Transcript Section 16-3 - sandsbiochem

Section 16-3
The Process of
Speciation
How does speciation occur?
• A genetic change in one individual
can spread through the population
as reproduction occurs
•If the change increases fitness it
will increase in the population
How does speciation occur?
• For new species to form, gene pools
must be separated
• As new species evolve, reproductive
isolation occurs
Reproductive isolation
can develop through:
• Behavioral isolation
• Geographic isolation
• Temporal isolation
Behavioral Isolation
Differences in reproductive behaviors
prevent interbreeding
Example: Birds that
use different songs
to attract mates won't
interbreed
Geographic Isolation
Populations are separated by geographical
barriers
– If barrier is removed (ex. lakes connect during
a flood) and populations can still interbreed,
it's still one species
– Barriers may not separate all organisms (a
river doesn't separate bird populations)
Temporal Isolation
Species reproduce at different
times
Example: Plants that
flower at different
times
Darwin's Finches
Darwin's Hypothesis: Natural selection
shaped the beaks of different finch
populations
Peter and Rosemary Grant
The Grants researched
two testable
assumptions that
Darwin's theory was
based on
Testable Assumptions:
• Enough heritable variation in
beak size and shape for natural
selection to act upon
• Different beaks cause different
fitnesses (allowing natural
selection to occur)
Variation
• They measured and banded individual
birds on one of the Galapagos islands
– Data showed great variation of heritable
traits
Natural selection
• Birds with different-sized beaks had
different fitnesses during drought
– Finches with large beaks could crack
large seeds that others couldn't
– Over time the average beak size of the
population increased
Steps of speciation in Darwin's
Finches
• Founding of new populations
• Geographic isolation
• Changes in the gene pool
• Reproductive isolation
• Ecological competition
How we think it happened:
Founders Arrive
• Long ago, a few finches from the
South American mainland flew or
were blown to one of the
Galapagos islands (SPECIES A)
How we think it happened:
Geographic isolation
• Some birds from species A crossed
to another island
• The two populations were isolated
(finches do not normally fly over
open water)
How we think it happened:
Changes in gene pool
• Natural selection caused the
populations to evolve different beaks
depending on available food sources
How we think it happened:
Reproductive Isolation
• Finches mate with birds with the
same-sized beaks as they have
(behavioral isolation)
– The populations are now separate
species (species A on first island,
Species B on second island)
How we think it happened:
Ecological Competition
• If species B returns to the first island, A
and B must compete for food
• Differences increase
• Species B evolves into species C
How we think it happened:
Continued evolution
Repeated isolation on a island, genetic
change and reproductive isolation
produced 13
species of
finches
Studying Evolution Since
Darwin
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New data from genetics, physics and
biochemistry have supported evolution
New discoveries have led to new
hypotheses that refine and expand
Darwin's original theory
Many questions remain presently
unanswered