Download Chapter 15.3

Chapter 15.3
 Biologist
now know that natural selection
is not the only mechanism of evolution
 Inheritable
variation may come from any
of the following:
 1.
Genetic drift
 2. Gene flow
 3. Mutation
 4. Non-random mating
 5. Natural selection
Genetic Drift: when the same
genes are combined in different ways
to produce different results
 Due to chance with independent
assortment
 In
smaller populations, the effects of
genetic drift become more pronounced,
and the chance of losing an allele
becomes greater.
Founder Effect
 Occurs when a small sample of a
population settles in a location
separated from the rest of the
population
 Alleles that were uncommon in the
original population might be common in
the new population.
 Ex.
Blue People of the Appalachian
Mountains
Bottleneck
 Occurs when a population declines to
a very low number and then rebounds
 The
rebound group has traits that are
most similar to the smallest group
Gene Flow:
 Increases genetic variation as individuals
move in and out of a population
Mutations: Random changes in the
genetic code
Nonrandom Mating
Promotes inbreeding and could lead to a
change in allelic proportions favoring
individuals that are homozygous for
particular traits
Natural Selection
 Acts to select
the individuals
that are best
adapted for
survival and
reproduction
 Stabilizing selection eliminates extreme
expressions of a trait when the average
expression leads to higher fitness.
 Ex.
Babies who have too low birth weights
are less likely to survive
 Directional selection when an extreme
trait makes organisms more fit it
becomes more common.
 Ex.
Body color of the peppered moth
found in England
 Originally,
brown
most were white, now most are
 Why?
 Disruptive selection a process that splits
a population into two groups.
 Tends
to remove individuals with average
traits but retains the extreme traits to
better suit them for their environment
 Ex.
Same snake species found in two
areas
Speciation
Speciation: The development of new
species through evolution
A population must diverge and then be
reproductively isolated
 Two
Types of Speciation:
 1. Allopatric speciation
 2. Sympatric speciation
Allopatric Speciation
 A physical barrier divides one
population into two or more
populations.
Abert squirrel
Kaibab
squirrel
Sympatric Speciation
 A species evolves into a new species
without a physical barrier.
 The ancestor species and the new
species live side by side during the
speciation process but cannot
reproduce.
Patterns of Evolution
 Speciation
takes a very long time
(compared to a human life) but there is
evidence that it occurs:
 Adaptive
Radiation
 Coevolution
Adaptive Radiation (divergent evolution)
 Can occur in a relatively short time when
one species
gives rise to
many different
species
 Follows large-scale extinction events
Coevolution
 The relationship between two species
might be so close that the evolution of
one species affects the evolution of the
other species.
 Mutualism
Convergent
Evolution
 Unrelated species
evolve similar
traits even
though they live
in different parts
of the world.
Rate of Speciation
 Evolution proceeds in small, gradual
steps according to a theory called
gradualism.
 Punctuated equilibrium explains rapid
spurts of genetic change causing
species to diverge quickly.