Evolution__3rd_Edition

4 PART 1 / Introduction
Evolution is a big theory in biology
Evolution can be defined ...
1.1 Evolution means change in living things by descent
with modification
Evolutionary biology is a large science, and is growing larger. A list of its various subject
areas could sound rather daunting. Evolutionary biologists now carry out research in
some sciences, like molecular genetics, that are young and move rapidly, and in others
like morphology and embryology, that have accumulated their discoveries at a more
stately speed over a much longer period. Evolutionary biologists work with materials
as diverse as naked chemicals in test tubes, animal behavior in the jungle, and fossils
collected from barren and inhospitable rocks.
However, a beautifully simple and easily understood idea a evolution by natural
selection a can be scientifically tested in all these fields. It is one of the most powerful
ideas in all areas of science, and is the only theory that can seriously claim to unify biology.
It can give meaning to facts from the invisible world in a drop of rain water, or
from the many colored delights of a botanic garden, to thundering herds of big game.
The theory is also used to understand such topics as the geochemistry of life’s origins
and the gaseous proportions of the modern atmosphere. As Theodosius Dobzhansky,
one of the twentieth century’s most eminent evolutionary biologists, remarked in an
often quoted but scarcely exaggerated phrase, “nothing in biology makes sense except
in the light of evolution” (Dobzhansky 1973).
Evolution means change, change in the form and behavior of organisms between
generations. The forms of organisms, at all levels from DNA sequences to macroscopic
morphology and social behavior, can be modified from those of their ancestors during
evolution. However, not all kinds of biological change are included in the definition
(Figure 1.1). Developmental change within the life of an organism is not evolution in
the strict sense, and the definition referred to evolution as a “change between generations”
in order to exclude developmental change. A change in the composition of an
ecosystem, which is made up of a number of species, would also not normally be
counted as evolution. Imagine, for example, an ecosystem containing 10 species. At
time 1, the individuals of all 10 species are, on average, small in body size; the average
member of the ecosystem is therefore “small.” Several generations later, the ecosystem
may still contain 10 species, but only five of the original small species remain; the other
five have gone extinct and have been replaced by five species with large-sized individuals,
that have immigrated from elsewhere. The average size of an individual (or
species) in the ecosystem has changed, even though there has been no evolutionary
change within any one species.
Most of the processes described in this book concern change between generations
within a population of a species, and it is this kind of change we shall call evolution.
When the members of a population breed and produce the next generation, we can
imagine a lineage of populations, made up of a series of populations through time.
Each population is ancestral to the descendant population in the next generation: a
lineage is an “ancestor–descendant” series of populations. Evolution is then change
between generations within a population lineage. Darwin defined evolution as
“descent with modification,” and the word “descent” refers to the way evolutionary
modification takes place in a series of populations that are descended from one
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