Biology Reference
In-Depth Information
TWO
Naturalist
“
DESCENT WITH MODIFICATION,
” as Charles Darwin succinctly characterized evolution, encap-
sulates two indisputable facts. First, all organisms share ancestors from whom they have
descended. My beloved Riley comes from a long line of Labrador retrievers, but his herit-
age extends back more than ten thousand years to Eurasian gray wolves. My father's lin-
eage traces to Nathaniel Greene, a Revolutionary War general, and the most recent fore-
bears I have in common with Jesús Sigala, my Mexican Ph.D. student, lived in Europe,
many centuries ago. The second fact is that traits like Riley's yellow coat color and the
blue-gray hue of my eyes arose as mutations in individuals whose parents lacked them,
and those changes were inherited genetically by their offspring.
Darwin's clever phrase thus accounts for similarities (inherited from parents) as well
as differences (new traits in offspring) among individuals, and its implication, that strik-
ingly dissimilar organisms share a family tree, extends to all of life. Twisted-tooth nar-
whals, brightly splotched orcas, and their Eocene kin with small but obvious legs look
more like each other than they resemble sharks, reflecting joint heritage as cetaceans
rather than cartilaginous fishes. White bats making tents from leaves, western pipis-
trelles sheltering in crevices, and exquisitely preserved, sixty-million-year-old bat-winged
fossils resemble each other more than birds, just as expected if chiropterans diversified
from a single furry ancestor rather than a feathered reptile. And mammals as different
as whales, bats, and people, whose hairy collective progenitor nursed young more than
a hundred million years ago, share more similarities with each other than with sharks or
birds.
“Descent with modification” doesn't in itself explain a third observation, that organ-
isms seem designed for particular lifestyles. It doesn't tell us why limbs changed into
wings in one lineage and flippers in another, or how chameleons, woodpeckers, and
anteaters independently acquired astonishingly long tongues. In 1858, however, Darwin
and another Englishman, Alfred Russel Wallace, rocked the literate world by proposing
that natural selection drives evolution. Unchecked breeding, they observed, would lead
to overpopulation except that many die young; moreover, individuals with particular col-
ors, tooth shapes, and so forth are better suited than others for local conditions. As a res-
ult, those individuals are “selected” in the sense of contributing more offspring to future
generations, and populations diverge as environments change over time. Overwhelming
