Whale Hunting ■ 207a number of genes relatively late in embryonic
development (Figure 11.16). The loss of hind
limbs in the embryo corresponds to the disap-
pearance of hind limbs in the fossil record. “It
was an awesome way to combine embryology
with fossils,” says Thewissen. In 2015, research-
ers at the Smithsonian Institution likewise
found that the development of fetal ear bones
in the womb paralleled changes observed in the
whale fossil record.
With their discovery of Indohyus, Thewissen,
Cooper, and their team bridged a 10-million-year
gap in the fossil record, identifying an important
transition species to whales. It is another rock
in the mountain of evidence surrounding whale
evolution, which was further fortified in April
2015 with the discovery of fossils from a new
species of extinct pygmy sperm whale found in
Panama.
Evolution is supported by mutually rein-
forcing, independent lines of evidence: direct
observation, fossils, shared characteristics
among living organisms, similarities and
divergences in DNA, biogeographic evidence,
and common patterns of embryo development.
Just as the theor y of gravity forms the founda-
tion of physics, so evolution is the central tenet
of biology.
There is no question that evolution happens.
The intriguing, fascinating question is how
does it happen? Through scientif ic research,
we know that whales descended from a group
Forelimb Hind limb Dolphins maintain their front limb
buds, which become flippers.Dolphins’ hind limb
buds do not develop
into limbs or flippers.Figure 11.16
Dolphin embryonic development
The embryos of dolphins from weeks 4–9 of
development show the formation and then
subsequent loss of the hind limb buds. (Source:
Based on a photo by Thewissen Lab, NEOMED,
permission granted.)of land-living mammals including the petite
Indohyus, munching on freshwater plants
and splashing through the shallows. But
how did a population of small, furr y animals
become the mammoths of the sea? Next we
investigate the mechanisms of evolution—
how and why it works.● (^) Biological evolution is the change in characteristics of
a population of organisms over inherited generations.
● (^) Artificial selection results in biological evolution.
Humans choose which organisms survive and
reproduce—a process known as selective breeding.
● (^) Natural selection is the process by which individuals
with advantageous genetic characteristics for a
particular environment survive and reproduce at a
higher rate than competing individuals with other,
less useful characteristics.
● (^) Adaptation is an evolutionary process by which
a population becomes better matched to its
environment over time. An adaptation is also a trait—
an adaptive trait—that has evolved as a result of this
process.
REVIEWING THE SCIENCE
● (^) Fossils are the preserved remains (or their
impressions) of formerly living organisms. The fossil
record enables biologists to reconstruct the history
of life on Earth, and it provides some of the strongest
evidence that species have evolved over time.
● (^) Many similarities among organisms are due to the
fact that the organisms evolved via common descent
from a common ancestor. When one species splits
into two, the two resulting species share similar
features, called homologous traits. If a homologous
trait is no longer useful, it is called a vestigial trait.
● (^) The fact that organisms as different as bacteria,
redwood trees, and humans show DNA sequence
similarity is evidence that the great diversity of living
things evolved from a common ancestor.