evolution

Question 1

Distinguish between microevolution and macroevolution

LINK: Microevolution will eventually lead to macroevolution if given enough time

Answer 1

Microevolution :
•occurs due to a change in allele frequencies in a population over successive generations
•occurs over shorter periods of time(few generations)
•NO NEW species form

evidence (ans key)
1. Microevolution occurred over a few generations after the ancestral population with variation in beak
length arrived.
2. As beak lengths best adapted to the different food sources (fig.9.1) were selected for resulting in
changes to the allele frequencies within this ancestral species;
Hence microevolution occurred.

Macroevolution:
•occurs due to a change in allele frequencies in a population over successive generations
•occurs over longer periods of time (thousands/millions of generations)
•new species formed

evidence;(anskey)
1. Due to rising sea levels, more islands formed (fig.9.2), and geographical isolation of the ancestral
finch sub populations occurred that prevented interbreeding and hence disrupted gene flow;
2. the isolated subpopulations were subjected to genetic drift and accumulation of mutations* and
natural selection* and over many generations evolved independently into new species.
Hence macroevolution occurred;

Question 2

What are the 5 agents of evolutionary change

Answer 2

(1) Natural selection(2)Disruption of gene flow(3)Mutations(4)Non-random mating(5)Genetic drift
5. genetic drift = random change in allele frequencies due to chance events

a. founder effect** – A small group of individuals separate from a larger population and establish a colony in a new location. Because the
founder individuals of the new colony are just a few, certain alleles are overrepresented or underrepresented. Thus,
genetic variation is usually reduced.

b. bottleneck effect – A population size is dramatically reduced** due to catastrophe and then rebounds (in size after a few generations). The
initial reduction leads to certain alleles being overrepresented** or underrepresented** among the survivors and even
though the population numbers may rebound to the original, genetic variation is usually reduced.

Question 3

Explain the role of natural selection in evolution

Answer 3

(1) Overproduction of offspring: All organisms produce a large number of offspring. This can lead to an exponential increase in population size if all of the offspring survive
(2) Constancy of numbers: However the sizeof most populations stay relatively constant as many offspring die before they reach reproductive age
(3) Struggle for existence: This is because individuals of a species are constantly competing with each other for limited resources (eg:food, mates & shelter). Other factors such as disease and predators also impose a limit on the numbers
(4) Variation within a population: Individuals in a population differ from each other genetically(due to the presence of different alleles) and hence phenotypically. These variations, are a pre-requisite for evolution by natural selection
(5) Survival of the fittest by natural selection:Among the variety of individuals, some individuals will have favourable characteristics/phenotypes that are better adapted to the environment and will be selected for by the environment. They will be able to survive to reproductive age & produce viable, fertile offspring. Individuals less well adapted (i.e. with unfavorable characteristics) will be eliminated (i.e. selected against)
(6) Like produces like:Individuals with advantageous characteristics are likely to breed successfully and produce offspring similar to themselves. Thus alleles coding for these advantageous characteristics are passed onto the offspring and so the favourable allele frequencies will increase in the population(i.e. microevolution occurs)
(7) Formation of a new species:Over hundreds and thousands of generations, reproductive isolation can occur and a new species can form(i.e.macroevolution can occur)

Question 4

Explain, with example, how environmental factors act as forces of natural selection

(black and white peppered moth)

Answer 4

Example: Black and white peppered moth

❖Before 1848i.e. before the industrial revolution

•there were 2 formsof peppered moths (Biston betularia), the lighter form and the melanic form

• there was a greater proportion of the lighter form than the melanic form
• the melanic form arose by spontaneous mutation

❖By 1895 (take note of time period)
•98%of all peppered moths in industrial areas were the melanic form

❖Variation (black and white peppered moths) exists in a population

• Lighter form of moths were well-camouflaged from predators when they landed on light coloured, lichen-covered tree barks.

Thus they had a selective advantage** and were selected for**. Hence their numbers increased.

❖Selection pressure***(predation by birds)
With industrialisation, lichen on bark of trees were killed. The darker coloured barks which were once covered with white lichen were now exposed.Thus lighter forms of moth became easy prey to birds& their numbers declined.The melanic form of moth were camouflaged against the darker coloured bark& thus proliferated.

❖Differential reproductive success**

❖Thus the melanic form of moths had a selective advantage *in polluted areas & frequency of alleles coding for melanic form increased and frequency of alleles coding for lighter form decreased. Microevolution occurred.

Note:It is incorrect to imply that industrialisation caused the melanic form of peppered moth to appear.The melanic form arose by spontaneous mutations** which existed before the industrial revolution.

Question 5

Explain, with example, how environmental factors act as forces of natural selection

(e.g. antibiotics)

Note:
It is incorrect to imply that the antibiotics cause the bacteria to mutate into resistant strains. The resistant strains arose by spontaneous mutations or
gaining an antibiotic resistance gene via conjugation, transformation or transduction.

NOTE:
Sometimes a combination of antibiotics is given to a patient which act have different sites of action (e.g. prevent transcription or translation or
formation of peptidoglycan cell wall in bacteria). Bacteria that arise through mutations are unlikely to be resistant against all the antibiotics (as
chances of mutations occurring in a few genes in the same bacterium that make it antibiotic resistant to all the antibiotics is very small) and so
antibiotic resistance does not spread as the bacteria are killed by the combination treatment.

Answer 5

❖1940s: antibiotics first used to kill bacteria.
❖Variation (resistant and non-resistant bacterial strains) exists

❖Selection pressure (exposure to antibiotics)

❖Differential reproductive success
Antibiotics kill most of non-resistant bacteria. Resistant mutant strains survive.
Resistant mutant strains are selected for** as they have a selective advantage** in the presence of antibiotics.
These survive and pass on the allele for antibiotic resistance to offspring. Antibiotic-resistant allele frequency increases.***

Question 6

Explain why the population is the smallest unit that can evolve [3m]

Answer 6

1. Define population:
   ❖ A population is a group of interbreeding** individuals of the same species*
   Natural selection acts on individuals in a population and results in the perpetuation of favourable characteristics** (determined by favourable alleles).
   Thus favourable allele frequencies will increase over many generations.
2. Define microevolution:
   ❖ (Micro) Evolution is a measure of changes in allele frequencies* in a population over successive generations**

❖ Since a change in allele frequencies**
➔ can only be measured in a population over successive generations over time, and not an in an individual, the population is the smallest
unit that can evolve.

Question 7

Define biological species

Answer 7

A species is a
group of
organisms
capable of
interbreeding*
and producing
fertile*, viable*
offspring and are reproductively isolated** from other such groups

• Advantage:
Organisms being studied can be interbred to see if they produce fertile, viable offspring.
• Limitation:
This definition cannot be applied to asexually reproducing organisms and extinct species
whose breeding behavior cannot be observed.

Question 8

Define genetic species

Answer 8

A species is a group of genetically* compatible interbreeding** organisms in a natural population that is genetically isolated **from other such groups
- organisms in a species have sufficient similarity in their DNA sequences and share the same number of chromosomes

• According to this species concept, different genetic species
- are genetically distinct and evolve independently of each other
- have undergone genetic changes that lead to behavioural changes or changes in the type of
pheromones produced that are associated with species recognition

Thus different genetic species do not interbreed** in nature.

• However, if two different genetic species are mated, it is possible to produce fertile, viable
offspring. Hence two genetic species can be GENETICALLY ISOLATED but NOT REPRODUCTIVELY ISOLATED. e.g. polar bear and grizzly bear

• Advantage:
Genetic data from mitochondrial and nuclear DNA to identify species can be unambiguous in
deducing evolutionary relationships.
• Limitation:
Technology required to study DNA sequences is relatively expensive.

Question 9

Define ecological species / morphological species / phylogenetic

Answer 9

ecological :A species is a
group of
organisms
sharing the
same ecological
niche*.

Morphological: A species is a
group of
organisms
sharing similar
body shape**,
size* and other
structural
features*.

phylogenetic: A species is the
smallest group
of organisms**
that share a
most recent
common
ancestor** and
can be
distinguished
from other such
groups. They
occupy a branch
on a phylogenetic
tree.

Question 10

Define Niche [2m]

Answer 10

1. place/habitat *where an organism lives;
2. and roles* of an organism / interactions* with other organisms in its habitat and environment e.g.
   predator-prey relationship, decomposer etc.;

Question 11

Define species [3m]

(use biological species)

Answer 11

1. a group of organisms of the same species are capable of interbreeding* and producing fertile**,
   viable offspring*;
2. are reproductively isolated* from other species;
3. have a common gene pool and same chromosome number;
4. usually have similar morphological, physiological and behavioural features;

(pts 1&2 are compulsory)

Question 12

What is gene flow and when does it occur?

Answer 12

Gene flow is the transfer of alleles** from one population to another, due to the movement of fertile individuals or their gametes.

If members of
a population migrate and interbreed* with members of another population, gene flow has occurred**.

Question 13

COMMMMONNN QN:
concept: macroevolution
Explain how natural selection could have caused the evolution of the six closely related species in the primary radiation [3m]

Answer 13

1. When lake first filled, ancestors of Tropheus occupied new niches;

DISRUPTION OF GENE FLOW
2. and became geographically isolated* (A:a reproductive barrier formed). Sub-populations of ancestral
fish were prevented from interbreeding and gene flow was disrupted***;

DIFFERENT SELECTION PRESSURES
3. Different environments/niches presented different selection pressures** and so individuals with
favourable traits and were best adapted had a selective advantage and were selected for, which led
to an increasing frequency of favourable alleles**;

CHANGE IN ALLELE FREQUENCY
4. As the different sub populations evolved independently** of each other, their allele frequencies changed
as they accumulated different genetic mutations, and were subjected to genetic drift and natural
selection.

REPORODUCTIVE ISOLATION
Over a long period of time this led to reproductive isolation* and formation of 6 distinct
but closely related species (i.e. macroevolution occurred);

Question 14

Take note of change in phrasing of qn
concept: microevolution

Suggest how the lowering of the water level in the lake to form three separate lake basins could have
caused the evolution of so many subspecies. [2]

Answer 14

1. Lowering of water level to form 3 separate lake basins that are geographically isolated* will allow
   subpopulations to evolve independently as gene flow is disrupted*;
2. Lower water level also created new niches** that presented different selection pressures* and so fishes
   with favourable traits/better adapted and hence a selective advantage* were selected for, increasing
   frequency of favourable alleles;
3. As different sub populations of fishes evolved independently* of each other, their allele frequencies
   changed as they accumulated different genetic mutations, and were subjected to genetic drift* and
   natural selection*. Hence they eventually became different sub species. (i.e. microevolution occurred);

!!!!!!!!!!!!R: reproductive isolation because they are still the same species

Question 15

Describe briefly why these four ecomorphs have such different adaptations despite all the
species of Anolis lizards in these ecomorphs being closely related. [3]

(take note of variation in questions of the same concept)

Answer 15

VARIATION:
1. Random mutations which resulted in new alleles as well as sexual reproduction, meant
that there was variation* in size, leg length, size of toepads, tail length and colour in the
population of lizards;

SELECTION PRESSURE : (in this case since question gave a figure with the different ecologies and structural adaptations; describe how the traits are selected for)
2. The different habitats had different selection pressures, possibly the availability of
insects (food) (briefly explain:) in different sections of the tree /or presence of ground-based predators that
the lizards need to escape from resulted in selection of different leg lengths or toe pads; or
possibly the different size and colour of lizards allowed them to be better camouflaged and
they could hide from predators/ catch prey more easily(Give at least one context specific
explanation)**

SELECTIVE ADVANTAGE:
3. The lizards that were best adapted to a particular habitat/ part of the tree survived were
selected for* and survived and reproduced and passed on their alleles* their offspring;
R:genes
4. Over time frequency of alleles responsible for the various favourable adaptations in the 11
species of lizard, increased resulting in the 4 ecomorphs with different adaptations;

Question 16

Give a named example of physiological isolation

type of speciation: sympatric

Answer 16

- Two species of palms in
Lord Howe island, Howea
forsteriana and Howea
belmoreana
One palm species grows in
calcareous soil* while the
other grows in volcanic soil*
in close proximity* to each
other but they flower at
different times.

• a difference in flowering time
  occurred in ancestral palms
  growing in different soil types - arose as a physiological response* to growing on different soil
• prevented interbreeding between the 2 subpop
• disruption of gene flow resulted in evolutionary changes occurring independently within each subpop from accumulation of mutation as well as change in allele frequencies through genetic drift and natural selection
• over 100 and 1000s of successive generations, each sub pop becomes genetically distinct species
• reproductively isolated and unable to interbreed to form fertile viable offspring

Question 17

Give a named example of behavioural isolation

type of speciation: sympatric

Answer 17

Eastern meadowlark and
the western meadowlark
Although the regions where
both species can be found
overlap in the central United
States, they do not mate due
to a differences in their bird
call**

the emergence of a new bird
call such that birds only
mated with other birds with a
similar call

Question 18

Define speciation

&

types of speciation

Answer 18

process by which one or more new species arise from a previously existing species

Allopatric speciation and sympatric speciation

Evidence for allopatric:
1. The 3 Carribean islands are geographically isolated* as they are surrounded by water
that acts as a physical barrier that prevented interbreeding. This resulted in the disruption
of gene flow;
2. The islands due to their differing habitats / environments/ niches, presented different
selection pressures ;
3. that resulted in ancestral lizards with the best adapted size/leg length/tail length as seen in
Fig.9.1, surviving and reproducing which eventually led to different reproductively
isolated* species in the 3 different islands over time as seen in Fig 9.2;

evidence for sympatric:
1. A reproductive barrier formed in the ancestral lizards of the twig and canopy ectomorphs
that lived in the trees (as seen in Fig.9.1) which disrupted gene flow***** and prevented
interbreeding between the two subpopulations;
2. In Jamaica, the 2 sub populations evolved independently and the best adapted survived
and reproduced until the differences between them became so great that speciation
occurred (as seen in Fig.9.2) leading to the formation of the twig and canopy ectomorphs;

Question 19

difference between allopatric and sympatric speciation

Answer 19

allopatric: members are geographically isolated due to a physical barrier; gene flow disrupted ; evolutionary changes occurring independently within each subpopulation ; different genetic changes from accumulation of mutations

sympatric; members are not geographically isolated but a REPRODUCTIVE BARRIER emerges that isolates a subset of a population from the remainder of the population in the same area ;; same process as allopatric

Question 20

What is adaptive radiaiton

Answer 20

rapid increase in the number of species produced from a common ancestor upon introduction into new environment

Question 21

Define biological classification

Answer 21

Biological classification is the organization of species according to their shared characteristics in a hierarchical manner, into increasingly
inclusive groups/taxons based on anatomical and more recently molecular data.

Question 22

Define phylogeny

Answer 22

❖ Phylogeny is the organization of species to show their evolutionary relationships (i.e. ancestor-descendent relationships)

Question 23

Taxon: a recognisable group of organisms at any particular level of classification.

Organisms are broadly categorised into a taxonomic hierarchy. They are:

Answer 23

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
(Eukarya) (Animalia) (Chordata) (Mammalia) (Primates) (Hominidae) (Homo) (sapiens)
(Dumb King Phillip Came Over For Great Spaghetti )

Question 24

Disadvantages of Linnaean classification

Answer 24

• cannot infer the evolutionary relationship between members of each category
• cannot tell how distantly related one species is to another
• name des not tell us the evolutionary history of the species

Question 25

Difference between shared ancestral character and shared derived character

Answer 25

shared ancestral character - originated in an ancestor; shared by all its descendent

shared derived character - a unique character of the group but not found in the ancestors

Question 26

Advantages of molecular methods in classifying organisms and determining their phylogenetic relationships

Answer 26

* 3. Nucleotide data is objective* as molecular character states are unambiguous*** (e.g. A, C, G & T) unlike some morphological characteristics
such the shape or colour of a structure which can be difficult to distinguish objectively.

**4. Nucleotide data is quantitative as molecular data can be converted into numerical form and is amenable to statistical analysis. Hence the
degree of relatedness can be quantified and inferred by calculating nucleotide differences between species.

1. Since all known life is based on nucleic acids, nucleotide data can be used to compare all organisms which share common genes.
2. Nucleotide data can be used to compare organisms that are morphologically indistinguishable due to convergent evolution or because
   they are closely related.
3. Changes in nucleotide sequences accumulate over time with clockwork regularity. Thus we can thus estimate the time of speciation
   of modern to ancient species.
4. Some molecular differences may not be reflected as a morphological difference while small genetic differences may not result in a major phenotypic difference. This means that molecular data does not underestimate nor exaggerate differences unlike morphological analysis.
5. Molecular methods offer a large set of characters that can be studied quickly. Each nucleotide position can be considered a character to
   distinguish between species.
6. Nucleotide sequences for a rapidly increasing number of genomes & amino acid sequences for many proteins can be accessed from
   electronic databases for comparative study & classification of all life.
7. Specimens need not be complete (can be incomplete fossil evidence) or alive (can be dead) for comparative analysis so long as the
   molecules (e.g. mitochondrial DNA) survive degradation.

Question 27

COMPARISON of phylogeny and linnean classification

points of comparison
1. basis of grouping organisms

2. systems of organising organisms
3. how species are presented
4. types characteristics used
5. strengths and weaknesses
6. inference of speciation events
7. inference of common ancestors
8. inference of relationships

Answer 27

1. basis of
   grouping
   organisms

CLASSIFICATION
organisms grouped based on overall
morphological similarities and may or may
not consider their evolutionary history

PHYLOGENY
organisms grouped based on their evolutionary history of organisms
(i.e. based on ancestor-descendent relationships)

2. systems of organising organisms

C: a naming system** where each organism is given a binomial name and grouped into domain, kingdom, phylum, class, order,family, genus and species using a hierarchical classification system**

P: is a system which assigns each organism a position on a phylogenetic tree based on its evolutionary relationship with other organisms on the tree

3. how species are presented
   C: uses binomial nomenclature***
   P: uses a phylogenetic tree where more closely related
   organisms are grouped closer together in the different
   branches of the tree
4. types characteristics used
   C: organisms grouped using only morphological characteristics**

P: uses (1)morphological, (2)anatomical as well as
(3)molecular characteristics (such as DNA, RNA, amino
acid sequences) and (4)fossil record

5. strengths and weaknesses
   C; can easily place** an organism into its well
   defined group
   ➔may wrongly classify organisms that are not
   related but look similar due to convergent
   evolution**

P:cannot immediately place** an organism into the
phylogenetic tree as evolutionary history need to be
established from multiple sources
➔rarely classifies wrongly* as convergent evolution will be
placed in separate branches

6. inference of speciation events
   C: does not allow inference of speciation events

P:indicates speciation events as the node of the phylogenetic tree

7. inference of common ancestors
   C; doe not allow

P; allows inference of common ancestors. Descendants of a
common ancestor are represented in the same branch

8. inference of relationships
   C: cannot infer of how closely related 2 species are
   especially since they are grouped together in
   the same hierarchy, “species”

P: allows accurate inference of how closely related 2
species are by looking at how recently they diverged from
their common ancestor

Question 28

Define homology;

Answer 28

➔ refers to similar anatomical & molecular characteristics found in different species due to common ancestry

Question 29

Name 2 examples of anatomical homologies

Answer 29

Example 1: pentadactyl limbs
1. The pentadactyl limb is a homologous structure in forelimbs of all tetrapods. e.g. humans, cats and whales.
2. The forelimbs have the same arrangement of bones but have different functions and superficially look different.
(e.g. walking in cats and swimming in whales)
3. The 5 digit pentadactyl limb structure in the common ancestor was altered by natural selection in the different organisms to suit
their specialised functions/environments, resulting in variations of the pentadactyl limb structure

Example 2: Vestigal structures
1. Anatomically homologous structures that are greatly reduced in size or have little to no function are called vestigial structures.
2. Organisms with vestigial structures share common ancestry with organisms in which the structure is still functional.
3. e.g.1: The hind limbs in whales are reduced to small bones (i.e. vestigial structures) as they are no longer beneficial to whales
which swim. However, their presence in whales suggest common ancestry with tetrapods.
4. e.g.2: The appendix in humans is also a vestigial structure as it is reduced from the cecum of its primate ancestors which was
involved in digestion of plant material. Thus, the presence of the appendix in humans, suggests common ancestry with
primates.

Question 30

(figure showing arrangement of bones in the pentadactyl forelimbs of 4 vetebrates)

Explain how the relationship between structures in Fig4.1 provides evidence to support the theory of evolution [3m]

Answer 30

SAME ARRANGEMENT OF BONES
1. Forelimbs of bat, human, lizard and whale show the same arrangement of bones** from shoulder
to tips of digits, even though these appendages have very different functions: flying,
lifting/grasping, walking, swimming and don’t resemble each other;

NATURAL SELECTION
2. Natural selection** had resulted in the different forms of the pentadactyl limb to suit their
specialised functions/environment*;

DESCENT WITH MODIFICATION
3. and that it was a modification* of five-digit forelimb of common ancestor* which they all descended
from;

Question 31

Suggest how natural selection could have brought about the evolution of the skeleton of a bat’s wing.
[4]

Answer 31

1. Random mutations resulting in new alleles as well as sexual reproduction, meant that there
   was variation* in shape, size and length of forearm bones in the population;
2. Selection pressure*, possibly the availability and type of food or a need for escaping ground based predators resulted in selection of a forearm that facilitated flight; (Give at least one
   context specific explanation)
3. These ancestral bats survived and reproduced and passed on their alleles* conferring an
   ability to fly to their offspring; R: genes
4. Over time frequency of allele responsible for ever increasing ability to fly increased and bat
   wing eventually evolved;

Question 32

❖ Fossils ➔ are relics or impressions of organisms that lived in the past that are preserved in rock.
(The deeper the strata the organism is found in, the earlier it existed.)

name examples of fossils

• describe what is meant by transitional fossils

Answer 32

Example 3: Horse fossils
1. When horse fossils are studied we see an ordered sequence of progression in terms of lengthening of limbs, toe reduction and increase in tooth size over time that coincided with the change in environment from dense forest to open grasslands.
2. Through natural selection, these adaptations transformed the ancestral horse into the horses we see today that is best suited for
open grasslands. (descent with modification)

• Example 4: Transitional fossils
Qn: Describe what is meant by transitional fossils [2m]
1. any fossilised remains of a life form
2. exhibits traits common to both an ancestral group and its derived descendent group

1. Tiktaalik is an example of a transitional fossil animal between fishes & tetrapods.
2. It provides strong evidence that fish are the ancestors to modern tetrapods.
3. It was similar to its fish ancestors as it had fish gills & scales, & was similar to its tetrapod descendants as it had tetrapod leg bones,
   lungs, upward positioned eyes & a mobile neck.

Question 33

❖ Continental drift: (not v impt)

with named examples

Answer 33

When related species are not distributed in the same geographical region (when it should), continental drift can explain this discrepancy.

So fossilized remains that once might have been found on a supercontinent may have split up into a multitude of smaller continents
distributed over a wide area after millions of years.

Example 5: Mesosaurs
1. In the case of the extinct mesosaurs, their fossils are distributed across Africa and South America which are separated by the vast
Atlantic ocean. Based on continental drift, the 2 continents were once joined together and that was the time when the ancestor of
mesosaur originated.
2. This ancestral species gave rise to a variety of mesosaur species that radiated out in this ancient supercontinent.
3. When the continents broke up and separated, their fossilized remains were transported to where they are today.
4. The concept of descent with modification from a common ancestor still holds true but time and continental drift has separated the
descendants.

Question 34

❖ Island Biogeography: (not v impt)

Answer 34

Example 6: Galapagos finches
1. The Galapagos islands are a group of small islands that are found off the coast of Ecuador, South America. The 13 different finch species in the on the Galapagos islands bear similarity to the finches found on the coast of the South American mainland.
2. This is because the finches from the mainland migrated to the islands and those that were best adapted to a variety of different
niches in the different islands survived, reproduced there.
3. Since the finches were geographically isolated (which disrupted gene flow) from the mainland & each other, evolutionary changes (e.g.
natural selection, genetic drift, mutations) occurred independently in each sub-population and over time they evolved into different
species.
4. Thus, the biogeographic distribution of the finches supports the evolutionary deduction of descent with modification from a
common ancestor because the finch species didn’t emerge from the Galapagos islands but came from an ancestral species from
the mainland, which then evolved into different species through adaptive radiation.

Question 35

What is molecular homology between organisms?

Answer 35

Organisms with molecular homology have similar DNA, RNA & amino acid sequences as they share a common ancestor that had these
molecules.

Question 36

Name an example of molecular homology

Answer 36

Example: Cytochrome C and p53 are homologous genes
1. Homologous genes share significant sequence homology and when expressed produce proteins** that have the same function** in
all organisms that possess them.
2. Nucleotide sequences* in the ancestral genes were modified* due to accumulation of mutations* that occurred over many
generations and selection pressures favoured some mutations over others.
3. The greater the sequence similarity* between homologous genes, the more closely related* the 2 species are.

Question 37

Suggest advantage of using mtDNA [2m]

Answer 37

1. There is no recombination/crossing over in mitochondrial DNA** from parent to offspring (idea of no
   recombination) unlike nuclear DNA, hence changes in DNA sequence is SOLELY due to the
   accumulation of mutations over time** at a regular rate. This allows estimation of time of speciation**;
2. Faster mutation rate (key idea) compared to nuclear DNA and hence it is useful for comparing
   individuals within a species or species that are closely related* as you require discernible differences
   between the DNA of organisms being compared;

Question 38

List ways in which genetic variation arises

Answer 38

❖ How genetic variation arises in natural populations

1. Mutations (Gene mutations, chromosomal mutations)
   a) Gene mutations
   - substitution, deletion or insertion of a nucleotide that changes the triplet code & hence the amino acid.

Mutations in
non-coding regions such as the promoter & enhancer can result in phenotypic variation as well.

b) Chromosomal mutations (may involve changes in chromosome structure and number)
Number:
Polyploidy - when more than 2 homologous sets of chromosomes are present e.g., triploids: 3n, tetraploid: 4n.

Aneuploidy - when one or more chromosomes are over-represented or represented due to non-disjunction during meiosis I or II.
e.g Trisomy 21

Structure:
Deletion - when a segment of a chromosome is missing e.g. cri-du-chat disease where there is a deletion of chromosome
Duplication - when an extra segment of a chromosome is present.
Inversion - when a chromosome segment is detached, flipped around 180 degrees & reattached to the rest of the chromosome
Translocation - when a segment from one chromosome is detached & reattached to a different chromosome.
Note: When a horse (2n=64) and donkey (2n=62) interbreed, they produce a mule (2n=63) (hybrid) which is sterile as gamete formation cannot
occur in mules as homologous chromosomes do not exist and hence cannot pair up during prophase 1 of meiosis

2. Meiosis
   a) Independent assortment & segregation of homologous chromosomes during metaphase I & anaphase I respectively

Independent arrangement & separation of sister chromatids during metaphase II and anaphase II respectively
➔ results in gametes with numerous combinations of maternal & paternal chromosomes.

b) Crossing over between non-sister chromatids of homologous chromosomes results in more allelic combinations.

3. Sexual Reproduction
   Random fusion of gametes add to the variety of genotypes. Different genotypes will result in different phenotypes and these will act as
   raw materials for natural selection.

Question 39

Describe heterozygote advantage

Explain with named example

Answer 39

2. Heterozygote advantage
   This is a form of balancing selection** which occurs when heterozygotes have greater fitness than both kinds of homozygotes.

e.g. In a region where malaria is prevalent, heterozygous individuals with the HbAHbS genotype do not develop sickle cell anaemia and at the saame time have less chance of contracting malaria. They are able to survive and reproduce in malaria-infected regions.

Therefore, BOTH
the HbA and HbS alleles of these people remain in the population. Thus, the HbS allele confers a survival advantage* on people who have one
copy of the allele*, and is therefore maintained in the population at a relatively high frequency.

Question 40

Describe Heterozygote protection

Answer 40

A gene can be dominant/recessive. Expression of the dominant alleles mask the expression of recessive alleles. Thus even if recessive
alleles may be less favorable in the current environment, they persist because they are propagated in heterozygous individuals where the
disadvantageous trait does not manifest and hence is not selected against.

Question 41

Describe frequency-dependent selection

Answer 41

This is a form balancing selection within a population which is able to maintain stable frequencies of two phenotypic forms and so the
alleles coding for them are preserved.

EXAMPLE
e.g. Scale eating fish in Lake Taganyika are either “left-mouthed” or “right-mouthed”. The “left-mouthed” fish attacks its prey’s right while the
“right-mouthed” fish attacks its prey’s left. The prey guards itself against attack from whatever phenotype of scale-eating fish is most common
in the lake.
So from year to year, selection favours whichever mouth phenotype is least common**. As a result the frequency of “left-mouthed”
fish and “right-mouthed” fish oscillates over time and frequency-dependent selection keeps the frequency of each phenotype close to 50%.

Question 42

read up on 3 types of selection

Answer 42

directional selection
 Phenotype at one extreme is
repeatedly selected for.
disruptive selection
▪ Intermediate phenotypes are
selected against and favours
individuals on both extremes of a
phenotypic range.
stabilising selection
▪ Extreme phenotypes are selected against and
favours more common intermediate variants in
a population.

Question 43

Comparison of amino acid sequences have been used to determine evolutionary relationships in the primates. Suggest why such a comparison was made with haemoglobin? [2m]

Answer 43

1. Homologous protein*** found in all the 4 primates
2. Hence amino acid sequence is highly conserved* over time
3. But with sufficient variation** to distinguish between the 4 different primate species

Question 44

Suggest why it is often impossible to determine evolutionary relationships from the structural features of the fossil skulls

Answer 44

1. fossil skulls are incomplete/damaged
2. often there are insufficient morphological characters that they all share that can form a basis of comparison and so we can’t draw any evolutionary relationships

Question 45

Suggest and explain why, in a rapidly changing environment, small animals may be at an
evolutionary advantage compared to large animals. [10]

Answer 45

1. A) Small animals populations put only a small investment of resources into each offspring;
   B) so that there will be enough resources to produce more offspring;
2. A)Small animals populations have high reproductive output;
   B) so that there is greater chance of survival in a rapidly changing environment;
3. A) Small animals populations are also generally not very invested in protecting or rearing their
   young for many years;
   B) Often, the eggs are fertilized and then cared for only a short period of time. The benefit of
   this strategy is that if resources are limited or unpredictable, you can still produce some young;
4. A) Small animal populations grow rapidly, with shorter lifespans;
   B) this short generation time allow the fittest to survive, reproduce and pass on their favourable
   alleles to the next generation, allowing faster rate of evolution to adapt to the changing
   environment;
5. A)The young of small animals tend to be more rapidly maturing and develop early
   independence;
   B) This allows less dependence on parents so even if parents do not survive the changing
   environment, there is a chance for their offspring to adapt and survive;
6. A) Small animal populations require less food as compared to large animals or maybe able to
   avoid predation due to their size;
   B) hence allow them a selective advantage over the larger animal populations;

Question 46

State how information needed to construct a phylogeny tree can be obtained through molecular techniques

Answer 46

1. Amplify DNA sequences of a homologous gene** common to all the species of organisms being compared via polymerase chain reaction
2. Sequence the DNA fragments obtained from the different organisms
3. Align the DNA sequences and calculate the differences between them
4. Species that are closely related have more similar nucleotide sequences than do distantly related species and should be shown to diverge from a more recent common ancestor in the phylogenetic tree