Page 211 - Science Course 3 (Book 1)

Mo5-L3b: Why Traits Change?

Let’s Begin Key Concept

Why Traits Change?

Evolution of Populations—Why Traits Change Why do traits change over time?
Evolution is change in inherited characteristics over
time.

Ambulocelus natans
Between 50-10 million years ago, this mammal
breathed air and walked clumsily on land. It spent a
lot time in water, but swimming was difficult because
of its rear legs, individuals born with variations that
made their rear legs smaller lived longer and
reproduced more. This mammal is an ancestor of
modern whales

A population of bacteria Nonresistant bacteria
Modern Toothed Whale can evolve antibiotic Resistant bacteria
After 10-15 millions more years of evolution, the resistance.
ancestors of modern whales could not walk on land.
They were adapted to an aquatic environment.

Modern whales have two small vestigial pelvic bones
that no longer support legs.

Evolution by natural selection is a way that
populations change over time. An antibiotic is added to a The antibiotic kills most of
population of bacteria. the nonresistant bacteria.
A few of the already have
As the environment changes, different inherited traits mutation that enable them The surviving bacteria
reproduce.
might enable survival, and the population can evolve to resist the antibiotic
again.

New resistant and nonresis- The antibiotic now affects
tant bacteria are treated with only a small percentage of
the same antibiotic bacteria. The surviving bac-
teria continue to reproduced.
Most of the bacteria are
resistant to the antibiotic
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