Genetic Diversity

Sources of genetic diversity
Forces decreasing population diversity
Forces increasing population diversity
Levels of weedy biodiversity within a habitat

    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.
    The unit of selection by which this process occurs is the individual organism, although some like Richard Dawkin argues cogently that it may be the allele (within the context of the individual.. Viewing weed evolution from the point of view of several levels of organization is sometimes helpful: allele, trait, individual. Group selection is a no-no, but community assembly of different weed species will be discussed in Life History.
    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.

Sources of genetic diversity
External. 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.
Internal. 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. The frequency of a gene, or allele, in a population is due to number of forces: forces increasing variability, and forces decreasing variability.

Forces increasing population variability
Four important allelic forces drive population heterogeneity. Viewed from a different perspective, Harper (1977) presents several other important selection forces driving population variability. It is interesting to see that Harper presents forces of selection as increasing population variability, while other regard selection as decreasing heterogeneity.

Allelic forces
1: 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

2: Recombination
Recombination (1): 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 (2): Event, occurring by crossing over of chromosomes during meiosis, in which DNA is exchanged between a pair of chromosomes of a pair. Thus, two genes that were previously unlinked, being on different chromosomes, can become linked because of recombination, and linked genes may become unlinked.

Recombination doesn't change gene frequency, but it does lead to combinations of different genes are better than others; and the number of recombinations is infinitely larger than the possible number of mutations. Most new types in populations arise from recombination.

3: Gene Flow
Gene flow: The exchange of genetic factors within and between populations by interbreeding or migration; incorporation of characteristics into a population from another population

4: Segregation Distortion
Segregation distortion: The unequal segregation of allelles in a heterozygote due to:

An aberrant meiotic mechanism. E.g. meiotic drive: Any mechanism operating differentially during meiosis in a heterozygote to produce the two kinds of gametes with unequal frequencies.
Other phenomena that result in altered gametic transmission ratios. E.g. in pollen competition where one allele results in a more slowly growing pollen tube than an alternate allele. Gametes bearing this allele will therefore show up in zygotes at a frequency less than 50%, as will all genes linked to the lousy allele (Wendel, pers. comm., 1998).

Other forces driving variability
Six categories of selective forces that drive diversity as presented in Harper (1977; pp. 753-776).

1. Genetic variation within species with different mating/breeding systems (selection) p.753
2. Forces of selection acting within populations of plants: speciation, r and k selection; p.758
3. Selection for ecological combining ability selection; p. 762
4. Selection by the activity of predator and pathogens; p. 767
5. The evolutionary consequences of crashes, cycles, and catastrophes (disturbance selection) p. 769
6. Selection in a patchy environment; p. 774

Forces decreasing population variability

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 occurance 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

Levels of Weedy Biodiversity within a Habitat
Weed genetic biodiversity exists at several levels of plant organization: all plants of the world all the way down to an individual plant of one species. Below find a presentation of this type of genetic diversity:

Levels of Weedy Biodiversity within a Habitat
--->Floral Community
------>Species-Group
--------->Species
------------>Population
--------------->Variant: ecotype, biotype, mutant
------------------>Individual
--------------------->Allele

--->Floral Community
Every agricultural field has, or has the potential, to produce a wide variety of weeds from its seed and bud bank. Even when we control these weeds for many years, they have the potential to come back because of the long soil life they have, seed dormancy. Below are some extreme weed infestations.

------>Species-group
    One of the best adapted and known weedy species-groups are the foxtails. Another related way closely related species group is the polyploid species cluster (Zohary), which we will discuss later in the course.

    The pigweeds are another infamous species-group. There is considerable diversity within this superspecies, maybe because promiscuous outcrossing between plants (and maybe species) makes firm species identification problematic. Is it any wonder that most weed management tactics miss at least some of these diverse pests?

    The smartweed species-group consists of several species, including ladysthumb and Pennsylvania smartweed. Below are a variety of leaf shapes and colors from several plants in this group. They could be from the same or different species, it is hard to tell from just this picture.

--------->Species
Much biodiversity exists within a single species, intra-specific diversity. Below is a range of common lambsquarters leaves that could have come from either the same plant or different plants in this species.

Hedge bindweed has several different colors of flowers within members of the species:

------------>Population
Populations within a species can consist of one or many different genotypes. Below are two different genotypes (and phenotypes) of quackgrass. The one on the left is a prostrate growth habit type, the one on the right has upright leaves. These two genotypes exist within the same field, forming the quackgrass population of that field. Because the prostrate type exposes more leaf surface to spray applications, often it is the first to get killed by foliar herbicide applications.

Wild proso millet genotypes produce different colored seeds, some crop seed, some weed seed. Below you can see this variation. Some fields have both, or several different colored seed producing variants. Together they compose the wild proso millet population.

--------------->Variant
Variants of a species can exist together in the same field and comprise that field's population of that species. Variants can also exist in a field all by themselves, in which case the population of that species in that field is only one variant, or one genotype. Below is a collection of different panicle (seedhead) variants of green foxtail. The panicles on the far left are from the crop foxtail millet. Human selection for types with big fat seedheads has produced these variants. The panicles on the middle left are from robust purple green foxtail, very colorful. The picture on the middle right was taken on a roadside near Madrid, Boone Co., Iowa. Several different colored panicles make up that site's population. Some are purplish, maybe on their way to being purple robust with time and selection. The panicle on the far right is from a salt-tolerant green foxtail variant known as pachy-stachys. It is located in a cozy cove in southern Japan on the Sea of Japan. Quite a range of variants, don't you think?

------------------>Individual

--------------------->Allele

Genetic Diversity

Šjdekker-2005