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Evolution by natural selection
How do species change over
Charles Darwin
• Born in England in 1809
• Took a 5-year voyage on the HMS Beagle
in 1831
• Observed that species vary:
– Globally
– Locally
– Over time
• Based on his observations, Darwin
developed the theory of biological
– Explains how modern organisms evolved over
LONG periods of time through descent from
modern ancestors
Ideas that influenced Darwin
• Hutton- 1785
– Geological change and “deep time”
• Lyell- 1830
– Laws of nature are constant over time and scientists
must explain past events in terms of processes they
observe in the present
• Lamarck- 1809
– Organisms could change during their lifetime by
selectively using or not using parts of their body
– Acquired traits could be passed on to offspring,
leading to change in species over time
Lamarck’s Evolutionary Hypothesis
More Ideas that Influenced Darwin
• Malthus- 1798
– Population growth of humans could lead to a
lack of living space and food for some people
– Provided a possible mechanism for Darwin’s
• Artificial Selection
– Nature provides variation
– Humans select for those they find useful
– Used in agriculture and with live stock
Evolution by Natural Selection
• Conditions under which natural selection
could occur:
– Struggle for existence: more individuals are
born than can survive in the environment
– Variation and adaptation: there is variation
among individuals and some variations are
more beneficial than others (adaptations)
– Survival of the fittest: Differences in
adaptations affect how well an organism can
survive and reproduce (fitness)
Adaptations and Fitness
• Adaptations can
involve body parts
or behavior
• Adaptations that
help an organism
survive AND
increase its fitness
How does evolution occur?
• Mechanism for evolution= natural
• Natural selection: process by which
organisms with variations most suited to
their local environment are more likely to
survive and leave more offspring
• Over time, populations change as they
become better adapted to their