Bat wings and bird wings evolved independently and are considered analogous structures. Genetically, a bat wing and a bird wing have very little in common; the last common ancestor of bats and birds did not have wings like either bats or birds.
Wings evolved independently in each lineage after diverging from ancestors with forelimbs that were not used as wings terrestrial mammals and theropod dinosaurs, respectively. It is important to distinguish between different hierarchical levels of homology in order to make informative biological comparisons. In the above example, the bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm not a wing in the last common ancestor of tetrapods.
Analogy is different than homology. Although analogous characteristics are superficially similar, they are not homologous because they are phylogenetically independent. Analogy is commonly also referred to as homoplasy. Convergent evolution occurs in different species that have evolved similar traits independently of each other. Sometimes, similar phenotypes evolve independently in distantly related species.
For example, flight has evolved in both bats and insects, and they both have wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry.
Convergent evolution describes the independent evolution of similar features in species of different lineages.
The two species came to the same function, flying, but did so separately from each other. Both sharks and dolphins have similar body forms, yet are only distantly related: sharks are fish and dolphins are mammals.
Such similarities are a result of both populations being exposed to the same selective pressures. Within both groups, changes that aid swimming have been favored. Thus, over time, they developed similar appearances morphology , even though they are not closely related. One of the most well-known examples of convergent evolution is the camera eye of cephalopods e.
Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye. Eye evolution : Vertebrates and octopi developed the camera eye independently. In the vertebrate version the nerve fibers pass in front of the retina, and there is a blind spot 4 where the nerves pass through the retina.
This means that octopi do not have a blind spot. Convergent evolution is similar to, but distinguishable from, the phenomenon of parallel evolution. Parallel evolution occurs when two independent but similar species evolve in the same direction and thus independently acquire similar characteristics; for example, gliding frogs have evolved in parallel from multiple types of tree frog. Traits arising through convergent evolution are analogous structures, in contrast to homologous structures, which have a common origin, but not necessarily similar function.
The British anatomist Richard Owen was the first scientist to recognize the fundamental difference between analogies and homologies. Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions. The opposite of convergent evolution is divergent evolution, whereby related species evolve different traits.
On a molecular level, this can happen due to random mutation unrelated to adaptive changes. Some organisms possess structures with no apparent function which appear to be residual parts from a past ancestor. For example, some snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs.
Another example of a structure with no function is the human vermiform appendix. These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings which may have other functions on flightless birds like the ostrich, leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals.
Vestigial appendix : In humans the vermiform appendix is a vestigial structure; it has lost much of its ancestral function. There are also several reflexes and behaviors that are considered to be vestigial. The arrector pili muscle, which is a band of smooth muscle that connects the hair follicle to connective tissue, contracts and creates the goose bumps on skin. Vestigial structures are often homologous to structures that function normally in other species.
Therefore, vestigial structures can be considered evidence for evolution, the process by which beneficial heritable traits arise in populations over an extended period of time. The existence of vestigial traits can be attributed to changes in the environment and behavior patterns of the organism in question. In some cases the structure becomes detrimental to the organism.
Whale Skeleton : The pelvic bones in whales are also a good example of vestigial evolution whales evolved from four-legged land mammals and secondarily lost their hind legs.
Letter c in the picture indicates the undeveloped hind legs of a baleen whale. If there are no selection pressures actively lowering the fitness of the individual, the trait will persist in future generations unless the trait is eliminated through genetic drift or other random events. Although in many cases the vestigial structure is of no direct harm, all structures require extra energy in terms of development, maintenance, and weight and are also a risk in terms of disease e.
The vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of the structure. Homologous structures indicate common ancestry with those organisms that have a functional version of the structure.
Vestigial traits can still be considered adaptations because an adaptation is often defined as a trait that has been favored by natural selection. Adaptations, therefore, need not be adaptive, as long as they were at some point. The biological distribution of species is based on the movement of tectonic plates over a period of time. Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their distribution.
Abiotic factors, such as temperature and rainfall, vary based on latitude and elevation, primarily. As these abiotic factors change, the composition of plant and animal communities also changes. Ecologists who study biogeography examine patterns of species distribution.
No species exists everywhere; for example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size.
Other species are generalists: species which live in a wide variety of geographic areas; the raccoon, for example, is native to most of North and Central America. Since species distribution patterns are based on biotic and abiotic factors and their influences during the very long periods of time required for species evolution, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century.
Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers. Biologists estimate that Australia, for example, has between , and , species of plants and animals. Australia : Australia is home to many endemic species.
The a wallaby Wallabia bicolor , a medium-sized member of the kangaroo family, is a pouched mammal, or marsupial. The b echidna Tachyglossus aculeatus is an egg-laying mammal. The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time.
Broad groups that evolved before the breakup of the supercontinent Pangaea about million years ago are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana.
Biogeography : The Proteacea family of plants evolved before the supercontinent Gondwana broke up. Today, members of this plant family are found throughout the southern hemisphere shown in red. Privacy Policy. Skip to main content. Evolution and the Origin of Species. Search for:. Evidence of Evolution. The Fossil Record as Evidence for Evolution Fossils tell us when organisms lived, as well as provide evidence for the progression and evolution of life on earth over millions of years.
Learning Objectives Synthesize the contributions of the fossil record to our understanding of evolution. Key Takeaways Key Points Fossils are the preserved remains or traces of animals, plants, and other organisms from the past. Fossils are important evidence for evolution because they show that life on earth was once different from life found on earth today. Usually only a portion of an organism is preserved as a fossil, such as body fossils bones and exoskeletons , trace fossils feces and footprints , and chemofossils biochemical signals.
Paleontologists can determine the age of fossils using methods like radiometric dating and categorize them to determine the evolutionary relationships between organisms. Key Terms biomarker : A substance used as an indicator of a biological state, most commonly disease. Fossil Formation Fossils can form under ideal conditions by preservation, permineralization, molding casting , replacement, or compression. Learning Objectives Predict the conditions suitable to fossil formation.
Key Takeaways Key Points Preservation of remains in amber or other substances is the rarest from of fossilization; this mechanism allows scientists to study the skin, hair, and organs of ancient creatures. Permineralization, where minerals like silica fill the empty spaces of shells, is the most common form of fossilization. Molds form when shells or bones dissolve, leaving behind an empty depression; a cast is then formed when the depression is filled by sediment.
Replacement occurs when the original shell or bone dissolves away and is replaced by a different mineral; when this occurs with permineralization, it is called petrification. In compression, the most common form of fossilization of leaves and ferns, a dark imprint of the fossil remains.
Decay, chemical weathering, erosion, and predators are factors that deter fossilization. Fossilization of soft body parts is rare, and hard parts are better preserved when buried. Key Terms amber : a hard, generally yellow to brown translucent fossil resin permineralization : form of fossilization in which minerals are deposited in the pores of bone and similar hard animal parts petrification : process by which organic material is converted into stone through the replacement of the original material and the filling of the original pore spaces with minerals.
Gaps in the Fossil Record Because not all animals have bodies which fossilize easily, the fossil record is considered incomplete. Learning Objectives Explain the gap in the fossil record. Because hard body parts are more easily preserved than soft body parts, there are more fossils of animals with hard body parts, such as vertebrates, echinoderms, brachiopods, and some groups of arthropods. Key Terms transitional fossil : Fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.
Carbon Dating and Estimating Fossil Age The age of fossils can be determined using stratigraphy, biostratigraphy, and radiocarbon dating. Can I find a dinosaur bone in my backyard? That depends.
If a paleontologist wants to find dinosaur fossils, he or she must first go to a place where the rocks are the right age. In other words, the rocks at the surface must have first formed during the age of dinosaurs. In the United States, rocks of this age are very common throughout the Rocky Mountain region. So, the rocks found in any area depend on the rocks at the surface. If the rocks were first deposited in a shallow sea, you might find fossils of corals and other sea creatures.
If the rocks formed in a river during the age of dinosaurs, you might find a dinosaur bone! Why do we study fossils? Fossils help us understand life's past. Without fossils, we'd know nothing of the mighty dinosaurs, tiny three-toed horses, and thousands of other prehistoric forms that are long extinct. Fossils also provide clues to the interrelationships of all species. The life that we see today is just a snapshot in time, but that life has a history. One of the ways we learn about the ancestors of today's species and their ancestors, and their ancestors, and so on is by studying fossils.
Fossils of walking whales, for example, tell us that whales evolved from four-footed mammals that lived on land, and fossils of ancient humans show us our own roots in Africa. Finally, fossils can also provide more information regarding changing climates. Fifty million years ago, areas of Wyoming were covered with large lakes that were inhabited by fishes and crocodiles, with tall palms along the shore.
Together, the occurrence of these plant and animal-types tell us that Wyoming was much warmer in the past than it is today. Fossils can therefore be used as "thermometers" for determining ancient climates. Not long after the tropical lakes covered areas of Wyoming, global temperatures cooled.
Grasses flourished in these cooler and drier conditions, while thick tropical forests shrank. This climate-change induced floral turnover had an incredible effect on mammals; many mammal types evolved to possess a diet of grasses instead of leaves such as horses and rhinos , while others became extinct such as titanotheres.
The pattern of abrupt appearance of fully formed and diverse species within groups is repeated on a less spectacular scale in other organisms such as dinosaurs, and flowering plants. Gradual evolution also fails to explain adequately the shortage of potential transitional links between the different types of organisms throughout the column.
The majority, if not all, of animal and plant forms appear abruptly in the fossil record without known ancestors or intermediate forms. For example, bats, ants, dragonflies, cockroaches and many others appear in the fossil record without any apparent ancestors, and look much like their modern counterparts. This pattern of abrupt appearance does not fit models that invoke gradual evolution, and may fit better in a creation and global catastrophe model.
Moreover, Darwinian theory fails to explain how changes in genetic information can produce new types of organs and body plans.
Because of these failures, Darwinian theory is not a satisfactory explanation for the fossil sequence. The New Diluvialism. Angwin, CA: Science Publications, How do Bible-believing scientists think about data differently?
Brand served for many years as chair…. Why is it that when it comes to the big questions on origins e. How did a mischievous boy, who thought God wanted him to be priest, end up with a PhD in molecular biology….
Articles October 31, Shells and fragments of shells are a common type of fossil encountered in sedimentary rocks. Their hardness favors preservation. What Are Fossils? Dinosaur trackways are an example of ichnofossils. The main trackway in this picture is thought to have been left by an ankylosaurid hammer for scale.
Torotoro, Bolivia. Well developed fossil trackways imply a special set of conditions, with sediment wet enough for footprints to remain impressed followed by prompt induration and burial. How Do Fossils Form? A fossilized instance of cannibalism in Dastilbe crandalli , a Cretaceous fish from the Crato Fm. Unique environmental conditions are required for spectacular preservation of fossil fish.
This form of cannibalism is also interpreted as indicative of stressful environmental conditions. Specimen on display at the Paleontological Museum in Santana do Cariri. Fish is about 10 cm in size. What Do We Learn from Fossils?
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