ENVIRONMENTAL SELECTION PRESSURES, GENETIC DRIFT AND GENE FLOW

Define environmental selection pressures

• External agents which influence the ability of an individual to survive in their environment

Define fitness

• the ability to survive and pass genetic material on to the next generation 

Define selective advantage

• relative higher genetic fitness of a phenotype compared with other phenotypes controlled by the same gene

Define natural selection

• the process in which organisms better adapted for an environment are more likely to pass on their genes to the next generation

Define genetic drift

• random changes in allele frequencies from one generation to the next owing to the action of chance events

What are the two types of genetic drift

• bottleneck effect

• founder effect

Define bottleneck effect

• a major reduction in population size causes allele frequencies that are an unrepresentative sample of the gene pool in a population

Define founder effect

• chance effects on allele frequencies in a population that is formed from a small unrepresentative sample of a larger population

Define founder population

• a small group of organisms that starts a new population

Define gene flow

• the movement of individuals and their genetic material between populations

Define immigration

• the movement of individuals and their alleles into a population, and thus into a gene pool

Define emigration

•  the movement of individuals and their alleles exit out of a population, and thus out of a gene pool

Four mechanisms to change allele frequency

• mutation

• gene flow

• genetic drift

• natural selection

Natural selection

•  where there is variation within a population and those with the most beneficial traits survive and reproduce better

• leading to shifts in allele frequency

What does natural selection result from

• changing environmental pressures

What are the main processes involved in natural selection

• variation

• isolation

• selection

• (time)

AND/OR

• variation

• survival

• reproduction/passing on of genetic information

• (and time)

Steps of natural selection

• There is variation in the population

• Some individuals have a selective advantage, increasing their chance of survival & reproduction

• These individuals pass on their genes to the next generation increasing the frequency of this trait

Environmental selection pressures

•  external agents which influence the ability of an individual to survive in their environment

Natural selection organisms are…

• better adapted for an environment are more likely to pass on their genes to the next generation

Physical, biological and chemical examples of environmental selection pressures

yes

Fitness

•  a measure of the contribution a certain individual can make to the next generation

How is fitness achieved

• through a combination of the ability to survive and reproductive success

Selective advantage

• relative higher genetic fitness of a phenotype compared with other phenotypes controlled by the same gene

What is considered to be a selective advantage

• A phenotype that makes the greater contribution to the gene pool in the next generation has a higher fitness value

What is said to be selected against

• Phenotypes that make lesser contributions to the gene pool of the next generation are ‘less fit’

Alleles with vs without selective advantage

• Alleles with a selective advantage will increase in frequency

• alleles which do not give an advantage will decrease in frequency.

What will the with and without selective advantage lead to

• a decrease in genetic diversity

• alleles that lead to a beneficial trait may become fixed, while others that confer a selective disadvantage may be lost.

• If given enough time, mutations lead to more variation.

Does genetic drift favour one allele over another

• No

• both alleles are equally subject to being affected by genetic drift

Is genetic drift random

• Yes

What do chance factors cause

• allele frequencies in a population to randomly change over time

In genetic drift, is the resulting population representative or not representative of an initial population

• not representative

Founder effect

•  a reduction of genetic variation when a small group of individuals starts a new population

(founder effect)

Which group is the founder population

• smaller group

(founder effect)

the smaller group can be as small as a…

• mating pair or an individual inseminated female

(founder effect)

Gene pools are highly likely to

• have reduced genetic variation 

• be a non-random or unrepresentative sample of the original population

Founder effect diagram

yes

(founder effect)

Reduction of population may be the result of

• a natural disaster, e.g. bushfire or flood 

• a new disease 

• human activity e.g. destruction of habitat or large scale poaching.

(founder effect)

Allele diversitty in original large population vs small post-disaster population

• Original large population = diverse set of alleles in its gene pool

• Small post-disaster population = likely, by chance, to have a much less diverse set of alleles in its gene pool

Bottleneck effect diagram

yes

Founder effect versus bottleneck effect

• DEFINITION

FOUNDER EFFECT

• The phenomenon, which occurs when a small group of individuals becomes isolated from a large population

BOTTLENECK EFFECT

• The phenomenon, which occurs when a population rapidly decreases in size

Founder effect versus bottleneck effect

• CAUSES

FOUNDER EFFECT

• Causes: Migration of very small individuals from the main population

BOTTLENECK EFFECT

• Causes: A sharp reduction of the population size by environmental events such as droughts, floods, fires, earthquakes, diseases, etc.

Founder effect versus bottleneck effect

• AFFECTS/DOES NOT AFFECT THE ORIGINAL POPULATION

FOUNDER EFFECT

• Does not affect the original population

BOTTLENECK EFFECT

• Affects the original population

Founder effect versus bottleneck effect

• PROBABILITY OF INBREEDING

FOUNDER EFFECT

• Probability of Inbreeding: High

BOTTLENECK EFFECT

• Probability of Inbreeding: Very high

Founder effect versus bottleneck effect

• RANDOM/NON RANDOM SAMPLE OF GENES FROM ORIGINAL POPULATION

FOUNDER EFFECT

• Produces a population with a non-random sample of genes of the original population

BOTTLENECK EFFECT

• Occurs due to the random sampling of genes from the original population

Genetic drift

• Genetic drift is the change in the frequency of alleles in a population due to random sampling of organisms

How does population size affect genetic drift

• Smaller populations are more greatly impacted by genetic drift than larger populations.

What can genetic drift lead to in very small populations

• In very small populations, genetic drift can lead to the decrease and eventual loss of favorable alleles from the gene pool.

What is the risk for species reduced to one or a few small populations

• Species reduced to one or a few small populations are at great risk of extinction due to genetic drift.

Gene flow

• Gene flow is the movement of alleles between interbreeding populations

How does gene flow occur

• Gene flow occurs through the migration of individual organisms between populations, transferring their alleles from one gene pool to another (if they interbreed)

What are the two types of migration involved in gene flow

• The two types of migration involved in gene flow are immigration (movement into a population) and emigration (movement out of a population)

How does gene flow affect genetic diversity in a recipient population

• Gene flow can increase genetic diversity in a recipient population if new alleles arrive with immigrating individuals

What happens when there is no gene flow between populations

• When there is no gene flow between populations, they become isolated, and any new alleles that arise through mutations will remain in the one population

How can gene flow affect genetic variation in the source population

• Gene flow may decrease genetic variation in the source population if alleles leave with emigrating individuals