Unit 4 AOS 1: Mutations & Evolution

Question 1

Block mutations

Answer 1

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.

Question 2

4 types of block mutations: DDIT

Answer 2

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.

Question 3

Point mutations

Answer 3

A mutation that alters one nucleotide

Question 4

3 types of point mutations: SMN

Answer 4

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

Question 5

Frameshift mutations

Answer 5

A mutation that involves the insertion or deletion of one or two nucleotides, affecting every codon from that point forward

Question 6

Mutagen

Answer 6

Agents that can cause mutations in DNA

Question 7

Gene pool

Answer 7

All the genes in a population

Question 8

Mutations

Answer 8

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.

Question 9

Allele frequency

Answer 9

The proportion of certain alleles in a gene pool

Question 10

The effect of mutations on allele frequencies

Answer 10

Mutations introduce new alleles into a population and this increases genetic diversity

Question 11

Natural selection

Answer 11

A mechanism for evolution in which the individuals best adapted to the selection pressures in their environment survive and pass on their alleles

Question 12

Natural selection occurs via the following steps:

Answer 12

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.

Question 13

Effect of natural selection on allele frequencies

Answer 13

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.

Question 14

Natural selection summary

Answer 14

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.

Question 15

Evolution

Answer 15

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

Question 16

Gene flow

Answer 16

The introduction or removal of alleles from a population due to migration

Question 17

Why does migration occur

Answer 17

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.

Question 18

The effect of gene flow on allele frequencies

Answer 18

Gene flow can introduce or remove alleles from a population. Therefore, it can increase or decrease genetic variation.

Question 19

Genetic drift

Answer 19

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

Question 20

Bottleneck effect

Answer 20

When a large portion of a population is wiped out by a random event (such as natural disasters or overhunting).

Question 21

Founder effect

Answer 21

When a small group of individuals colonise a new population.

Question 22

Why does the bottleneck effect occur?

Answer 22

Because the new population has lower genetic diversity than the pre-disaster population.

Question 23

Why does the founder effect occur?

Answer 23

Because they have reduced genetic variation from the original population

Question 24

The effect of genetic drift on allele frequencies:

B + F effect

Answer 24

The founder and bottleneck effects both decrease genetic diversity in a population.

Question 25

Gene flow and genetic drift summary

Answer 25

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.

Question 26

Speciation

Answer 26

The process which populations genetically diverge until they become different species
e.g. Allopatric speciation

Question 27

Allopatric speciation

Answer 27

The geographic separation of a population from a parent population resulting in the evolution of a new species

Question 28

Allopatric speciation steps

Answer 28

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.

Question 29

Isolating mechanisms

Pre-reproductive

Answer 29

• 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.

Question 30

Isolating mechanisms

Post-reproductive

Answer 30

• 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.

Question 31

Speciation summary

Answer 31

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.

Question 32

Artificial selection

Answer 32

The alteration of a population’s gene pool due to direct human action, usually selecting for a desired trait. Also known as selective breeding

Question 33

How does artificial and natural selection differ?

Answer 33

• 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.

Question 34

3 steps of artificial selection

Answer 34

1. Selecting for the trait you want
2. Selecting against the trait you want
3. Selecting against the trait you don’t want

Question 35

Effect of artificial selection on allele frequencies

Answer 35

Smaller gene pools and over-expression of deleterious alleles can reduce adaptability and fitness within an artificially selected population.

Question 36

Reduced genetic variation has 2 major impacts on the population:

Answer 36

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.

Question 37

Continuous variation

Answer 37

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

Question 38

Discontinuous variation

Answer 38

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

Question 39

Homologous structures

Answer 39

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

Question 40

Analogous structures

Answer 40

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

Question 41

Fossilisation

4 main types of fossils

Conditions reducing the change of decomposing and becoming a fossil

Answer 41

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.

Question 42

Radioisotopic dating

Answer 42

A dating technique used to determine the absolute age of a fossil by measuring the relative amounts of radioisotopes to their products

Question 43

Absolute and relative dating

Answer 43

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.

Question 44

Convergent and divergent evolution

Answer 44

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.

Question 45

Molecular evidence for evolution

Answer 45

• 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