The diversity of the animals on our planet is breathtaking. Millions of species adapted to all kinds of habitats. Ever since Darwin, understanding how so many species evolved has been a major quest of biologists around the world. In the Caribbean, there is a remarkable group of lizards and Biologists who have been searching for clues to their origins, their bodies, their lifestyles, and their DNA.
These lizards are providing fresh insight into both how new species formed and why our world is filled with so many creatures.
Puerto Rico’s Anoles all feed on similar food, mostly small prey like spiders and crickets. But they divide up their habitats in a clever way. The long-tailed slender species that can be found lives in the grasses and bushes and called a grass Bush Anole. On the low parts of tree trunks and the ground, a longer-legged stockier species forages called trunk ground anoles. And higher up the tree lives another Anole species which is found on twigs and branches. It is very small and has very short legs. This slender lizard is called a twig Anole. Further up the tree is yet another species. High up in the canopy there lives a large green lizard that has large toepads and lives high off the ground.
The same way we humans occupy apartment blocks, each species lives in its own vertical space. The only difference is here each floor offers the Anole unique evolutionary opportunities.
The fact that lizards differ in leg length and toepad size depending on where they live suggests that these differences in traits are adaptations to the lizard’s habitats. To test whether that is, in fact, the case, biologists have been conducting tests.
The shorter-legged twig lizard is not nearly as fast. It seems like a disadvantage. Why arent their legs longer? Instead of speed, the twig lizard legs provide a firm grasp. On twigs, while long legs only increase the chance of falling. So ground lizards have evolved long legs and twig lizards, short ones to enable their lifestyles.
Next, let’s look at how well two different species can climb the slick surfaces of leaves. Anoles have different sized toepads on their feet. Let’s look at how these help them navigate different environments. The small ground lizard can barely grip onto a large leave when it’s in the vertical position, while the much larger green canopy lizard can move up and down with ease because of its larger toepads.
Even though the larger green lizard weighs much more than the smaller ground lizard, it uses little microscopic hairs in its toe-pads to bond to the surface and this is what holds them up.
Scientists wanted to see how Lizards evolve and realized they could use some very small islands in the Caribbean as laboratories as hurricanes occasionally scrub these tiny islands free of any lizards.
They began their experiment by capturing tree-dwelling anoles from the larger islands. Then they visited seven islands that a hurricane had cleared of all lizards. On each island, they placed a female and male Anole. These islands had no trees, only small bushes. How would the long-legged lizards fair on thin branches?
The next year, the scientists returned. They found that the main pairs that they had introduced not only survived, but reproduced. Amazingly the new population had grown and taken to living on thin branches.
The scientists collected the Lizards and measured exactly how far off the ground each one was, the width of the surface and what direction the lizard was facing, such as horizontal, head up or head down.
The brought the Lizards back to their field lab, took Xrays to precisely measure the length of their legs, and scan their toe pads. Then they returned each lizard to the exact spot where they had found them. Now they had baseline data on the new populations of Anoles.
A year later, they came back and discovered that the average lizard leg had shortened in just two generations. They thought at first it was just a fluke when in fact, over four years, the populations all got shorter and shorter legs. It seems that evolution can occur very rapidly when natural selection is strong.
Adaptations like these explain how different body types evolve but they do not explain how new anoles species arise.
Its changes in other traits that play a key role in speciation. Two groups of animals are defined as different species when individuals from one group don’t made and reproduce from the other. So for a population to become a new species, something has to prevent its members from breading with members of closely related populations. This is called reproductive isolation.
One way a species can split into two is for a population to separate geographically. Over many generations, they can undergo enough changes in their respective habitats that if or when they come back together again, they do not mate. So what kind of changes keep Anoles from mating?
Anoles have a flap of skin under their throats that is called the dewlap which males display to attract females. And remarkably, every species in the same area has a different dewlap. So a change in the dewlap is a critical step in the formation of a new Anoles species. Why would the color of the dewlap change?
Well, consider a grass lizard that lives in the forest where it is relatively dark and has a light-colored dewlap. Now suppose a population of these lizards ended up in an area which was much more open and sunnier. In this environment, a light-colored dewlap will not be as effective, so over time, the population would evolve a natural selection to have darker dewlaps.
If for some reason these two populations come back together, the females would no longer recognize the male as the same species and wouldn’t mate. They would be reproductively isolated.
There is a simple connection between changes within populations or microevolution and their formation of new species or macroevolution. When changes within populations include traits in mating, a dewlap color and the stage is set for the formation of new species.
Once new species are formed, competition drives the evolution of different body types. Species living in the same area compete for resources but if members of one species move into another habitat, they can use other resources not available to the other species.
Over many generations, natural selection favors traits that enable species to occupy other habitats. This process has led to the body types you see in Peurto Rico. And not just there, on each of the Caribbean’s largest islands, Peurto Rico, jamaica Cuba and Hispaniola, we find the same distribution of similar-looking lizards.
You would think all the different islands would look different but they don’t. Each island has the same basic body types.
Each island has slender grass bush anoles with long tails, long-legged trunk ground anoles, short-legged twig anoles and canopy anoles with large toepads. How did each of these islands end up with the same body types? Did each body type only evolve once and then spread to other islands? Or rather, did each type evolve independently on each island?
To find out, scientists sequenced the DNA of the anoles from each island. They examined the same stretch of DNA from many species to uncover their evolutionary relationships.
After the scientists determined which two species were most closely related, they joined them together with a node representing a common ancestor. They then went on to join these and the next most closely related until all the lizards were combined in a phylogenetic tree.
The DNA revealed the pattern consistent with this. The lizards on each island tend to be more closely related to each other than to similar-looking lizards on different islands.
This means that usually, the same types of lizards evolved independently on each of the islands.
On each of the large Caribbean islands, the same traits have evolved over and over again. Body-color, limb length and toepad size.
Moreover, this repeated filling of habitats of each island by anoles explains why this planet has so many different species.
The simple reason why there are so many species in the world is that there are so many habitats. And each habitat provides numerous ways to survive.
In the Saran Getty, Zebras eat the tallest coarsest grass, wilder beast the medium-high grass and Thomson’s gazelles the shortest. In the Galapagos, some finches primarily eat seeds on the ground and others, insects in the trees. Look around your back yard or the world. There are so many different environments each full of creatures making a living differently.