BIOLOGY
Lesson 18.3: The Process of Speciation
Guided Notes
OBJECTIVES
1. Explain what types of __________________ lead to the formation of new species.
2. Describe a current hypothesis about Galapagos finch ______________________.
ISOLATING MECHANISMS
➢ Speciation: origin of a ______________ species
➢ What would happen if some members of a population were to ____________ breeding
with other members? If that occurred, the species gene pool could split into two
populations.
○ Over time, if members of those two populations stopped interbreeding
_____________________, changes in one gene pool could not spread to the
other— this is ______________________________________.
■ If reproductive isolation lasts long enough, the populations may split into
_____________ separate species.
➢ Reproductive isolation can develop in several ways— ___________________________l
isolation, ________________________ isolation, and ___________________________
isolation.
○ Behavioral isolation can occur if two populations that were once able to
interbreed evolve differences in _______________________________________
or other behaviors.
■ Example: Eastern and western meadowlarks are similar birds whose
habitats overlap, but the two species won’t mate partly because they use
different _________________________________________.
○ Geographic Isolation occurs when two populations are separated by
______________________________________________, such as rivers,
mountains, or bodies of water.
■ Example: The Galapagos Islands are far enough from the mainland of
South America that small birds like finches very rarely make the
_______________.
○ Temporal Isolation happens when two or more species reproduce at
__________________ times.
■ Example: If three similar species of orchids live in the same rainforest, but
bloom at different ___________ of the ____________, they can’t
pollinate one another.
SEPARATION IN DARWIN’S FINCHES
➢ How might the _______________________________________________ and
_______________________________________________ have led to reproductive
isolation and speciation among Galapagos finches?
➢ Recall: Peter & Rosemary Grant documented that the traits of finch populations originally
formed a ____________________________ distribution curve typical of polygenic traits.
After a _______________________, finches with ____________________________
were able to survive and reproduce more due to their ability to crack thicker seeds. Over
many generations, the proportion of large-beak finches __________________________.
➢ Founders Arrive
○ Many years ago, a few finches from South America (we’ll call them species “M”)
arrived on one of the Galapagos Islands. Why? — They may have gotten lost or
been blown off course by a storm.
■ Once on the island, they survived and reproduced.
○ Due to ____________________________, allele frequencies of this founding
finch population could have ______________________ from allele frequencies
in the South American population.
➢ Geographic isolation
○ The ____________________ in the Galapagos was different than mainland
South America.
○ The combination of the founder effect, geographic isolation, and natural selection
resulted in differential reproductive success, adaptation, and evolution of this
island population into a ________________________________.
○ Later, a few birds from species A ______________________________ to
another island. Now the finch populations on the two islands are
_____________________________________ and no longer share a common
gene pool.
➢ Changes in gene pools
○ Over time populations on each island further __________________ to local
environments.
○ Plants on the first island may have produced ____________________________
seeds, while plants on the second island may have produced _______________
______________________________________ seeds.
■ Birds with ___________________ beaks would be better able to survive
on the ______________________________ island and over time, natural
selection would have caused that population to _____________________
larger beaks, forming a new population _____ with a new phenotype.
➢ Behavioral isolation
○ Now, imagine that a few birds from the second island ______________________
to the first island.
■ Will population A birds breed with population B birds?
_____________________________. During courtship, these finches
closely inspect a potential partner’s __________________. Finches
prefer to mate with birds that have similar beak size.
○ ___________________________________________ could occur due to
differences in _______________________ and _______________ behavior. The
gene pools of the two bird populations _____________________________ and
the populations have now become __________ new species.
➢ Competition and Continued Evolution
○ The two species living together on the first island ________________ for seeds,
but during the _____________ season, the birds with the highest
__________________ are the ones that are the most different from each other.
More specialized birds experience ______________ competition for certain kinds
of seeds and other foods.
○ Over time, the species continue to evolve in a way that __________________
the differences between them. Birds on the first island that once belonged to
species B may evolve into a new species _______.
○ Over many generations, natural selection could have produced the diversity
among the __________ different finch species found there today.
