Unit 4 AOS 1: Mutations & Evolution Flashcards
Block mutations
A mutation that affects a large chunk of DNA, or an entire gene
Involve altering the structure of a chromosome by inserting, deleting, or swapping a cluster of nucleotides, potentially involving multiple genes.
These mutations usually occur during the process of meiosis.
4 types of block mutations: DDIT
1 Deletion mutation – when a section of DNA is removed from a chromosome, shortening the DNA.
2 Duplication mutation – when a section of DNA is replicated, lengthening the DNA.
3 Inversion mutation – when a section of DNA has its sequence reversed.
4 Translocation mutation – when two sections of DNA on different chromosomes switch places.
Point mutations
A mutation that alters one nucleotide
3 types of point mutations: SMN
Silent mutation: a mutation in which a nucleotide is substituted for another, changing the codon, while coding for the same amino acid. Therefore, there is 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 is an effect on protein structure
Nonsense mutation: a mutation in which a nucleotide is substituted for another, changing the codon to a stop codon, ceasing transcription on the gene. Therefore, there is an effect on protein structure
Frameshift mutations
A mutation that involves the insertion or deletion of one or two nucleotides, affecting every codon from that point forward
Mutagen
Agents that can cause mutations in DNA
Gene pool
All the genes in a population
Mutations
Mutations can be as small as the substitution of a base or as large as the addition of an entire set of chromosomes. However, they both share the similarity of increasing the genetic variation in a population. Mutations are a key source of genetic variation and are an important process in evolution.
Allele frequency
The proportion of certain alleles in a gene pool
The effect of mutations on allele frequencies
Mutations introduce new alleles into a population and this increases genetic diversity
Natural selection
A mechanism for evolution in which the individuals best adapted to the selection pressures in their environment survive and pass on their alleles
Natural selection occurs via the following steps:
1 There is physical, biochemical, or behavioural variation in phenotypes between individuals in a population.
2 This variation is heritable.
3 A specific selection pressure exists that causes a struggle for survival. Some individuals survive and reproduce better, as they have an advantageous phenotype that helps them overcome the selection pressure.
4 Organisms with the ‘fitter’ - or advantageous - phenotype pass their alleles onto the next generation. This changes the allele frequencies in the population as the trait becomes more common.
Effect of natural selection on allele frequencies
Natural selection can reduce the genetic diversity in a gene pool as only the fittest individuals with alleles that code for advantageous phenotypes survive and reproduce.
Natural selection summary
Natural selection is a mechanism for evolution in which individuals best adapted to their environment are more reproductively successful and pass on their genes.
The selection pressure determines which phenotype makes organisms fitter.
As the fit trait becomes more common in the population, the allele frequencies in the population change.
Evolution
The change in the genetic makeup of a population over successive generations
Gene flow
The introduction or removal of alleles from a population due to migration
Why does migration occur
Migration can occur because populations are physically close together, or due to external forces such as the clearing of a geographical barrier between populations.
The migration into and out of a population is known as immigration and emigration respectively.
The effect of gene flow on allele frequencies
Gene flow can introduce or remove alleles from a population. Therefore, it can increase or decrease genetic variation.
Genetic drift
The dramatic change in allele frequencies due to a chance event. The random occurrences often result in genetic drift occurring via either the bottleneck effect or the founder effect
Bottleneck effect
When a large portion of a population is wiped out by a random event (such as natural disasters or overhunting).
Founder effect
When a small group of individuals colonise a new population.
Why does the bottleneck effect occur?
Because the new population has lower genetic diversity than the pre-disaster population.
Why does the founder effect occur?
Because they have reduced genetic variation from the original population
The effect of genetic drift on allele frequencies:
B + F effect
The founder and bottleneck effects both decrease genetic diversity in a population.
Gene flow and genetic drift summary
Gene flow and genetic drift are processes that result in the genetic diversity of a population changing.
Whilst genetic drift only decreases the gene pool of a population, the effects of gene flow can both increase and decrease genetic variation.
Speciation
The process which populations genetically diverge until they become different species
e.g. Allopatric speciation
Allopatric speciation
The geographic separation of a population from a parent population resulting in the evolution of a new species
Allopatric speciation steps
1 Initially a population (or populations) of the same species becomes isolated by a geographical barrier.
2 The isolated populations are exposed to different selective pressures.
3 Over time, sufficient differences accumulate in the two populations until they form new species.
Once these steps have occurred, the two populations could be reunited and would be unable to interbreed to produce viable and fertile offspring.
Isolating mechanisms
Pre-reproductive
- Geographical - individuals may be too far away within or between populations.
- Ecological - individuals may inhabit different ecological niches and not interact.
- Temporal - the time of the day or year when individuals are ready to breed may differ.
- Behavioural - the type of mating behaviours, such as a mating call, of individuals may vary.
- Structural - the physical characteristics of individuals may drastically vary which would make breeding impossible. For example, a Great Dane, and a Chihuahua cannot breed due to their size despite both being the same species.
Isolating mechanisms
Post-reproductive
- Gamete mortality - the sperm may be unable to penetrate the ovum for fertilisation.
- Zygote mortality - fertilisation may occur and a zygote may be formed, however it will not survive.
- Hybrid sterility - a viable offspring may be formed and may survive until adulthood, however, this offspring will not be fertile.
Speciation summary
Allopatric speciation involves the separation of a population by a geographic barrier.
This reproductively isolates the populations on either side of the barrier preventing gene flow, until enough genetic differences accumulate that the populations become distinct species.
Remember that a species is defined as individuals that can interbreed and produce viable and fertile offspring.
Artificial selection
The alteration of a population’s gene pool due to direct human action, usually selecting for a desired trait. Also known as selective breeding
How does artificial and natural selection differ?
- Artificial selection – the selection pressure is human–induced, and there is a desired trait that humans are selecting for or removing from the population.
- Natural selection – the selection pressure is naturally occurring.
3 steps of artificial selection
- Selecting for the trait you want
- Selecting against the trait you want
- Selecting against the trait you don’t want
Effect of artificial selection on allele frequencies
Smaller gene pools and over-expression of deleterious alleles can reduce adaptability and fitness within an artificially selected population.
Reduced genetic variation has 2 major impacts on the population:
1 Increased inbreeding – this increases the amount of homozygous alleles in offspring. While high homozygosity is not always damaging, it can lead to expression of deleterious recessive alleles.
2 Lower adaptive potential – this means that the population is less likely to have alleles that will help individuals survive under new selection pressures.
Continuous variation
Variation measured on a continuum rather than in discrete units or categories (eg height in human beings)
E.g. eye colour is produced by not a single gene, but many
Discontinuous variation
Variation that falls into discrete categories (eg the colour of garden peas).
E.g. ear lobe alleles, there are only two options, attached or free, and is controlled by a single gene
Homologous structures
A structure present in two or more species that may look and function very differently in each species, but is derived from a common ancestor
Similar species, different function
Analogous structures
A structure present in two or more species that fulfils the same function but does not originate from a common ancestor
Same function, different species
Fossilisation
4 main types of fossils
Conditions reducing the change of decomposing and becoming a fossil
The process by which an organism becomes a fossil
Fossils are the preserved remains of ancient life. There are four main types of fossils: permineralised, impression, trace, and mummified.
Conditions that reduce the rate of decomposition typically increase an organism’s chance of becoming fossilised. These conditions include:
• areas of rapid sediment accumulation
• constant cool temperatures
• low light availability
• physical protection from scavengers and decomposers (e.g. fungi, bacteria).
For example, as aquatic systems regularly deposit large amounts of sediment, many aquatic animals and plants are preserved.
Radioisotopic dating
A dating technique used to determine the absolute age of a fossil by measuring the relative amounts of radioisotopes to their products
Absolute and relative dating
Absolute dating techniques reveal the age of a fossil in years by analysing the breakdown of radioisotopes.
Relative dating techniques indicate a fossil’s age by analysing its position in sedimentary rock compared to other fossils.
Convergent and divergent evolution
Divergent evolution:
When a common ancestor speciates into two or more descendant species
Two populations of the same species, but exposed to different selection pressures, can accumulate differences.
Convergent evolution:
Evolution of analogous traits due to similar selective pressures
Two unrelated species can evolve structures with similar functions when exposed to similar selection pressures.
Molecular evidence for evolution
- Fossils:
Provide evidence of long-term evolutionary changes, documenting the past existence of species that are now extinct. - The same biochemical building blocks:
DNA, RNA, proteins, ATP
