population genetics population genetics and patterns of evolution

Population Genetics

Population Genetics and Population Genetics and Patterns of EvolutionPatterns of Evolution

Are these organisms the same Are these organisms the same species?species?

Are these organisms the Are these organisms the same species?same species?

Species

a group of similar organisms that are capable of producing fertile offspring.

Population

A population is a localized group of a species in a defined area.

Biodiversity

the sum total of the genetically based variety of all organisms in the biosphere

Genes and Variation

What are genes?

Inheritable traits coded for in DNA

What are different forms of a gene called?

alleles

Genes and Variation

Although each organism has only 2 alleles for each gene, more than two alleles may exist in a population.

There exists variation within a population for many of these alleles.

The gene pool consists of all the alleles for each gene present in a population.

We can figure outwhat the frequency of aparticular allele is bycalculating the numberof times that alleleappears in thatpopulation compared toothers in the entire

gene pool.

Relative frequency of an allele in a population is expressed in a

percentage or a decimal

(95% = 0.95)

1. What is the frequency of the black allele?

– 20 out of 50– 0.4

2. What is the frequency of the brown allele?

– 30 out of 50– 0.6

In this sample

population, is the

most common allele

the dominant one?

The most common

allele does not

have to be dominant!!

When a change in the relative frequency of an allele occurs in a population, “change over time” has occurred, and this is evolution on a small scale.

Polydactyly

Consider alleles for in the polydactyly gene pool, the allele coding for extra digits, the polydactyly allele (P), is only 1% of the population, the frequency is 0.01.

The allele for 5 fingers and toes (p) is 99% of the population, or a frequency of 0.99.

If over time, extra fingers was an advantage, and natural selection selected FOR individuals with extra digits, a shift in that allele frequency might happen, and evolution on a small scale would have occurred!

Sources of Genetic Variation

1. Mutations- a change in the DNA sequence makes a new form of gene (and new proteins).

2. Gene shuffling- because of independent

assortment of chromosomes and crossing over during gamete formation.

(No change in a frequency)

Selection on a Single-Gene trait

A single-gene trait with two alleles will show two phenotypes (if it is not codominant or incomplete dominance). A change in frequency is easy to see in a population. Example: See Pg. 397

Genetic Genetic Equilibrium

Allele frequencies in a population don’t change from generation to generation (constant)

Gene frequencies will not change as long as certain factors (called Hardy-Weinberg Principles) are met.

Hardy-Weinberg Principles:

No movement in or out of population Large population size No mutation Random Mating No selection (natural or artificial)

Do you think that these principles Do you think that these principles are met for most populations?are met for most populations?

Hardy-Weinberg Formulas:Hardy-Weinberg Formulas:

p is the frequency of the dominant allele

q is the frequency of the recessive allele

p + q = 1

p2 + 2pq + q2 = 1

Homozygous dominant Heterozygous Homozygous recessive

Example:Example:

In these pigs, the allele for pink coat is dominant and the allele for black coat is recessive.

What is p? What is q?

Determine the percent of the pig population that is heterozygous for pink coat.

Hardy-Weinberg Formulas:Hardy-Weinberg Formulas:

You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:

1. The frequency of the “aa” genotype: _____

36% (As stated in the question)

Hardy-Weinberg Formulas:Hardy-Weinberg Formulas:

You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:

2. The frequency of the “a” allele: _____

60% (We know “aa” (or p2) is .36, then just P = .6 or 60%!)

Hardy-Weinberg Formulas:Hardy-Weinberg Formulas:

You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:

3. The frequency of the “A” allele: ____

40 % (P + Q = 1 so .6 + x = 1)

Hardy-Weinberg Formulas:Hardy-Weinberg Formulas:

You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:

4. The frequency of the alleles “AA” and “aa”:

16% and 48% (AA = p2 and Aa = 2pq)

Genetic DriftGenetic Drift

Genetic drift is the change in a population’s allele frequencies due to chance.

There are 2 situations in which a population is shrunk and genetic drift can take place:

The Bottleneck EffectThe Bottleneck Effect

Disasters such as earthquakes, floods, droughts, and fires can greatly reduce the size of a population. Those that survive may not be representative of the original gene pool.

This can greatly reduce genetic variability.

The Founder EffectThe Founder Effect

Takes place when a few individuals from a larger population colonize an isolated habitat.

There is very little genetic variety in the gene pool because not all genes from the original population are represented.

Read Bottleneck & Founder Read Bottleneck & Founder Effect ArticlesEffect Articles

Selection on a Polygenic Trait

Polygenic trait - controlled by more than one gene.

Examples: human height, weight, beak size

If you were to graph the frequencies of the phenotypes, you would get a bell shaped curve.

(Label your axes)

Number vs. Running speed of Rabbits

Individuals at one end of the curve are advantaged

(Higher biological fitness) Individuals at the other end are disadvantaged

(Lower biological fitness) Over time the population will shift in its

phenotypes to one direction.– Example: Food becomes scarce and one type

of beak is most efficient

Directional Selection

On your graph, draw the line that shows the change!

Directional SelectionDirectional Selection

Number of spiders vs. body size

Increasing body size

Individuals in the middle of the curve are more advantaged than individuals at the ends.

This causes the frequency of the mid-phenotypes to increase, and the ends to decrease

Example: Birth weight in humans

Stabilizing selection

Draw the lines!

Individuals at the ends of the curve are more advantaged than the individuals at the middle of the curve.

Less common. A single curve will appear to split in two.

– Example: Larger and smaller seeds become more common

Disruptive Selection

3. Disruptive Selection- occurs when individuals at the ends of the curve are more advantaged, or have a higher fitness, than the individuals at the middle of the curve. This is less common. A single curve will appear to split in two.

– Example: Larger and smaller seeds become more common

Selection on a Polygenic Trait

Selection on a Polygenic TraitSelection on a Polygenic Trait

Draw the line!

Summary: types of selection on polygenic traits

Speciation

What causes new species to arise?

Natural selection acts upon a population as a whole. Reproductive isolation must occur to separate the

population into distinct populations for natural selection to act on them separately.

The way this occur is called an isolating mechanism. The population must be separate and no longer be

able to produce fertile offspring, or become reproductively isolated, in order to become officially a different species. This is speciation.

Types of IsolationTypes of Isolation

1. Behavioral Isolation- two populations of one species are capable of mating, but they do not because of differences in mating behavior. If they do not mate, they are not interbreeding!

Ex. The western meadowlark (left) and eastern meadowlark (right) have overlapping ranges. They do not interbreed because they have different mating songs.

Types of Isolation:

2. Geographic Isolation - two populations of the same species are separated by some geologic or geographic feature and are prevented from mating.

Types of Isolation

3. Temporal Isolation- two populations do not “mate” at the same time of year, time of day, etc.

How did speciation occur in the How did speciation occur in the Galapagos?Galapagos?

How did speciation occur in the How did speciation occur in the Galapagos?Galapagos?

What type of beak would each bird have? Notice the beaks’ structure fits their functions

Patterns of EvolutionPatterns of Evolution

ExtinctionExtinction

99% of all species that have ever lived are now extinct.

In the struggle for existence, species compete for resources - some lose, and die.

Sudden changes in the environment or natural disasters can cause mass extinctions.

A mass extinction allows for a new radiation of species to fill all the empty niches.

The dodo bird has been extinct for several hundred years after humans introduced predators to their habitat

Adaptive RadiationAdaptive Radiation

several vastly different species arise from a single species to fill available niches.

Convergent EvolutionConvergent Evolution

unrelated organisms come to resemble each other because of similar environmental pressures

Structures with the same functions, but not on related animals, are called analogous structures.

CoevolutionCoevolution

Two species evolve along with each other based on a close relationship with each other.

Plants and their pollinators, parasites with their hosts, etc.

Punctuated EquilibriumPunctuated Equilibrium

Long periods of time with stable species broken with rapid period of change.

Complexity of the Cell

Molecules needed in metabolic processes

Energy conversions in organisms Evidence of the formation of simple

organic molecules Development of complex molecules into

DNA Cells for self-replicating