Scientists don't draw much of a distinction between macroevolution and microevolution because macroevolution is ultimately produced by microevolution — lots and lots of microevolution over long periods of time. Macroevolution is essentially microevolution. The basic mechanisms of macroevolution are also the basic mechanisms of microevolution. There is a difference, but it's primarily a difference of degree (the amount of evolution and the amount of time), not a difference of kind or essence.
Collected here are explanations of macroevolution producing microevolution in scientific works: basic biology texts found in high school or college biology classes and scientific reference works. This is how scientists explain microevolution producing macroevolution when teaching science to students and when explaining the underlying science to other scientists.
Microevolution & Macroevolution in Biology Texts
Because there is so much misunderstanding about what evolution is and how evolution works, it's critical for any school text on biology or evolution to make it absolutely clear that microevolution and macroevolution are not fundamentally different processes — that microevolution does produce macroevolution given enough time. Below are some examples of how some standard biology texts explain the relationship between macroevolution and microevolution.
Changes in gene frequency that occur within a population without producing a new species are called microevolution. As microevolution continues, a population may become so different that it is no longer able to reproduce with members of other populations. At that point, the population becomes a new species. As the new species continues to evolve, biologists might eventually consider it to be a new genus, order, family, or higher level of classification. Such evolution at the level of species or higher is called macroevolution.
Microevolution can occur very quickly; indeed, it is probably always occurring. For example, in less than half a century after the discovery of antibiotics, many bacteria evolved resistance to them. Resistance to antibiotics evolves when antibiotics are used improperly, allowing the survival of a few bacteria with mutated genes that confer resistance. Natural selection then leads to the evolution of antibiotic-resistant strains. Pesticide-resistant insects and herbicide-resistant weeds are additional examples of rapid microevolution.
Macroevolution occurs over much longer periods and is seldom observed within the human life span. Occasionally, however, scientists do see evidence that new species have recently evolved. There are species of parasitic insects, for example, that are unable to reproduce except in domesticated plants that did not even exist a few centuries ago. The pace of evolution can be quite variable, with long periods in which there is little change being punctuated by relatively brief periods of tens of thousands of years in which most changes occur.Biology, Richard Robinson ed.
The origin of new species, or speciation, is at the focal point of evolutionary theory because the appearance of new species is the source of biological diversity. It is not enough to explain how adaptations evolve in a population (...such changes, confined to a single gene pool, are described as microevolution). Evolutionary theory must also explain how new species originate and develop through the subdivision and subsequent divergence of gene pools.
The fossil record reveals the cumulative effects of speciation over vast tracts of time. The term macroevolution refers to such evolutionary change above the species level for example, the appearance of feathers during the evolution of birds from one group of dinosaurs, and other such "evolutionary novelties" that can be used to define higher taxa. ...
Speciation can result from differences as seemingly small as the color on a cichlid's back. However, as species diverge and speciate again and again, these differences can accumulate and become more pronounced. Thus, speciation constitutes the beginning of macroevolutionary change. Macroevolutionary transformations, like the microevo!ulionary changes that take place within a single gene pool, accumulate through the processes that we examined in Chapter 23 — natural selection, mutation, genetic drift, and gene flow. It is the cumulative change during thousands of small speciation episodes that accounts for sweeping evolutionary changes.Biology, 7th ed., Neil A. Campbell & Jane B. Reece [emphasis added]
Darwin's critics raise seven principal objections to teaching evolution: ...
6. Natural selection does not imply evolution.
"No scientist has come up with an experiment where fish evolve into frogs and leap away from predators."
Is microevolution (evolution within a species) the mechanism that has produced macroevolution (evolution among species)? Most biologists that have studied the problem think so. Some kinds of animals produced by artificial selection are remarkably distinctive, such as Chihuahuas, dachshunds, and greyhounds.
While all dogs are in fact the same species and can interbreed, laboratory selection experiments easily create forms that cannot interbreed and thus would in nature be considered different species. Thus, production of radically different forms has indeed been observed, repeatedly. To object that evolution still does not explain really major differences, like between fish and amphibians, simply takes us back to point 2—these changes take millions of years, and are seen clearly in the fossil record.Biology, 6th ed., Raven Johnson
I'm not sure which is the best tactic: to address misunderstandings about evolution directly and explain why they are wrong, or to embed these explanations within the overall text without specifying in so many words what myths it refutes.
The former might sound better because it draws students' attention to it, but that has to be done carefully because studies have shown that attempts to refute myths which repeat the myth itself actually reinforce acceptance of the myth in the long run.
