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Monohybrid Crosses01:20

Monohybrid Crosses

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Frequency-dependent Selection01:21

Frequency-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.Positive Frequency-Dependent SelectionIn positive...
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Hardy-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.In the early 20th century,...
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Speciation is the evolutionary process resulting in the formation of new, distinct species—groups of reproductively isolated populations.The genetics of speciation involves the different traits or isolating mechanisms preventing gene exchange, leading to reproductive isolation. Reproductive isolation can be due to reproductive barriers that have effects either before or after the formation of a zygote. Pre-zygotic mechanisms prevent fertilization from occurring, and post-zygotic mechanisms...
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Hybrid zones are narrow regions where two closely related species interact, mate, and produce hybrids. Relative to either parent species, hybrids may possess distinct phenotypic or genetic differences that impact their survival and reproductive success. The genetic variances introduced by hybridization influence species diversity and speciation processes within the hybrid zone.Gene flow and natural selection are evolutionary mechanisms that shape the outcome of a hybrid zone. Gene flow...
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Trihybrid Crosses

Trihybrid Crosses
Some of Mendel’s crosses examined three pairs of contrasting characteristics. Such a cross is called a trihybrid cross. A trihybrid cross is a combination of three individual monohybrid crosses. For example, plant height (tall vs. short), seed shape (round vs. wrinkled), and seed color (yellow vs. green).
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The fitness effects of outcrossing in Calylophus serrulatus, a permanent translocation heterozygote.

David A Heiser1, Ruth G Shaw

  • 1Peabody Museum of Natural History, Yale University, New Haven, Connecticut 06520, USA. david.heiser@yale.edu

Evolution; International Journal of Organic Evolution
|March 30, 2006
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Small prairie plant populations face extinction risks. Calylophus serrulatus, a unique heterozygote, shows reduced fitness with distant mating (outbreeding depression), not inbreeding depression, maintaining fitness despite fragmentation.

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Area of Science:

  • Ecology
  • Evolutionary Biology
  • Genetics

Background:

  • Small, isolated populations in fragmented habitats are vulnerable to local extinction.
  • Mating distance can impact progeny fitness, often through inbreeding depression, but specialized genetic systems may alter these effects.

Purpose of the Study:

  • To investigate the effect of mating distance on progeny fitness in Calylophus serrulatus, a permanent translocation heterozygote.
  • To determine if inbreeding depression or outbreeding depression influences fitness in this species within fragmented prairie habitats.

Main Methods:

  • Hand-pollination of Calylophus serrulatus flowers using pollen from various distances within and between populations.
  • Measurement of progeny fitness-related characters in a greenhouse setting.
  • Analysis of fitness consequences associated with self-mating, near-neighbor mating, and long-distance mating.

Main Results:

  • Progeny from self-mating did not show significantly reduced mean fitness compared to progeny from crosses between plants.
  • Crosses between distant populations (20 km) resulted in reduced progeny fitness, indicating outbreeding depression.
  • Fitness of self-pollinated and near-crossed progeny (<2 m) was generally higher than that of progeny from mid (10-25 m) and far (>35 m) crosses within populations.

Conclusions:

  • The specialized genetic system of Calylophus serrulatus prevents inbreeding depression, allowing fitness to be maintained despite habitat fragmentation.
  • Outbreeding depression occurs at larger spatial scales, but such distant matings are likely rare in the current fragmented landscape.
  • The species' genetic system is crucial for maintaining mean fitness in severely fragmented populations.