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I. ENVIRONMENT.
The environment is always changing:In human terms, climate and environment seems pretty much the same but, over thousands and millions of years (the time evolution takes) major climatic and environmental changes have occurred.
A. The Ice Ages are an example of massive, long-term changes. The genetic shifts in grazed pasture weeds are an example of a rapid, short term environmental change.
B. Plants dispersed to the Hawaiian Islands encountered different environments than those from which they came. This created a major environmental "change" for them.
II. ISOLATION.
Isolation is a major factor in evolution of Hawaiian plants and animals. (See figure 15-A)
A. Only one individual plant may immigrate or disperse to the islands.
B. It would have only certain genetic traits from a larger, more diverse population. It could not genetically "carry" all the traits. (Sometimes called the "Founder's effect").
C. Its physical separation from its "home" or founding group is almost impossible to overcome. It is very unlikely to cross with a plant from its original population once isolated in the middle of the Pacific. Therefore, different genetic traits arising in either cannot be interchanged.
D. It is also in a different environment. Certain traits may be passed on to its offspring or may disappear. Some of those traits could be linked to the environment as negatives or positives.
III. HYBRIDIZATION.
The ability to form hybrids between genetically different groups is highly developed in plants, much more so than in animals.A. Plants can form fertile hybrids between species, even between genera. It is rarely the case with animals. A horse may be bred with a donkey to get a mule,but mules are infertile and cannot produce mule offspring!
B. Plants may undergo a genetic change in which the hybrid develops a doubled set of chromosomes. Doubling of chromosomes is called polyploidy. (See figure 15-B)
(Normally individuals with different chromosome numbers, like in figure 15-B, with numbers of 20 and 40, may be able to form hybrids but these cannot produce fertile offspring. However, if the chromosomes double to 60, the resulting hybrid can produce gametes with 30 chromosomes. In having a complete single chromosome set from each parent in the gametes, normal pairing now occurs in meiosis, and the plant can produce viable gametes (egg and sperm) which will produce a fertile next generation.)
As a result, plants can become fertile and reproduce. These plants will be a new combination of both parents, self-fertile, and genetically isolated from the parents. They may even form a new species.
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