A century and a half ago, Charles Darwin was inspired to develop
a scientific explanation for these three broad observations.
When he published his hypothesis in The Origin of
Species, Darwin ushered in a scientific revolution—the era of
evolutionary biology.
For now, we will define evolution as descent with modification,
a phrase Darwin used in proposing that Earth’s many
species are descendants of ancestral species that were different
from the present-day species. Evolution can also be defined
more narrowly as a change in the genetic composition of a
population from generation to generation, as discussed further
in Chapter 23.
Whether it is defined broadly or narrowly, we can view evolution
in two related but different ways: as a pattern and as a
process. The pattern of evolutionary change is revealed by data
from a range of scientific disciplines, including biology, geology,
physics, and chemistry. These data are facts—they are observations
about the natural world. The process of evolution
consists of the mechanisms that produce the observed pattern
of change. These mechanisms represent natural causes of the
natural phenomena we observe. Indeed, the power of evolution
as a unifying theory is its ability to explain and connect
a vast array of observations about the living world.
As with all general theories in science, we continue to test
our understanding of evolution by examining whether it can
account for new observations and experimental results. In this
and the following chapters, we’ll examine how ongoing discoveries
shape what we know about the pattern and process
of evolution. To set the stage, we’ll first retrace Darwin’s quest
to explain the adaptations, unity, and diversity of what he
called life’s “endless forms most beautiful.”