What Might Be a Disadvantage of Self-Fertilization for Plants?

Self-Fertilization

• Flowering plants reproduce sexually. They require pollen from a male flower to come into contact with and fertilize a female flower before that flower can mature into seed-bearing fruit. Many plants -- more than 70 percent of flowering plant species, according to botany course material from the University of Toronto -- bear both male and female flowers on the same organism, allowing the plant to pollinate itself. This is called self-fertilization.

Diminishing Genetic Diversity

• When a plant self-fertilizes, it is both mother and father to its offspring, who therefore only receive copies of one plant's genes. Individual pollen ovules may carry different alleles from the parent plant, but the pool of possible genetic combinations from a single plant's alleles is limited. Over generations of self-fertilization, the genetic variability in the plant population decreases. With reduced genetic variability, a plant population has reduced chances of successfully adapting to a changing environment. Over generations, the organisms grow too much alike in their potential vulnerabilities and specific requirements, so that one change to their surroundings could wipe out the entire population.

Inbreeding Depression

• Inbreeding is any form of reproduction between relative organisms. In animals, inbreeding usually refers to mating between primary cousins or siblings. In plants, inbreeding occurs when one flower pollinates another flower on the same individual. With inbreeding, there is a chance of inbreeding depression, a term describing the noticeable reduction in reproductive fitness of the offspring of the inbreeding. Inbreeding depression most likely springs from the expression of deleterious recessive alleles. All plant and animal populations carry deleterious alleles, harmful genetic products resulting from DNA mutation. But if they are recessive, the child organism will not exhibit the unhealthy traits unless it receives a copy of the allele from each of its parents. The pairing and expression of deleterious recessive alleles becomes much more likely if the father and mother are closely related or, as in the case of self-pollinating plants, the same individual.

Surviving Self-Fertilization

• Because many plant species can self-fertilize, certain mechanisms are required to prevent their self-compatibility from leading them into inbreeding depression and evolutionary dead ends. One is separation of male and female sex organs. Either the structure of the flower separates them physically, or the life cycle of the flower -- such that the male and female organs operate at different times -- separates them chronologically. This separation reduces the frequency of self-pollination as compared to cross-pollination. And among the inbreed offspring, individual organisms expressing traits of deleterious recessive alleles die off, leaving only offspring that carry the healthier dominant alleles to procreate. The die-off of lethal recessive combinations is the mechanism whereby inbreed lines from self-compatible populations stabilize and even, over future generations, go on to thrive.