Evolution [part 1]

VARIATION & MUTATION

Variation:

describes the differences in characteristics shown by organisms of the same species

can be: discontinuous continuous

Discontinuous Variation:

Produces:individuals with clear-cut differences

with no intermediates between them

examples: blood group in humans wing length in Drosophila sex tongue rolling

Discontinuous Variation: Characteristics are usually controlled by:

1-2 major genes which have: two or more allelic forms

their phenotypic expression is relatively unaffected by environmental conditions

Continuous Variation:

many characteristics in a population show a complete gradation from one extreme to the other without a break

Continuous Variation: Examples:

Mass Height Intelligence Colour of organs & organisms

Continuous Variation:

Characteristics are produced by the combined effects of:

many polygenes and the environment

Influence of the environment

Sources of variation inasexually reproducing organisms :

replication of DNA is nearly perfect - i.e. little possibility of variation in

genotype

any apparent variation between organisms is thus almost certainly the result of environmental influences

Sources of Variation: Meiosis

Allows for new combinations of genes

Creates variation within a population

Four Sources of variation in:

sexually reproducing organisms:

1. Crossing over 2. Random orientation of bivalents

Four Sources of variation in:

sexually reproducing organisms:

4. Random

fertilisation

3. Random orientation of sister chromatids

Metaphase plate

Metaphase plate

OR

DO NOT:Crossing over

generate the major changes in genotype necessary to give rise to new species

Random orientation

Random fertilisation

MUTATIONSgenerate changes in genotype

necessary to give rise to new species

MUTATIONS

A Mutation is:

a change in the amount, arrangement or structure of the DNA of an organism

A mutation produces a change in the genotype & is passed on: when a cell nucleus divides by:

mitosis or meiosis from the mutant cell

Mutant daughter cellsMutant daughter cells

Mutant cell

Mutant cell

Which type of mutation can be inherited by the offspring?

germinal

somatic Occur in somatic cells:

are NOT passed on the offspring

Occur in gamete cells:

are passed on to the offspring

Normal progeny can be produced if mutations occur in gamete cells because:

the effect produced by defective allele is masked by the dominant allele

DOMINANT RECESSIVE

A Mutation may result in:

the change in appearance of a characteristic of a population

e.g. red eyes in Drosophila appeared in 1909

e.g. dark-coloured moth appeared in 1848

The "typica" form of the moth.

The "carbonaria" form.

Mutations can:

occur in: any gene at any timebe:

1. SPONTANEOUS

2. INDUCED

Spontaneous Mutations:

are permanent changes in the genome that occur without any outside influence

occur because the machinery of the cell is imperfect

Spontaneous Mutations arise because:

a) DNA polymerase makes errors in replication

b) meiosis is not perfect:

– non-disjunction

- translocationscan occur

One chromatid goes to each

daughter cell.

Both chromatids are sent to one daughter cell, the other gets

none. TranslocationChromosome 21 fuses

with 14.

Induced Mutations:

occur when some outside agent causes a permanent change in DNA

mutagens: anything that causes a mutation examples:

• Asbestos

• Tar from tobacco

• UV radiation

• Pesticides

• Caffeine UV light causes adjacent

thymines to cross link

Mutations can be:

1. Chromosomal mutations

2. Gene mutations or point mutations

MUTATION

CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of

the DNA

GENE/POINT MUTATIONSDescribe a change in the

structure of DNA at a single locus

CHANGES IN CHROMOSOME

NUMBER

CHANGES IN CHROMOSOME

STRUCTURE

ANEUPLOIDY EUPLOIDY / POLYPLOIDY

AUTOPOLYPLOIDY

ALLOPOLYPLOIDY

DELETION

DUPLICATION

INVERSION

TRANSLOCATION

INSERTION

INVERSION

DELETION

SUBSTITUTION

CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of the DNA

CHANGES IN CHROMOSOME

NUMBER

CHANGES IN CHROMOSOME STRUCTURE

ANEUPLOIDYLoss or gain of a

single chromosome

EUPLOIDY/POLYPLOIDYThe increase in entire

haploid sets of chromosomes

AUTOPOLYPLOIDYResults from an increase in

the number of chromosomes within the same species

ALLOPOLYPLOIDYChromosome number within

a sterile hybrid doubles, producing fertile hybrids

DELETION

DUPLICATION

INVERSION

TRANSLOCATION

Changes in Chromosome structure [not in syllabus]

MUTATION

CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of

the DNA

GENE/POINT MUTATIONSDescribe a change in the

structure of DNA at a single locus

CHANGES IN CHROMOSOME

NUMBER

CHANGES IN CHROMOSOME

STRUCTURE

ANEUPLOIDY EUPLOIDY / POLYPLOIDY

AUTOPOLYPLOIDY

ALLOPOLYPLOIDY

DELETION

DUPLICATION

INVERSION

TRANSLOCATION

INSERTION

INVERSION

DELETION

SUBSTITUTION

Aneuploidy:

loss or gain of a single chromosomeresults from non-disjunction

Down Syndrome: A meiotic error

Cause: non-disjunction at either:

anaphase I anaphase II

anaphase I anaphase II

Homologous chromosomes fail to

separate

Sister chromatids fail

to separate

Eggcell

Spermcell

n + 1

n (normal)

Zygote2n + 1

Fertilisation after non-disjunction in the mother results in a zygote with an extra

chromosome

Down’s Syndrome is a common form of chromosomal mutation in humans:

resulting from non-disjunction

Why is Down’s syndrome also called

trisomy 21?

Trisomy 21

Three copies of Chromosome 21

Causes of Down Syndrome:

1. 96% of the cases: non-disjunction of

chromosome 21 during anaphase of meiosis

2. 3-4% of the cases: translocation (movement) of:

chromosome 21 to chromosome 14, or less commonly to chromosome 22

Relationship Between Age and Aneuploidy

Older mothers more likely to produce aneuploid eggs

Sex Chromosome AbnormalitiesGenotype Gender Syndrome Physical Traits

XXY, XXYY, XXXY maleKlinefelter syndrome

sterility, small testicles, breast

enlargement

XYY male XYY syndrome normal male traits

Klinefelter syndrome

Sex Chromosome AbnormalitiesGenotype Gender Syndrome Physical Traits

XO femaleTurner

syndrome

sex organs do not mature , sterility,

short stature

XXX female Trisomy Xtall stature, learning disabilities, limited

fertility

Turner syndrome

Question: [SEP, 2009]

Suggest explanations for each of the following observations.

1. Meiosis generates biological diversity in eukaryotes. (2)

New combinations of alleles arise through crossing-over and random orientation of both bivalents and sister chromatids.

Question: [SEP, 2009]

Suggest explanations for each of the following observations.

2. Klinefelter’s Syndrome, where individuals would be characterised by a XXY combination of sex chromosomes, is caused by an anomalous event during meiosis. (2)

Due to non-disjunction of sex chromosomes, one cell would result in having two copies of the X chromosome rather than one. This can happen at either anaphase I or II.

MUTATION

CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of

the DNA

GENE/POINT MUTATIONSDescribe a change in the

structure of DNA at a single locus

CHANGES IN CHROMOSOME

NUMBER

CHANGES IN CHROMOSOME

STRUCTURE

ANEUPLOIDY EUPLOIDY / POLYPLOIDY

AUTOPOLYPLOIDY

ALLOPOLYPLOIDY

DELETION

DUPLICATION

INVERSION

TRANSLOCATION

INSERTION

INVERSION

DELETION

SUBSTITUTION

Euploidy (Polyploidy)

polyploids are gamete and somatic cells containing multiples of the haploid number of chromosomes

2n = 6 = 2x (diploid)

2n = 9 = 3x (triploid)

2n = 12 = 4x (tetraploid)

Homologous chromosomes

One extra set

Two extra sets

Plants commonly exhibit polyploidy: 95% of ferns 70% of angiosperms many of the fruits & grain are polyploid plants

are polyploid

Polyploidy is often associated with:

advantageous features such as

increased:- Size- Hardiness- Resistance to

disease

having advantageous features is called hybrid vigour

DiploidPolyploid

Hybrid vigour in corn

Polyploids with odd numbered chromosome sets are usually sterile

as they produce mostly aneuploid gametes

Somatic cell:Full set of chromosomes = 9

Gamete:Will contain the INCORRECT number of chromosomes

Benefit of Odd Ploidy-Induced Sterility

Seedless fruit watermelons and bananas

Not all watermelons are red. This triploid, seedless variety has sweet, yellow flesh.

Polyploidy is more common in:

plants than animals

Reason:

the increased number of chromosomes makes normal gamete formation during meiosis much more prone to errors

Why are polyploid plants still capable of reproduction?

As most plants are capable of vegetative propagation.

MUTATION

CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of

the DNA

GENE/POINT MUTATIONSDescribe a change in the

structure of DNA at a single locus

CHANGES IN CHROMOSOME

NUMBER

CHANGES IN CHROMOSOME

STRUCTURE

ANEUPLOIDY EUPLOIDY / POLYPLOIDY

AUTOPOLYPLOIDY

ALLOPOLYPLOIDY

DELETION

DUPLICATION

INVERSION

TRANSLOCATION

INSERTION

INVERSION

DELETION

SUBSTITUTION

Autopolyploidy:

• a condition that may arise naturally or artificially

• results from an increase in the number of chromosomes within the same species

2n = 64n = 12

2n

4n

Failure of cell divisionin a cell of a growing

diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.

Gametes produced by flowers on this

branch will be diploid.

Offspring with tetraploid karyotypes may be viable

and fertile—a new biological species.

Autopolyploidy can be induced by:

the drug colchicine – spindle formation is inhibited and chromatids fail to separate during anaphase

How is a 4n cell produced from a 2n one?

DNA replicates but cytoplasm does not split

Allopolyploidy

chromosome number within a sterile hybrid doubles,

producing a fertile hybrid

Let’s explain ‘Allopolyploidy’ using Spartina as an example

Spartina alterniflora

Spartina (cord grass): an example of allopolyploidy

Spartina alterniflora2n = 62

Spartina maritima2n = 60

1n 1870 in Townsend harbor in southern England:

a sterile hybrid of two Spartina species was found and was called Spartina townsendii

Spartina townsendii2n = 61

Gametes: n = 30 Gametes: n = 31 STERILE HYBRID

X

Spartina townsdendii went through a process of genome duplication

22 years later, a new vigorous and fertile form was recorded – it was named S. anglica S. maritima

2n = 60

S. alterniflora

2n = 62

n = 30 n = 31Gametes:

S. townsendii

2n = 61

4n = 122

S. anglica

Sterile hybrid:

Allopolyploidy

A NEW speciesS. anglica

Fertile hybrid :

Allopolyploidy has been described as ‘instant evolution’. Explain.

A new species is produced in a short time – over one generation

S. maritima

2n = 60

S. alterniflora

2n = 62

n = 30 n = 31Gametes:

S. townsendii

2n = 61

4n = 122

S. anglica

Sterile hybrid:

Allopolyploidy

How can S. townsendii reproduce, if it is sterile?

Vegetatitve propagation by producing:

Why is S. townsendii sterile?

Meiosis cannot occur properly as chromosomes are NOT

homologous in S. townsendii

Consider this example:

No homologous chromosomes in sterile hybrid:

Species 1 Species 2

2n = 4 2n = 6

Sterile hybrid: 2n = 5

Gametes:

X

X

n = 2 n = 3

BUT what if chromosome

number doubles in a sterile hybrid?

Now homologous chromosomes are present!!

Sterile hybrid: 2n = 5

Chromosome doubling:

Allopolyploidy

4n = 10

Can this new organism produce viable gametes?

YES

Why is the new species fertile?

Sterile hybrid:

2n = 5

Fertile new species:

2n = 10

Meiosis can occur properly as homologous chromosomes are

present

Individual becomes fertile after chromosome doubling

Gametes:

n = 5

2n = 10

X

X

a new species is produced which is fertile with polyploids like itself but infertile with parental species

2n = 10

Meiotic error in anther during pollen grain formation

Sterile hybrid

AB

Meiotic error

AABB

X

ABD

Meiotic error

T. turgidum(Emmer wheat)

(2n = 28)

X

Sterile hybrid

Spot where allopolyploidy

occurred during the

development of T. aestivum.

a b c d e

Primitive wheat (a) crossed withwild grass (b) to produce an infertile hybrid.

Chromosome doubling produces a fertile hybrid (c)

which is crossed with wild grass(d) to produce an infertile hybrid

Chromosome doubling results in fertile hybrid (e)

This hybrid is a cultivated wheat used for flour production

Many crop plants result from doubling of chromosome number

Question: [SEP, 1999]

1. What is polyploidy? (2)

Gamete and somatic cells containing multiples of the haploid number of chromosomes.

2. Suggest TWO mechanisms that may result in polyploidy. (4)

Allopolyploidy – chromosomes double in a sterile F1 hybrid.

Autopolyploidy – duplication of chromosomes of a single species after fertilisation.

Question: [SEP, 2003]

Two closely-related plants, A and B, have chromosome sets AA and BB respectively. When Plant A is crossed with Plant B a hybrid, possessing chromosome set AB, is produced. Such hybrids are sterile.

a) Why are hybrid plants sterile? (3)

As there are no homologous chromosomes which is a prerequisite for meiosis to occur.

Question: [SEP, 2003]b) Fertile polyploidy hybrids might develop from

these sterile plants. Describe a sequence of events that would enable this to occur. (3)

Chromosomes replicate but cytokinesis does not occur. This results in a cell having homologous chromosomes and so meiosis can occur.

c) Given the results observed in these crosses, would you conclude that Plant A and Plant B belong to the same species or to different species? Justify your answer. (3)

Belong to different species. Offspring produced are sterile.

MUTATION

CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of

the DNA

GENE/POINT MUTATIONSDescribe a change in the

structure of DNA at a single locus

CHANGES IN CHROMOSOME

NUMBER

CHANGES IN CHROMOSOME

STRUCTURE

ANEUPLOIDY EUPLOIDY / POLYPLOIDY

AUTOPOLYPLOIDY

ALLOPOLYPLOIDY

DELETION

DUPLICATION

INVERSION

TRANSLOCATION

INSERTION

INVERSION

DELETION

SUBSTITUTION

MUTATIONChange in the amount, structure or arrangement of the DNA of

an organism

GENE/POINT MUTATIONSDescribe a change in the structure of DNA at a single locus

INSERTION

INVERSION

DELETION

SUBSTITUTION

Fig. 6 Gene or point mutation

1) INSERTION: the addition of an extra nucleotide

A GT G C A TA TT G A C A G

2) DELETION: involves the loss of a nucleotide

A GT G C A T A TT C A G

Fig. 6 Gene or point mutation

4) SUBSTITUTION:

a particular base is substituted by another (e.g. sickle-cell anaemia)

A GT G C A T A TT G T A G

3) INVERSION: two nucleotides become arranged in the wrong order

A GT G C A T T TA G C A G

What is a:

Silent mutation:

an alteration in a DNA sequence that does not result in an amino acid change in a polypeptide

Frameshift mutation:

is caused by insertions or deletions of nucleotides which shifts the codon triplets of the genetic code of mRNA and causes a misreading during translation

A silent mutation: no effect on amino acid sequence

A frameshift mutation:

Sickle Cell Anaemia:

in humans is an example of base substitution

affects a base in one of the genes involved in producing haemoglobin

at position 14 in the DNA: the base thymine is

replaced by adenine

Amino acid sequence

Sickle Cell Anaemia:

at low oxygen tensions, haemoglobin S crystallises in the red cells distorting them into a sickle shape

Sickle Cell Anaemia:

many sickle cells are destroyed in the circulation

Result: oxygen-carrying capacity of the blood is lowered

incidence is very high in Africa and Asia

Sickle Cell Trait is the heterozygous condition

heterozygotes are resistant to some forms of malaria

Incidence of Malaria Incidence of Sickle cell anaemia

Inheritance of sickle cell anaemia:

Carrier parents : HbAHbS x HbAHbS

F1 phenotypes: 25% normal : 50% carriers : 25% sufferers

F1 genotypes: HbAHbA HbSHbA HbSHbA HbSHbS

Gametes:  

HbAHbAHbS

HbSx

Essay Titles

1. Variation among organisms and between individuals of the same species is purely the result of genetic differences. Discuss this statement. [SEP, 2003]

2. “Random mutations are the driving force of evolutionary change”. Discuss. [SEP, 2009]

3. Compare and contrast the roles of genetic mutations and meiosis in generating genetic diversity. [MAY, 2010]

END OF SECTION