Comparative Genomics: Homology

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  • 0:01 What Is Homology?
  • 0:41 Evolutionary Divergence
  • 1:48 Homologous Structures
  • 2:38 Vestigial Structures
  • 3:23 Analogous Structures
  • 3:40 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
Evolutionary heritage links many modern species by way of a common ancestor. Homology examines the structural similarities between species that reveal their ancestry. Here we will examine the concepts of homology, look at examples, and finish with a brief quiz.

What Is Homology?

We know that organisms we see today evolved from extinct species, and that their ancestry can be traced using the fossil record and, sometimes, genetic analysis. Many existing organisms share a recent common ancestor, and so they are genetically closely related. This is just like how you and your cousins share a common ancestor - one set of grandparents. You and your cousins may even share a distinct physical trait that came from those grandparents, linking you together by way of your common ancestry. Different but closely related species have these types of traits as well. The study of homology examines related evolutionary structures and uses them as pieces of the evolutionary puzzle.

Evolutionary Divergence

The common ancestor of closely related species existed at a kind of evolutionary 'fork in the road,' where a species divergence occurred. An evolutionary divergence is when one species separates into two separate species over time. It could have happened because of a rapid shift in the environment, or perhaps because one population of a species migrated to a new geographical location and had to adapt to new conditions, but either way, over millions of years, one species became two. For example, humans and chimpanzees share an extinct, ape-like ancestor that was distinct from both modern species but had some of the same characteristics that we see today.

Homologous structures are the physical characteristics that are common among ancestor species and existing 'cousin' species. As you might expect, organisms that are closer to each other in evolutionary distance tend to have more homologous structures than organisms that are more distantly related.

Homologous Structures

Although homologous structures were passed down from a common ancestor, they don't necessarily have the same functions among the modern cousin species. The best example of this is the series of forelimb bones that we see in human arms, bat wings, and dolphin pectoral fins. All three animals share a common land mammal ancestor who also had a humerus, radius, ulna, carpals, metacarpals, and phalanges that make up the forelimb structure, though they do not all use them in the same way. Bats use their wings for flying, dolphins use their pectoral fins for lateral stability and steering, and we humans use our arms for, well, a variety of tasks, including balance when we walk. While phalanges, or finger bones, are very useful on human hands, the phalanges found on dolphin fins are not so articulated.

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