Most higher organisms (both plants and animals) reproduce
sexually—that is, they produce offspring through the union of reproductive cells from
two different parents. Think back to our example of the violet. Violets produce showy
flowers that attract insects that carry pollen from one plant to the next. Offspring
resulting from this cross-pollination are genetically distinct from either parent. Violets
also produce flowers that never open, and are self-pollinated. The resulting offspring are
genetically similar, though not identical, to the parent. And, finally, violets send out
creeping stems. Plants sprouting from these runners are genetically identical to the
parent plant.We’ve talked about genetic
diversity—but why is it important, and how does it fit in with our general topic of
plant sexual reproduction?
Sexual reproduction is critical for maintaining genetic
diversity within a species because it combines the parents’ genetic material,
resulting in offspring with unique genetic blueprints—different from either parent.
Genetic diversity is important for two reasons.
First of all, when a population of an organism contains a
large gene pool—that is, if the genetic blueprints of individuals in the population
vary significantly—the group has a greater chance of surviving and flourishing than a
population with limited genetic variability.
Why is this so? Because some of the individuals may have
inherited traits making them particularly resistant to disease or tolerant of cold, for
example. Or they may possess other traits that increase their chance for survival. In
nature, the "fittest" individuals succeed and go on to reproduce—Darwin
termed this process "natural selection." Suppose there’s an outbreak of a
disease that threatens to wipe out an entire species. The more genetic variability there
is within that species, the higher the likelihood that at least some of the individuals
will be resistant, and will survive.
In the lab, plant breeders take advantage of these genetic
variants to improve existing plants and create new varieties. Through cross breeding they
strive to breed in disease resistance, superior fruit production, increased cold
tolerance, or other desirable traits.
Genetic diversity also reduces the incidence of
unfavorable inherited traits. In a small, isolated population of organisms, individuals
may be forced to breed with close relatives. When this happens, the genetic makeup of the
individuals becomes more and more uniform, and genetic flaws become increasingly more
common. This phenomenon is called inbreeding.
When closely related organisms (siblings or cousins, for
example) interbreed, any genetic weaknesses that are hidden in the parents can be
multiplied in the offspring. For example, animals can be carriers of a gene for an
inherited disease, but not show any symptoms. If they mate with a partner who is also a
carrier, then the offspring may exhibit symptoms of the disease. (We’ll talk more
about inherited traits next week.) In an inbred population, chances are greater that
carriers will interbreed. Over time, the entire population is weakened.
In summary, genetic diversity strengthens a population by
increasing the likelihood that at least some individuals will be able to survive major
disturbances, and by making the group less susceptible to inherited disorders.
