Download Darwin Presents His Case

Darwin Presents
His Case
• Darwin waited 20 years to publish his ideas
• 1858 Darwin read Alfred Russel Wallace’s
essay, Wallace’s thoughts were almost
identical to Darwin’s
• 1859 published On the Origin of Species
The Struggle For Existence
• More individuals are produced than can
survive
• Members of a populations must compete to
obtain
– Food
Struggle for
– living space
existence
– other necessities of life
Who wins?
Variation and Adaptation
• Individuals have natural variations among
their traits
• Darwin hypothesized that some of those
variants are better suited to life in their
environment than others.
– Predators that are faster or have longer claws or
sharper teeth can catch more prey
– Prey that are faster or better camouflaged can
avoid being caught
Adaptation
• Adaptation = any heritable characteristic that
increases an organism’s ability to survive and
reproduce in its environment
– Body structures: tiger’s claw
– Colors: camouflage or mimicry
– Physiological functions: photosynthesis
– Behavior: avoidance
Mimicry
Camouflage: scorpion fish
Defensive behavior
Survival of the Fittest
• Fitness = describes how well an organism can
survive and reproduce in its environment
High Fitness
Low Fitness
Adaptations well
Characteristics not
suited to environment well suited to
environment
Survive and reproduce Die without
reproducing or
produce few offspring
Survival of the Fittest
• Survival is not just staying alive
• In evolutionary terms, survival means
reproducing and passing adaptations on to the
next generation
Natural Selection
• Darwin named his mechanism for evolution
natural selection because of its similarities to
artificial selection
• Natural selection = the process by which
organisms with variations most suited to their
local environment survive and leave more
offspring
Natural Selection
• Both natural and artificial selection: only
certain individuals in a population reproduce
• Natural selection: the environment, not the
farmer, influences fitness
Natural Selection
• Natural selection occurs in any situation in
which more individuals are born than can
survive (the struggle for existence), there is
natural heritable variation (variation and
adaptation) and there is variable fitness
among individuals (survival of the fittest)
Natural Selection
• Well adapted individuals survive and
reproduce
• From generation to generation, populations
continue to change as they become better
adapted, or as their environment changes
• Natural selection acts on inherited traits- only
characteristics that parents can pass on to
offspring
Natural Selection
• Natural selection does not make organisms
“better”
• Adaptations don’t have to be perfect- just
good enough to enable an organism to pass it
genes to the next generation
• Does not move in a fixed direction- no one
perfect way of doing something
Natural Selection
• Process that enables species to survive and
reproduce in a local environment
• If local environmental conditions change,
some traits that were once adaptive, may no
longer be useful, and different traits may
become adaptive
• If environmental conditions change faster
than a species can adapt, the species may
become extinct
Dark moths on light colored bark are
easy targets for hungry birds but are
hidden on pollution darkened trees.
Common Descent
• Natural selection depends on the ability of
organisms to reproduce and leave
descendants
• Every organism alive today is descended from
parents who survived and reproduced. Those
parents descended from their parents and so
on…
Common Descent
• Well adapted species survive over time
• Darwin proposed that over many generations,
adaptations could cause successful species to
evolve into new species
• Descent with modification = living species are
descended with modifications from common
ancestors
• Implies that life has been on Earth for a very
long time