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Natural Science - Year II

Unit 43: Darwin and Natural Selection

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Science Web Assignment for Unit 43


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Science Lecture for Unit 43: The Case Made for Evoluion Theory

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Outline/Summary

Modern Theories of Evolution

Modern evolution draws much of its evidence from genetics, a theory and set of evidence which Darwin did not know about. We will discuss genetics in another unit, when we study the work of Gregor Mendel.

Evidence for evolution used by biologists

Biologists draw on evidence from many different sources in order to establish the relationships of different species to common ancestors according to evolutionary theories.

Populations and evolution

Modern evolutionists are careful to emphasize that they discuss the evolution of populations, not individuals. A population is any well-defined set of organisms. One of the most common populations to consider is the species, which is defined in biology as any group of organisms that will breed and produce fertile offspring under natural conditions. In some ways, this is an artifical distinction: animals which may be able to produce in the lab might not meet to produce in nature, because they live in different areas.

In most cases, interbreeding is what produces the range of characteristics available to each generation of the population. If the species is isolated by its mating practices and opportunities, no variations in characteristics will be introduced from outside organisms, although new ones may be created by mutation. As the population becomes adapted to its environment, the frequency of occurance (or the number of individuals with a particular characteristic) shifts. If the shift continues over several generations with a measurable or significant change in the frequency, biologists say that microevolution is occuring. Note that microevolutionary changes are litmited to within species; a single microevolutionary event does not result in the emergence of new species.

Biologists use a mathematical relationship called the Hardy-Weinberg principle to determine whether microevolution is occuring. If successive generations have a distribution of characteristics that follow this principle, then the biologists are reasonably certain the following conditions apply:

  1. All matings are random.
  2. No mutations are occuring.
  3. The population is large, so that statistical deviations are muted.
  4. No migrations are occuring, so no new types of characteristics are being added
  5. No mechanisms for natural selection are occuring.

Generally, studies of populations show some deviation is occuring in the population.

Factors in microevolution

Five different causes are usually listed as sources of microevolutionary change. Notice that some factors increase genetic diversity while other factors reduce it.

Macroevolution

The more controversial aspects of evolution involve the theories of macroevolution that are used to explain changes in an entire species.

An existing species can die out or become extinct; such events are usually associated with a catastrophe that suddenly or gradually destroys the species habitat. We are constantly being warned that the trend of global warming, or the destruction of tropical rain forests or our own old growth forests will result in the loss of many species of insects, plants, and animals. Many biologists think that the fossil record shows several periods of mass extinction, such as the abrupt disappearance of the dinosaurs, dated to about 65 million years ago.

A new species (a population which can no longer mate with pre-existing species) may emerge from existing species. The sudden appearance of fossils with particular traits in later rock strata is used as evidence of the appearance of a new species. One of the biggest challenges to current evolution theory is that no "intermediate forms" exist for fossils which appear related except for one or two major characteristics.

The emergence of new species (using the above definition) is actually a relatively common occurance, particularly with hybrid plants, when the offspring of the hybrids cannot cross-pollinate with the parent plants or their normal offspring any longer. It also occurs in some animal forms, although rarely because of the more complex structures of animals.

One of the criteria for a successful "new species" is its ability to reproduce healthy offspring. Hybrid plants can reproduce and often become staples of new food, such as the triticale wheat we use in bread. In contrast, as an example of an unsuccessful mating, consider the mule. Mules are the hybrid offspring of horses and donkeys, but are sterile and so not considered a true species.

Study/Discussion Questions