Download Chapter 15 Section 3

Chapter 15
Section 3
The nuts and bolts of change
2 Sources of Variation for Evolution
Genetic variation / shuffling due to
sexual reproduction using gametes
produced by meiosis
 Remember, meiosis concepts of crossing over,
segregation, and independent assortment.
Mutation of a gene in a gamete
 1 in a million rate
Mechanisms of Evolution
(change over time)
Not all changes in a population are due
to natural selection.
 Genetic Drift: changes due to chance
 Founder effect: small group separates and then becomes isolated
 Bottleneck: a very few members of a species survive and then numbers rebound
with a limited gene pool
 Gene Flow: movement of genes by immigration and emigration
 Non-random mating: inbreeding
 Natural Selection: those organisms more fit for an environment
will survive and pass on genes.
Genetic Drift.
 In this small population of frogs, if the three brown frogs die
by chance (that is, not due to predation or due to being unfit),
then this species of frog evolves to be only a shade of green.
Founder Effect
 The best example would be an isolated island. Lets say you have a population of
birds that either have green feathers, red feathers, or blue feathers. In the main
population lets say that the genetic variation is evenly distributed as 1/3 green
feathers, 1/3 blue feathers, and 1/3 red feathers. However, now lets say that only
5 of those birds fly off to that new and isolated island. These 5 birds are the
"founders" of the new population.
 Lets say out of those 5 birds, 4 of them were green birds and only 1 of them was a
blue bird. Now the genetic diversity in this new population of birds on this isolated
island is 80% green feathers, 20% blue feathers, and 0% red feathers. After this
new population of birds begins reproducing, you may discover that their genetic
variance results in 95% green birds and only 5% blue birds with 0 red birds.
For example, the Afrikaner population of Dutch
settlers in South Africa is descended mainly
from a few colonists. Today, the Afrikaner
population has an unusually high frequency of
the gene that causes Huntington’s disease,
because those original Dutch colonists just
happened to carry that gene with unusually high
frequency. This effect is easy to recognize in
genetic diseases, but of course, the frequencies
of all sorts of genes are affected by founder
events.
An example of a bottleneck:
 Northern elephant seals have
reduced genetic variation probably
because of a population bottleneck
humans inflicted on them in the
1890s. Hunting reduced their
population size to as few as 20
individuals at the end of the 19th
century. Their population has since
rebounded to over 30,000—but their
genes still carry the marks of this
bottleneck: they have much less
genetic variation than a population
of southern elephant seals that was
not so intensely hunted.
The Main Types of Natural Selection
 Stabilizing Selection – becoming more
average
 Directional Selection – changing to one
different type
 Disruptive Selection – changing to two
distinctly different types
 Sexual Selection – changes due to
attractiveness
Natural Selection
 Acts to select
the individuals
that are best
adapted for
survival and
reproduction
 Stabilizing selection operates to eliminate
extreme expressions of a trait when the
average expression leads to higher fitness.
 Directional selection makes an organism
more fit.
 Disruptive selection is a process that splits
a population into two groups.
Which type of selection is shown
here? Use one photo only (a or b)
 Sexual selection operates in populations
where males and females differ
significantly in appearance.
 Qualities of sexual attractiveness
appear to be the opposite of qualities
that might enhance survival.
In order for change to occur and
a new species to form, species
must become isolated from each
other.
2 Main Types of isolation
Prezygotic
(before baby)
Geographical
Mechanical
Behavioral
Temporal
(timing)
Postzygotic
(after baby)
 Prezygotic isolation
prevents reproduction
by making fertilization
unlikely.
 Prevents genotypes
from entering a
population’s gene pool
through geographic,
mechanical, behavioral,
or other differences
Eastern meadowlark and Western meadowlark
 Mechanical
http://www.youtube.com/watch?v=rjYPJUBcoN8
 Geographic
 Behavioral
http://www.youtube.com/watch?v=cTGnQZ-1CDw
 Temporal
 Postzygotic isolation occurs when fertilization
has occurred but
a hybrid offspring
cannot develop
or reproduce.
 Prevents
offspring survival
or reproduction
Liger
Once a population is isolated, it
can undergo changes leading to a
new species: SPECIATION
 COPY THE FLOW CHART FROM THE BOARD HERE
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.
Adaptive Radiation (Divergent evolution)
 Can occur in a relatively short time when one species
gives rise to
many different
species in
response to the
creation of new
habitat or some
other ecological
opportunity
 Follows large-scale extinction events
Convergent
Evolution
 Unrelated species
evolve similar traits
even though they
live in different
parts of the world.
Coevolution
 The relationship between two species might
be so close that the evolution of one
species affects the evolution of the other
species.
 Mutualism
 Coevolutionary arms race
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.