variations?
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Phenotypic variations are caused by genotypic variations and environmental conditions.
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Why is phenotypic variation necessary for natural selection?
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Phenotypic variations lead to different survival and reproduction rates among organisms. Natural selection--where individuals with traits that are more adapted to the environment have more chances of survival and reproduction--can only take place when these differences are present.
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The phenotype of an organism refers to its observable traits.
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A genotype is the combination of alleles (gene variants) that an organism has. The genotype contributes to the phenotype of an organism.
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When two alleles of a gene are identical, this is referred to as:
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When two alleles of a gene are different, this is referred to as:
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Mutation is a change in the sequence of genes in DNA. It is the ultimate source of new alleles.
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Gene flow is the movement of genes from one population to another. This can occur when organisms migrate and reproduce with a different population or when pollen or seeds are dispersed to a population that is geographically separated.
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How does sexual reproduction promote genotypic variation?
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Sexual reproduction promotes genotypic variation by creating new combinations of genes.
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How does the environment contribute to phenotypic variation?
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Environmental conditions such as climate, availability of food, and interactions with other organisms can influence the development of inherited traits, thus contributing to phenotypic variation.
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What are discrete variations?
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Discrete variations are traits with qualitative differences. These are distinct and separate categories with nothing ‘in between’.
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What are continuous variations?
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Continuous variations are traits expressed as quantitative differences with a wide range of values.
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What are polygenic traits?
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Polygenic traits are traits that are determined by more than one gene.
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What is the difference between a dominant and a recessive allele?
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A dominant allele affects the phenotype, while the recessive allele does not. |