Evolution Chapter 9

• Evolution: The change in the genetic makeup (allele frequency) of a population over time.

  Gene pool: the collection of all the genes/alleles present in a population
• Genetic diversity:

  • A larger and more diverse gene pool indicates high amounts of genetic diversity, increasing the chances of biological fitness and survival.
  • A small and less diverse gene pool indicates low amounts of diversity, decreasing the likelihood of and more chance of extinction in changing environments. 

  Allele frequency can be affected by

  • Mutation
  • Natural selection
• Mutations:

  • The only way that new alleles can be created
  • Mutations are permanent changes in a DNA sequence
  • Point mutations means one change 
• Natural selection: a mechanism through which organisms that are better adapted to their environment have an increased chance of surviving and passing on their alleles. 

  This process is one mechanism by which allele frequency changes and evolution can occur

  • Variation
  • Selection pressure
  • Selective advantage
  • heritability
• Variation: for natural selection to occur there must be variation in the traits/genetics of the population. 

  Sources of variation:

  • Mutation: new genetic information
  • Sexual reproduction: new combinations of genetics
• Environmental selection pressure: a factor in the environment that impacts an organism’s ability to survive and reproduce (limited resources, deforestation, changing temperature, predation)
• Selective advantage: an organism conferred a beneficial allele, which increases its chances of survival against a specific environmental pressure
• Heritability: the advantageous trait must be able to be passed from parent to offspring (heritable). Over time the frequency of the advantageous allele will increase.
• Genetic drift: In response to random events, allele frequencies can change drastically and affect a population’s overall genetic diversity.

  Random changes in allele frequencies in populations as a result of sudden and chance events.

  The two main examples of genetic drift are the bottleneck effect and the founder effect.

  Both types of genetic drift decrease genetic diversity
• Bottleneck effect

  • The reduction in genetic diversity that occurs when a large proportion of a population is removed due to a chance event.
  • The bottleneck effect occurs when the size of a population is severely reduced.
  • Events like natural disasters can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors.
  • dramatically decreasing population size significantly impacts allele frequencies.
  • The allele frequencies in this group may be different from those of the population prior to the event, and some alleles may be missing entirely.
• Founder effect

  • The reduction in genetic diversity occurs when a population is derived from a small unrepresentative sample of the original sample.
  • When a small number of individuals from a population move to a new area and become isolated they take only a sample of the gene pool with them. This new population will then interbreed and the alleles' ratio will be different from that of the larger original population.
• Reduction in genetic diversity have two major risks:

  • Inbreeding - this keeps harmful alleles in the gene pool.
  • Lower adaptive potential - populations become vulnerable to new selection pressures that could challenge and potentially wipe out the entire population due to the absence of advantageous alleles.
• Smaller populations are more susceptible to genetic  drift as they typically already have lower genetic diversity when compared to larger populations.
• Gene flow

  • Migration can occur when populations are physically close together or due to external forces
  • Migration into and out of a population is known is immigration (into) and emigration (out).
  • When individuals enter via immigration, their alleles are added to the gene pool
  • When individuals exit via emigration, their alleles are removed from the gene pool
  • Individuals can temporarily enter a population and interbreed with local individuals before leaving again, they contribute to the gene pool of that population.
• Effect of gene flow on genetic diversity

  • Immigration increases genetic diversity; more prominently in smaller populations since they have smaller gene pool to begin with, whereas in larger populations gene flow does not significantly affect the gene pool.
  • Emigration removes alleles from a population’s gene pool, decreasing genetic diversity. Effects are more pronounced in smaller populations than larger populations.
  • Individuals carrying unique alleles from different populations inbreed and increase genetic diversity, these alleles can be permanently added to the population’s gene pool
• Species: a group of individuals who are to breed with each other and produce viable and fertile offspring.
• Speciation when genetic differences accumulate through evolutionary processes (mutation, natural selection, genetic drift and gene flow), speciation can occurs, which involves the formation of a new species.

  Speciation can be separated into:

  Allopatric speciation and sympatric speciation
• Allopatric speciation the geographic separation of a population from a parent population resulting in the formation of a new species.

  • A parent population is divided by a geographic barrier
  • There is no gene flow between two daughter populations
  • Mutations may arise in each population randomly and/or
  • Different selection pressures operate in each population
• Examiner tip for allopatric speciation questions:

  1. state that a geographical barrier has isolated a population/s of the same species from each other, preventing gene flow
  2. state that the isolated populations are subjected to different selection pressures, accumulating genetic differences.
  3. explain that once sufficient genetic differences accumulate and the two populations can no long interbreed to form viable and fertile offspring, a new species has been formed
• Ecological niche the specific environmental conditions and resources or selection pressures within a particular environment.
• Galápagos finches - allopatric speciation:

  • Geographical barrier: separation by the ocean, preventing gene flow between them which suggests the formation of different species has largely been a result of allopatric speciation.
  • Genetic differences accumulate due to different food sources and selection pressures, selecting for different phenotypes.
  • Once sufficient differences accumulate and viable and fertile offspring could no longer be produced through interbreeding, new species of finches were formed.
• Sympatric speciation: the divergence of a species from an original species without a geographical barrier.

  • Can arise from genetic abnormalities that occur during gamete formation, producing polyploid variants - differences in the number of sets of chromosomes compared to the original parent.
  • Example: a diploid (2n) parent consisting of two sets of chromosomes may produce a diploid gamete (2n) instead of a haploid gamete (n). Therefore, fertilisation where two diploid gametes fuse to make a tetraploid organism, if it is viable and fertile then it would be considered a new species.
• Howea palms - sympatric speciation:

  • Howea Belmoreana inhabits neutral and acidic soils (low pH)
  • Howea Forsteriana inhabit alkaline soil (high pH)
  • H. Forsteriana diverged from its sister species H. Belmoreana after the initial population colonise the alkaline soil (selection pressure)
  • Physiological differences developed - flowering times (a reproductive isolation mechanism)
  • a new species formed once they could no longer interbreed to produce viable and fertile offspring.
  • The small size of Lord Howe Island made it unlikely that the plants were geographically isolated from one another
• Selective breeding the changing of a population’s gene pool due to humans altering the breeding behaviour of animals and plants to develop a selected trait. Aka artificial selection
• Natural selection a mechanism through which organisms that are better adapted to their environment have an increased chance of surviving and passing on their alleles.
• Selective breeding and natural selection share similarities such as:

  • Requires variation
  • Presence of selection pressure
  • Heritability of the trait

  Key difference: origin of selection pressure
• The requirements for selective breeding

  • Variation: Individuals in a population vary genetically, which leads to phenotypic differences.
  • Selection pressure: Direct human intervention places artificial selection pressure upon a population, only allowing certain individuals with desirable traits to breed together.
  • Heritability: The trait selected must be heritable, allowing it to be passed on from the parents to their offspring. Therefore, after the breeding population reproduces, the frequency of the selected allele will increase.
• A comparison between selective breeding and natural selection

  • Selective breeding (artificial): Involves human-induced selection pressures in the form of humans directly selecting desirable traits or removing particular traits from a population.
  • Natural selection (environmental): Involves naturally occurring environmental selection pressures such as predation, disease, and climate change, which select individuals with a selective advantage within their environment.
• While the primary method of selective breeding is to simply select for and breed individuals with a desirable trait together, it is also possible to select against an unwanted trait to remove it from the population. 

  An example could be selecting against large body size in a population of fish by overfishing large-bodied fish. Eventually, after many generations, there would only be small-bodied fish left.
• The effect of selective breeding on genetic diversity:

  
  

  Selective breeding can lead to smaller gene pools and overexpression of deleterious alleles, which can reduce adaptability and fitness within a population.
• Deleterious allele: an allele that has an overall negative effect on individual's fitness when expressed
• Adaptive potential the ability for a population to adjust to new environmental selection pressures.
• Antimicrobial agent an agent that kills or slows the growth of microorganisms. Examples: antiseptics, disinfectants, antifungals, and antibacterial agents.

  Antimicrobial agents play an important role in protecting us from harmful pathogens.
• Antimicrobial resistance the ability of a microorganism to survive exposure to an antimicrobial agent
• Antibiotic-resistant bacteria are attributed to natural selection where exposure to antibiotics serves as an environmental selection pressure.

  If antibiotic-resistant bacteria are present in the population, they will have a selective advantage, allowing them to continue living and replicating within their host and increasing the allele frequency for antibiotic resistance.

  Conversely, bacteria that are susceptible to antibiotics will die.

  Bacteria can exchange genetic material with each other through bacterial conjugations, spreading the alleles for antibiotic resistance.
• The development of antibiotic-resistant bacteria:

  • Variation: a population has individuals resistant and susceptible to an antibiotic
  • Selection pressure: exposure to an antibiotic serves as an environmental selection pressure
  • A selective advantage is conferred to bacteria with resistance to an antibiotic
  • Heritability: bacteria resistant to the antibiotic can continue replicating and pass on their allele for resistance to other bacteria via bacterial conjugation, increasing its allele frequency.
• Examiners tip for questions about the development of antibiotic-resistant bacteria

  1. outline that variation exists
  2. identify the presence of a new selection pressure (exposure to antibiotics)
  3. identify that group that is conferred an advantage
  4. highlight the increased heritability of the antibiotic-resistant alleles
• • The evolution and development of antibiotic-resistance is an example of natural selection.
  • Antibiotics do not cause bacteria to evolve resistance.
  • Rather, resistance to an antibiotic already exists within the population.
  • This allows bacteria with genes conferring antibiotic resistance to have a higher chance of surviving in an environment that has exposure to that antibiotic compared to bacteria that don't have those genes.
• Silent; point mutation

  • Substitution mutation that has no effect on the resulting amino acid sequence.
  • Due to the degenerate nature of genetic codes, multiple different codons code for the same amino acid
  • The same amino acid is incorporated into the protein
• Missense; point mutation

  • Substitution mutation which codes for a different amino acid, altering the primary structure of the polypeptide
  • Affects the folding of the polypeptide and function of the protein