Population Genetics

Genotype

Genotype

The letter representation of an allele/trait

• Tt, TT, tt

• What the genes say

Phenotype

Physical expression of an allele

• A flower being purple

Allele

A version/variant of a gene

• Long or short hair

What are the three types of genotypes?

Homozygous dominant, homozygous recessive, heterozygous

What does homozygous dominant mean?

Two of the same allele, and they are both dominant

• TT

What does homozygous recessive mean?

Two of the same allele, and they are both recessive

• tt

What does heterozygous mean?

Two different alleles, one dominant, and one recessive

• Tt

What are the two major Hardy-Weinberg Formulas?

p² + 2 pq + q² = 1      p + q = 1

What does each piece of the Hardy-Weinberg Formulas mean?

p = frequency of the A allele

q = frequency of the a allele

p² = frequency of AA in the population

q² = frequency of aa in the population

2pq = frequency of Aa in the population

What happens to the frequency of alleles over time?

The change (over generations)

How many chromosomes do humans have?

46 = 23 sets

What is the difference between diploid and haploid?

• Examples?

Diploid = 2 copies of a gene (alleles)

Haploid = 1 copy of a gene

• Diploid = somatic cells

• Haploid = reproductive cells (gametes)

How many alleles code for a gene?

Two or more

Natural Selection acts on ________ but only __________ evolve

Natural Selection acts on individuals but only populations evolve

Microevolution

A change in allele frequencies in a population over generations

What are the causes of microevolution?

• What are their opposites??

1. Natural Selection = no selection

2. Genetic Drift = large breeding population

3. Gene Flow = no immigration or emigration

4. Non-Random Mating = random mating

5. Mutations = no mutations

What causes genetic variation in individuals?

Differences in genes or other DNA segments

What is a phenotype a product of?

Inherited genotype and environmental influences

Can natural selection act on variation without a genetic component?

Nope

Point mutation

A change in one base of a gene

In what cases can a mutation be harmless/neutral?

When they are in the noncoding regions of the DNA or due to redundancy in the genetic code

When can mutations be harmful

When they change protein production → in a negative way

When can mutations be beneficial

When they change protein production → in a positive way

What is the only way mutations can be passed on?

If they are in the sex cells

What is gene duplication?

The duplication of a small piece of DNA(won’t generally do too much damage → compared to the duplication of a chromosome portion), this gene can take on new functions through further mutations

What does sexual reproduction due in terms of alleles?

Shuffles existing alleles into new combinations

• Recombination is more important than mutation for genetic differences

What type of characters contribute to variation within a population?

Discrete (having a widow’s peak or not, either you have it or not) and quantitative characters (skin color = a gradient; controlled by many genes)

How can genetic variation be measured?

Through gene or nucleotide variability

What is gene variability

The average heterozygosity measures the average percent of loci that are heterozygous in a population

What is nucleotide variability

The measurement by comparing the DNA sequences of pairs of individuals

What is chromosomal variation among populations caused by?

Genetic drift, not natural selection

Do species exhibit geographic variation?

Yes, many

What is the Hardy-Weinberg equation used to test?

Whether a population is evolving

Population

a localized group of individuals capable of interbreeding and producing fertile offspring

What does a gene pool consist of?

All the alleles for all loci in a population

When is a locus fixed?

If all individuals in a population are homozygous for the same allele

What is a locus

A location

• Where on a chromosome a specific gene is

What is the total number of alleles at a locus for diploid organisms?

The total number of individuals divided by 2

What is the total number of dominant alleles at a locus?

• Recessive?

2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual

• Same principle for recessive

What does the Hardy-Weinberg Principle describe?

A population that is not evolving

What does the Hardy-Weinberg Principle state?

That frequencies of alleles and genotypes in a population remain constant from generation to generation

In a given population where gametes contribute to the next generation randomly, will allele frequencies change?

No

What does Mendelian Inheritance preserve?

Genetic variation in a population

What is the condition for the hypothetical population in a Hardy-Weinberg theorem?

• What a real populations like?

A population that is not evolving

• Real populations have changing allele and genotype frequencies over time

Can natural populations evolve at some loci and be at Hardy-Weinberg equilibrium at other loci?

Yes, they can

What does genetic drift describe?

How allele frequencies fluctuate unpredictably from one generation to the next

What are two methods by which genetic drift may occur?

Founder effect, and Bottleneck effect

Founder Effect

• Describe

Occurs when a few individuals become isolated from a larger population

• Allele frequencies in the small founder population can be different from those in the larger parent population

• For example, the minority of a population mat migrate, which does not represent the go population, but they will evolve and grow from that

Bottleneck Effect

• Describe

A sudden reduction in population size due to a change in the environment 

• Usually associated with a natural disaster

What are the main ideas of genetic drift?

1. Genetic drift is significant in small populations

2. Genetic drift causes allele frequencies to change at random

3. Genetic drift can lead to a loss of genetic variation within populations

4. Genetic drift can cause harmful alleles to become fixed

What does gene flow consist of?

The movement of alleles among populations

What does gene flow tend to do?

• Why?

REDUCE variation among populations overtime?

• If you have two species, A and B, and they reproduce together, both populations will almost merge, and get more and more similar to each other, reducing the variation between the two

True or False: Gene flow decreases the fitness of a population only

No, it can both increase or decrease fitness

What is the only mechanism that consistently causes adaptive evolution?

• How?

Natural Selection

• By acting on a organisms phenotype

Are the terms “struggle for existence” and “survival of the fittest” accurate?

• Why or why not?

No, because they imply direct competition

• Reproductive success is generally more subtle and depends on many factors

What is relative fitness

The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

What are the modes of selection?

• Bell curves

1. Directional selection

2. Disruptive selection

3. Stabilizing selection

Directional selection

The curve shifts to the left or the right, toward one side of the spectrum

Disruptive selection

The middle of the curve collapses, both extremes have higher frequencies

Stabilizing selection

The middle is the only frequency, other extremes are virtually non-existent

How does adaptive evolution occur?

• Is it continuous, why?

It occurs as the match between an organism and its environment increases

• Yes, it’s continuous because the environment can change

What is sexual selection?

Natural selection for mating success

What are the types of sexual selection?

1. Intrasexual selection

2. Intersexual selection

What can sexual selection result in?

Sexual dimorphism

What is intrasexual selection?

Fighting between the same sex for territory

What is intersexual selection?

Females selecting a male to mate with(usually)

• They usually mate with males who have won territory as they are better fit

What is sexual dimorphism

The differences between males and females of a species

• Appearance, behaviors(fighting)

What is balancing selection?

• What is included in it?

When multiple alleles are maintained in a population, which results in their preservation over long evolutionary time periods

• Heterozygote advantage

• Frequency-dependent selection

What is an example of heterozygote advantage?

The sickle-cell alleles causing mutations in hemoglobin but also confers malaria resistance

Frequency-Dependent Selection

Mimicking for example; a non dangerous animal with the pattern of a poisonous one) = from google

Frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish

Why can’t natural selection create perfect organisms?

1. Selection can act only on existing variations

2. Evolution is limited by historical constraints

3. Adaptations are often compromises

4. Chance, natural selection, and the environment interact

What is a cline?

A measurable gradient in a single characteristic (or biological trait) of a species across its geographical range.