Evolution – Analagy and Homology Part 2

Similarities and differences: understanding homology and analogy In everyday life, people look like one another for different reasons. Biological similarity It works the same way in biology. Some traits shared by two living things were inherited from their ancestor, and some similarities evolved in other ways. These are called homologies and analogies. Homology - traits inherited by two different organisms from a common ancestor Analogy - similarity due to convergent evolution not common ancestry

Homology -Tetrapod Limb Notice how these tetrapod limbs are similar to one another: They are all built from many individual bones. They are all spin-offs of the same basic bone layout: one long bone (the humerus) attached to two other long bones (the radius and ulna), with a branching series of smaller bones (carpals, metacarpals and phalanges) on the end. Homology - traits inherited by organisms from a common ancestor

Inherited Homology Whales, lizards, humans, and birds all have the same basic limb layout. They inherited it from a common ancestor, just as cousins might inherit the same trait from their grandfather.

Inherited Homology In the same way, an evolutionary "family" tree shows relationships over much longer periods of time. This 350 million year old animal, the first tetrapod, had limbs with one long bone (the humerus) attached to two other long bones (the radius and ulna) with a branching series of smaller bones (carpals, metacarpals and phalanges) on the end.

Homologies Not all homologies are obvious. For example, the chomping front teeth of a beaver look quite different than the tusks of an elephant. The beaver uses its teeth for chewing through tree trunks, and the elephant uses its tusks for a number of tasks including digging, peeling bark from trees, and fighting. Each is a modification of the basic incisor tooth structure. Over time, evolution adapted each of these animals' incisors to perform different functions. They are homologous structures, inherited from a common ancestor with incisor teeth.

Evolution - Analagy Both of these skulls belong to extinct animals, and in each the upper canine teeth have evolved into long, curving saberteeth with serrated edges. homologous or analogous?

Evolution - Analogy These two skulls certainly look like the animals could have inherited their saberteeth from a common ancestor. However, despite their similarities, the unusual length of these teeth is NOT homologous. One skull belongs to Thylacosmilus, a marsupial mammal. The other belongs to Smilodon, the saber-toothed cat, which is a placental mammal. Marsupial and placental mammals are very different, and diverged from each other a long time ago on the evolutionary tree.

Evolution - Analogy Thylacosmilus is more closely related to other marsupials such as kangaroos and koalas than it is to Smilodon. Smilodon is more closely related to other placentals such as housecats and elephants than it is to Thylacosmilus. Saberteeth is not a common trait in the marsupials or the placentals . Their common ancestor certainly had canine teeth, but they were probably not adapted into fierce "sabers."

Evolution - Analogy As they weren't inherited from a common ancestor, the saberteeth in Smilodon and Thylacosmilus evolved independently from one another. Analogous evolution That means that the evolution of saberteeth occurred more than once. One lineage on one part of the tree of life evolved saberteeth from normal length teeth, and a different lineage somewhere else on the tree also evolved saberteeth from normal length teeth.

How do analogies evolve? You have probably noticed that dolphins and sharks both have a streamlined body shape with a triangular fin on the back and two side fins. However, the two animals also have many differences. skeleton made of cartilage skeleton made of bone use gills to get oxygen from the water in which they swim go to the surface and breathe atmospheric air in through their blowholes don't nurse their young do nurse their young don't have hair do have hair — they are born with hair around their "noses"

How do analogies evolve? Analogous Evolution If two species face a similar problem, challenge or opportunity, evolution may end up shaping them both in similar ways. Both dolphins and sharks swim after prey in the ocean. Streamlined bodies and fins provide a big advantage for them, allowing them to swim faster. We know that dolphins and sharks are not closely related, and they didn't inherit their similar body shapes from a common ancestor. Their streamlined bodies, dorsal fins and flippers are the result of convergent /analogous evolution. Since dolphins and sharks occupy similar niches and face similar challenges, similar adaptations have been advantageous to them, resulting in their analogous structures. They may share the same basic shape, but underneath their skins, sharks and dolphins are very different! The tree shows the relationship between various groups, including cartilaginous fishes (sharks) and mammals (dolphins). They are not very closely related to one another. So how did they end up looking so much alike?

Test your understanding!!! Sugar gliders and flying squirrels look amazingly similar. They are both furry animals of about the same size, with big eyes and a white belly. And they both glide from treetops using a thin piece of skin that is stretched between their legs. This piece of skin helps keep them stable while gliding. Was this a homologous or analogous development?

More information….. Sugar gliders live in Australia, and flying squirrels live in North America. Sugar gliders have a pouch (like a kangaroo does), which provides shelter and safety for their tiny babies — at birth, a baby sugar glider is smaller than a peanut! Flying squirrels, on the other hand, have much larger babies and no pouch. By studying their genes and other traits, biologists have figured out that sugar gliders and flying squirrels are probably not very closely related. Sugar gliders are marsupial mammals and flying squirrels are placental mammals. Considering all of the evidence, are the "wings" of sugar gliders and flying squirrels homologous or analogous structures?