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.
3. Generational change
in gene frequencies:
Individuals within a population that carry 'favorable' characteristics
are more likely to survive to reproductive age than those that do not.
Those that carry 'unfavorable' characterists are least likely to survive
to reproductive age. If these characteristics have a genetic basis
(Mendel demonstrated that most do) then, through NATURAL SELECTION, sequential
generations will carry MORE favorable characteristics and LESS unfavorable
characteristics, i.e. natural selection works to CHANGE gene frequencies
with each generation.
from Jones and Lucksinger, Plant Systematics, fig. 7-2, p. 162
Examples of directional selection: human-imposed
positive pressures ('artificial' selection) to produce domesticated
animals and plants over the past 20,000 years and human-imposed
negative pressures to produce various types or organisms resistant to chemical-based
eradication efforts. Regardless of source, selective pressures act
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