Hhmi Lizard Evolution

[Kimbery Challacombe]

TheCaribbean is home to about 150 species of anole lizards. In this lab, students investigate how so many species of anoles evolved. The lab includes four modules that cover different concepts in evolutionary biology, including adaptation, convergent evolution, phylogenetic analysis, reproductive isolation, and speciation. In each module, students engage in key science practices, including taking measurements to collect data, constructing and interpreting graphs, and performing statistical analyses.

This film explores the adaptation of anole lizards (genus Anolis) to habitats common across the islands of the Caribbean. The anoles are excellent examples of adaptive radiation, convergent evolution, and speciation through reproductive isolation.

Working in the islands of the Caribbean, biologist Jonathan Losos discovered traits that enable dozens of anole species to live in different vertical niches in the forest. Differences in limb length, body shape, and toepad size reflect adaptations to life on the ground, on thin branches, or high in the canopy. Remarkably, similar combinations of these traits have evolved independently on different islands, a phenomenon known as convergent evolution. The film also describes how new species of anoles are formed due to variations in the color of their dewlaps, a reproductive trait that determines their mating behavior.

Anolis is a genus of lizards belonging to the family Polychrotidae. Anoles, as they are commonly known, are distributed from southeastern North America to central South America, including the West Indies. Anoles are vertebrates (have a backbone or spine), have four limbs and a long tail, and are characterized by having adhesive toe-pads, and in most species, brightly colored throat fans, called dewlaps.

Often studied as an example of evolutionary diversification and adaptive radiation, the genus Anolis is an important research subject in areas like physiology, behavior, and community ecology, among others. The scientific importance of this group is reflected in that the North American green anole lizard was selected to be the first non-avian reptile to have its genome sequenced.

Like something out of a reality-TV show, scientists released pairs of small lizards onto tiny uninhabited islands in the Bahamas and watched what happened. Rather than playing for money or fame, the reptiles played for survival, allowing the voyeuristic researchers to witness the interaction between evolutionary processes rarely observed in nature.

"We were actually able to see these processes and document them happening in a natural environment," Jason Kolbe, a biologist at the University of Rhode Island who led the study, told LiveScience. "We know that islands are colonized by new species over time, but we are rarely there to see it happen."

When a few individuals of a species colonize a new area, their offspring undergo what is known as the founder effect, which is a change in genetics or physical characteristics. Because of the small number of founding individuals, the new population experiences a loss in genetic variability, often resulting in individuals that are physically and genetically different from their source population.

In addition to random processes like the founder effect, which has everything to do with the random genes that get passed down from the first individuals on the island, populations also experience natural selection, where they adapt to their environment and pass on beneficial traits to their offspring.

To find out, Kolbe and his colleagues randomly selected male-female pairs of brown anole (Anolis sagrei) lizards from Iron Cay, an island in the Bahamas, and released them on seven smaller islands in 2005. The smaller islands, whose lizard populations had been wiped out by a recent hurricane, are very similar to one another, populated by the same types of insects, birds and vegetation (short scrubs), but very different from Iron Cay, which is forested.

The researchers predicted that over time, the lizards in their experiment would develop shorter hind limbs than those of the lizards on Iron Cay, but they didn't know what role the founder effect would play in the matter.

"There were also significant differences in hind-limb length among the islands, even though the lizards were all from same source population," Kolbe said. Since the founder effect is a random process independent of the environment, there was no pattern to the length of the lizards' hind limbs and apparently no relationship between limb length and perch diameter, he explained.

Over the next few years, however, a pattern did emerge for the lizards on the experimental islands. With each generation, their hind limbs got shorter, making them better suited for their environment. But the founder effect wasn't completely snuffed out: Lizard populations with the longest limbs in 2006 still had the longest limbs three years later.

Andrew Hendry, an evolutionary biologist at McGill University in Quebec, who was not involved in the research, was impressed with the study and its findings. "There are very few experimental studies that have looked at these processes in nature," Hendry told LiveScience. "I would have done exactly the same study had I thought of it."

But, Hendry notes, "I'm not sure how much it informs us about real situations." The researchers set up an experiment where they would see the maximum effects of the evolutionary processes, which isn't always the case in real life, he said. Hendry is interested in seeing what would happen if more than two animals were used to create a founding population.

David Reznick, an evolutionary biologist at the University of California, Riverside, was intrigued that all of the experimental populations survived throughout the course of the study (on average, the populations actually grew 13-fold over the first two years, before leveling off). When a population only starts with a few individuals, there is always the risk of inbreeding, which decreases the fitness of the population and their ability to survive and reproduce, he said.

"It would mean that a small number of founders is enough, so long as the populations grow well after they've been introduced," Reznick told LiveScience. "Restoring species and their habitats are important issues we are now confronting."

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The evolution of snakes involved dramatic modifications to the ancestral lizard body plan. Limb loss and elongation of the trunk are hallmarks of snakes, although convergent evolution of limb-reduced and trunk-elongated forms occurred multiple times in snake-like lizards. Advanced snakes are completely limbless, but intermediate and basal snakes have retained rudiments of hindlimbs and pelvic girdles. Moreover, the snake fossil record indicates that complete legs were re-acquired at least once, suggesting that the potential for limb development was retained in some limb-reduced taxa. Recent work has shown that python embryos initiate development of a transitory distal leg skeleton, including a footplate, and that the limb-specific enhancer of the Sonic hedgehog gene, known as the zone of polarizing activity regulatory sequence (ZRS), underwent gradual degeneration during snake evolution. In this article, we review historical and recent investigations into squamate limblessness, and we discuss how new genomic and functional genetic experiments have improved our understanding of the evolution of limblessness in snakes. Finally, we explore the idea that pleiotropy of cis-regulatory elements may illuminate the convergent genetic changes that occurred in snake-like lizards, and we discuss a number of challenges that remain to be addressed in future studies.

Virtual Lab Student Handout Lizard Evolution Virtual Lab LIZARD EVOLUTION VIRTUAL LAB Answer the following questions as you finish each module of the virtual lab or as a final assessment after completing the entire virtual lab. Module 1: Ecomorphs 1. At the beginning of the virtual lab, you were asked to sort eight lizards into categories. What criteria did you initially use to make your groups? Did you revise your criteria later? Why? 2. An adaptation is a structure or function that is common in a population because it enhances the ability to survive and reproduce in a particular environment. Provide one example and an explanation of one adaptation in the Anolis lizards. 3. Provide one evolutionary explanation for why lizards living in the same part of the habitat (i.e., grass) would have similar characteristics. 4. What is an ecomorph? Provide one example from the virtual lab. 5. How is an ecomorph different from a species? 6. Explain how a particular body feature of one of the lizard ecomorphs from the virtual lab is an adaptation to their particular niche. www.BioInteractive.org February 2015 Page 1 of 5

Virtual Lab Student Handout Lizard Evolution Virtual Lab Module 2: Phylogeny 1. In module 1, you identified which species of lizards were most similar to one another based on relative limb length and toe pad size. In this module, you determined which lizards are more similar to one another based on what type of information? 2. Are the species of lizard that are more similar to one another according to body type also more closely related based on the results obtained in this module? Explain your answer. 3. The figures below show two phylogenetic trees similar to the one you constructed in the virtual lab but with more lizards. The trees below show the evolutionary relationships among species from four ecomorphs from the four largest Caribbean islands. www.BioInteractive.org Figure 1. Phylogeny of anole lizards on four of the major Caribbean islands color-coded according to geographical distribution. Light dotted line, Puerto Rico; small dashed line, Cuba; large dashed line, Hispaniola; and solid line, Jamaica. Figure 2. Phylogeny of anole lizards in the four major Caribbean islands colored in according to ecomorph. Light dotted line, twig; small dashed line, trunk-ground; large dashed line, trunk-crown; solid line, grass- bus. February 2015 Page 2 of 5

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