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01:21Frequency-dependent Selection
When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.
01:46Types of Selection
Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an...
01:49Hardy-Weinberg Principle
Diploid organisms have two alleles of each gene, one from each parent, in their somatic cells. Therefore, each individual contributes two alleles to the gene pool of the population. The gene pool of a population is the sum of every allele of all genes within that population and has some degree of variation. Genetic variation is typically expressed as a relative frequency, which is the percentage of the total population that has a given allele, genotype or phenotype.
01:09Mutation, Gene Flow, and Genetic Drift
In a population that is not at Hardy-Weinberg equilibrium, the frequency of alleles changes over time. Therefore, any deviations from the five conditions of Hardy-Weinberg equilibrium can alter the genetic variation of a given population. Conditions that change the genetic variability of a population include mutations, natural selection, non-random mating, gene flow, and genetic drift (small population size).
01:38Limits to Natural Selection
Organisms that are well-adapted to their environment are more likely to survive and reproduce. However, natural selection does not lead to perfectly adapted organisms. Several factors constrain natural selection.
01:49Law of Segregation
When crossing pea plants, Mendel noticed that one of the parental traits would sometimes disappear in the first generation of offspring, called the F1 generation, and could reappear in the next generation (F2). He concluded that one of the traits must be dominant over the other, thereby causing masking of one trait in the F1 generation. When he crossed the F1 plants, he found that 75% of the offspring in the F2 generation had the dominant phenotype, while 25% had the recessive phenotype.
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The effects of sexual selection on functional and molecular reproductive divergence during experimental evolution in seed beetles.
Evolution letters·2026
Related Experiment Video
Updated: May 26, 2026
07:54
Manipulation of Ploidy in Caenorhabditis elegans
Published on: March 15, 2018
Ploidally antagonistic selection maintains stable genetic polymorphism.
Simone Immler1, Göran Arnqvist, Sarah Perin Otto
1Department of Ecology and Genetics, Evolutionary Biology, Uppsala University, Norbyvägen 18D, 752 36 Uppsala, Sweden. simone.immler@ebc.uu.se
Evolution; International Journal of Organic Evolution
|January 7, 2012
Summary
Opposing selection during different life stages (diploid and haploid) can maintain genetic diversity. Negative interactions between ploidy and sex-specific selection are crucial for preserving genetic variation.
Area of Science:
- Evolutionary Biology
- Genetics
- Population Genetics
Background:
- Maintaining genetic variation despite natural selection is a central problem in evolutionary biology.
- Biphasic life cycles with distinct diploid and haploid stages are common in eukaryotes.
- Opposing selection pressures in haploid and diploid phases, or between sexes, may preserve genetic diversity.
Purpose of the Study:
- To investigate how ploidy-antagonistic and sex-specific selection maintain stable genetic polymorphisms.
- To examine the influence of allele location (autosomal or sex-linked) on polymorphism maintenance.
- To identify conditions favoring the persistence of genetic variation in biphasic life cycles.
Main Methods:
- Mathematical modeling of selection pressures across diploid and haploid life stages.
- Analysis of autosomal, X-linked, and Y-linked gene dynamics.
- Simulation of allele frequency changes under various selection scenarios.
Main Results:
- The most favorable conditions for polymorphism maintenance involve negative ploidy-by-sex interactions.
- Stronger selection in female diploids coupled with weaker selection in female haploids promotes variation.
- Ploidy-by-sex interactions drive differences in allele frequencies between sexes.
- Ploidaly antagonistic selection maintains polymorphism for autosomal and X-linked genes, but not Y-linked genes.
Conclusions:
- Opposing selection across ploidy levels and sexes can effectively maintain genetic variation.
- Allele location significantly impacts the potential for polymorphism maintenance.
- The findings have implications for understanding evolutionary dynamics in organisms with biphasic life cycles.