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Homologous structures

Content objective:

What are we learning and why are we learning this? Content, procedures, or skills.

Vocabulary objective

Tier II: High frequency words used across content areas. Key to understanding directions & relationships, and for making inferences.

Tier III: Low frequency, domain specific terms.

Building on what we already know

Make connections to prior knowledge. This is where we build from.


What are homographs? Homographs are words that have the same spelling, but different meanings.


What does homologous mean? It derives from the Greek ὁμόλογος homologos – from ὁμός homos “same” – and λόγος logos “plan.”

What are homologous structures? They are structures derived from a common, ancestral structure.

The limbs of humans, cats, whales and bats aren’t just similar in function and shape. They are literally built with similar bone structure, because they are programmed by the same basic set of genes.

homologous_forelimbs Human Cat Whale Bat

Look at the similarities of ankle bones: dogs, ancient whales, pigs and deer.

This similarity is no coincidence – they are homologous structures.

ankle bones of dog Pakicetus (whale) pig deer

The first vertebrates on land had four limbs, with five fingers on each. Over time, vertebrates evolved into many different groups, and most of them retain homologous structures, as seen here:

adaptive radiation from five fingered hand

Analogous structures

What is an analogy? An analogy is a comparison between two objects, that highlights ways in which they are similar.

Types of analogies

The opposite of homologous structures are analogous structures.

Analogous structures look similar, and do similar jobs, but they did not evolve from a common structure – they evolved independently.

Consider the evolution of wings in bats, birds, and ancient flying reptiles. They evolved through convergent evolution.

“It is important to distinguish between different levels of homology, in order to make informative biological comparisons: bird and bat wings are analogous as wings, but homologous as forelimbs. Why? Because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods.” {http://en.wikipedia.org/wiki/Homology_(biology)

Homologies from Understanding Evolution

Images from

1 Comment

  1. […] Similar structures like the one described above can be found throughout various different species for many millions of years. Here are some links to get you started. […]


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