History of Life Biogeography | Homologies

History of Life Learning Objectives Describe how biogeography and homology provide evidence for evolution Distinguish between anatomical, molecular, and developmental homologies After this lesson you will be able to describe how biogeography and homology provide evidence for evolution. You will also be able to distinguish between anatomical, molecular, and developmental homologies.

Fossil Record Fossil – the preserved traces or remains of living organisms from the past See progressive change in organisms Scientists use the fossil record, biogeography, and homologies to understand the history of life on Earth. This fossil record consists of fossils, preserved traces or remains of living organisms from the past. There is a wide variety of fossils found across the Earth, including preserved footprints, insects in amber, animals in ice, and mineralized remains. Under the right conditions, some organisms’ remains have the ability to be preserved. This picture shows a dinosaur’s body that has been covered with sediments. Over time, the bones are replaced by mineral deposits. When an event such as a landslide or earthquake removes the top levels of earth, scientists can more easily find and remove these fossils. Scientists can use geological and chemical data to date these fossils. When scientists arrange fossils chronologically, they are able to see progressive change in organisms.

Fossil Record Fossil record from Equus genus Almost complete Supports theory of common ancestry *Simplified fossil record Some fossil records are almost complete and support the theory of common ancestry. These records allow scientists to trace adaptions through different environments. A good example is the Equus genus, which includes the horse. The illustration shows how, over time, the limbs have elongated, the number of digits has been reduced to facilitate speed, and the teeth have become longer and wider for grazing grass. This almost complete record provides evidence that all organisms in the Equus genus, such as horses, zebras, and donkeys share a common ancestor.

Fossil Record Stasis – the periods of time in the fossil record that show limited change When species exhibit limited morphological change over long periods of time, stasis is occurring. Remember that populations change through the process of natural selection. If no trait is actively being selected, there will be no natural selection. This lack of natural selection creates stasis. In stasis, the fossil record for one or a group of species does not change for large stretches of time. These species are sometimes called “living fossils” because of the lack of changes. One example of this stasis is the crocodile, which has seen little morphological change over time. Because crocodiles are highly adapted to their environment, there are little selection pressures for change, creating stasis.

Fossil Record Limitations of fossil record Fossilization requires specific conditions Organisms can appear suddenly in fossil record Incomplete fossilization results in gaps in fossil record Conditions needed for fossilization are very specific, creating a limitation of the fossil record. Some organisms, such as ones with soft bodies or small bones, do not fossilize well. If the species eventually evolves into a form that is more favorable for fossilization, organisms may appear suddenly in the fossil record. Irregular fossil preservation may also be responsible for a lack of transitional forms in the fossil record. This would make it look like an organism rapidly changed from one form to another. Gaps in the fossil record continue to be filled in as new fossils are discovered. However, because of the specific conditions needed for fossilization, it is likely that fossils of some species will never be discovered.

Fossil Record Fossil record supports theory of sequential evolutionary change Older fossils located in older strata Younger fossils located in younger strata The fossil record provides evidence for the theory of sequential evolutionary change within groups over time. This is evidenced by fossil analysis, in which the older fossils are located in the lower (older) stratum while the younger fossils are located closer to the surface.

Fossil Record Ex) Sequential evolution of the coiled oyster Coiled oyster shells became Larger Thinner Flatter An example of sequential evolution is seen in the evolution of the coiled oyster. The coiled oyster population evolved over 12 million years during the Early Jurassic Period. Since these animals remained primarily on the ocean floor, large, flat shells were beneficial because of increased stability. Over time, the shells of this group became larger, thinner, and flatter. This advantageous shell shape was a product of natural selection, leading to sequential changes in this group.

Biogeography Biogeography – the study of the distribution of species, organisms, and ecosystems through geologic space and time Earth’s land masses have moved over time Biogeography is the study of the distribution of species, organisms, and ecosystems through geological space and time. Throughout geological time, Earth’s land masses have moved.

Biogeography Fossils provide evidence of species that were present across Earth’s original land mass Allopatric speciation occurred as land mass split South American and African monkeys share common ancestor As the Earth changes, populations adapt to their new environment. Fossils have been found across the world that date to when the continents were still connected. These fossils provide evidence that some species were located across the original land mass. As the land mass split apart, members of each species became isolated from each other, resulting in allopatric speciation. Similar environments are found across the world. For example, there are tropical rainforests in both South America and Africa. While monkeys on both of these continents share many features, there are distinct traits within South American monkeys that are not found in African monkeys. Similar early primate fossils have been found in both Africa and South America, dated to when the two continents were connected. This provides evidence that South American monkeys share a common ancestor with African monkeys. Their differences can be attributed to allopatric speciation that occurred after South America and Africa split apart.

Homologies Homology – the similar features between different organisms resulting from common ancestry Anatomical Molecular Developmental Homologies are similar features between different organisms resulting from common ancestry. For example, the oak, maple, and sweetgum trees all have leaves. Since it is unlikely that leaves evolved independently in each species, the shared trait probably resulted from a common ancestor. There are three different types of homologies: anatomical, molecular, and developmental.

Homologies Anatomical homology – a group of similar structures between different organisms resulting from common ancestry Anatomical homologies are similar structures between different organisms as a result of common ancestry. The illustration compares the forelimbs of humans, alligators, and cats. Humans grip and manipulate things, cats walk, and alligators swim with their forearms. However, if you look at the components of the three different forearms, they are composed of the same skeletal parts. While the number and shape of bones change, they are all represented. This shared structure is evidence that all vertebrates share the same common ancestor.

Homologous Structures ARM LEG Wing Flipper grasping walking swimming flight

Homologies Vestigial structure – nonfunctional structure left over from a common ancestor Ex) Pelvis in whale which lacks hind limbs Sometimes structures are no longer needed as populations change and diversify. When a structure is no longer needed, it often slowly degenerates in populations until it is nonfunctional. These are called vestigial structures. The illustration shows a vestigial structure in a Baleen whale. Whales have forearms modified into front flippers with no hind limbs, yet there are remains of a pelvis in their skeletons. The remains are degenerated but still homologous to other vertebrate pelvises, which normally support hind limb movement. Whales still have these bones because they are relics from a vertebrate common ancestor.

Homologies Anatomical homology Convergent evolution Result of common ancestor with that trait Convergent evolution Creates similar structures/functions Are not anatomical homologies When studying anatomical homologies, scientists must be careful to differentiate between homologies and convergent evolution. Remember that convergent evolution occurs when unrelated organisms evolve similar adaptations. The illustration shows the wings of a bat and a bird. Notice that the wings contain the same bones, but they are arranged differently. The skeletal structure of the forelimb is an anatomical homology due to a common vertebrate ancestor. The wing and the ability to fly, however, evolved separately in each species and is the result of convergent evolution. The bird and the bat do not share a common winged ancestor.

Homologies Molecular homology – the similar stretches of genetic material between different organisms resulting from common ancestry Shared genetic code indicates common ancestor Organisms likely to be related have retained same stretches of DNA Molecular homologies are similar stretches of genetic material in different organisms. The genetic code is used by all living organisms. Since it is unlikely that this code evolved separately in different species, the shared genetic code indicates that all organisms evolved from a common ancestor. When scientists study the relationships between groups of organisms, they analyze the amount of difference in their DNA. Organisms likely to be related and share a common ancestor have retained the same stretches of DNA.

#5 Molecular and Genetic Evidence AKA Biochemical Evidence Two closely-related organisms will have similar DNA, RNA, and protein (amino acid) sequences. This also gives evidence of a common ancestor.

Homologies Developmental homology – the similar features in the embryos of different organisms resulting from common ancestry Hox genes – the sections of the genome that allow embryos to develop structures in the correct place Similarities in development in different species indicate common ancestry Developmental homologies are similar features in embryos of different species. Since the same feature evolving twice is unlikely, similarities in development also provide clues for common ancestry. For example, adult baleen whales have baleen filters instead of teeth. However, their embryos have teeth before they develop baleen. Baleen whales are related to a group of teethed whales, where they share a common ancestor that most likely had teeth. Hox genes are another piece of evidence for common ancestry. These sections of related genes in the embryo control the body plan to ensure the right structure development in the correct body segment. These sections are similar in related organisms. For example, the region coding for eye development in mice is very similar to the region in flies, indicating that mice and flies have a common ancestor.

#3 Embryology Embryo (early developmental stage) gives evidence of evolution Identical larvae, different adult body forms Similar embryos, related but diverse organisms Shows common ancestry Larva Adult barnacle Adult crab

#3 Embryology Vertebrates all share gill slits and a tail in their early embryo stage; Share a common ancestor

History of Life Learning Objectives Describe how biogeography and homology provide evidence for evolution Distinguish between anatomical, molecular, and developmental homologies You should now be able to describe how biogeography and homology provide evidence for evolution. You should also be able to distinguish between anatomical, molecular, and developmental homologies.