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Bringing Homologies Into Focus Evo Edu Outreach (2008) 1:498–504 DOI 10.1007/s12052-008-0080-5 VIEWS FROM UNDERSTANDING EVOLUTION Bringing Homologies Into Focus Anastasia Thanukos Published online: 26 September 2008 # Springer Science + Business Media, LLC 2008 Abstract Anyone who has skimmed a high school biology Subsequent evolution in the lineage leading to modern textbook will be familiar with the iconic examples of humans and in the lineage leading to modern lizards has homology that seem inseparable from any explanation of resulted in differences between our two appendages: bones the term: the limb structure of four-legged animals, the with slightly different shapes, orientations, and functions. human tailbone and the more elaborate tail of monkeys, and Nevertheless, the common evolutionary origin of the two the remarkable similarities among the embryological appendages is evident in their deep similarities. Though development of fish, birds, and humans. These same differently shaped, corresponding bones are present in the examples make their way from edition to edition, along two groups of organisms and develop in similar ways. The with the classic illustration of an analogous structure: the same set of arguments applies to human and lizard eyes. wings of butterflies, birds, and bats. But is that really all Both lineages bear the same type of complex lens eyes that there is to say about homologies and analogies? Several we inherited from our common ancestor (Fig. 2). articles in this issue discuss these concepts more deeply in Analogies, on the other hand, are similar traits that the context of eye evolution (Gregory 2008; Oakley and evolved through convergent evolution. In the classic Pankey 2008; Piatigorsky 2008). Homologies and analo- example, evolution independently shaped the forelimb of gies, it seems, are not a black and white issue—especially a dinosaur, the forelimb of an ancient mammal, and the gill- when it comes to vision. like appendage of an ancient insect into wings that could carry their bearers (birds, bats, and insects) through the air. Keywords Homology. Homoplasy. Analogy. Teaching The fact that modern birds, bats, and most insects have wings reflects, not common ancestry, but similar processes of exaptation and subsequent adaptation. In the same way, A Textbook Review of Homologies and Analogies the complex lens eyes of humans and squid are remarkably similar (Fig. 3) but analogous, since they evolved inde- Homologies are traits present in two or more organisms that pendently in our ancestral lineages (Fig. 4). were inherited from the common ancestor of those organisms. The human five-fingered hand and the five-toed foot of a lizard, for example, were both inherited from our Beyond Analogies common ancestor that lived more than 300 Mya (Fig. 1). So far, so good. This is the view presented by standard biology textbooks. However, biologists often depart from high school texts in their analysis of analogies (e.g., Hall A. Thanukos (*) 2007; Gregory 2008). The counterpart of homology is University of California Museum of Paleontology, usually considered to be homoplasy, a much broader 1101 Valley Life Sciences Building, concept than analogy. A homoplasious trait is a similarity Berkeley, CA 94720-4780, USA e-mail: [email protected] among organisms that was not inherited from the common URL: http://evolution.berkeley.edu ancestor of those organisms. Homoplasies can evolve in Evo Edu Outreach (2008) 1:498–504 499 Fig. 1 Humans and lizards inherited appendages with similar structures from a common ancestor whose limbs human also had this structure. Illustra- ancestral tetrapod tion adapted with permission from the Understanding Evolution website lizard three ways (though the lines between these categories are and then regains them, producing a species similar to a often blurry): closely related lineage that never lost its stripes in the first place. A real-life example of evolutionary reversal & Convergent evolution. This process produces analogies, involves stick insects, which evolved from a winged as discussed above. Two lineages that begin with ancestor but then lost those wings. Stick insects different traits evolve a similar characteristic indepen- diversified into many different species in their new dently of one another, often because both lineages face wingless form. However, a few stick insect lineages similar environmental challenges and selective pres- seem to have independently “reevolved” wings by sures. For example, two distantly related plant lineages reactivating an ancient genetic program that produces might evolve analogous tubular red flowers under wings (Whiting et al. 2003). These newly winged selection from hummingbird nectar feeders. Or, as in insects now have wings similar to one another and to Fig. 5a, fish lineages that originally had different body more distantly related winged insects—insects which patterns might independently evolve analogous vertical never lost their wings in the first place. However, the stripes, perhaps because of selection for a particular wings in these different groups are homoplasious since camouflage pattern. they were not directly inherited from a common & Parallel evolution. In this process, two traits that are ancestor: the winged stick insects went through an already similar (usually because of common ancestry) intermediary wingless stage. independently evolve the same set of changes—generally meaning that the same set of underlying genes are involved. This is illustrated in Fig. 5b in which two Eye evolution, in fact, exhibits both convergent and closely related fish lineages evolve in the same way, parallel evolution. The similarity in structure between squid resulting in two lineages with a striped body pattern. A and vertebrate eyes (Fig. 3) is a classic example of real-life example of parallel evolution also involves fish. convergent evolution, since our most recent common The ancestral stickleback fish was marine-living and ancestor may have borne nothing more complex than a heavily armored by sturdy plates. However, several few light-sensitive cells. In a remarkable example of stickleback lineages invaded new, freshwater environ- parallel evolution, the same protein (known as zeta- ments. Many of these now-freshwater lineages indepen- crystallin) appears to have been independently recruited to dently evolved the same sort of genetic changes which go to work as part of the lens in two distantly related groups produce a lightly armored body form. Scientists are not of vertebrates: the llama and guinea pig families (Gonzalez sure why the lightly armored trait was favored in so many et al. 1995). The zeta-crystallin in the lenses of these different lineages, but it may have involved selection for relatively remote relatives represents the legacy of parallel increased body flexibility (Colosimo et al. 2005). & Evolutionary reversal. In this process, a lineage evolves Fig. 2 Humans and lizards humans lizards toward one of its ancestral traits, effectively losing a inherited their complex, lens- based eye from a common more recently evolved trait. This is generally thought to ancestor that also had this sort involve genetically “reactivating” the ancestral trait of eye structure (e.g., see Marshall et al. 1994). If a related lineage has retained the ancestral trait, the two lineages will share a similar feature, not because of inheritance from a common ancestor but because of an evolutionary reversal in one lineage. This is illustrated in Fig. 5cin Complex lens eyes evolved which an originally striped fish lineage loses its stripes 500 Evo Edu Outreach (2008) 1:498–504 Fig. 3 Human and squid eyes iris are structured similarly but are pigment iris alternating analogous since they evolved cells photoreceptor independently in the two line- & pigment cells ages. Illustration adapted with lens lens permission from the Under- standing Evolution website retina (photoreceptors) human eye squid eye evolutionary events, not recent common ancestry. These Partly because of this hierarchy, meaningful statements processes, as well as evolutionary reversal, can produce about homology must include several details: traits that are strikingly similar—though not inherited directly from a common ancestor. 1. Which organisms are being compared? The complex lens eye is homologous among humans, lizards, and fish, but the same trait is homoplasious between Homologies at Many Levels humans and squid, having evolved independently in vertebrates and mollusks. Simply identifying a trait as An examination of eye evolution also highlights another, homologous or homoplasious is meaningless unless we less familiar aspect of homology. Though we tend to think know which lineages are being considered. of homologies in terms of anatomy (e.g., the tetrapod limb, 2. What specific aspect of the trait is being compared? insect wing, or vertebrate eye), any heritable trait— Taking another example from eye evolution, we need to anything that can be directly or indirectly encoded in know whether we are considering the entirety of a DNA—can be a homology. Coding and noncoding DNA complex structure (e.g., a lens-type eye), the lens itself, sequences, the proteins and gene regulation systems encoded the proteins that make up that lens, the genes that by DNA sequences, simple traits (like eye color), compo- encode those proteins, or the genetic triggers that cause nents of complex organs (like the lens of an eye), entire those genes to be turned on in developing eyes. Each complex structures, and even behaviors
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