Basic Notions (Darwin, 1859):
1. Natural Variation: There is variation for just about any characteristic in any characteristic of sexually reproducting organisms. Much of this variation has a GENETIC basis. THIS WAS NOT CONFIRMED UNTIL THE WORK OF MENDEL BECAME GENERALLY KNOWN - CA. 1900.
2. Differential Survival: All progeny do not live to reproductive age, i.e., is a given plant population, all seeds produced by one generation do not survive to contribute to the next generation.
in gene frequencies:
Individuals within a population that carry 'favorable' characteristics
are more likely to survive to reproductive age than those that do
Those that carry 'unfavorable' characterists are least likely to
to reproductive age. If these characteristics have a genetic
(Mendel demonstrated that most do) then, through NATURAL SELECTION,
generations will carry MORE favorable characteristics and LESS
characteristics, i.e. natural selection works to CHANGE gene
with each generation.
from Jones and Lucksinger, Plant Systematics, fig. 7-2, p. 162
Examples of directional selection:
positive pressures ('artificial'
plants over the past 20,000 years and human-imposed
negative pressures to produce various types or organisms resistant to
eradication efforts. Regardless of source, selective pressures
on genetic variation this is expressed at various levels:
from Jones and Lucksinger, Plant Systematics, fig. 7-6, p. 173
from Jones and Lucksinger, Plant Systematics, fig. 7-7, p. 174