Evolution Evolution is change in organisms over time. Evolution does not have a goal; it is often shaped by natural selection (see below). Species an interbreeding population of organisms that can produce healthy, fertile offspring. If evolutionary change between organisms is great, it may not allow for interbreeding between divergent groups. A key characteristic of life is adaptation to environment. This is a fundamental aspect of evolution. Structure: Scientists use organismal structures and fossils to help them determine the degree of relatedness (how closely related two species are). They do this through the examination of fossils and the anatomy and molecular profiles of species present today, and are able to stipulate as to how long ago they share a common ancestor. Homologous structure these traits are similar because they are inherited from a common ancestor. The trait may be modified through evolution to fit different environments, but the structure is the same. The classic example of this is the arm of a human, the fin of a whale the wing of a bird and the leg of a dog. The bone structure is the same in all of them because they derived from a common ancestral arm, but they are clearly used for different functions in present day species. Turtle Alligator Bird Mammal Ancient lobe-finned fish Analogous structure these traits have similar function but different structure that is not inherited from a common ancestor. This suggests they have very distant ancestors. An example of this is the wing of a bird and an insect. They are both used for flight but are clearly very different in structure. Another example is the body shape of a ichthyosaurus, penguin and porpoise (dolphin). It is streamline to help them move quickly in the water, but it has evolved independently each time. The trait was not inherited from a common ancestor. Vestigial structure these traits are inherited but the size is reduced and the structure is usually unused. These can be used to guess the structure of the species ancestor. An example of this is the leg and hipbones of a python. The bones are homologous structures to reptiles, suggesting that pythons (and snakes) evolved from other reptiles that had legs at one 1
point. Another example is in humans. The appendix is used only slightly in immunology, but is not critical. Also the nipples in males, they are not used to feed young. Our canine teeth are not used to pierce flesh and our tailbone does not support a tail. Charles Darwin: As a boy in England, he had an interest in nature. His father wanted him to be a doctor, but he did not want to. He dropped out of medical school at the age of 17 to become a naturalist (biologist). When he was 22, he went on a voyage to the Galapagos Islands. They began in Great Britain, and skirted around South America. Darwin closely observed the flora and fauna of S.A. on the way to the islands. On this trip to the Galapagos Islands in 1831, Darwin came up with the idea of natural selection. He saw there were many types of not only finches but tortoises too, each having its own niche, he wondered if each of these were a different species or just variation within the finch and tortoise population. He hypothesized that each type of finch and each type of tortoise, had adaptations to their particular environment (each island has it s own unique environmental conditions) and this eventually lead to the emergence of a new species. These hypotheses along with his observations lead him to write a book describing the process of natural selection. There are 4 main points to the theory of natural selection: 1) There is variation within a population. Variations within a species are what allow the process of evolution to occur. Some of these differences are physical and some are chemical, some are large and some are small, but they non-the less allow for evolution, and natural selection to take place. 2) Some variations are favorable, they lead to the increased success of the population 3) Not all young produced in each generation can survive, many will die from starvation, disease or predation 4) Individuals that survive and reproduce are those with favorable variations survival of the fittest Over time, the accumulation of these small changes can result in a new species. Natural selection may result in the evolution of a new species. Adaptation is what allows for the success of a species. These are heritable traits that are evolutionarily beneficial by increasing an individual s survival and reproductions in a particular environment. An example of this is the anteater. It has a long snout, which allows it to search the ant mounds for food; it has a sticky tongue allowing it to trap the ants and a large tail for balance, it is perfectly suited for its niche. Adaptations allow species to be perfectly suited for a particular niche (its role in an ecosystem). A few examples of Natural selection: - This is not a real scenario; it is just to show how natural selection works: Way back in the day, let s say, humans had both short and long legs. In the wild, there were predators to humans, so we needed to run. The predators would kill those with shorter legs, because they could not run as fast. This meant that those with long legs, and the DNA that codes for that, survived and had offspring. So, now, 10-50,000 years lager, the human race has longer legs, instead of shorter. In other words, there are natural pressures that eliminate the least favorable trait and the 2
advantageous ones will survive. The advantageous trait will thus increase in frequency and be passed on. - There is a moth called the peppered moth. It can range from black to very light gray in its coloration and typically has spots on it, to camouflage it to the trees it lives on. Before the industrial revolution in England, the prominent color of these moths was light gray because of the color of the bark. Black moths would be produces, but were usually picked off by the birds more easily, thus were not very prominent. Once industrialization began, the smog began to collect on the trees, making them a dark, sooty color. This made it so the black moths were now the camouflaged ones and the light gray ones were easily seen. Slowly, the black ones lived to pass on their genes and the gray ones did not. The population shifted to a different color through natural selection. Of course, we have increased our technology and that much smog is no longer produced in England. What do you think happed then? The trees were not longer sooty, they were darker than originally though, so slowly, the moths are now a middle gray color. Natural selection can also be seen in our every day life. This is called Artificial Selection. Take our farm animals or our domestic pets, for example. We have miniature poodles, which, in the wild would die, but we have bread them to our liking. We bread pigs to be fat and cows to produce a lot of milk and meat. We select the qualities we want and breed the organisms to enhance this trait. The same is done with corn and other produce. It was done with the mustard plant, (pictured here) for various aspects for many of our common vegetables. Also on Darwin s trip he noticed that there many finches, all with different characteristics. It was obvious that they had a common ancestor and then each species of finch adapted to its conditions on the island. This created 13 different species of finches. This is adaptive radiation. Adaptive radiation is when many species arise from one common ancestor. Each of the new species is well adapted for their environment, their niche. With these finches, they all had one common ancestor that came from South America. Due to the different conditions on each island, each population evolved into its own species. There are even some finches now that will drink blood from larger birds during droughts. They pluck the feathers off of larger birds and lick up the blood. They consider them to be seasonal-vampire finches. It is unknown if they are going to continue with this habit or remain seasonal. Hood Island Saddle-backed shell Speciation occurs when a populations has been isolated from the original population and must change to meet the demands of that environment. This isolation can be geographical, behavioral or Pinta Island Intermediate shell 3 Isabela Island Dome-shaped shell
temporal. A geographical barrier is something like a river or a mountain range, and a behavioral barrier is something like different matting rituals, and a temporal barrier is something like different mating times of year. If two species remain separated long enough, they will speciate/evolve into their own species, causing them to be genetically different. This may prevent successful interbreeding, one of the characteristics of a species, thus they have become their own species. Once again, speciation is when new species arise when one or more species has evolved from a single ancestor. We can look at the Galapagos tortoises for this. Depending on what was available on the island for food, they adapted shells were best shaped for that environment and mouths that were best for their type of diet. Adaptive radiation is a type of speciation. Thus, the same can be said of the finches. Separated groups may respond differently to the selective pressures of the new environment. If the species are apart long enough, they become reproductively isolated. This creates a new species, thus speciation has occurred. Evolutionary directions: Divergent evolution when a population is split through isolation, the two populations will evolve independently to their conditions, creating two completely new species. They have the same ancestor but evolve independently, away from each other. An example of this is with bears. They were once the ancestral species. A barrier (probably due to migration) slowly caused one set of bears to darken, because they lived in the forest, and the other group to lighten, because they lived in the snow. This lead to many species of bears including the black bears and the polar bears. Convergent evolution when two different species, with different ancestors evolve similar characteristics/traits. The two converging species can never evolve into the same species, due to the difference in ancestry. An example of this is the wings of birds, bats and insects are traits that occurred through convergent evolution. They are all used for flight but evolved independently. Another example of this might be the webbed feet of the turtles, alligators, ducks and frogs in the swamps. This is usually due to the fact that the two species have similar niches. You can compare this to two people who come up with the same invention independently, at the same time. Co-evolution - This happens when two organisms evolve together due to a close relationship they have with one another. They interact closely so they adapt together. Some examples: There is a flower that has evolved the perfect shape of flower to fit the humming birds beak and tiny little body. This co-evolution was possible because the 4
birds were fed and the flowers were pollinated. Another, not so friendly example, is with squirrels and rattlesnakes. In parts of the southwest US, there is a species of squirrel that is immune to the venom of a particular species of rattlesnake. This venom has become so potent, that to any other animal, it would be affected immediately. This co-evolution has occurred because the squirrels are the prey of the rattler, but, those that were not killed by the venom, had offspring, which are now not as affected by the venom either. The rattler then looses out, so those with stronger venom were able to eat, and had offspring. Down the line, this led to the venom being strong as well as the immunity to it from the squirrels. 5