1. Mechanisms of Evolution
- Allelic frequency
- Hardy-Weinberg
- Polymorphism
- Patterns of natural selection
- Mechanisms of isolation
- Rates of evolution
Chapter 11 in text

2. Individuals do not evolve –
Populations do.
Individuals may be selected,
and contribute differentially
to the future gene pool.
Gene mutations provide the genetic variability.
Natural selection acts on the collective genes
in the gene pool of a population.
An individual has the genes he was born with
(nothing will change that),
but the frequency of a given allele in a population may change over time.
This is evolution.

3. Allelic frequency

4. NB: We are seeing the frequency of these phenotypes
in the entire population… 3:1 assumes p = 0.5, q = 0.5

5. - Hardy-Weinberg
For a gene with two alleles,
Hardy-Weinberg equation:
p2 + 2pq + q2 = 1
The sums of the genotype frequencies equals 1...
p + q = 1
The sum of the allelic frequencies = 1, too
An example…
1 of every 375 African-Americans has sickle cell anemia.
What proportion of this population has sickle cell trait?
ss homozygous = 1/375, or = q2
______
q = √ = 0.052,
sooo p = 1 – = 0.948
Ss is 2pq, = 2(0.948)(0.052) = 0.099 or
1- (p2 + q2) = 1- ( ), = 1- ( ) = 0.099,
or so, I rounded along the way.

6. to study something, it is often helpful to know what it isn’t…
Hardy-Weinberg equilibrium:
The Hardy-Weinberg equation will describe
the allelic frequency in a population
if no evolution is happening.
Assumptions of such an equilibrium:
1. Extremely large population
2. No gene flow
3. No mutations
4. Random mating
5. No natural selection
We will look at each of these in turn.

7. 1. Extremely large population
If a population is small, allelic frequency may change significantly
through random chance or sampling error.
Picture 6 birds of one species arriving on an island,
If they have the same color distribution as the parent population,
but the two lightest happen to have fewer offspring,
the subsequent population will be darker.
This is called
genetic drift.

8. 1. Extremely large population (cont.)
Let’s say these first six birds are all equally successful in breeding,
but they happened to be a darker group to begin with,
compared to the parent population.
This is called
the founder effect.
faculty.clintoncc.suny.edu/.../evolutio.htm

9. 1. Extremely large population (cont.)
Let’s say the original, diverse morphs populated the island,
but a hurricane took out all but a few,
which happened to be lighter ones…
This is called the
bottleneck effect.
These three are
interconnected
(small founder group
is more subject to drift…)

10. 2. No gene flow
This means no significant immigration or emigration…
No gene flow into or out of the population.
Naturally, adjacent
populations may experience
integration occasionally→
This could change the allelic frequencies.

11. 3. No mutations
We have discussed gene and chromosome mutations:
- These tend to occur at a predictable frequency.
- They may be advantageous (adaptive),
disabling (maladaptive),
or inconsequential (silent or neutral variation).
Without the underlying variety that mutations enable,
no evolution would be possible.

12. Darwin was one of the first to propose sexual selection,
4. Random mating
Darwin was one of the first to propose sexual selection,
because the displays otherwise seem maladaptive -
(wastes of energy and unnecessary exposure).
Intrasexual selection:
competition between members of the same gender,
like elk rutting displays.
Intersexual selection:
choosing from one of the opposite gender,
like peahens and peacocks (sexual dimorphism).
- The displays may reflect overall
health and vigor, apart from
simply the apparent foppery.
- Note: doesn’t apply to asexually
reproducing organisms…

13. 5. No natural selection
Natural selection (for an adaptive trait) is the poster child of evolution.
The key is that
those traits that best allow
individuals to
survive to adulthood
and successfully reproduce,
will be passed along to the
next generation at the
highest frequency.
positive selection
Less favorable traits will
become less frequent.
negative selection

14. 5. No natural selection (cont.)
Sometimes an advantage
seems obvious
(disease resistance),
sometimes less so
(altruism).

15. So, does Hardy-Weinberg equilibrium ever apply?
- Some organisms are preserved relatively intact
for huge amounts of time,
as evidenced in the fossil record.
- Probably many species have come and gone
too quickly
to be preserved.
If the Hardy-Weinberg equation is found to not describe a population,
it is time to look at what mechanisms of evolution are at play.

16. caused by different alleles for different genes.
- Polymorphism
Within a population there is a range of forms (morphs) for a variety of traits,
caused by different alleles for different genes.
Some traits are polygenic, and exhibit a continuum of morphology.
Why isn’t it better to have one trait, or the other?
Two (or more) alleles cooperating is sometimes better than either one alone.
e.g. HsHs → sickle-cell anemia
HHs → OK (heterozygous advantage)
HH → more susceptible to malaria

17. Transient polymorphism:
One version is favored sometimes, the other at other times,
e.g. industrial melanism and the peppered moth.
Note: this is microevolution,
not speciation, but provides a
testable example of natural
selection.

18. - Patterns of natural selection
stabilizing selection:
The central-most morph is most
successful, and extreme forms are
reduced. Results in fine-tuned,
but potentially fragile species. (Why?)
disruptive selection:
The central form is less adaptive,
and the population splits into two.
Due to competition, loss of original
resource... Easy step to speciation.
directional selection:
One extreme is favored over the other. Generally leads to speciation
only if two populations are already separated.

19. divergent evolution:
The result of divergent selection,
in which one species faces selective pressure for
two different niches:
Recognized by presence of homologous structures.
e.g. Australian apple maggot fly:
originated from the
Australian hawthorn maggot fly,
but when apples were brought in,
the new food source was optimized...
Now two distinct species exist.

20. adaptive radiation
is a specific case of divergent evolution,
in which many niches are available for exploitation
e.g. the post-Mesozoic explosion of mammals
after mass extinction of dinosaurs

21. Adaptive radiation
also happens
when organisms
colonize a new island
grouping…
This resembles the
process of the
finches in the
Galapagos islands.

22. convergent evolution:
A specific type of directional selection, in which two species
are long separated, but selected towards a common adaptation:
Often recognized by the presence of analogous structures.

23. - Mechanisms of isolation
In order for actual speciation to occur,
there has to be some sort of boundary
preventing the mixing of genes…
Reproductive isolation… self explanatory.
- prevention of successful interbreeding, ….somehow.
Two main groupings:
1. Geographic (“allopatric”) – Organisms are physically separated.
This could be across an ocean,
or a stream,
or a spit of land,
or adjacent puddles.
2. Sympatric – (“same country”)
Various mechanisms that isolate neighbors…

24. 1. Geographic isolation
Your text has an example of squirrels
and the Galapagos finches.

25. 2. Sympatric isolation
habitat isolation: different use of an ecosystem.
e.g. water vs. land garter snakes
behavioral isolation: courtship, communication
e.g. blue-footed boobies, Pacific killer whales
temporal isolation: time of day, year, cycles…
e.g. E spotted skunk (winter) vs. W (summer)
mechanical isolation: structural incompatibility…
2 monkey flowers attract different pollinators
gametic isolation: repro. tract or gamete surface
sea urchins

26. - Rates of evolution
Microevolution by selection or genetic drift
can happen in the course of a few generations.
Speciation by polyploidy happens in a single generation.
More commonly, the accumulation of adaptive mutations
can be very slow (thousands of years? tens of thousands?),
and if an organism is well adapted,
there may be little change in allelic frequency
over millions of years.
HOWEVER, after a disruption,
there is generally astonishingly rapid evolution
as newly freed or created niches are filled,
depending on existent mutations.
- Punctuated equilibrium

27. If a given dominant trait is expressed
in 450 of every 1000 individuals in a population,
what is the expected frequency of heterozygotes?
Distinguish among bottleneck effect,
founder effect, and
genetic drift.
Propose illustrations
for the three types of selection -
stabilizing, directional, and disruptive.
What is Hardy-Weinberg equilibrium?
Describe the evolutionary factors
that contributed to the speciation
of “Darwin’s” finches.
What is evolution?

28. Benchmark SC.912.L.15.13
Describe the conditions required for natural selection,
including overproduction of offspring,
inherited variation,
and the struggle to survive,
which results in differential reproductive success.

29. Life on Earth is very diverse, and even within species
Enduring Understandings
Life on Earth is very diverse,
and even within species
there is great diversity.
Essential Questions
What mechanisms have allowed
for diversity in organisms?

30. gene pool
sexual dimorphism
evolution
natural selection
Hardy-Weinberg equation
positive selection
p2 + 2pq + q2 = 1
negative selection
p + q = 1
polymorphism
Hardy-Weinberg equilibrium
heterozygous advantage
1. Extremely large population
stabilizing selection
genetic drift
disruptive selection
founder effect
directional selection
bottleneck effect
divergent evolution
2. No gene flow
adaptive radiation
3. No mutations
convergent evolution
4. Random mating
reproductive isolation
5. No natural selection
geographic isolation
sexual selection
sympatric isolation
intrasexual selection
behavioral isolation
intersexual selection
temporal isolation
speciation
punctuated equilibrium
mutation
genetic recombination
diversity
extinction
allele
gene pool
genetic drift
gene migration/gene flow
Hardy-Weinberg principle