Human evolution or natural selection means that our ancestors of up to 3 million years ago in their various strains of say australopithecus or even Neanderthal man would be selected to flourish if they could resist disease better than say other strains of homo sapiens. An example of a case where foreign disease wss introduced was when the Spanish conquistadores came to Peru they brought smallpox which decimated the population or in British Columbia it was deliberately introduced to remove resistance to gold mining in the BC interior. The Incas and the BC First Nations simply had no immunity to the disease having had no contact with it before.
The statement that best explains why each river has its own set of characteristics is that Rivers interact with the ecosystems through which they pass.
Some of the characteristics of rivers that we can mention are :
- Length
- pH value
- chemical makeup
- aquatic life
As rivers flow downhill and flows into the ocean, It interacts with the environment, stripping it's minerals from the rocks and affecting the pH and chemical makeup of the water. The water that flows through the soil will erode the land and absorb the minerals in the soil as well. By affecting the pH of the river, this absorption will change the <u>diversity of aquatic life present in the river</u>. The length of the rIver can also depend on the limiting factors such as rocks or boulders present in the ecosystem through which it passes. Since every river pertains to its ecosystem, each will have it's own characteristics.
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In the eastern parts of Australia. Good luck.