Study Guide 10.1-10.5
Charles Darwin and the Voyage of the H.M.S Beetle
Charles Darwin, as part of his early scientific endeavors, was offered a place on the British Navy ship known as The H.M.S Beetle. His journey, which took place in , led him through the Galapagos, a group of isolated islands. During this voyage, Darwin made critical observations regarding the local fauna, specifically focusing on the finches and tortoises. He discovered that the finches on different islands varied significantly in their beak size and shape. Based on these observations, Darwin concluded that there must have originally been just one type of finch on the Galapagos which diversified over time. Similar observations were made regarding tortoises, which exhibited variations in shell shape and size depending on which specific island they inhabited. These discoveries led Darwin to the idea that all species were connected, a concept he referred to as the tree of life.
Two central concepts emerged from Darwin’s work: descent with modification and variation. Descent with modification was defined as the process where one type of organism transformed or evolved into something utterly different. Darwin realized that variation must be the starting point for change in nature. This variation serves as the fundamental mechanism that leads to evolution through the process of natural selection.
Scientific Contributions to Evolutionary Theory (10.1)
Several scientists prior to and contemporary with Darwin contributed to the development of evolutionary theory. Linnaeus proposed that some changes in organisms occurred through hybridization, which is the breeding of different species. In doing so, he transitioned away from the idea that organisms were fixed and did not change, and he proposed a new system of organization for plants, animals, and minerals. Buffon proposed that species shared ancestors instead of arising separately. He also suggested that the earth was a lot older than the years it was commonly thought to be at the time. Buffon discussed extensive ideas regarding the relationships among organisms, sources, and biological variation.
Erasmus Darwin, or E. Darwin, proposed that all living things were descended from a common ancestor and that more complex forms of life arose from less complex forms. He believed that species could evolve through competition. Additionally, Lamark proposed that all organisms evolved towards perfection and complexity. Contrary to the idea of extinction, he thought that organisms evolved to become less complex if needed. He proposed that the environment had an effect on the behavior of organisms, leading to his idea known as the inheritance of acquired traits. While influential, this specific idea was eventually found to be flawed.
In the context of these biological theories, a species is defined by two specific conditions that must be true for a group of animals to be considered the same. First, they have to be similar enough that they can reproduce with one another. Second, they must be capable of producing fertile offspring.
Theories of Geologic Change
Geological theories provided a necessary framework for Darwin's evolutionary concepts. Catastrophism states that natural disasters, such as floods and volcanic eruptions, have happened often during the long history of the earth and have shaped its landforms. Gradualism suggests that changes in landforms resulted from slow changes occurring over a long period of time. Uniformitarianism, which incorporates the observations of Lyell, posits that the geological processes that shape the earth are uniform through time. This theory combines the idea of gradualism with the observation that changes on earth are constant and ongoing.
Variation and Adaptation (10.2)
Evolution is defined as the process of biological change by which descendants come to differ from their ancestors. Central to this process is the study of fossils, which are the traces of organisms that existed in the past. Variation is defined as the difference in the physical traits of an individual from those of other individuals in the group to which it belongs. There are two types of variation: interspecific variation, which occurs among different species, and intraspecific variation, which occurs among individuals of the same species.
An adaptation is a feature that allows an organism to better survive in its environment. Adaptations are significant because they can lead to genetic change in a population over time. During his voyage, Darwin found evidence for these concepts in both the fossil record and the geological landscape. He discovered the fossil of a Glyptodon, a giant armadillo, which provided evidence that modern animals could have connections with extinct fossil forms. Geologically, he experienced an earthquake and observed that land that had previously been underwater was moved above sea level. These observations led Darwin to conclude that over long periods of time, gradual biological or geological processes can add up to a great change.
Mechanisms of Evolution: Selection and Population (10.3)
Selection acts as the primary mechanism for evolution and can occur through two different processes. Artificial selection is the process by which humans select specific traits through breeding. Natural selection is the mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than other individuals. In natural selection, the environment acts as the selective agent, and it acts specifically on existing variation. A key component of this is heritability, which is the ability of a trait to be passed down from one generation to the next.
Darwin was inspired by the ideas of Thomas Malthus, who proposed that resources such as food, water, and shelter were natural limits to population growth. Darwin reasoned that a similar struggle for survival took place in nature. This leads to overproduction, which is the practice of having many offspring to raise the chance that some will survive. However, overproduction also results in competition for resources. Fitness is defined as the measure of the ability to survive and produce more offspring relative to other members of the population. A population consists of all the individuals of a species that live in a specific area.
The Four Principles and Evidence of Evolution (10.4)
Natural selection is governed by four principles: overproduction, variation, adaptation, and descent with modification. Descent with modification in this context refers to a heritable trait becoming common in a population over generations. Evidence for evolution is drawn from four primary sources: fossils, geography, embryology, and anatomy. Geologists found that fossils in bottom layers were more primitive than those in the upper layers, allowing scientists to study the change in organisms over time. For example, while scientists suspected whales were once land animals, the discovery of the Basilasourus fossil confirmed this transition.
Geographical evidence involves looking at where animals show up or do not show up to see how environmental pressures favor certain adaptations. Finches on the Galapagos islands showed variation from one island to another in beak size and shape, and these island species most closely resemble the nearest mainland species. Embryology reveals shared traits between species by showing that embryos can be hard to tell apart, implying a distant common ancestor. It demonstrates the importance of specific genes being turned on or off. An example of this is seen in crab and barnacle larvae, which look alike and both swim despite growing up to look completely different.
Anatomical evidence involves comparing body structures. Homologous structures are features that are similar in structure but appear in different organisms and have different functions, such as a whale fin and a human hand. These structures are evidence of divergent evolution and a common ancestor. Analogous structures perform a similar function but are not similar in origin, such as bird wings and insect wings. These represent convergent evolution and do not show a common ancestor. Vestigial structures are remnants of organs or structures that had a function in an early ancestor but no longer do. Examples include the human appendix, a whale's hindlimb bones, and ostrich wings.
Paleontology and Molecular Evidence (10.5)
Paleontology is the study of fossils or extinct organisms and continues to supply new information to support current hypotheses about how evolution occurs. For instance, the Basilasauris fossil confirmed whales were once land animals. The fossil record is considered incomplete for two reasons: most living things do not form fossils after they die, and fossils have not been searched for in many areas of the world. Transitional fossils, often referred to as missing links, are important because they show how species change over time and connect a common ancestor, such as the Basilasauris.
Molecular and genetic evidence further support fossil and anatomical findings. DNA sequence analysis depends on the fact that more closely related organisms have more similar DNA. Because even simple organisms have thousands of genes, DNA contains a huge amount of information on evolutionary history. Pseudogenes are sequences of DNA nucleotides that act like vestigial structures; they can change while being passed through generations, providing a way to figure out evolutionary relationships and reflecting a distant common ancestor. Homeobox genes control the development of specific structures and are found in many organisms, suggesting a very distant common ancestor.
Protein comparisons can be conducted through molecular fingerprinting, which reveals similarities among cell types across organisms. Cells from different species that share the same protein are likely to have common ancestors. The underlying theme of all these molecular techniques is that they reveal common ancestors. The modern synthesis of evolutionary biology is defined as the theory of natural selection supported by genetic evidence.