Question

explain in detail the condition that should exist by natural selection to occur in population of organisms. include how evolutionary change will occur as a result

Answer 1

Four 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

1]  Weed Biodiversity
    a] Weed diversity and weed management.
2]  Definition of Biodiversity
3]  Genotypes & Phenotypes
4]  Somatic Variation
    a]  Phenotypic plasticity
    b]  Somatic polymorphism
5]  Genetic Variation
    a] Sources of genetic diversity
    b] Forces decreasing population diversity
    c] Forces increasing population diversity
    d] Levels of weedy biodiversity within a habitat

Weed Biodiversity

"The more diversified the descendants from any one species become in structure, constitution and habits, by so much will they be better enabled to seize on many widely diversified places in the polity of nature, and so enabled to increase in numbers."

-Charles Darwin

    Although Charlie's prose was a bit stiff for the 21st century, what I think he was saying was the more ecotypes or biotypes a weed species has (or the more species present in a field), the better it will be in taking advantage of opportunities left open by our weed management systems, and thus able to produce seed ensuring future survival. Charlie was no weed scientist, but he knew then what I want you to know: weeds are diverse; their diversity allows them to have at least a few variants (if not many) in the seed bank ready to take advantage of any opportunities left open in our weed management systems. These opportunities mean crop losses, and more weed seed returned to that field.

Biodiversity within plants and weeds can be defined narrowly or broadly. Broadly, weed biodiversity includes genetic, somatic, habitat, temporal (time) and floral community diversity. To understand more about this broad view of biodiversity, read my perpective on Weed Diversity and Weed Management [.pdf].

Biodiversity
1: the variety of organisms considered at all levels, from genetic variants of a single species through arrays of species to arrays of genera, families and still higher taxonomic levels;
2: includes the variety of ecosystems, which comprise both the communities of organisms within particular habitats and the physical conditions under which they live;
3: the totality of biological diversity

1:Somatic polymorphism of the parts of a genet
Genet: unit or group derived asexually from a single zygote: seedling, clone.
Phenotypic diversity: phenotypic plasticity, somatic polymorphism

2: The diversity of age-states within the community  
In a broader sense: time:
-flowering times,
-seed bank ages,
-temporal separation of activity;
-life cycle (annual (summer and winter), perennnial, biennial)
-emergence time

3: The genetic variants within a species: intra- and inter-specific

4: The diversity of microsites within the habitat

5: Groupings at a higher level than the species
Spp-groups, genera, families.  Examples:
-species-groups working overlapping niches
-in time: e.g. different parts of the season; erect and tall knotweed
-in space: same field
    The closer a neighbor is genetically, the more intense the competition.  The closer the genetic distance the more intense the struggle.  Genetic distance reveals the niche overlap between species. Competition within a species results in niche differentiation, formation and selection for separate species within genus; forces that drive creation of new species in a species-group.

Genotypes & Phenotypes
    In this section we will look at weed biodiversity in a somewhat narrower way: genetic and floral diversity, as well as somatic diversity:  Individual weed plants are the units of natural selection and adaptation. The diversity found among individual weed plants in populations and agricultural floral communities arises from both genetic and somatic variation. This diversity of individual plants and plant characters is revealed in the weed genotype and phenotype.

Genotype:
1: The hereditary or genetic constitution of an individual; all the genetic material of a cell, usually referring only to the nuclear material.
2: All the individuals sharing the same genetic constitution; biotype.

Phenotype can be viewed in two slightly different ways:
Phenotype (1): The sum total of observable structural and functional properties of an organism; the product of the interaction between the genotype and the environment.
Phenotype (2): The characters of an organism, whether due to the genotype or environment.

The relationship between the genotype (G), the phenotype (P) and the environment (E):

P = G x E