Chapter 9

What is a gene pool?

the complete set of alleles present in a population

What are allele frequencies?

the proportion of certain alleles in a gene pool

Factors that can lead to a change in allele frequency

• Mutations

• Environmental selection pressures

• Genetic drift and gene flow

• Artificial selection pressures

Alleles tend to stay constant when theres:

• No mutation

• Random mating

• The population is closed (no gene flow)

• The population is large (no genetic drift)

• No selection

What are mutations?

a permanent change to a DNA sequence

What is a mutagen?

an agent that can cause mutations in DNA

What are the types of Point mutations

silent, missense, nonsense

Silent mutation

a mutation in which a nucleotide is substituted for another, changing the codon, but still coding for the same amino acid. Therefore, there’s no effect on protein structure

Missense mutation

a mutation in which a nucleotide is substituted for another, changing the codon and coding for a different amino acid. Therefore, there can potentially be an effect on protein structure

Nonsense mutation

a mutation in which a nucleotide is substituted for another, changing the codon to a stop codon, prematurely ceasing translation of the gene’s mRNA. Therefore, there is an effect on protein structure

What’s a frameshift mutation?

a mutation that involves the insertion or deletion of one or two nucleotides, affecting the reading frame of all the following nucleotides.

How can a mutation be heritable

mutation must occur in an individual’s germline cells

if in somatic, then not heritable

What are the types of block mutations?

Deletion, Duplication, Inversion, Translocation

What’s a deletion block mutation

when a section of DNA is removed from a chromosome, shortening the DNA

What’s a duplication block mutation?

when a section of DNA is replicated, lengthening the DNA.

What’s a inversion block mutation?

when a section of DNA has its sequence reversed.

What’s a translocation block mutation?

when two sections of DNA on different chromosomes switch places.

Euploidy

the usual number and sets of chromosomes

Aneuploidy

when a cell or organism varies from the usual number of chromosomes in its genome by the addition or loss of a chromosome.

Polyploidy

when an organism contains additional sets of chromosomes.

What are the chromosomal abnormalities

aneuploidy and polyploidy

Types of aneuploidy

monosomy - one additional chromosome

trisomy - one additional chromosome

Types of polyploidy

triploidy - 3 sets of chromosomes

tetraploidy - 4 sets of chromosomes

What is evolution

change in the genetic makeup of a population over successive generations. 

What is natural selection

the process by which organisms that are better adapted to their local environmental selection pressures are more likely to survive and pass on their genes

Steps for natural selection

• Variation in phenotypes

• Specific environmental selection pressure

• Advantageous trait

• Heritable

Environmental selection pressures

factors in the environment that impact an organism’s ability to survive and reproduce

How does natural selection affect the genetic diversity of a population?

the genetic diversity will decrease

What’s fitness

a measure of how well an organism survives and reproduces in its environment

What’s genetic diversity

the variation in genetic makeup or alleles within a population

Explain how the likelihood of extinction of a species is affected by genetic diversity

a population with a greater variation in alleles has a higher chance of possessing a favourable allele that will help them survive if a new selection pressure arises.

What is gene flow

the introduction or removal of alleles from a population due to migration or interbreeding of individuals between populations.

Types of migration

immigration, emigration

What is immigration

where individuals move into a population

increases genetic diversity

What is emigration

where individuals move out of a population

decreases genetic diversity

How can migration occur

when populations are in close proximity or when a barrier between populations is removed

Effects of interbreeding

increase genetic diversity within a population

decrease genetic diversity between populations

What is genetic drift

the change in allele frequency due to a random chance event that has a dramatic effect on the population’s gene pool

Types of genetic drift

bottle neck effect, founder effect

What are the effects of genetic drift on genetic diversity?

causes a decrease in genetic diversity

What is the bottleneck effect?

a reduction in genetic diversity that occurs when a large proportion of a population is removed by a random event (e.g. natural disaster or human intervention)

What is the founder effect?

when the reduction in genetic diversity that occurs when a new population is started by a small unrepresentative sample of the original population.

Effects of low genetic diversity

Inbreeding - keeps harmful alleles in gene pool

Lower adaptive potential - populations become vulnerable to new selection pressures due to the absence of advantageous alleles

Inbreeding

the production of offspring from parents who are closely related or genetically similar

What’s a species

a group of organisms that are able to breed with each other and produce viable and fertile offspring.

What’s speciation

the process of populations genetically diverging until they become distinct species.

Genetic isolation (reproductive isolation)

when alleles are no longer exchanged between populations. 

Prezygotic isolation

things that prevent formation of a zygote

e.g. geographical, ecological, temporal, behavioural, structural

Postzygotic isolation

occuring after the zygote has formed

e.g. gamete mortality, zygote mortality, hybrid sterility

What is allopatric speciation?

involves the formation of a new species as a result of a geographical barrier

What are the steps of allopatric speciation

1. A population (or populations) of the same species becomes isolated by a geographical barrier.

2. The isolated populations are exposed to different selection pressures.

3. Over time, differences accumulate in the two populations until they form new species.

Can you describe allopatric speciation in the context of Galápagos finches?

• The islands are separated by the ocean, so there is no gene flow

• Different islands have different food sources

• Each island has different selection pressures, selecting for different phenotypes (such as beak shape) which has allowed genetic differences to accumulate

• Once sufficient differences accumulated and viable and fertile offspring could no longer be produced through interbreeding, new species of finches were formed.

What is sympatric speciation?

involves the formation of a new species in populations located in the same geographical location

Can you describe sympatric speciation in the context of Howea palms on Lord Howe Island? 

• Howea forsteriana diverged from its sister species Howea belmoreana after the initial population colonised the alkaline soil, which acted as a selection pressure

• After inhabiting the alkaline soil, physiological differences began to develop such as changes in flowering times.

• This served as a reproductive isolation mechanism. 

• Given the relatively small size of Lord Howe Island, it is unlikely that the Howea palms were ever geographically isolated from one another, further supporting the idea that the speciation event occurred sympatrically.

  
  

What is selective breeding (artificial selection)?

the alteration of a population’s gene pool due to humans altering the breeding behaviour of animals and plants to select for a desired trait

How does artificial selection compare with natural selection?

natural selection - the selection pressure is natural occuring

artificial selection - the selection pressure is human induced and there is a desired trait to be selected or removed

What are the steps of artificial selection?

- Determine the desired trait.

- Interbreed parents who show the desired trait.

- Select the offspring with the best form of the trait and interbreed these offspring.

- Continue this process until the population reliably reproduces the desired trait.

How does artificial selection affect genetic diversity?

reduces genetic diversity

Effects of artificial selection

increased genetic abnormalities, lowers adaptive potential, increases biodiversity loss

Antimicrobials

agents that are used to prevent and treat infections in humans, animals and plants

What is antimicrobial resistance?

the ability of a pathogen (e.g. bacterium) to survive exposure to an antimicrobial agent (e.g. antibiotics)

Types of antimicrobials

disinfectants - non living surfaces

antiseptics - living tissue

antibiotics - kill bacteria

antifungals - kill fungi

antivirals - kill viruses

How does bacterial resistance against antibiotics occur?

through mutations of bacteria and natural selection, where the exposure to antibiotics is an environmental selection pressure

What are some factors that contribute to bacterial resistance against antibiotics?

• Non-compliance when taking antibiotics

• Inappropriate use of antibiotics

• Widespread use of antibiotics

What is antiviral antigenic shift?

sudden and significant mutations in the genes encoding for viral surface antigens.

What is antiviral antigenic drift?

small and gradual mutations in the genes encoding for viral surface antigens.

How does antigenic shift occur

when multiple different strains of a virus combine when coinfecting the same host to form a completely new subtype

viral recombination

How does antigenic drift occur?

Mutations gradually accumulate and a new subtype of virus may form