Evidence of Relatedness Between Species: Morphology & DNA

1. Structural Morphology

1.1 Homologous Structures

• Definition: Structures in different species that have a similar underlying anatomy due to shared ancestry, but may have different functions.
• Origin: Derived from a common ancestor.
• Evidence of: Divergent evolution; species evolving in different directions from a common point.
• Examples:
  • The pentadactyl limb (five-fingered limb) found in vertebrates like humans, bats, whales, and birds. The basic bone structure is the same, but the limb is modified for different functions (grasping, flying, swimming, walking).
  • The forelimbs of vertebrates:
    • Human arm: used for lifting and manipulating objects
    • Bat wing: used for flight
    • Whale flipper: used for swimming
• Diagram:
  • Description: A diagram showing the skeletal structure of the pentadactyl limb in different vertebrates (human, bat, whale, bird) highlighting the similar bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges) despite the different overall appearance and function.

KEY TAKEAWAY: Homologous structures indicate common ancestry, regardless of current function. They are evidence of divergent evolution.

1.2 Vestigial Structures

• Definition: Structures in an organism that have lost most or all of their original function through evolution.
• Origin: Remnants of structures that were functional in ancestral species.
• Evidence of: Evolutionary change; structures once useful are no longer needed due to environmental changes or adaptation.

• Examples:

  • Human appendix: A small, pouch-like structure attached to the large intestine. It likely aided in digesting cellulose-rich diets in our ancestors, but now has little to no function.
  • Human coccyx (tailbone): A small bone at the base of the spine, representing a reduced tail.
  • Wings of flightless birds (e.g., emus, ostriches): These birds have wings, but they are too small to allow for flight.
  • Pelvic bones in whales: Whales evolved from land-dwelling mammals. Their pelvic bones are remnants of the hind limbs of their ancestors.
  • Pilomotor reflex in humans: The reflex that causes goosebumps. In animals with fur, this reflex raises the fur to provide insulation or make the animal appear larger. Humans have lost most of their fur, so the reflex is less useful.

• Diagram:

  • Description: A diagram showing the human appendix and coccyx, and the wings of a flightless bird (emu), highlighting their reduced size and lack of original function.

EXAM TIP: When describing vestigial structures, always mention the likely function in the ancestor and the current lack of function.

1.3 Analogous Structures

• Structures that have similar functions and appearances but do not share a common ancestry. (Butterfly wings and Bird wings)

COMMON MISTAKE: Confusing homologous and analogous structures. Homologous structures share a common ancestry but not necessarily function, while analogous structures share a function but not a common ancestry.

2. Molecular Homology

2.1 DNA Sequences

• Definition: Comparing the nucleotide sequences of DNA molecules to determine the degree of relatedness between species.
• Principle: Closely related species will have more similar DNA sequences than distantly related species.
• Method:
  • DNA sequencing: Determining the order of nucleotides in a DNA molecule.
  • Sequence alignment: Arranging DNA sequences from different species side-by-side to identify regions of similarity and difference.
  • Calculating sequence similarity: Determining the percentage of nucleotides that are identical between two sequences.
• Evidence:
  • High sequence similarity indicates a recent common ancestor.
  • Low sequence similarity indicates a more distant common ancestor.
• Example: Comparing the DNA sequences of humans and chimpanzees reveals a high degree of similarity (around 98%), indicating a close evolutionary relationship.

STUDY HINT: Focus on understanding the principles behind molecular homology rather than memorizing specific percentages of sequence similarity.

2.2 Amino Acid Sequences

• Definition: Comparing the amino acid sequences of proteins to determine the degree of relatedness between species.
• Principle: Closely related species will have more similar amino acid sequences in their proteins than distantly related species.
• Method:
  • Protein sequencing: Determining the order of amino acids in a protein molecule.
  • Sequence alignment: Arranging amino acid sequences from different species side-by-side to identify regions of similarity and difference.
  • Calculating sequence similarity: Determining the percentage of amino acids that are identical between two sequences.
• Evidence:
  • Proteins with highly conserved functions (e.g., cytochrome c, hemoglobin) are often used for comparison because changes in their amino acid sequences are less likely to be detrimental.
  • High sequence similarity indicates a recent common ancestor.
  • Low sequence similarity indicates a more distant common ancestor.
• Example: Comparing the amino acid sequence of cytochrome c (a protein involved in cellular respiration) in different species shows that humans are more closely related to chimpanzees than to yeast.

REMEMBER: DNA codes for proteins, so similarities in DNA sequences generally lead to similarities in amino acid sequences.

2.3 Interpretation of Molecular Data

• Phylogenetic Trees: Molecular data is used to construct phylogenetic trees, which visually represent the evolutionary relationships between species.
• Branch Lengths: The length of the branches in a phylogenetic tree can be proportional to the amount of genetic difference between species, providing an estimate of the time since they diverged from a common ancestor.
• Nodes: Represent common ancestors.

APPLICATION: Molecular homology is used in various fields, including medicine (understanding disease origins), agriculture (improving crop yields), and conservation biology (identifying endangered species).

2.4 Differences Between DNA and Amino Acid Sequences

VCAA FOCUS: Be prepared to explain how both structural morphology (homologous and vestigial structures) and molecular homology (DNA and amino acid sequences) provide evidence for evolution and relatedness between species. Be able to compare and contrast them.