The name "Charles Darwin" is essentially synonymous with the concept of biological evolution. Indeed, "Darwinism" and "Darwinian evolution" are common terms in scientific literature.
A contemporary of Darwin's, however, named Alfred Russel Wallace, independently arrived at many of the same conclusions as did his English compatriot, and in proposing the same basic mechanism, natural selection, he added strength to the idea. The two presented their ideas together in a conference in 1858.
Today, evolution remains the foundation on which biological science rests. The work of Gregor Mendel on the specific pathways of inheritance and advent of molecular biology, including the discovery of DNA, have broadened and deepened the field. Along the way, evolution has come to encompass two basic forms, or subtypes: microevolution and macroevolution.
These are integrated concepts that have important similarities and differences.
The theory of evolution describes how organisms change and adapt over time as a result of inherited physical and behavioral characteristics that are passed down from parent to offspring, a process dubbed "descent with modification."
All living things on Earth share a common ancestor dating back to the earliest life forms, which appeared about 3.5 billion years ago. Organisms that are more closely related, such as humans and gorillas, share more recent common ancestors; both of these species share common ancestry with other mammals, and so on up the family tree of life.
The mechanism that drives evolutionary change is natural selection. Organisms both within a species and between species that have traits that enable them to more easily survive and reproduce, such as the fastest land predators (e.g., cheetahs), are more likely to pass on their genes to offspring that are similarly "fitter." These organisms become more prevalent because their genes are naturally selected for within their environment, whereas less-fit organisms die off.
This is not a random process, but it is also not a conscious one; the chance genetic mutations in DNA that originally created the favorable traits are the material on which natural selection acts in a systematic way.
Microevolution vs. Macroevolution
Microevolution, as the name suggests, is evolutionary change on a small scale, such as evolution or selection occurring on a single gene or a few genes in a single population over a short period of time. An instance of microevolution may turn out to contribute to macroevolution, but this does not necessarily occur.
More formally, microevolution is simply a change in gene frequency within the gene pool, or the range of available genes organisms may inherit, of a given population.
Macroevolution, in contrast, is evolutionary change on a large scale that happens over a longer period of time. Examples include a species diverging into one or more different species, or the formation of brand new groups of organisms; these represent the long-term culmination of many instances of microevolution.
Similarities: "Microevolution versus macroevolution" is in many ways a false dichotomy, and it is often invoked by opponents of the theory of evolution to suggest that the former may be true while the latter is false. Both, in fact, are types of evolution.
To propose that microevolution is possible but macroevolution is not is rather like saying that one can drive from Maine to New York, and from New York to Ohio, and so on in small steps all the way to California, but that driving all the way across the United States is impossible.
Both happen through the same overall processes of natural selection, mutation, migration, genetic drift and so on. Microevolutionary changes that accumulate, sometimes but not always over long periods, can and do produce major evolutionary changes.
Differences: The main difference between microevolution and macroevolution is simply the time scales over which they occur. Microevolution happens over short periods of time, while macroevolution is more gradual, adding up many instances of microevolution over time.
Accordingly, there are differences in what is specifically affected in each case. Microevolution usually only happens on one or a few genes at a time in a small population, while macroevolution is a large-scale change of many things in larger groups, such as species diverging to create new species.
Examples of Microevolution
A vast number of examples of microevolution in animals species provide the most easily demonstrated and understood examples of the process, because they can often be directly observed.
For example, house sparrows arrived in North America in 1852. Since then, these sparrows have evolved different characteristics in different habitats in accordance with the environmental pressures the different sparrow populations face. Sparrows in more northern latitudes are larger-bodied than sparrow populations in the south.
Natural selection readily accounts for this: Larger birds can typically survive lower temperatures better than smaller-bodied counterparts, who do better to the south.
Sometimes, the time scales of microevolution are very short.
This occurs, as one would predict, in species that reproduce rapidly, such as bacteria (which can quickly evolve resistance to antibiotics as those that happen to be naturally resistant to a given antibacterial drug are selected for and continue to reproduce in large numbers) and insects (which can quickly develop pesticide resistance for the same molecular reasons).
Getting from "Micro" to "Macro": Watch and Wait
Macroevolution cannot be "seen" as handily because it happens over such a long period, allowing people who resist the theory of evolution a token foothold for their claims. Nevertheless, the evidence is very solid and rests mostly in comparative studies of the anatomical features of related organisms and, crucially, the fossil record.
Some of the many small microevolutionary changes building up over time that sum to macroevolution include insects developing a new color, pesticide resistance, larger mandibles and resistance to cold. These can all build up over time to create a macroevolutionary change in the entire species, not just in one small, localized population of that species.
The underlying causes of evolution – mutation, migration, genetic drift and natural selection – all result in macroevolution, given sufficient time. 3.5 billion years is certainly a long time, and is very hard for even astute and willing human minds to wrap itself around.
Gene drift, reproductive isolation (i.e., groups within a species tending to reproduce only with its own members) and the geographical relocation of a population are some of the factors leading to microevolutionary changes that add up over time and lead to the creation of a new species from the original species.
Examples of Macroevolution
Macroevolution, though necessarily involving small changes within the gene pool of a species, occurs above the species level rather than within it. Speciation, the term for the emergence of new species, is synonymous with macroevolution.
The emergence of mammals as a larger-than-species group and the diversification of flowering plants into many species are both examples of macroevolution. Other examples are the evolution of vertebrate fish from invertebrate marine species over long periods of time and the development of multicellular organisms from unicellular ones.
If one considers these to be instantaneous events, of course macroevolution seems intuitively implausible.
In addition to the fossil record, scientists have molecular evidence of common ancestry, implying that macroevolution is not only a way for all life on Earth to have come to its present state, but literally the only way.
For example, all organisms use DNA as their genetic material, and use glucose and adenosine triphosphate (ATP) as a nutrient and an energy source respectively in complex metabolic reactions. If individual species had more or less winked into being independently, this state of affairs would represent both a tremendous coincidence and, again literally, a waste of energy.