Sources of genetic variations

Ch. 21

Mutations1. mutations:

Is a change in the sequence of nucleotides in the DNAMay result in new alleles or new genesIs usually harmfulMust be in the gametes to be passed on to offspring

Mutations are more likely to be favorable when the environment is changing

Duplications Duplication mutations can lead

to an expanded genome with more loci that can take on new functions

Duplication mutations are thought to be the reason why certain mammals have more genes for detecting odors than others

Mutation rates Plants and animals

average about 1 mutation for every 100,000 genes per generation

Mutations happen more quickly in organisms that reproduce quickly, like bacteria

2. Sexual Recombination of DNA

During crossing over and fertilization

Produces variations that make adaptation possible

Sexual reproduction results in unique combinations of genes every generation

Quick Checks What is “sexual recombination”? What processes need to occur in cells

for “sexual recombination” to occur?

Shifts in allele frequency

Most evolutionary change the result of…1. Natural selection2. Genetic drift3. Gene flow

Natural Selection Results in the

differential success of organisms in being able to reproduce and pass on their genes to the next generation

Genetic Drift Fluctuations in allele frequencies in a small population from

one generation to the next The smaller the size of the population, the more likely there is to be a

major shift in allele frequencies

Genetic DriftEvolution by drift is aimless, not adaptive, because it is chance alone

Drift is common in two population events: Genetic bottlenecks and Founder events.

3 initial breeding pairs - shift in gene pool

The Bottleneck Effect when a population

undergoes a drastic reduction in size as a result of chance events (fire, flood, drought).

It is completely arbitrary and unrelated to phenotype.

The Bottleneck Effect

Lack of variation means less adaptability

Some alleles may be overrepresented in the survivors, some underrepresented, and some not represented at all.

Humans sometimes create bottlenecks in other species

The Founder Effect

a small group of individuals becomes separated from the larger population. They may establish a new population with a gene pool that is not representative of the larger population. Helps account for the high frequency of certain disorders in isolated populations

Gene Flow

Gene flow - movement of genes between populations. Gain or loss of alleles from a population due to migration of fertile individuals, or from the transfer of gametes.

Tends to reduce differences between populations

Gene flow increases the variability of the gene pool by adding new alleles.

Bioflix directions Natural selection Genetic Drift Gene Flow Title, explanation,

example

Natural selection

Explanation:

Example:

Genetic Drift

Explanation:

Example:

Gene Flow

Explanation:

Example:

Natural Selection in more detail

21

Variation in a population

Discrete characters - “either/or” traits, usually controlled by a single gene E.g. - widow’s peak or no widow’s

peak Phenotypic polymorphism - the

population has 2 or more distinct morphs for a trait that are fairly common in the population

Genetic polymorphisms - the presence of 2 or more distinct alleles in the gene pool of a population

May result in phenotypes that vary along a continuum, like height in humans

Geographic variation Most species

have geographic variations in their gene pools between different populations Due to

environmental differences

Survival of the fittest

Fitness - the contribution an individual makes to the gene pool of the next generation for advantageous adaptations

Relative fitness - the contribution of a genotype to the next generation. It is dependent upon the genetic and environmental context in which it was expressed (is it still an advantage to have that gene?)

Modes of Selection Directional

Most common during times of change or migration Favors one end of the phenotype range

Modes of Selection Disruptive

Occurs when conditions favor the two extremes in the phenotype range

Most likely to result in speciation

Modes of Selection Stabilizing -

Favors the average phenotype Reduces variation in a population

Why don’t “negative” phenotypes eventually disappear?

Diploids - variation “hides” in recessive alleles of heterozygotes

Balancing selection - natural selection that favors two or more phenotypes in a population, called balanced polymorphism

The Heterozygote Advantage

When individuals that are heterozygous for a trait have greater fitness than homozygotes

Example: sickle cell anemia AA - normal RBCs but prone to the

worst effects of malaria aa - have sickle cell anemia Aa - do not have sickle cell, are

protected against the worst effects of malaria

Frequency Dependent Selection The fitness of any one morph declines if

that morph becomes too common in the population Common in predator-prey relationships

Sexual Selection Natural selection for mating success Sexual dimorphism - the obvious

differences between males & females in a population

Intrasexual selection - members of the same sex compete with each other for mates Antlers, large size, etc

Intersexual selection - mate choice - individuals of one sex are choosy in selecting their mate Peacock tails, etc. Reflects upon the overall

fitness of the individual (only healthy males can grow that big tail)

Natural Selection cannot create perfect organisms

1. New phenotypes are the result of many small changes to previous phenotypes

2. As the environment changes, previous adaptations may not be favorable anymore

3. Selection can only act on variations that already exist in the gene pool