Brent Cornell

Key Knowledge: Evidence of relatedness between species: structural morphology – homologous and vestigial structures

Evolution can be demonstrated by comparing the anatomy of groups of organisms to identify similarities (i.e. structural evidence)

• The presence of shared structural morphologies in different species would indicate a common ancestry (and demonstrate relatedness)

Homologous Structures

Anatomical features that are similar in basic structure despite being used in different ways are called homologous structures

• Homologous structures arise as a result of divergent evolution, where common structures evolve into a variety of distinct forms due to the presence of different selective pressures
• The more similar the homologous structures between two species are, the more closely related the two species are likely to be (as less time has passed since the point of evolutionary divergence)

A classical example of homologous structures is the pentadactyl limb in a variety of different animals

• Mammals, birds, amphibians and reptiles all share a similar arrangement of bones in their appendages based on a five-digit limb 
• Despite possessing similar bone arrangements, animal limbs may be highly dissimilar according to the mode of locomotion:
  • Human hands are adapted for tool manipulation (power vs precision grip)
  • Bird and bat wings are adapted for flying
  • Horse hooves are adapted for galloping
  • Whale and dolphin fins are adapted for swimming

Analogous Structures

Not all morphological similarities represent relatedness – unrelated species can share common structures due to convergent evolution

• Convergent evolution is the independent evolution of similar features in species with distinct lineages
• It may occur when different species occupy the same habitat and are thus subjected to the same selection pressures
• The shared conditions cause common adaptations to be selected in different species, resulting in structural similarity

Traits that are superficially similar but were derived through separate evolutionary pathways are termed analogous structures

• They are essentially the opposite of homologous structures (traits that are similar because they are derived from common ancestry)
• An example of analogous structures can be seen in the streamlined body shape of a dolphin (mammal) and a shark (fish)

Vestigial Structures

Some species show the presence of functionless and reduced remnants of structures that were once present in their ancestors

• Changes to the environment have rendered these structures redundant and so over time they have lost their functionality

These anatomical features are called vestigial structures and demonstrate the evolutionary divergence of a species from a past activity

• An example of a vestigial structure is the pelvic bone in whales – this bone suggests that whale ancestors were terrestrial mammals

Comparative Embryology

Studying the growing embryo in animals or plants shows that closely related species go through similar stages of development

• These similarities in embryonic development suggest that the organisms shared a common evolutionary pathway

Comparing the embryonic development of a range of diverse animal species, it can be seen that:

• All terrestrial animals have non-functioning gill slits (pharyngeal slits) as early embryos (suggesting an aquatic origin)
• Many vertebrates (including humans) demonstrate a primitive tail at an early stage of embryonic development