Agents of Evolutionary Change
The Five Forces of Evolution
and How We Measure Them
I. What is Evolution?
A. First, remember that…
Evolution is a two-stage process:
1. Production and redistribution of variation
(inherited difference between individuals).
2. Natural selection (whereby genetic
differences in some individuals lead to their
higher reproductive success.
I. What is Evolution?
B. A change in the frequency of alleles in a population
from one generation to the next.
1. An allele frequency is the percentage of all the alleles at
a specific locus accounted for by one specific allele.
2. For evolution to occur, a new allele must spread through
the population and change in frequency.
3. Still, the frequencies of all alleles ill add to 1.0 (100 %)
4. Evolution is driven by five known factors or agents.
II. Agents of Evolutionary Change
A.
B.
C.
D.
E.
Mutation
Gene Flow
Nonrandom (Assortative) Mating
Genetic Drift
Selection
II A. Mutation
1.
2.
3.
4.
Mutation is a molecular alteration in genetic
material:
For a mutation to have evolutionary significance it
must occur in a gamete (sex cell).
Such mutations will be carried on one of the
individual's chromosomes.
During meiosis the chromosome carrying the
mutation will assort giving a 50 percent chance of
passing the allele to an offspring.
II A. Mutation, continued
5. Mutation is the only way new variations can be
produced.
6. Since mutation occurs so infrequently at any
particular locus, it would rarely have an effect on
allele frequencies.
7. Most mutations are "hidden" as recessive alleles.
example: About 1 in in 12,000 babies carry the homozygous
form of the allele for PKU (phenylketonuria). Without
early detection and treatment, this disorder will lead to
mental retardation.
II B. Gene Flow
1.
2.
Gene flow is the exchange of alleles
between populations. Also called migration.
Long-term patterns of mate selection can
cause significant changes in gene frequency.
e.g. African Americans are largely of West
African descent, but there has been
considerable influx of alleles from non-African
populations.
II C. Assortative Mating
1.
2.
3.
4.
Purposeful selection of mates based on the
presence or absence of certain traits in potential
mating partners.
Also called non-random mating or sexual selection
Undesirable traits will diminish over time as the
organisms having those traits are less
reproductively successful.
One example of this in history was choice of antler
rack size among Red Deer in the British isles.
II D. Genetic Drift
1.
2.
3.
4.
Random fluctuation in allele frequencies
Can change in either direction.
Associated with small populations
Two variants.
a. Founder Effect occurs when a small migrant band of
"founders" colonizes a new area away from the parent
group.
b. Bottleneck Effect occurs when circumstances dictate
that only a select number of individuals will survive to
reproduce.
Tristan de Cunha: Example of Founder Effect
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An Island in the South Atlantic was settled in early 1800s
by one Scottish family of about 15 people.
Population was added to by shipwrecks and births
The 1987 population numbered 296 people with 7 family
names, and mtDNA studies have shown that they are
descended from five female founders.
The population has a high frequency of a hereditary eye
disorder (retinitis pigmentosa). Four islanders have the
disease and nine are carriers at last count.
Northern Elephant Seal: Example of Bottleneck
Hunted down to 20
individuals in 1890’s
Population has
recovered to over
30,000
No genetic diversity
at 20 loci.
Concern about
resistance to pathogens
II E. Natural Selection
1. Differential reproductive success in a population
due to heritable variants among the population
2. Malaria is found in areas where the sickle-cell
allele is found and it was discovered that
heterozygous carriers of sickle-cell anemia are
more resistant to malarial infection than the
homozygous "normals".
3. Natural selection did not cause the sickle cell allele
to appear; it merely acted to increase its frequency
in some populations.
III. Evolution in Population Genetics
A. Some things to remember:
1. Allele frequencies are the proportions of
alleles in a population.
2. Each population has a particular complement of
genes, a level of variability, called its gene pool.
3. A change in variability in the gene or allele pool
across generations is thus microevolution
4. As a species changes over time,
macroevolution (speciation) may occur
III. Evolution in Population Genetics
B. Physical anthropologists a formula called the
Hardy Weinberg Equilibrium to determine
whether allele frequencies in an isolated human
population are changing.
C. The Hardy Weinberg is good for two things:
1. Estimating allele frequencies for a population in which
actual counts are impractical
2. Acting as a model against which to test assumptions
about the evolutionary status of a known population.
D. The formula is: p2+2pq+q2=1.0
III. Evolution in Population Genetics
E. For the Hardy Weinberg to be true…
NONE OF THE FOLLOWING CAN OCCUR:
1. New variation (mutation).
2. Redistributed variation through gene flow
3. Redistributed variation through genetic drift
4. Select "advantageous" allele combinations that
promote reproductive success (natural selection)
5. Assortative mating
I. Types of Human Variation
Humans vary in their:
A. Physiology
B. Body proportions
C. Skin color
I A. Physiologic Variation
1.
Lactose Tolerance
1.
2.
2.
Ability to produce enzyme lactase throughout life
Found in populations with a pastoral heritage
Vasoconstriction and Vasodilation
1.
2.
Ability to control restriction of blood vessels
Found in populations in extreme environments
I B. Body Proportions
1. Bergmann’s Rule
Climate controls body mass to
surface ratio
2. Allens’ Rule
Climate controls limb lengths
I C. Skin Color
Factors affecting skin color
1. Sunlight
Vitamin D needs, cancer, and folate
2. Nutrition
Excess dietary pigment can change color
3. Environment
Short term responses (I.e. tanning)
4. Genes
What your parents pre-determined about you
II. Historical Views of Human Variation
A.
Biological determinism
cultural variations are inherited in the same
manner as biological variations
B.
Eugenics
Galton’s "race improvement" through forced
sterilization & encouraged reproduction
C.
Mono and Polygenism
One or many origins for humanity (A&E?)
III. The Concept of Race
A.
B.
All Modern Humans Belong to the Same
polytypic species, Homo sapiens.
A polytypic species consists of local
populations that differ from one another
in the expression of one or more traits.
III. The Concept of Race, cont.
C. Variation (race) is geographic in nature
D. Variation can be plotted on clines
IV. The Traditional Race Concept
A.
B.
C.
Since the 1600s race has been used to
refer to various culturally defined groups.
While race is usually used as a biological
term, it is also one with enormous social
significance.
In the 1950's the use of the term "race"
was replaced with the term "ethnicity "
V. Race and Intelligence
A.
B.
C.
D.
Both genetic and environmental factors contribute
to intelligence.
Many psychologists say that IQ scores measure
life experience.
Innate differences in abilities reflect individual
variation within populations, not inherent
differences between groups.
There is no convincing evidence that populations
vary with regard to cognitive abilities.
VI. Adaptive Significance of Variation
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Human variation is the result of past and present
adaptations to environmental conditions.
Physiological response to environmental change is
under genetic control and environmental influence
and operates at two levels:
1.
2.
Long-term (i.e. genetic) evolutionary changes
characterize all individuals within a population or species.
Short-term physiological response to environmental
change is called acclimatization; such physiological
change is temporary.