SC B-5.4: Explain how genetic variability and environmental
factors lead to biological evolution.
formation of a new species
Species: group of similar organisms that
can successfully breed & their offspring are
 Individuals of same species share a common gene
 genetic change in 1 member of species can spread
thru the population
▪ If that change increases fitness  many members of
the species
When a subset of a population becomes
isolated & no longer breeds with original
population   over time they will no
longer be able to interbreed …..new species
has formed
Occurs when 2 populations are capable of
interbreeding BUT have differences in
courtship rituals or reproductive strategies
that involves behavior
Eastern & Western
meadowlarks have
overlapping ranges.
 No interbreeding
because they have
different mating
2 populations physically separated by
geographic barriers
 May separate some species (non-flying insects,
small mammals) but not others (birds, flying
Does not guarantee formation of new species
2 or more species reproduce at different times
Darwin’s Finches:
 Once Darwin realized
all the birds on the
Galapagos were finches
he hypothesized that
they all had a common
finch ancestor
His hypothesis relied on 2 testable
1. Must have enough heritable variation to
provide raw material for natural selection
2. These variations must produce differences
in fitness
Variation studied by Peter & Rosemary
 Found lots of variation in a # of traits
Was there differences in their fitness?
Galapagos finches:
 Small birds
 Do not usually fly over open water
Founder Effect
a few finches from South America made it
to one of the Galapagos islands
once there, they managed to survive &
2. Geographic Isolation
once there, birds do not move from island to
3. Changes in the Gene Pool
over time populations on different islands
natural selection forces acted on the
different populations…which had slightly
different variations among the
population…leading to development of
different adaptations suited to the available
food sources on each island
3. changes in the Gene Pool
 Over time natural selection would have caused
birds on islands with large seeds or nuts as food
source to have larger and larger beaks… birds on
islands with plentiful small seeds would still
have small beaks
4. Reproductive Isolation
if finches return to original island after
natural selection has changed their beak size
they are not likely to interbreed with
original population of finches
the finch is very particular about beak
size when choosing a mate
 So population with large beak will live in
same community as small beaked finches
but never interbreed
5. Ecological Competition
different populations of finch on same
island will be competing for resources
during dry season when few seeds
available those with more specialized beaks
will have less competition from others so will
be more successful ….over time differences
between populations will increase
6. Continued Evolution
 This process of geographic
isolation..reproductive isolation…genetic
change would have been repeated many
times over the years on the Galapagos
Modern technology has confirmed Darwin’s
hypothesis that living species descended with
modification from common ancestors that
lived in ancient past
 DNA, RNA, proteins
There are “holes” in the evolutionary tree
 intermediate fossils helpful
Because it is still going on
 Drug-resistance in bacteria
Understanding it helps respond to changes
in ways that hopefully improve human life