Evidence of Evolution

Evidence ofEvolution In this program, you will learn more about the evidence that Charles Darwin and scientists since then have found that shows life has evolved.

At several points in the program: • You will be asked questions to check your understanding of the concepts. • To respond, click the correct answer. • Don’t worry if you get an answer wrong -- you will still learn something. Next

Here’s the first question: Q: Why is it called the Theory of Evolution? • It has been thoroughly tested many times by many people and has gained widespread acceptance. • It has not yet been proven well enough to satisfy most people.

Correct! • The term “theory” has multiple meanings. • People commonly use the word to mean a wild guess. • In science, though, it means almost the exact opposite! Next

Actually, A is correct. • Don’t feel bad, though -- the term “theory” has multiple meanings. • People commonly use it to mean a wild guess. • In science, though, it has almost the exact opposite meaning! Next

Scientific theories are testable hypotheses that explain observable data. • Atomic Theory, Germ Theory, the Cell Theory, the Theory of Relativity, Plate Tectonics and Gravitational Theory are all examples of accepted scientific theories. • Like evolution, each of these theories is the simplest explanation that fits with the available data. Each has been repeatedly tested by experts in the field and found to be acceptable. • People today don’t question the validity of these theories, but new data may force scientists to change them or even throw out them out. • For this reason, scientists prefer not to say that a theory is proven to be a fact. (Facts are the observations used to support theories.) Next

Testing a Theory is a Rough Business • For a long time, people accepted the Geocentric Theory. Their observations seemed to show that the sun, stars, and planets revolved around the Earth. • Careful observations made by later scientists didn’t fit this model, and it was eventually scrapped in favor of the Heliocentric Theory. Next

This is the Home Page Convergent Evolution Fossil Record Comparative Anatomy From the Home Page, you can investigate several lines of evidence that support the Theory of Evolution. Evidence of Evolution Fossil Record is a good place to start Click any box to enter that area. Molecular Comparisons Vestigial Structures Embryological Development

Convergent Evolution Fossil Record Comparative Anatomy Evidence of Evolution Molecular Comparisons Vestigial Structures Embryological Development

When you think fossil, what comes to mind? • I know what you were thinking… DINOSAURS!

Fossil Record • Fossil Facts: • A fossil is any part or trace of a once-living organism. • It may be bones, but there are other types of fossils. • Click one of the items below to learn more about the different types of fossils. Trace Fossils Imprints Amber Molds/Castings Petrified Wood Fossilized Bones Frozen Organisms Next

Trace fossils include burrows, tracks, even fossilized poop! Dinosaur tracks Dinosaur & Fish Poop Fossil burrows Back

Amber • Amber is fossilized tree sap. It often traps pollen, insects and other small animals. Insects trapped in amber. Back

Petrified Wood • When wood is buried in ash or sandy sediments, silica can form within the wood structure, eventually replacing the organic material. These petrified logs are found at Petrified Forest National Park in Arizona. Back

Molds/Castings • Molds and castings form when the hard shell or exoskeleton of an organism is buried in sediment. • The sediment hardens around the body, then the body dissolves or decomposes, leaving a void or mold. • The mold can fill with minerals to form a cast in the shape of the mold. Casting of a fossil trilobite Back

Frozen Organisms • In some places, animals and plants get trapped in snow and are preserved for thousands of years. • This obviously happens only in cold mountainous and polar areas. Above - Otzi, the famous “Ice Man” found frozen in the Italian Alps Left - Dima, a baby mammoth found preserved in the Siberian permafrost by Russian miners. Back

Imprint Fossils • When plants and animals fall in fine sediments, their bodies can sometimes leave an imprint that shows up when the resulting rock layers are split apart. Clockwise from left - The imprints of feathers can be seen in the Archaeopteryx fossil. A dragonfly imprint in the same fine sediment bed. The delicate imprint of an extinct fern’s frond. Back

Fossil Bones • Bones and teeth of vertebrates are resistant enough to remain intact when the rest of the body has decayed. • Dinosaur skeletons are among the most spectacular fossils ever found. Sue, the largest, most complete T. rex fossil ever found. It resides at the Field Museum in Chicago. A 12,000 year old mammoth skeleton Back

If evolution had not occurred, click the statement below that should be true about the fossil record. • Different rock layers should contain different fossil species. • Younger (shallower) rock layers should contain fossils that look more like modern organisms. • Most rock layers should contain fossils of species that are not alive today. • Every rock layer should contain fossils that look exactly the same.

That’s right. • If every species had always been present in its current form, then we should be able to find fossils of all modern species -- from bacteria to bats -- in every layer of rock. • It wouldn’t matter how deep you dig -- if modern species were living from the very beginning of earth’s history, then some fossil evidence should exist even in the oldest rocks. It doesn’t. • By itself, the fossil record provides strong evidence that evolution has occurred. Next

Actually, this statement should be correct if evolution has occurred. • Think about what it means for things to evolve. • Evolution refers to the change that happens in species over time. • Go back and try to pick out the statement that should be true if things NEVER changed. Back

Conclusion: • The fossil record shows a progression from simpler life forms to more complex, modern-looking forms. • The only logical explanation is that life has evolved from a single life form into the diversity we see here today. Home

Convergent Evolution • Convergent evolution refers to the tendency of different species to evolve similar adaptations • Convergence happens because these different species face similar selective pressures as they struggle to survive and pass on their genes. • There is no guarantee that similar selective pressures will lead to convergence, since all adaptations are due to mutations and/or new gene combinations. • Click the next button to see some examples of convergent evolution. Next

Examples of Convergence: (Click on the examples below for more information.) • Whales and Fish • Marsupials and Placental Mammals • Counter-shading coloration and other forms of camouflage • Mimicry • Hedgehogs and Porcupines • Beaver and Capybara Home

Whales and Fish • The ancestors of fish evolved to swim in the water, which supports the body’s weight, but is much more dense than air. • The ancestors of whales were 4-legged mammals living on land. • As ancestral whales evolved to live and move in the water, they faced the same selective pressures that favor fins and a streamlined shape. Back

Marsupials and Placentals • The first mammals to evolve laid eggs, like their reptilian ancestors. (Only two species of egg-laying mammals are alive today.) • Marsupials, pouched mammals, give birth to underdeveloped young, who then complete development in their mother’s pouch. Nearly all of them are found in Australia/New Zealand. • Placental mammals keep their young inside of the mother’s uterus, connected to her through the placenta. Most of the world’s mammals are placental. • Still, because the same types of selective pressures exist in Australia and different habitats in other parts of the world, we would expect some convergence to evolve. Next

Examples of Marsupials/Placentals: Marsupial and placental species may look the same on the outside and occupy similar niches in their environment, but all of the placental mammals are more closely related to each other than to any marsupial, and vice versa. (Click on animals to see videos.) Back Marsupials Placentals

Camouflage and Coloration • There is a strong selective pressure for most animals to blend in with their surroundings. It helps them hide from predators and their prey. This giraffe and grasshopper are not closely related animals, but the same selective pressure to blend in has led to similar color and markings. Back

Mimicry • Sometimes, it is good to stand out. Dangerous animals like poison arrow frogs, bees, and nasty-tasting insects have markings that warn potential predators. Bright, high-contrast patterns are universal signals to stay away. “Leave us alone!” Next

Sometimes, it pays just to look tough. See if you can pick out the wannabes in the examples below. Click on the dangerous insects. or… or… Back

The monarch butterfly on the left is poisonous and will make a bird that eats it sick. The viceroy on the right is harmless, but the birds that have made the mistake of eating a monarch won’t touch it. Back

The yellow jacket on the right can afford to be aggressive because it has a stinger to back up its behavior. The hoverfly on the left is very tame. It flies from flower to flower eating nectar, confident that its colors will protect it from any nosy predators. Back

Hedgehog & Porcupine • Hedgehogs are insectivores and porcupines are rodents, but both are mammals that defend themselves with spiny hairs. • Each had ancestors that experienced mutations that produced stiff, spiky hairs. It turns out that these mutants were successful and survived attacks by predators. • As a result, they were able to reproduce and pass on these traits to the next generation. Back

Beaver and Capybara • Beaver and capybara are large, distantly related rodents – the beaver lives in North America and the Capybara in South America. • They both evolved similar traits living in similar marshy environments and engaging in similar ecological roles. Back

Comparative Anatomy • Humans, cats, whales and bats are all mammals, but they engage in totally different lifestyles. On the outside, their bodies appear to be perfectly designed to perform different functions. Next

Comparative Anatomy • If all of these mammals evolved from the same ancestral mammal, we would expect to see underlying anatomical similarities despite the obvious differences. • Underneath the skin, all of these animals share the same skeletal anatomy – one that was inherited from their common ancestor. • The appearance of design is an illusion produced by the effects of natural selection. Home

Vestigial Structures • Mutations are always producing new body forms each generation. Sometimes, environmental changes and mutations combine to provide new opportunities for species. • As species evolve to fill different niches in their changing world, we would expect their bodies to change over many generations. • Vestigial structures are body parts that are reduced in size and function. Mutations that reduce the size of such structures are not harmful and may actually be of a benefit to the individuals that inherit them. • Click on one of the examples below to learn more. Fossil whale legs Human coccyx Snake leg spurs Reduced eyes Home

Fossil Whale Legs • Whales are mammals that have no hind legs. • They are thought to have evolved from 4-legged land animals that lived over 50 million years ago. • Paleontologists expected to find whale ancestors that showed reduced and vestigial legs by looking in sediments that were between 50 and 30 million years old. Sure enough, they did. Some examples of this transition are shown below. Ambulocetus 49 million years ago Pakicetus 42 million years ago Basilosaurus 40 million years ago Back

Coccyx in Humans & Apes • Humans and other hominiod apes evolved from primates that had an external tail. • In our hominoid ancestor, it was reduced to a small coccyx or tailbone that is not visible on the outside of the body. The coccyx or tailbone is visible in the chimp pelvis on the left and the human pelvis on the right. Back

Snake Leg Spurs • Snakes evolved from four-legged lizards, but mutations that reduced or eliminated their legs provided new opportunities for their ancestors. • The genes that produce legs are still there, however, and in some species of snakes, there are actually vestigial legs. Leg spurs are visible on the underside of this female ball python. Back

Reduced Eyes in Cave Dwellers • Mutations that reduce or eliminate eyes in species that dwell in caves are not harmful, and may benefit the individuals that experience them. • Many different cave species show these same mutations. The fish Astyanax comes in two varieties – the surface-dwelling varitey is on the left, the eyeless cave fish is on the right. The mutation that eliminates eyes also gives the fish a larger jaw and more taste buds. Several examples of eyeless cave-dwelling animals Back

Comparative Embryology • Species that reproduce sexually start out as a fertilized egg – a single cell. All fertilized eggs are basically the same. • As the egg divides again and again, master control genes activate to establish the basic body plan of the organism. • Although these genes were first discovered in fruit flies, they exist in other animals as well. • Other genes that control embryonic development take each species from the same basic starting point down their own path of development. Home

Comparison of Proteins & DNA • Darwin didn’t have access to knowledge about DNA and genetics like we do today, but he did understand that individuals passed down their traits to their offspring. • If genetic changes (mutations) have led to the changes that form new species, then we should expect species that share a recent common ancestor to have fewer differences. • Species that share a more distant common ancestor should have more genetic differences because there has been more time for random mutations to build up. Next

Comparison of Proteins & DNA • Chimpanzees are humans’ closest living relative – in other words, we share a more recent ancestor with them than any other living species. Our genes are 98.2% identical. • The humans and chimps last shared an ancestor with the gorillas at an earlier time. Human and chimp DNA is 97.7% identical to a gorilla’s. • Human, chimp, and gorilla genes are 96.3% identical to orangutan genes. Orangutan Gorilla Human Chimpanzee Next

Comparison of Proteins & DNA • Even when one considers a species as distantly related to animals as a yeast, there are still similarities to be found. • Half of the genes in a yeast are also found in humans. • Experiments have shown that mutant yeast can be fixed by inserting the human version of a gene to replace their defective gene. • These genes have been inherited from our common ancestor, which lived well over half a billion years ago. Yeast Primates Home

Evidence of Evolution