Evidence of Relatedness (VCE SSCE Biology): Revision Notes

Evidence of Relatedness

Scientists use various types of evidence to determine how closely related different species are to each other. This evidence falls into two main categories: structural morphology (comparing physical features) and molecular homology (comparing DNA and protein sequences). Understanding these methods helps us trace evolutionary relationships and identify common ancestors.

Structural morphology

Structural morphology is the study of physical structures to establish relatedness between different species. By examining the physical features of organisms, such as their skeletal structures, scientists can identify similarities that suggest shared ancestry.

Homologous structures

Homologous structures are features present in two or more species that may look and function very differently in each species, but are derived from a common ancestor.

Even though homologous structures may serve completely different purposes in different species, they share the same basic underlying structure. This similarity indicates that these species evolved from a common ancestor that possessed this structure.

A classic example is the upper limb structure found in humans, cats, whales, and bats. Each species uses its limbs differently:

• Humans use their arms to carry objects
• Cats use their legs to walk
• Whales use their flippers to swim
• Bats use their wings to fly

Despite these different functions, all four limbs share a remarkably similar bone structure. The same bones are present in each limb, just modified in size and shape.

This similarity suggests that humans, cats, whales, and bats all descended from a common ancestor that had this particular limb structure. Over time, through evolution, each species adapted the basic structure for different purposes.

Divergent evolution

Homologous structures provide physical evidence of divergent evolution, which is the process in which a common ancestor evolves into two or more descendant species. This occurs when populations of a single species become separated and adapt to different environments or selection pressures. Over extended periods, the accumulated genetic differences become so great that the populations are classified as separate species.

Analogous structures (for comparison)

It's important to distinguish homologous structures from analogous structures. Analogous structures are features present in two or more species that fulfil the same function but do not originate from a common ancestor.

Analogous structures arise through convergent evolution, which is the process in which distantly related species evolve similar traits over time due to the action of similar selection pressures. Species facing similar environmental challenges may independently evolve similar solutions, even though they don't share a recent common ancestor.

For example, bird wings and insect wings are analogous structures. Both are used for flight, but they evolved completely independently:

• Bird wings contain bones and are modified forelimbs
• Insect wings are thin membranes without any bones

The similarity in function doesn't indicate shared ancestry - it indicates that flight was advantageous in both lineages, so similar structures evolved separately.

Vestigial structures

Vestigial structures are features that have lost all or most of their usefulness as a result of evolution by natural selection.

These structures once served an important purpose in an organism's ancestors, but changing environmental conditions and selection pressures meant they were no longer necessary for survival. However, because they weren't harmful enough to be selected against, they remained in the species.

Vestigial structures provide evidence of evolutionary relationships because they reveal information about an organism's ancestral history.

The human coccyx

The human coccyx (commonly called the tailbone) is a vestigial structure that serves no significant function in modern humans.

The coccyx is a remnant of our ancestors' tails. Early human ancestors lived in trees and used their tails for balance. As humans evolved, more sophisticated features developed to help with balance, including the cerebellum (a brain structure) and the inner ear. These new features made the tail unnecessary for survival.

Although the tail lost its function, the small tailbone remained because there was no strong selection pressure against it. Its presence demonstrates our evolutionary connection to tailed ancestors.

Pelvic bones in snakes and whales

Vestigial structures aren't unique to humans. Many other animals possess structures that no longer serve their original purpose.

Both snakes and whales have pelvic bones, despite neither having legs:

• Snakes are descendants of reptiles that had legs
• Whales are descendants of land-dwelling mammals that had legs

The presence of these vestigial pelvic bones provides evidence of these species' evolutionary history and their descent from legged ancestors.

Molecular homology

While structural morphology examines physical features, molecular homology looks at the biochemical level. Molecular homology is the study of the similarities in the nucleotide sequences of DNA or amino acid sequences in proteins between organisms to establish relatedness.

Molecular evidence can reveal evolutionary relationships that aren't obvious from physical features alone. By comparing the sequences of specific genes or proteins across different species, scientists can determine how closely related they are.

Amino acid sequence similarity

Proteins are made of chains of amino acids. The sequence of amino acids in a protein is determined by the DNA sequence of the gene that codes for it. When studying relatedness, scientists focus on conserved genes—genes that have remained largely unchanged throughout evolution and are found across the genomes of many different species.

By comparing the amino acid sequences of the same protein in different species, researchers can assess how closely related those species are. Species with more similar amino acid sequences are more closely related and share a more recent common ancestor.

Haemoglobin analysis

Haemoglobin (Hb) is a protein found in red blood cells that is responsible for the transport of oxygen in the body. It's found in many different species, making it useful for comparison.

Haemoglobin consists of four polypeptide chains:

• 2 alpha chains (141 amino acids each)
• 2 beta chains (146 amino acids each)

The table below shows a portion of the haemoglobin amino acid sequence in humans and four other vertebrates:

Position	1	2	3	4	5	6	7	8	9	10	11	12	13	14
Human	Arg	Leu	Leu	Gly	Asn	Val	Leu	Val	Cys	Val	Leu	Ala	His
Chimpanzee	Arg	Leu	Leu	Gly	Asn	Val	Leu	Val	Cys	Val	Leu	Ala	His
Gorilla	Lys	Leu	Leu	Gly	Asn	Val	Leu	Val	Cys	Val	Leu	Ala	His
Kangaroo	Lys	Leu	Leu	Gly	Asn	Ile	Ile	Val	Ile	Cys	Leu	Ala	Glu

Cytochrome c analysis

Cytochrome c is an enzyme found in mitochondria that carries electrons in aerobic and anaerobic respiration reactions. It consists of 104 amino acids encoded by a conserved gene in mitochondrial DNA (mtDNA), which is circular DNA found in mitochondria.

The table below shows a segment of the cytochrome c amino acid sequence in three species:

Position	1	2	3	4	5	6	7	8	9
Human	Ser	Tyr	Thr	Gly	Ala	Asn	Ala	Lys	Asn
Rat	Ser	Tyr	Thr	Gly	Asp	Asn	Ala	Lys	Asn
Yeast	Ser	Tyr	Thr	Gly	Asp	Asn	Ala	Lys	Lys

In this segment, rats show more similarity to humans than yeast does, with only one difference compared to two differences for yeast.

When we examine the complete cytochrome c sequence, we can compare the total number of amino acid differences between humans and various species:

Species pairing	Number of amino acid differences
Human – chimpanzee	0
Human – rhesus monkey	1
Human – horse	12
Human – pigeon	12
Human – rattlesnake	14
Human – snapping turtle	15
Human – tuna	21
Human – fruit fly	29
Human – wheat	43

DNA sequence similarity

Just as amino acid sequences can be compared, DNA (deoxyribonucleic acid) sequences can also be used to determine evolutionary relatedness. DNA is a double-stranded nucleic acid chain made up of nucleotides. DNA carries the instructions for proteins which are required for cell and organism survival.

A nucleotide is the monomer subunit of nucleic acids. It's made up of a nitrogen-containing base, a five-carbon sugar molecule (ribose in RNA and deoxyribose in DNA), and a phosphate group.

DNA contains four different nitrogenous bases:

• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

Scientists compare DNA sequences between species by examining the order of these bases in corresponding gene regions. The more similar the sequences, the more closely related the species.

Comparing DNA sequences

The table below shows a comparison of a DNA sequence segment from the cytochrome c gene in three species:

Species	DNA Sequence (5' to 3')
Human	AGA ATA TGA CGG CGA TTG TTC TTA
Rat	AGA ATA TGA CTG CGT TTG TTT TTA
Yeast	AGA ATG TGG CTG CGT TTG TAT TTC

Advantages of using mitochondrial DNA

Mitochondria contain their own DNA, separate from the DNA in the cell nucleus. This mitochondrial DNA (mtDNA) is approximately 17,000 nucleotides long in humans and contains 37 genes, compared to over 24,000 genes in nuclear DNA (DNA that is located in the nucleus of a cell).

These advantages make mtDNA particularly useful for tracing human evolutionary history. Research using mtDNA has shown that all modern humans share a common female ancestor who lived in Africa approximately 160,000 years ago.

Choosing between amino acid and DNA sequence analysis

Both methods help determine relatedness, but each has advantages in different situations:

Whole genome comparison

Beyond individual genes, scientists can compare entire genomes (the complete set of DNA housed within an organism) between species. Higher similarity between genomes indicates closer evolutionary relationship and more recent divergence from a common ancestor.

For example, comparing the percentage of amino acids identical to human haemoglobin across species shows:

• Chimpanzees: ~99% similarity
• Mice: 87% similarity
• Chickens: 69% similarity
• Frogs: 54% similarity
• Lampreys: 14% similarity

This clearly illustrates the evolutionary distance between humans and each species.

Summary of evidence types

The table below summarises the two main approaches to determining relatedness:

Structural morphology	Molecular homology
Homologous structures: Features that have the same underlying structure but different functions. The presence of homologous structures in different species suggests they share a common ancestor.	Amino acid sequences: Researchers examine shared proteins between species and measure the degree of amino acid similarity to demonstrate relatedness. Higher similarity suggests closer relationship.
Vestigial structures: Structures that remain within a species despite losing their function and necessity. Like homologous structures, their presence suggests the organisms being compared share a common ancestor.	DNA sequences: Researchers examine corresponding gene regions (or entire genomes) between species and demonstrate relatedness based on nucleotide differences. Higher similarity suggests closer relationship.

Both structural and molecular evidence complement each other to provide a comprehensive picture of evolutionary relationships between species.