Natural Selection and its Four Conditions

a.  Reproduction
b.  Heredity
c.  Variation in fitness of organisms
d.  Variation in individuals and traits

The conditions inherent in our cropping systems makes weed success inevitable. Natural selection, diversification, evolution and adaptation are the important events that weed populations have experienced for the history of agriculture. In this unit we will explore these fundamental forces and processes and form weed communities and drive the appearance and changes in the weeds we have.

natural selection
1:  process by which forms of organisms in a population that are best adapted to the environment increase in frequency relative to less well-adapted forms over a number of generations.
2:  the non-random and differential reproduction of different genotypes acting to preserve favorable variants and to eliminate less favorable variants; viewed as the creative force that directs the course of evolution by preserving those variants or traits best adapted in the face of natural competition

Pre-conditions for natural selection.  The preconditions to natural selection are excess fecundity and the consequent competition for limited resources. Weeds produce many more seed than will survive. Many more seed germinate and form seedlings than will mature to produce their own seed. Only the successful competitors will reproduce, mortality is very high.

Four (4) conditions for natural selection.  Four conditions are needed for natural selection to occur: reproduction, heredity, variation in fitness or organisms, variation in individual characters among members of the population.  If they are met, natural selection automatically results.

1: Reproduction:  the act or process of producing offspring
    A condition necessary for evolution to occur is that a parent plant produces more offspring than can normally survive. The net (average) result of reproduction is that a parent plant leaves one descendant that reproduces, yet many more are produced that die. See Life History for full treatments of reproduction in weedy populations.

2: Heredity:  the mechanism of transmission of specific characters or traits from parent to offspring. 
inheritance: the transmission of genetic information from ancestors or parents to descendants or offspring.
    A condition necessary for evolution to occur is that the traits of the "fittest" phenotypes that survive are inherited by the successful progeny. The offspring must tend to resemble their parents.  Molecular genetics and biochemistry provide significant information about how this process occurs.

3: Variation in fitness of organisms.  Definitions of fitness: 
1:  the average number of offspring produced by individuals with a certain genotype, relative to the numbers produced by individuals with other genotypes.
2:  the relative competitive ability of a given genotype conferred by adaptive morphological, physiological or behavioral characters, expressed and usually quantified as the average number of surviving progeny of one genotype compared with the average number of surviving progeny of competing genotypes; a measure of the contribution of a given genotype to the subsequent generation relative to that of other genotypes
    A condition necessary for evolution to occur is variation in fitness of organisms according to the state they have for a heritable character. Individuals in the population with some characters must be more likely to reproduce, more fit. Organisms in a population vary in reproductive success. We will discuss fitness in Life History when we discuss competition, interference and the effects of neighbor plants.  See also pages on Fitness & Fecundity in the reproductive life history section.

4: Variation in individual characters among members of the population