A3.1 Diversity of Organisms

Define variation

Trait differences among individuals of the same species

What is the difference between polygenic & monogenic traits?

Polygenic traits - Inherited traits controlled by multiple genes

Monogenic traits - Inherited traits controlled by single genes

Compare the types of variation (3×2)

Type	Description	Heritability	Examples
Continuous	Traits vary gradually across a range	Inherited, polygenic	Height, skin colour
Discontinuous	Traits have distinct categories	Inherited, monogenic	Blood type, flower colour

Compare the causes of variation (2×2)

Variation	Description	Examples
Genetic	Arises from DNA differences	Mutations, meiosis
Environmental	Comes from external factors	Sunlight, nutrition

• Most traits are a combination of both genes & environment

Define species

A group of organisms that can interbreed and produce fertile offspring

Describe the Morphological Species Concept

• Carl Linnaeus developed a concept that defined species as:

“A group of organisms that look similar and can be distinguished from other groups based on shared, observable traits (morphology)” 

What is the binomial system?

• All species have a binomial name, a unique two-part Latinised label

  • The first word represents the genus & second word represents the specific species within the genus 

• Species within the same genus share a common ancestor & have similar genetic/structural traits 

Describe the Biological Species Concept (4)

• A concept that defines species as:

“A group of organisms that can interbreed and produce fertile offspring under natural conditions”

Principles of BSC: 

• Interbreeding - Members of the same species can successfully reproduce with one another 

• Shared gene pool - All individuals exchange genes within same population → maintains genetic continuity & diversity 

• Reproductive isolation - Different species are separated by barriers → inhibited gene flow & independent evolution is ensured 

Define speciation

The process by which one species splits into two or more new species

Describe how speciation occurs

• As populations of the same species stop interbeeding, they accumulate:

  • Genetic differences through mutation & genetic drift

  • Phenotypic differences through natural selection

Explain the difficulties in classifying speciation (4)

• It is hard to decide when diverging populations have become distinct species because:

Issue	Explanation
Gradual change	Speciation is a continuous process → cannot pinpoint time of divergence
Geographical isolation	Populations may not interbreed due to distance & not genetic incompatibility
Hybrid formation	Sometimes diverging populations can still produce hybrids = unclear if they’re truly separate species

Describe the diversity of chromosome number in organisms (3)

• Members of a species typically share the same number of chromosomes

• Most organisms have a diploid number of chromosomes in their somatic cells

  • Diploid cells always have an even number as chromosomes come in pairs (one from each parent)

• Chromosome number can change over time through fusion, fission, or duplication/deletion of segments

Explain the difference in chromosome number between humans and chimpanzees (2)

• Human chromosome 2 is the result of a fusion between two ancestral ape chromosomes after speciation 

• Hence explaining why humans have 46 chromosomes while chimpanzees, gorillas, & orangutans have 48 

Define karyotype

The complete chromosome set of an organism

What is a karyogram?

The visual chart of an organism’s homologous chromosome pairs

How are chromosomes ordered in a karyogram? (

• Length - longest to shortest

• Banding pattern - identified by staining w/ dyes like Giemsa

• Centromere position  - e.g. metacentric, submetacentric, acrocentric

Describe the uses of karyograms (3)

Purpose	Explanation
Sex determination	Identify XX or XY chromosomes
Diagnosing disorders	Identify extra/missing chromosomes
Evolutionary comparison	Compare chromosomes between species

Define genome

The entire set of genetic material in an organism

Describe the genome

• An organism’s genome includes both coding (encode proteins) & non-coding (regulate gene expression/other functions) regions 

• While genomes are largely uniform within a species, small variations create uniqueness

• Variation between species is much greater than within one species

Describe sources of genetic diversity (4)

1) Single-Nucleotide Polymorphisms (SNPs) 

• This is a change in a single DNA base at a specific position (e.g. G instead of A)

• There are millions of SNPs across the genome 

2) Insertions/Deletions (Indels) 

• Small stretches of DNA added or deleted → may affect gene function or expression 

3) Copy Number Variations (CNVs) 

• Sections of DNA may appear in multiple copies for some individual

Describe diversity of eukaryote genomes (4)

• All eukaryotes share a universal genetic code but their genomes vary in total size & base sequence:

  • Total DNA content (genome size) - Larger genomes contain more non-coding DNA (e.g. introns & repetitive sequences) 

  • Base sequence composition - Differences in nucleotide order accumulate with mutation, gene duplication, & deletion 

What is the C-value paradox? (2)

• Larger genomes do not necessarily mean greater biological complexity

• Many simple organisms have more DNA than complex ones — known as the C-value paradox 

Describe Whole Genome Sequencing (WGS)

A DNA sequencing technique that determines the order of all or most of an organism’s DNA bases

• It is now rapid & affordable, transforming biological & medical research 

Describe the current applications of Whole Genome Sequencing (WGS) (3)

1) Comparative genomics - Aligning DNA sequences identifies conserved genes (essential functions) & divergent genes (adaptive traits)

2) Molecular clocks - Mutation rates estimate divergence times between species

3) Horizontal gene transfer - Reveals genes transferred between unrelated species 

Describe the future uses of Whole Genome Sequencing (WGS) (3)

1) Personalised medicine - Tailoring medical treatment to individual genetic profile

2) Gene therapy - Designing treatments to correct faulty genes 

3) Preventative healthcare - Detecting genetic  risks for diseases before symptoms appear 

Explain why Biological Species Concept doesn’t apply to asexually reproducing species (2)

Feature	Why BSC doesn’t apply
No mating occurs	There’s no interbreeding so species can’t be defined by reproductive compatibility
Reproduction by cloning	Offspring are genetically identical so species boundaries are difficult to define

Define horizontal gene transfer

The direct exchange of genetic material between unrelated organisms

Explain why Biological Species Concept doesn’t apply to bacterial horizontal gene transfer (2)

Feature	Why BSC doesn’t apply
Bacteria can exchange genes across species lines	Blurs boundaries between species
New traits arise suddenly	Confuses define species based on shared traits