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Natural Selection and Fossils
An example where natural selection works in a non-changing environment, which is generally found in deep oceanic regions where the Lingula, a lamp shell, dwells. Certain populations of the Lingula (such as the Paleozoic ones) produced variants which were different from their ancestors which resulted in their extinction because they could not survive in the deep-sea regions. The only survivors were those that were similar to ancestors.
Let's see how natural selection works in an environment that does not change. Such an environment is found in deep-sea regions where a lamp shell called Lingula lives. Paleozoic populations of the Lingula must have produced a number of variants including those with, for example, differently shaped shells. These variants were not suited to the deep-sea environment and so perished, leaving no offspring. The only ones to survive were those that were similar to their ancestors. This process of selection for the same shell shape must have continued for millions of generations, for the shell is exactly the same today as it was many millions of years ago.
Gryphaea is another example of natural selection in which the
envronment is changing. The ocean-dwelling Gryphaea, though related
to the 
oyster, has a different shell, which is coiled. This shell
coiled increasingly during the Jurassic Period
and during this time, the seabed surface changed from a rocky to a
muddy one and thus the coiling was a suitable adaptation. Flat
Gryphaeas could live with their valve facing upwards to take in
food and water on a rocky substrata but on a muddy seabed, the
coiled Gryphaea could face its valve upwards and whereas the flat
Gryphaea would not have survived.
Darwinian fitness is the relative probability of the reproduction of a hereditary characteristic; the degree of fitness varies with the reproductive efficiency of the characteristic. All living organisms are constantly struggling for existence because of the problem of overpopulation. Thus nature selects the best traits from each individual organism and reproduces in them the next generation.