L earning Objective 25

A. Gene pool: All the genes in an interbreeding population at any one time. This is a more meaningful genetic unit to study, when looking at the "big picture" in evolutionary trends than the genetics of the individual.

B. The problem for Hawaiian plants has to do primarily with the restricted gene pool. A large gene pool provides genetic diversity that allows for "more genetic resources" for the demands of change that always face any group of individuals of a species (population) in survival over time. Contributing factors for the restricted gene pool of Hawaiian native plants is as follows:

1. One or only a very few individuals (the colonists) "made it" to isolated islands, thus creating a small gene pool from the start. (Also called Founders Effect.)

2. This group is cut off permanently, for all practical purposes, from the "mother population" from which it came. Little new "genetic infusion" is possible which could increase the gene pool.

3. Inherent in an island's structure is limited space for plants to colonize. This restricts numbers of individuals that can live at one time on the island, again resulting in a small gene pool.

4. The three factors above give rise to a genetic situation where the gene pool is limited. If the individuals of the population self-fertilize, or INBREED, this also decreases genetic variability. Inbreeding occurs most strongly when a plant is self-fertile.

A. Increase of OUTBREEDING

1. Most plants inbreed by self-pollination, which occurs when flowers are bisexual. The early colonists to the Hawaiian islands almost had to be self-fertile in this way. (Why?) However, this causes decrease in genetic variability in the long run, especially in small populations. Inbreeding is not a means to long-term survival on the islands.

2. There are several ways to insure outbreeding. If there are two different parent plants, the genetic variability of the offspring will be greater than if there is only one parent.

a. One way to increase outbreeding in plants having the common bisexual flower which produces both viable male gametes (in the pollen grain) and female gametes (egg cell in the ovules) is to be self-incompatible (cannot fertilize itself). This may occur in the following ways:

The pollen or egg cells of flowers of one plant do not mature at the same time, thus self-fertilization is not possible.

Or the pollen of a plant is incompatible and unable to fertilize the egg cells of the same plant.

b. Another way is having unisexual flowers, which have "male" flowers having only stamens or "female" flowers which have only pistils.

These unisexual flowers can be on the same plant (monoecy) or on different plants (dioecy). See Sohmer and Gustafson, pp. 23-4 for more discussion.

The breadfruit, or Ulu, a Polynesian introduced, is a good example of a monoecious plant. It has the slender, elongated "male" flowers and the rounded, bumpy "female" flowers which turn into the breadfruit, both on the same plant.

Kanawao, or Hawaiian hydrangea, Broussaisia arguta is an example of a native dioecious plant. It has "male" plants with flowers with only stamens being functional (staminate flowers), and "female" plants having only the pistils functional (pistillate flowers) and of course, producing the fruit.

Unisexual flowers, especially on different plants, clearly force outcrossing, because two genetically different parents are involved, and there will be more genetic variety in their offspring.

3. What is clear, however, is that plants initially "winning the lottery" (breaking the odds by getting here), would almost have to be bisexual in order to initially survive and thrive. Later, the tendency towards unisexual flowers or other modifications to increase outcrossing would develop for long-term survival.

4. The outbreeding mechanism is apparently effective because about 73% of Hawaiian species and 50% of the genera have special conditions which help increase outbreeding.

B. HYBRIDIZATION may occur between different species or clearly different "forms" of related plants.

Weeping Moa

1. Hybridization increases gene pool size and allows a way for new genetic material to move "between" groups of plants. The silverswords have demonstrated this tendency and more studies of Hawaiian plants would probably uncover more examples. 2. There is much variation in botanists' estimations of how many different plants there are in Hawai'i,and this may will be due, in part, to frequent, unrecognized hybridization. One example is moa, or Psilotum, which shows hybridization between an upright species and a weeping species.

Upright Moa

C. POLYMORPHY (literally "many forms") is the existence of many highly variable members of the same species or in genera.

1. The Metrosideros polymorpha, ohia lehua, is an example of a highly variable species with eight varieties and four other species.

2. Ohi'a variations are in the many different forms found in strikingly different environments: small shrubs in lava or bogs where living conditions are severe, cup-shaped leaf forms in moist areas, and large forest trees in the moist rain forest zones.

Click here for photo examples.

3. Polymorphy is found in many other groups of Hawaiian native plants, often making it very difficult for botanists to distinguish between species' visible characteristics.