What fundamental forces and processes acted to create the weeds we now have?
Minimum Conditions of Evolution
Weed populations are the adaptive product of long term selection in response to forces
in agroecosystems. Certain minimum conditions are required for evolution to occur. There
are several conditions to consider in this weedy adaptation:
1. Reproduction: A parent plant produces more offspring than can normally survive
2. Selection for fitness: "Fitter" phenotypes among those excess offspring are
more likely to survive
3. Inheritance: The "fitter" traits of those phenotypes are inherited
4. Genetic variation exists in the population
5. Increased probability of survival of offspring possessing a particular phenotypic
characteristic
6. There exists an environmental "uniformity" over time within which evolution
takes place
Reproduction
A parent plant produces more offspring than can normally survive.
Selection for Fitness
Correlation between probability of survival and phenotype:
-"Fitter" phenotypes among those excess offspring are more likely to survive.
-Non-random reproduction
-Differential contribution of offspring to next generation
-Proportionate contribution is called fitness or adaptive value:
Fitness: 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.
Adaptation:
1: The process of adjustment of an individual organism to environmental stress;
adaptability;
2: Process of evolutionary modification which results in improved survival and
reproductive efficiency;
3: Any morphological, physiological, developmental or behavioral character that enhances
survival and reproductive success of an organism
Inheritance: The "fitter" traits of those phenotypes are inherited.
Related concepts:
-Genes, chromosomes, mating systems, inbreeding, outcrossing.
-Introgression: The spread of genes of one species into the gene pool of another by
hybridization and backcrossing; introgressive hybridization
-Hybridization: Any crossing of individuals of different genetic compostion, typically
belonging to separate species, resulting in hybrid offspring
Genetic Variation
A condition necessary for evolution to occur is that there must be genetic variation
among the excess offspring produced. The most "fit" genotypes and phenotypes can
be selected from the genetic variation that exists in the population.
Population variability and diversity are dependent on more than one kind of individual in
the population in relation to one or more characteristics.
For any evolutionary change to occur there must be some genetic variation in the
population; the greater the amount of variability, the greater the possibilities for
change. Natural selection decreases variability in populations: in a uniform environment,
better adapted types should completely replace unfit types. But nature is very diverse,
why? A source of new variability is necessary for continued evolution to occur.
Index:
-Sources of variability in populations
-Gene frequency forces
Sources of Variability in Populations
Internal, genetic, sources of diversity:
The ultimate sources of variation, the source of new heritable characteristics in
populations and species, are due to mutation and recombination in chromosomes, genes, DNA.
-Linkage and crossing over: genes near each other go together; random breaks reconnect
-Linkage disequilibrium: genes on same chromosome cannot assort independently in meiosis,
crossing over breaks these connections and allows new linkages
External, environmental, sources of diversity:
Much of variation in populations, species, is retained due to changes in the environment (adaptive traits are only good in some environments, and not in others)
Hardy-Weinberg Law:
The original variability in a population will be maintained in the absence of forces that tend to decrease or increase this variability: changes in gene frequencies brought about by outside forces.
Gene Frequency Forces
The frequency of a gene, or allele, in a population is due to number of forces: forces
increasing variability, and forces decreasing variability.
Allele: Any of the different forms of a gene occupying the same locus (q.v. on homologous
chromosomes), and which undergo meiotic pairing (q.v. and can mutate one to another)
Forces that increase variability of populations:
Mutation: 1: A sudden heritable change in the genetic material, most often an
alteration of a single gene by duplication, replacement or deletion of a number of DNA
base pairs;
2: An individual that has undergone such a mutational change; mutant
Recombination: Any process that gives rise to a new combination of hereditary
determinants, such as the reassortment of parental genes during meiosis through crossing
over; mixing in the offspring of the genes and chromosomes of their parents. Recombination
doesn't change gene frequency, but it does:
1. combinations of different genes are better than others;
2. number of recombinations in infinitely larger than the possible number of mutations;
Most new types in populations arise from recombination.
Gene flow: The exchange of gentetic factors within and between populations by
interbreeding or migration; incorporation of characteristics into a population from
another population
Segregation distortion: The unequal segregation of allelles in a heterozygote due to an
aberrant meiotic mechanism;
-meiotic drift linkage:
-meiotic drive: Any mechanism operating differentially during meiosis in a heterozygote to
produce the two kinds of gametes with unequal frequencies
Forces that decrease variability of populations:
Selection: (gametic and zygotic differential mortality); Non-random differential
reproduction of different genotypes in a population; various modes of selection are:
-directional selection: selection for an optimum phenotype resulting in a directional
shift in gene frequencies of the character concerned and leading to a state of adaptation
in a progressively changing environment; dynamic selection; progressive selection
-disruptive selection: selection for phenotypic extremes in a polymorphic population,
which preserves and accentuates discontinuity; centrifugal selection; diversifying
selection.
-stabilizing selection: selecting for the mean or intermediate phenotype with the
consequent elimination of peripheral variants, maintaining an existing state of adaptation
in a stable environment; centripetal selection; normalizing selection. -artificial
selection: selection by humans; domestication; selective breeding -natural selection: 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 th course of evolution by preserving those variants or traits best adapted in
the face of natural competition Genetic drift: The occurence of random changes in the gene
frequencies of small isolated population, not due to selection, mutation or imigration;
drift; Sewall Wright effect; equivalent to static noise in system; adaptive alleles can be
lost in process, especially in small populations
Increased probability of survival of offspring possessing a particular phenotypic characteristic
Environmental "Uniformity" Over Time
There exists an environmental "uniformity" over time within which evolution takes place: -In a uniform environment, better adapted types should completely replace unfit types, but nature is very diverse. -Why?: see sources of variability, diversity