Directional selection is one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme.
In stabilizing selection, it is when selective pressures select against the two extremes of a trait, the population experiences stabilizing selection.
Disruptive selection is when selection pressures act against individuals in the middle of the trait distribution.
Example of directional: Breeding of the greyhound dog. Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest.
Example of stabilizing: The size of newborn human babies.
Example of disruptive: The evolution of Darwin's finches, the explanation being that each island applied different selection pressure on the same types of mainland finch, favouring slightly different bill shape.
Microevolution is simply a change in gene frequency within a population. Evolution at this scale can be observed over short periods of time, for example, between one generation and the next, the frequency of a gene for pesticide resistance in a population of crop pests increases. Such a change might come about because natural selection favored the gene, because the population received new immigrants carrying the gene, because some nonresistant genes mutated to the resistant version, or because of random genetic drift from one generation to the next.
Selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations.
Mutations are changes in the DNA sequence of a cell's genome and are caused by radiation, viruses, transposons and mutagenic chemicals, as well as errors that occur during meiosis or DNA replication. Errors are introduced particularly often in the process of DNA replication, in the polymerization of the second strand. These errors can also be induced by the organism itself, by cellular processes such as hypermutation.
Genetic drift is the change in the relative frequency in which a gene variant occurs in a population due to random sampling and chance. That is, the alleles in the offspring in the population are a random sample of those in the parents. And chance has a role in determining whether a given individual survives and reproduces.
The interpretation of the fossil record has been biased by differential preservation. Some species are underrepresented or have not yet been found. We are left with a somewhat blurred picture of the past, especially the early past. Whether an organism is preserved greatly depends on the local environment in which it died. Plants and animals from humid tropical forests are rarely preserved because they decay rapidly in these regions. Similarly, fossils from mountainous areas rarely survive due to high rates of erosion. Desert creatures generally become fossilized more often due to the preserving arid conditions. Likewise, aquatic organisms are often well preserved if their bodies ended up in deep water where there is little oxygen and life. Not all bones from the same animal survive equally well. Lightweight bones with relatively large surface areas deteriorate more quickly and are, therefore, less often fossilized. Small, delicate bones are also more likely to be crushed or carried away from the rest of a skeleton by running water. There is bias in the fossil record resulting from the fact that paleontologists have not equally searched all areas of the globe. Because of the inaccessibility of some regions, such as Central Asia and much of Africa, their fossil records are poorly understood compared to those of Europe and North America.
Theory in science means something else. It doesn't mean speculation, in science "theory" means explanation. A theory is an explanatory framework that incorporates all other data including laws. A law in science is descriptive of some kind of physical relationship for example the ideal gas law. Where as a theory is explanatory for example the ideal gas law would be explained as part of atomic theory. And you could quite easily prove that evolution is not real. Find a fossil mammal in the Silurian and you've done it. A law is a pattern that natural phenomena follow. So they're two different things. A law is great for predicting events, because we expect the events to follow the pattern. But the law won't explain why events follow the pattern. For that we need a theory.
