Evidence of Evolution: Videos & Practice Problems

Homology indicates shared traits among species due to common ancestry, evident in structures like upper limb bones across tetrapods. Key types include vestigial structures, embryologic homology, and molecular homology, which reveal evolutionary relationships. Fossils provide crucial evidence, showing changes over time and specific transitions, such as the evolution of whales from land-dwelling ancestors. Understanding these concepts is vital for grasping evolutionary processes and the interconnectedness of life forms.

Homology and Analogy

Understanding the evidence for evolution involves exploring the concept of common descent, which posits that species share common ancestors. A key method scientists use to determine shared ancestry is through the study of homology. Homology refers to traits that exhibit similar underlying structures across different species, indicating a common evolutionary origin.

For instance, examining the upper limb bones of tetrapods—organisms with backbones that live on land, including reptiles, amphibians, birds, and mammals—reveals striking similarities despite their diverse functions. Consider the limb structures of a whale, frog, horse, lion, human, and bird. Although these animals utilize their limbs in vastly different ways, their bone structures are remarkably alike. Each limb begins with a large bone known as the humerus, followed by two parallel bones, the radius and ulna. Even in species like horses and frogs, which may appear to have a single bone, this bone originates from the fusion of two bones during development.

This shared bone structure suggests that these organisms inherited it from a common ancestor, supporting the idea of descent with modification. Evolution does not create entirely new body plans but rather modifies existing structures, leading to variations that become more or less common over time. This principle of homology is crucial for understanding evolutionary relationships across large groups of organisms.

In contrast, the concept of analogy refers to functionally similar traits that arise from different evolutionary origins. For example, wings in bees, birds, and bats serve similar functions but have distinct structural differences, indicating that their common ancestor did not possess wings. This distinction is vital, as relying on analogy can lead to misconceptions about evolutionary relationships.

To remember these concepts, note that "homo" in homology derives from Greek, meaning "the same," highlighting the shared structures among different organisms due to common ancestry. Conversely, "ana" in analogy means "again," referring to traits that have evolved independently in different lineages. Understanding these terms and their implications is essential for grasping the complexities of evolutionary biology.

Evidence of Evolution Example 1

In the study of evolutionary biology, understanding the concepts of homology and analogy is crucial for interpreting the relationships between different species. Homology refers to traits that are inherited from a common ancestor, while analogy describes traits that have evolved independently in different lineages, often due to similar environmental pressures or functions.

To illustrate these concepts, consider the North American flying squirrel and the Australian sugar glider. Both species possess the ability to glide, facilitated by skin flaps that extend between their limbs. At first glance, their similarities might suggest a shared evolutionary origin. However, a closer examination of their evolutionary tree reveals a different story.

When analyzing the evolutionary tree, one must identify the common ancestor of both organisms. Tracing the branches back, it becomes evident that the flying squirrel and the sugar glider share a distant ancestor at the root of the tree, which represents the common ancestor of all placental and marsupial mammals. This ancestor likely did not possess the gliding trait, indicating that the ability to glide evolved independently in these two species.

Thus, the gliding ability in the flying squirrel and the sugar glider is an example of analogy, as it has arisen separately in each lineage rather than being inherited from a shared ancestor. This conclusion is supported by the reasoning that the common ancestor of these mammals did not have the trait of gliding with skin flaps.

In summary, recognizing the distinction between homology and analogy is essential for understanding evolutionary relationships. The ability to glide in the flying squirrel and sugar glider exemplifies analogy, highlighting the independent evolution of similar traits in response to environmental challenges.

A similar arrangement of internal organs is observed in birds, rodents, and frogs. This is likely an example of:

• A. Analogy.
• B. Use and Disuse.
• C. Homology.
• D. Uniformitarianism.

Answer: (C)

The potato (Solanum tuberosum) and the sweet potato (Ipomoea batatas) are both plants that form starchy tubers. A tuber is an underground part of plant used for nutrient storage and is the part of the potato and sweet potato that we eat. Most other plants of both the genus Solanum, which includes the tomato and the eggplant, and the genus Ipomoea, which includes the morning glory, do not form tubers. As described do you think the tubers found in potatoes and sweet potatoes are more likely an example of analogy or homology and why?

• A. Homology, because the tuber phenotype is extremely similar in both plants.
• B. Homology, because potatoes and sweet potatoes likely share a common ancestor.
• C. Analogy, because the tuber is a structure that likely first evolved in the common ancestor of these plants.
• D. Analogy, because the presence of a tuber in these plants likely arose separately in each population.

Answer: (D)

Types of Homologies

The concept of homology is fundamental in understanding evolutionary relationships, as it highlights how organisms share traits inherited from a common ancestor. Within this framework, there are specific types of homology that are essential to grasp: vestigial structures, embryologic homology, and molecular homology.

Vestigial structures are traits that have significantly diminished or lost their ancestral function. These structures may still serve a purpose, albeit a reduced one. For instance, snakes possess pelvic girdles and femurs, remnants of their legged ancestors, while ostriches have wings that are no longer functional for flight. A relatable example in humans is goosebumps, which were once useful for insulation or as a threat display in fur-covered ancestors but now serve little purpose due to our reduced body hair.

Embryologic homology refers to the similarities observed in the embryonic stages of different organisms, despite their adult forms being quite distinct. For example, fish, birds, reptiles, and mammals exhibit similar features, such as pharyngeal arches and post-anal tails, during early development. These shared traits suggest that evolution operates on existing variations, with early developmental changes having a more significant impact on the organism's eventual form than changes occurring later in development. This stability in early embryonic traits across diverse species indicates a common ancestry.

Molecular homology is increasingly significant in modern biology, particularly in constructing evolutionary trees. It posits that DNA and protein sequences are more similar among closely related organisms. For example, the DNA sequences of lions and tigers show greater similarity compared to those of house cats, reflecting their closer evolutionary relationship. This similarity in genetic material underscores the concept of common ancestry, as these sequences have been preserved through generations, while more distant relatives exhibit greater genetic divergence due to the passage of time and evolutionary modifications.

In summary, homologies provide critical insights into the evolutionary connections among organisms, revealing how traits have been inherited and modified over time. Understanding these concepts is vital for comprehending the intricate web of life and the processes that shape biodiversity.

Evidence of Evolution Example 2

In studying the genetic relationships among different species, the concept of homology plays a crucial role. Homology refers to the similarities in DNA sequences that arise from a common ancestor. By analyzing the DNA sequences of three species, we can determine which organisms are more closely related based on the degree of similarity in their nucleotide sequences. For instance, if two species share more identical nucleotides, they are likely to be more closely related than those with significant differences. In this case, species 1 and 3 exhibit more homology compared to species 2, indicating a closer evolutionary relationship.

When examining the traits or phenotypes of these organisms, the amino acid sequences derived from the DNA provide essential insights. Since proteins are responsible for various functions within cells, the amino acid sequence is critical in determining the traits of an organism. In this example, all three species have identical amino acid sequences, suggesting that there will be no observable differences in their phenotypes, despite the variations in their DNA sequences.

To identify regions of homology, one should focus on the nucleotides that remain consistent across the species. In this scenario, the first two nucleotides of each codon are the same, while differences appear in the third position. This phenomenon can be attributed to the redundancy of the genetic code, where variations in the third nucleotide often do not affect the resulting amino acid, a concept known as silent mutations. Thus, even if the DNA sequences show differences, the corresponding amino acid sequences can still be identical, highlighting the complexity of genetic information.

Overall, molecular homology is a powerful tool in modern biology, allowing scientists to analyze vast amounts of genetic data to establish evolutionary relationships. With thousands of nucleotides coding for each protein, researchers can uncover significant insights into the evolutionary history of organisms by examining their genetic similarities and differences.

Different alleles of the MC1R gene have been associated with hair/coat color variation in many different mammals, including mice, jaguars, cows, and humans. In humans, specific mutations of the MC1R gene are associated with red hair. Using the concept of homology, explain why the same gene would affect similar traits in multiple organisms.

• A. Because the MC1R gene affects hair/fur color variation, you can expect that the gene will change often as organisms adapt to different environments.
• B. Since hair color is not a trait that confers a survival or reproductive advantage in humans, the MC1R gene can be considered an example of a vestigial trait.
• C. The MC1R gene was likely involved in coat color determination in the common ancestor of these organisms and maintains that role in each of these lineages.
• D. It is likely that the MC1R gene evolved to control hair/fur color separately in each lineage, as the presence of hair/fur is a homologous trait in mammals.

Answer: (C)

Which of the following statements describes a vestigial structure?

• A. While cats have much better balance than humans, the inner ear structures responsible for balance are largely the same in both cats and humans.
• B. Owls’ arteries are located much closer to the spine than in other birds, allowing for extreme neck flexibility without diminished blood flow.
• C. A peacock’s tail is large and brightly ornamented to advertise its fitness to females.
• D. Cave salamanders live in complete darkness but still develop eyes with limited functionality.

Answer: (D)

Fossils

Fossils serve as a crucial piece of evidence for understanding evolution, illustrating how species have changed over time. These fossils are the preserved remains of organisms found in sedimentary rock layers, formed as sediment accumulates and compresses over time. The geological layers of the Earth create a timeline, where deeper layers contain older fossils, often revealing organisms that are significantly different from those alive today. For instance, fossils found near the surface may resemble modern species, such as woolly mammoths, which, while extinct, share similarities with contemporary elephants. In contrast, deeper layers may yield dinosaur fossils, indicating that these creatures thrived during a specific period in Earth's history.

As we delve deeper into the fossil record, we encounter even older organisms, such as trilobites, which were once abundant in ancient oceans but are now extinct. This stratification of fossils not only highlights extinction events but also allows scientists to track broad evolutionary changes over time. A compelling example of this is the evolutionary history of whales, which are believed to have evolved from four-legged terrestrial ancestors. By examining fossils, researchers can identify transitional forms that exhibit intermediate leg structures, supporting the hypothesis of whale evolution.

Modern whales possess vestigial pelvic bones, remnants of their land-dwelling ancestors. Fossils from approximately 40 million years ago, such as those of the organism Darwinus, show a whale-like appearance with diminished leg bones. Further back, fossils of Rodhocetus, dating to around 47 million years ago, reveal a creature with more developed limbs, suggesting it was adapted for swimming. Even deeper in the fossil record, Indohyus, which lived about 50 million years ago, had legs suited for walking on land, indicating a gradual transition from land to aquatic life.

This evolutionary process illustrates that species do not evolve with a predetermined goal; rather, variations that enhance survival in specific environments become more prevalent over time. As organisms like Indohyus adapted to life near water, traits that facilitated swimming became advantageous, leading to the development of modern whales. Thus, fossils not only provide evidence of broad evolutionary patterns but also allow for the testing of specific hypotheses regarding how species have transformed throughout history.

Sedimentary rock creates distinct layers that paleontologists can use to help interpret their findings. Use the image below to determine which of the following statements is true about fossils and sedimentary rock.

• A. The fossils found in layer B will be older than the fossils in layer A.
• B. More fossils are likely found in layer C than in layer B.
• C. Fossils are most likely to be found in the transitions between layers.
• D. Fossils in layer C are likely to be the ancestors of fossils in layer D.

Answer: (A)

Dinosaurs went extinct approximately 65 million years ago (mya). Fossils of the organism Archaeopteryx have been found in strata dating to approximately 150 mya. These fossils show a dinosaur-like skeleton with a bony tail, teeth, and claws, as well as clear impressions of feathers similar to those found in flying birds. It is thought that Archaeopteryx belongs to a group of dinosaurs from which birds evolved. Which statement below is consistent with the evidence presented above?

• A. Because Archaeopteryx had more dinosaur-like traits than bird-like traits, it should not be considered a transitional form.
• B. Flight is an analogous trait found in both dinosaurs and birds.
• C. Archaeopteryx should be considered a bird and not a dinosaur due to the presence of feathers.
• D. At least some traits common in modern birds evolved long before dinosaurs went extinct.

Answer: (D)

Here’s what students ask on this topic:

What is homology in the context of evolution?

Homology in evolution refers to traits shared by different species due to a common ancestor. These traits can be observed in various biological structures, such as bones, genetic sequences, and proteins. For example, the similar bone structures in the limbs of tetrapods (e.g., whales, frogs, horses, lions, humans, and birds) suggest a common ancestry. Homology helps scientists understand evolutionary relationships by indicating that these species inherited these traits from a shared ancestor, with modifications occurring over time.

How do fossils provide evidence for evolution?

Fossils provide crucial evidence for evolution by showing how species have changed over time. Found in layers of rock or sediment, fossils create a timeline where deeper layers contain older organisms. This reveals patterns of extinction and evolutionary changes. For instance, fossils demonstrate the transition of whales from land-dwelling ancestors to modern aquatic forms. By examining these fossils, scientists can track specific evolutionary histories and transitions, supporting the theory of evolution and illustrating the interconnectedness of life forms.

What are vestigial structures and how do they support the theory of evolution?

Vestigial structures are traits that have lost most or all of their ancestral function. Examples include the pelvic girdle in snakes, wings on ostriches, and human goosebumps. These structures indicate that the species evolved from ancestors that had fully functional versions of these traits. The presence of vestigial structures supports the theory of evolution by demonstrating how species adapt over time, retaining remnants of traits that were once useful in their ancestors but are no longer necessary in their current form.

What is molecular homology and why is it important in studying evolution?

Molecular homology refers to the similarity in DNA and protein sequences among related organisms. It is crucial in studying evolution because it provides evidence of common ancestry at the molecular level. For example, the DNA sequences of lions, tigers, and house cats show similarities, indicating a shared ancestor. Molecular homology allows scientists to construct evolutionary trees and understand the genetic relationships between species, offering a detailed view of how life forms are interconnected and have evolved over time.

How do embryologic homologies provide evidence for evolution?

Embryologic homologies are traits shared by embryos of different species that are not present in adults. For example, embryos of fish, birds, reptiles, and mammals all have pharyngeal arches and post-anal tails. These similarities suggest a common ancestry, as these traits are inherited from a shared ancestor. Embryologic homologies provide evidence for evolution by showing that early developmental stages are remarkably stable over time, and small changes in these stages can lead to significant differences in adult forms, supporting the idea of descent with modification.