Phys Pop Genetics

8/12/2019 Phys Pop Genetics

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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 theirhigher 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 ata 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. Mutation

B. Gene Flow

C. Nonrandom (Assortative) Mating

D. Genetic Drift

E. Selection



II A. Mutation

1. Mutation is a molecular alteration in geneticmaterial:

2.

For a mutation to have evolutionary significance itmust occur in a gamete (sex cell).

3. Such mutations will be carried on one of theindividual's chromosomes.

4. During meiosis the chromosome carrying themutation will assort giving a 50 percent chance ofpassing the allele to an offspring.



II A. Mutation, continued

5. Mutation is the only way new variations can beproduced.

6. Since mutation occurs so infrequently at anyparticular locus, it would rarely have an effect onallele frequencies.

7. Most mutations are "hidden" as recessive alleles.

example: About 1 in in 12,000 babies carry thehomozygous form of the allele for PKU(phenylketonuria). Without early detection and treatment,this disorder will lead to mental retardation.



II B. Gene Flow

1. Gene flow is the exchange of alleles

between populations. Also called migration.

2. Long-term patterns of mate selection cancause significant changes in gene frequency.

e.g. African Americans are largely of West

African descent, but there has beenconsiderable influx of alleles from non-African

populations.



II C. Assortative Mating

1. Purposeful selection of mates based on the

presence or absence of certain traits in potential

mating partners.

2. Also called non-random mating or sexual selection

3. Undesirable traits will diminish over time as the

organisms having those traits are less

reproductively successful.4. One example of this in history was choice of antler

rack size among Red Deer in the British isles.



II D. Genetic Drift

1. Random fluctuation in allele frequencies

2. Can change in either direction.

3. Associated with small populations4. 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.



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 familynames, 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 thedisease and nine are carriers at last count.

Tristan de Cunha: Example of Founder Effect



Northern Elephant Seal: Example of Bottleneck

Hunted down to 20

individuals in 1890’s

Population hasrecovered to over

30,000

No genetic diversityat 20 loci.

Concern about

resistance to pathogens



II E. Natural Selection

1. Differential reproductive success in a populationdue to heritable variants among the population

2. Malaria is found in areas where the sickle-cellallele is found and it was discovered thatheterozygous carriers of sickle-cell anemia aremore resistant to malarial infection than thehomozygous "normals".

3. Natural selection did not cause the sickle cell alleleto 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 microevolution4. As a species changes over time,

macroevolution (speciation) may occur



B. Physical anthropologists a formula called theHardy Weinberg Equilibrium to determinewhether 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 whichactual counts are impractical

2. Acting as a model against which to test assumptionsabout the evolutionary status of a known population.

D. The formula is: p2+2pq+q2=1.0

III. Evolution in Population Genetics



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. Ability to produce enzyme lactase throughout life

2. Found in populations with a pastoral heritage

2. Vasoconstriction and Vasodilation

1. Ability to control restriction of blood vessels

2. Found in populations in extreme environments



I B. Body Proportions

1. Bergmann’s Rule

Climate controls body mass tosurface 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. All Modern Humans Belong to the Same

polytypic species, Homo sapiens.

B. A polytypic species consists of localpopulations 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. Since the 1600s race has been used to

refer to various culturally defined groups.

B. While race is usually used as a biologicalterm, it is also one with enormous social

significance.

C. In the 1950's the use of the term "race"was replaced with the term "ethnicity "



V. Race and Intelligence

A. Both genetic and environmental factors contributeto intelligence.

B. Many psychologists say that IQ scores measurelife experience.

C. Innate differences in abilities reflect individualvariation within populations, not inherentdifferences between groups.

D. There is no convincing evidence that populationsvary with regard to cognitive abilities.



VI. Adaptive Significance of Variation

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. Long-term (i.e. genetic) evolutionary changes

characterize all individuals within a population or species.

2. Short-term physiological response to environmental

change is called accl imatizat ion ; such physiological

change is temporary.

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