EVOLUTION

Population genetics 
"In a large population in which random mating occurs and in the absence of forces that change the proportions of alleles the original ratio of dominant alleles to recessive alleles will be retained from generation to generation"

This statement of the Hardy Weinberg Law describes a state of equilibrium in the genetic makeup of populations that is more interesting for its exceptions than the comparitively rare occasions when it occurs.

Looking at the gene frequencies in the dihybrid cross we see that the frequency of dominant and recessive alleles remains constant at 50% from P1(parental) to F1 and F2 generations. Even if we were to select for the phenotype of the dominant genes, recessive alleles would persist in the population for several generations because they would be concealed by the dominant alleles in the heterozygous state.

When recessives occur at low frequency the chances of them coming together in the homozygous state become very low, so that even a lethal recessive can persist for many generations.

Exceptions to the rule lead to genetic change, or evolution in populations:

• When a small population breaks away or gets isolated, some alleles may be lost from the gene pool.
• Populations can become separated in their breeding as well as geographically. A mutation causing flowering at a different time than in the main population would block the flow of genes between them.
• Mutation is continually introducing new variation into populations, and this is another disturbance to the equilibrium.
• Usually there is some kind of selection, natural or human that results in change of gene frequencies.

So it is doubtful that the Hardy Weinberg equilibrium exists for long periods of time in real-life populations. Even in the absence of direct selection, organisms can be subject to"genetic drift".

Problems of genetic uniformity
Plants are more vulnerable to the environment than most animals that can move away from hostile situations. Also sexual reproduction is less certain for a static organism than one that can actively seek a mate. Throughout the kingdom, plants have retained a capability for asexual reproduction that may enable them to colonize a habitat but can lead to a narrow base of genetic variability. The organism trades in future adaptibility for short term gain of habitat.

Similar considerations apply to human manipulation of crop plants. The production of uniform stands of crops with consistent composition and quality is attractive but brings with it the risk of uniform susceptibility to a particular disease. A famous example is the problem of early hybrid corn varieties that were all susceptible to a new race of Southern corn blight (Helminthosporium maydis) that arose in 1970. About 15% of the US corn crop was lost.

Selection
Natural selection occurs over long time scales so that generally we cannot observe it in action. Sometimes we can deduce the path that selection has taken from observation of the variation in closely related species in relation to their environment. A famous example is the relation of beak form and food source amongst the Galapagos finches, studied by Charles Darwin.

Human beings have become major agents of selection, both conscious and unconscious. Many of our crop plants are far-removed from their wild ancestors so that it can even be difficult to say where they came from. Corn (Zea mays) is so unlike any wild grass that we are still trying to explain its origin.

An example of unwitting plant selection is the development of metal tolerance in bent grass (Agrostis tenuis) in the Upper Swansea Valley. This is an area where soils became heavily polluted with copper and other metals in the nineteenth century. Bent grass from nearby unpolluted areas is unable to colonize these soils, but the species is found growing there. In the laboratory the grass from polluted areas can be shown to be unusually tolerant to copper. It does not persist when transplanted to unpolluted areas where it cannot compete with non-tolerant strains.

Evolutionary divergence
When populations become very large and spread over a large area, groups of plants can be under different environmental conditions and reproductively isolated. Trees such as red maple (Acer rubrum), bur oak (Quercus macrocarpa) and green ash (Fraxinus pennsylvanica) grow from Canada to the Gulf of Mexico. Their times of flowering, bud break and leaf fall and their temperature tolerance is adapted to the local environment. If trees are moved form one zone to another (North to South or South to North) they may be injured because their development is not attuned to the new environment. This continuous variation in physiological adaptation in a species over distance is called a"cline".

In mountainous parts of the US, such as California the environment changes substantially over short distances. Some plants, such as sticky cinquefoil (Potentilla glandulosa) occur from sea level up to 5,000 feet. There are distinct ecotypes that are adapted to different altitudes. A low altitude form transplanted to the mountains fails to make enough vegetative growth in the short summer season and dies in the following winter.

When plants colonize a new environment, such as an island, with little existing vegetation they have an unusual opportunity to develop a variety of forms to fill different ecological niches. This leads to adaptive radiation, exemplified particularly well in plants like the tarweeds or silverswords (Dabautia, Wilkesia, and Argyroxiphium) on Hawaii and other Pacific islands.

Hybridization
Plants are more likely to mate with close relatives or even themselves than animals.

• Inbreeding stabilizes a genotype that may be adapted to a particular location, and we may find this desirable for uniformity in cultivated plants (or animals).
• However, the loss of variation may be at the expense of future adaptibility and inbreeding can lead to loss of vigor.
• Lethal recessive alleles are more likely to come together in the homozygous state under inbreeding; they are soon eliminated but sublethal alleles may be retained with a depressing effect on plant growth and reproduction.

The development of mechanisms to encourage or ensure outbreeding can be understood in relation to the advantages of heterozygosity. Deliberate crossing of inbred lines of crop plants often leads to an increase in plant growth and yield (Hybrid vigor or heterosis). This is exploited in the production of F1 hybrids of crop and ornamental plants.

Hybrid vigor may be particularly pronounced when different species or even genera are crossed. The London plane (Platanus x acerifolia) is a cold-tolerant urban tree, produced by crossing the North American sycamore (Platanus occidentalis) with the tender oriental plane (Platanus orientalis).

The London plane is fully fertile but many interspecific hybrids are sterile. Although the genomes of the parental species are related, differences in chromosome structure prevent normal meiotic paring and separation so that functional spores are not formed. Hybrids of annual plants would inevitably die because of this, but hybrids of perennial plants can persist and even spread vegetatively. An example is the common horsetail.

Equisetum x ferrisii is a sterile hybrid of E. laevigatum and E. hyemale. The hybrid has spread along rivers in the midwest because pieces of rhizome get carried along on the stream.

Fertility can be restored by doubling the number of chromosomes; then every chromosome can find an exact match with which to pair during meiosis. This can happen by chance in nature as in the example of the grass Spartina anglica a tetraploid derivative of a hybrid between a North American species (S. alterniflora) and the European S. maritima.

Spartina anglica has colonized large areas of coastal Britain since its origin at the turn of the century.

Many crop plants are allopolyploids originating from duplication of the genomes of interspecific hybrids. Examples of allotetraploids among the Brassica vegetables have already been discussed in lab. Wheat, Triticum aestivum and strawberry, Fragaria ananassa are examples of allohexaploids. More recently the deliberate induction of polyploidy by colchicine treatment has become a standard practice in plant breeding in order to obtain fertile hybrids. Hybrid vigor is stabilized in a plant with two or more genomes between which there is no recombination. Polyploids are also more vigorous than their diploid ancestors, even if there is no mixing of genomes (autopolyploids contain multiple sets of a single haploid genome.)

Another escape from sterility in hybrid plants is apomixis, the production of an embryo from a diploid cell without meiosis and gamete fusion. This occurs in many hawthorns and brambles and has the same advantages and disadvantages as vegetative reproduction. Seed production is assured and a successful genotype can rapidly invade a suitable habitat but there is little prospect of future variation to adapt to changing circumstances unless the sexual process can be restored.

Copyright © Michael Knee,
The Ohio State University
All rights reserved.