postzygotic barrier

Examples of postzygotic barrier in the following topics:

• Reproductive Isolation

  • Scientists classify reproductive isolation in two groups: prezygotic barriers and postzygotic barriers.
  • A postzygotic barrier occurs after zygote formation; this includes organisms that don't survive the embryonic stage and those that are born sterile.
  • Some types of prezygotic barriers prevent reproduction entirely.
  • Other prezygotic barriers work when differences in their gamete cells prevent fertilization from taking place; this is called a gametic barrier.
  • When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction.

• Physical and Chemical Barriers

  • The innate immune response has physical and chemical barriers that exist as the first line of defense against infectious pathogens.
  • Before any immune factors are triggered, the skin (also known as the epithelial surface) functions as a continuous, impassable barrier to potentially-infectious pathogens .
  • The skin is considered the first defense of the innate immune system; it is the first of the nonspecific barrier defenses.
  • Some pathogens have evolved specific mechanisms that allow them to overcome physical and chemical barriers.
  • Once inside, the body still has many other defenses, including chemical barriers.

• Hybrid Zones

  • Over time, two species may further diverge or reconnect, depending on the fitness strength and the reproductive barriers of the hybrids.
  • Over time, the hybrid zone may change depending on the fitness strength and the reproductive barriers of the hybrids .
  • If the hybrids are as fit or more fit than the parents, or the reproductive barriers weaken, the two species may fuse back into one species (reconnection).
  • Reinforcement, fusion, or stability may result, depending on reproductive barriers and the relative fitness of the hybrids.

• Plant Defenses Against Pathogens

  • Plants defend against pathogens with barriers, secondary metabolites, and antimicrobial compounds.
  • The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle.

• Plant Defenses Against Herbivores

  • Plants defend against herbivores with mechanical wounding, barriers, secondary metabolites, and attraction of parasitoids.
  • The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle.

• Contraception and Birth Control

  • There are many methods of birth control, including barriers to sperm, hormones that prevent ovulation, sterilization procedures, and "natural" methods.
  • Barrier methods include those that prevent the sperm from reaching the egg.
  • Barrier methods such as condoms, cervical caps, and diaphragms serve to block sperm from entering the uterus, thereby preventing fertilization.
  • Combinations of spermicidal chemicals and barrier methods achieve lower failure rates than do the methods when used separately.

• Enzyme Active Site and Substrate Specificity

  • Enzymes catalyze chemical reactions by lowering activation energy barriers and converting substrate molecules to products.

• The Complement System

  • The innate immune system serves as a first responder to pathogenic threats that bypass natural physical and chemical barriers of the body.

• Glia

  • Astrocytes also form the blood-brain barrier: a structure that blocks entrance of toxic substances into the brain.

• Genetic Variation

  • Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or when they are under selection pressures from a different environment.