Universal Common Descent

Are all living things commonly related? One scientific approach called universal common descent tries to answer that question. It is the hypothesis that all living, earth bound organisms are genealogically related. In other words, all existing species originated gradually by biological, reproductive processes on a geological timescale. Modern organisms come from a common gene pool; they are the genetic descendants of some original species. That species could have been found in one location, but it may also have been found in multiple locations around the earth.

Genetical "gradualness" is a mode of biological change that is dependent on population phenomena. To be sure, it is not a statement about the tempo of evolution modification. Genetically gradual events instead are changes which occur within the range of biological variation that is normally expected between two consecutive generations. On a geological timescale morphological change may appear fast but nevertheless still be genetically gradual. How does this impact macroevolutionary events? Gradualness is not a mechanism of evolutionary change, but it imposes severe constraints on possible macroevolutionary events. Likewise, the requirement of gradualness necessarily restricts the possible mechanisms of common descent and adaptation

How are Microevolutionary theories related to Macroevolutionary theories?
Microevolutionary theories are gradualistic explanatory mechanisms that biologists use to account for the origin and evolution of macroevolutionary adaptations and variation. These mechanisms include such concepts as natural selection, genetic drift, sexual selection, neutral evolution, and theories of speciation.

The Basic Mechanisms of Evolution

Natural Selection
This is a process in which some individuals have genetically-based traits that will improve survival chances or reproduction abilities thus having more offspring which in turn will survive to reproductive age than other individuals. Now as the offspring also carry the genes with these traits, this process gives the genes with these advantageous traits to become more common in populations and the genes with disadvantageous traits to become less common in populations.

Genetic Drift
Some individuals in each generation may leave behind a few more descendents and their genes than other individuals. The genes of the next generation will be the genes of the “lucky” individuals. This does not necessarily mean that they are healthier or “better” individuals. That, is genetic drift. It is a random effect. Some genes survive, others don't. It happens to all populations—there’s no avoiding it. Genetic drift affects the genetic makeup of the population through an entirely random process. It is a mechanism of evolution, it doesn’t work to produce adaptions.

Sexual Selection
Sexually acting on an organism’s ability to obtain or successfully copulate with a mate. This process may produce traits that seem to decrease an organism’s chance of survival, while increasing its chances of mating. It is a “special case” of natural selection.
Sexual selection is often powerful enough to produce features that are harmful to the individual’s survival. Think Peacock, for example. It's colorful and extravagant tail feathers are as likely to attract predators as well as interested members of the opposite sex.

Neutral Evolution
If we look around it might appear that everywhere we look, we will see evidence of natural selection - where organisms seem to make do pretty well to their environments; they have adapted successfully. But the neutral theory of molecular evolution suggests that most of the genetic variation in populations is the result of mutation and genetic drift and not selection.

Speciation
A species is a group of individuals that actually or potentially interbreed in nature. So a species would be the biggest gene pool possible under natural conditions.
Now speciation is a lineage-splitting event that produces two or more separate species. Imagine that you are looking at a tip of the tree of life that constitutes a species of beetle. Move down the phylogeny to where your beetle twig is connected to the rest of the tree. That branching point, and every other branching point on the tree is called a speciation event. And it is at that point where genetic changes have resulted in two separate beetle lineages. Previously there had just been one lineage.