Introduction to Evolution and Natural Selection: Videos & Practice Problems

Evolution is the gradual change of populations over time, defined by "descent with modification," where species share common ancestors. Natural selection, a key mechanism of evolution, favors traits that enhance survival and reproduction, leading to adaptations that suit organisms to their environments. Understanding species relationships through phylogenetic trees emphasizes common descent rather than a hierarchy of complexity, highlighting the importance of variation in defining species. This framework is essential for grasping evolutionary processes and the diversity of life.

Evolution and Natural Selection

Evolution is a fundamental concept in biology that describes the gradual change of a population over time. This process can be summarized by the phrase "descent with modification," coined by Charles Darwin, which emphasizes that as offspring are produced, they inherit traits from their ancestors, and these traits can change over generations. This leads to the idea of common descent, where species today share common ancestors, illustrated by evolutionary trees that depict relationships among organisms, such as the big cats in the genus Panthera.

Natural selection serves as the primary mechanism driving evolution. It can be simply defined as the process by which traits that enhance an individual's ability to survive and reproduce become more prevalent in a population. This concept is often encapsulated in the phrase "survival of the fittest," which, while popularized during Darwin's time, requires clarification. In this context, "fitness" refers not to physical strength but to an organism's ability to produce offspring that survive to the next generation. Thus, an organism that is considered "fit" is one that successfully contributes to the gene pool of future generations.

Natural selection leads to adaptations, which are traits that enhance an organism's suitability to its environment. These adaptations arise because individuals with advantageous traits are more likely to survive and reproduce, thereby passing those traits on to their offspring. This ongoing process explains why organisms are often well-suited to their environments, as they have evolved through natural selection to thrive in specific conditions.

In summary, understanding evolution and natural selection provides insight into the dynamic processes that shape the diversity of life on Earth, illustrating how species adapt and change over time through mechanisms that favor survival and reproduction.

Introduction to Evolution and Natual Selection Example 1

Evolution and natural selection are interconnected concepts in biology, often leading to confusion. Understanding their distinctions is crucial for grasping how species change over time. Natural selection refers to the process by which certain traits become more common in a population due to their advantages in survival and reproduction, while evolution encompasses the broader changes in species over generations, including descent with modification.

For instance, consider the statement: "A caterpillar that is camouflaged is more likely to reproduce because it is less likely to be eaten by a bird." This scenario illustrates natural selection, as the camouflage trait enhances survival and reproductive success, allowing the caterpillar to pass this advantageous trait to its offspring.

In contrast, the statement: "The closest living relatives of birds are alligators and crocodiles. Therefore, we expect birds to share many traits with reptiles," relates to evolution. It highlights the concept of common descent, where species that share a common ancestor exhibit similar traits due to their evolutionary history.

Another example is the observation that "fossil data indicates that early ancestors of modern horses were less than a half a meter tall, and a trend in horse evolution is that horse ancestors got larger over time." This reflects evolution through descent with modification, as it shows how species change over time based on their ancestral traits.

When discussing fitness, the statement "When thinking of fitness, attracting mates can be just as important as surviving" emphasizes natural selection. Here, fitness is defined by the ability to reproduce, which can be influenced by traits that attract mates, thus affecting the likelihood of passing on genes to the next generation.

Lastly, the statement "Dandelion seeds are able to travel great distances in the wind. The dandelions whose seeds travel the farthest have the best chance of spreading their offspring widely" also pertains to natural selection. It illustrates how certain traits, like seed dispersal ability, enhance reproductive success and survival in varying environments.

In summary, while natural selection focuses on the mechanisms that drive the survival and reproduction of individuals based on advantageous traits, evolution encompasses the broader changes in species over time, including their shared ancestry and modifications. Understanding these concepts is essential for studying biological diversity and the processes that shape life on Earth.

Explain the relationship between natural selection and evolution.

• A. Evolution is an explanation for how natural selection happens.
• B. Evolution and natural selection explain the same phenomena but use different timescales.
• C. Evolution explains how life begins, while natural selection explains how life changes.
• D. Natural selection is an explanation for how evolution happens.

Answer: (D)

Which organism will have the highest fitness?

• A. Organism A lives for 1 year, is relatively large, and produces 1 offspring.
• B. Organism B lives for 5 years, is relatively small, and produces 1 offspring.
• C. Organism C lives for 1 year, is relatively small, and produces 3 offspring.
• D. Organism D lives for 3 years, is medium-sized, and produces 2 offspring.

Answer: (C)

Which statement would be supported by the concept of common descent?

• A. Adaptations allow species to survive well in the environments in which they live.
• B. Many similarities between species exist because they were inherited from the same ancestor.
• C. Organisms that reproduce the most are considered to be the most fit in an evolutionary context.
• D. Species change over time in response to environmental pressures.

Answer: (B)

Typological vs. Population Thinking

Understanding organisms and their classification can be approached through two distinct frameworks: typological thinking and population thinking. Typological thinking, rooted in the philosophy of Plato, posits that there exists an idealized form of a species against which all variations are measured. For instance, when envisioning a lion, one might picture a specific ideal male lion. Any observed differences in mane color or size are seen as deviations from this ideal. This perspective implies that species descriptions remain static over time and overlook the inherent diversity within a species.

In contrast, population thinking emphasizes the significance of variation among individuals within a species. Instead of relying on a singular ideal, this approach advocates for examining a representative sample of the population to understand what defines a species. For example, when studying lions, one would consider the range of mane variations and recognize that there is no single 'average' lion that embodies all traits. Instead, the average is a statistical representation that acknowledges the diversity present within the population. This perspective allows for a dynamic understanding of species, where descriptions can evolve over time to reflect the actual diversity observed.

While typological thinking can simplify certain biological discussions, such as referring to "the gene for hemoglobin" or "the human heart," it is crucial for the study of evolution and natural selection to adopt population thinking. This approach highlights the importance of variation, which is essential for understanding how species adapt and evolve over time. By focusing on the diversity within populations, we gain a more accurate and comprehensive view of biological processes.

Introduction to Evolution and Natual Selection Example 2

Modern biology recognizes that species are not fixed entities, and understanding the distinction between typological and population thinking is crucial for grasping evolutionary concepts. Typological thinking often involves defining species based on a set of fixed characteristics, while population thinking emphasizes the variation within species.

For instance, when stating that the "normal height for a human female is 5 foot 5 inches tall," this reflects typological thinking. The term "normal" implies a standard, disregarding the natural variation in height that exists within the population. In contrast, a statement like "mature white oak trees often live for more than 200 years" acknowledges variation, making it more aligned with population thinking. It recognizes that while many trees may live to this age, there are exceptions, which is a key aspect of population dynamics.

Similarly, describing the eastern gray squirrel as having "uniform gray fur weighing between 406-600 grams" exemplifies typological thinking. This definition excludes the natural variations that can occur within the species. On the other hand, referring to a jaguar or leopard with dark coloration as a "black panther" also reflects typological thinking, as it categorizes a variation as abnormal rather than recognizing it as part of the species' diversity.

In contrast, stating that "the mean length of a birch tree leaf is 7 ± 2.5 centimeters" is a clear example of population thinking. This statement not only provides an average but also incorporates the variability inherent in the population, highlighting the importance of understanding statistical measures in biological contexts.

Overall, while all statements can be true, recognizing the difference between typological and population thinking enhances our understanding of evolution and the diversity of life. Emphasizing population thinking allows for a more nuanced view of species and their characteristics, which is essential for studying evolutionary processes.

What is one reason population thinking is so important in biology?

• A. Population thinking requires scientists to consider the full variation that is present in nature.
• B. Population thinking allows for fast and accurate species identification.
• C. Population thinking makes identifying abnormalities and diseases more straightforward.
• D. Population thinking simplifies making comparisons between different species.

Answer: (A)

Dog breeds are defined by denoting breed standards, a list of agreed-upon traits that individuals of that breed should possess (such as size, coat, coloring, temperament, etc.). Dog shows rank dogs based on their adherence to these breed standards. Based on this information, which statement about dog breeds below is correct?

• A. Breed standards describe many different traits; because many traits are considered, this reflects population thinking.
• B. Because there are many breeds of dogs, dog breeds reflect population thinking.
• C. Dog breeds reflect typological thinking because through breeding, humans control what traits a dog inherits.
• D. Breed standards use an idealized form to which dogs are compared; this reflects typological thinking.

Answer: (D)

Ladder vs. Tree Thinking

Before delving into the concepts of evolution and natural selection, it's essential to understand different ways of conceptualizing organisms, particularly through ladder versus tree thinking. Historically, ladder thinking has dominated our understanding of species, originating from Aristotle's idea of a hierarchy of complexity. Aristotle proposed the Scala Naturae, or the Great Chain of Being, which ranked species based on their complexity, placing plants at the bottom and humans at the top, often with gods above them. This perspective suggests a linear progression of life forms, where each species is seen as more or less complex than others.

However, this hierarchical view is misleading. While humans may exhibit complexity in certain traits, such as the central nervous system, other organisms demonstrate remarkable adaptations that challenge the notion of a simple hierarchy. For instance, octopuses possess advanced camouflage abilities, and plants can perform photosynthesis, a process humans cannot replicate. This highlights that complexity is not a definitive measure of an organism's evolutionary success.

In contrast, tree thinking emphasizes the relationships among species through common descent, presenting an evolutionary tree that illustrates how species are interconnected rather than ranked. This model indicates that all living organisms have been evolving for the same duration, and their differences arise from how well they adapt to their environments. The focus shifts from complexity to the fit of species within their ecological niches, where variation among species can be traced back to shared evolutionary histories.

For example, when examining why certain organisms possess bones, tree thinking allows us to trace their ancestry and understand that these traits emerged from a common ancestor. This approach fosters a deeper understanding of adaptations and evolutionary processes, moving beyond simplistic notions of superiority or inferiority among species.

It's crucial to be mindful of language that may inadvertently reinforce ladder thinking, such as referring to bacteria as "lower organisms." Such terminology can subtly influence our understanding of evolution and biodiversity. Embracing tree thinking encourages a more accurate and nuanced appreciation of the complexity and interrelatedness of life on Earth.

Introduction to Evolution and Natual Selection Example 3

In the study of evolutionary biology, different types of thinking help us understand the relationships and characteristics of species. These include ladder-based thinking, population thinking, tree-based thinking, and typological thinking. Each type of thinking offers a unique perspective on how we view species and their variations.

Ladder-based thinking posits that species are unchanging and can be arranged in a hierarchy, often referred to as the "great chain of being" or scala naturae. This perspective places humans at the top as the most complex organisms, reflecting a belief in a linear progression of life forms.

In contrast, population thinking emphasizes that species can change over time, recognizing the importance of variation within species. This approach suggests that understanding a species requires acknowledging its diversity rather than adhering to a single, unchanging type.

Tree-based thinking focuses on the evolutionary relationships among organisms, highlighting common ancestry as a means to explain similarities. This perspective visualizes the connections between species as a branching tree, illustrating how different organisms share common ancestors.

Typological thinking, rooted in the ideas of Plato, views species as ideal types, where variations are seen as imperfections or deviations from a perfect model. This approach often leads to a rigid classification of species based on fixed characteristics.

Overall, these frameworks provide valuable insights into the complexities of biological classification and the evolution of species, each contributing to our understanding of life's diversity.

True or False: if false, choose the answer that best corrects the statement.


In tree thinking, species are organized from low complexity to high complexity.

• A. True.
• B. False; in tree thinking, species are thought of as unchanging.
• C. False; in tree thinking, species are organized from high complexity to low complexity.
• D. False; in tree thinking, species are organized based on evolutionary relationships.

Answer: (D)

The Latin term Scala Naturae reflects which argument?

• A. Darwin’s argument for descent with modification.
• B. Aristotle’s argument that organisms can be arranged hierarchically by complexity.
• C. Plato’s argument that all individuals deviate from a divine ideal form.
• D. The modern biological argument that organisms should be organized by evolutionary relationships.

Answer: (B)

Reading Trees

Phylogenetic trees, also known as evolutionary trees, are essential tools for understanding the relationships between different species based on their common ancestry. These trees illustrate how populations evolve over time, highlighting the concept of common descent, where organisms share a common ancestor. When interpreting a phylogenetic tree, two primary components are crucial: branches and nodes. The branches represent populations as they change and evolve, while the nodes signify points where branches meet, indicating a common ancestor of the populations represented.

Phylogenetic trees can be presented in various formats, such as vertical, horizontal, or circular arrangements. Regardless of the style, the key to reading these trees lies in understanding how branches connect at the nodes. For instance, in a tree depicting the genus Panthera, which includes big cats, one can trace the lineage from a common ancestor through various splits that lead to modern species like the jaguar, lion, leopard, snow leopard, and tiger. Each split represents a divergence in the evolutionary path of these populations.

Time is an inherent aspect of phylogenetic trees, often represented by an arrow indicating the progression from past to present. The depth of a node in the tree correlates with the relatedness of the species; the closer the node is to the present, the more closely related the species are. For example, while leopards and snow leopards may appear similar and be positioned near each other on the tree, their common ancestor is located much deeper in the tree, indicating they are not closely related. In contrast, the leopard shares a more recent common ancestor with the lion, making them more closely related.

Understanding these relationships and the structure of phylogenetic trees enhances our comprehension of evolutionary biology and the interconnectedness of life on Earth.

Introduction to Evolution and Natual Selection Example 4

In studying the evolutionary relationships among various bird species, we can utilize a phylogenetic tree to understand their connections. The tree includes the ostrich, rhea, tinamu, cassowary, emu, and kiwi, each representing different evolutionary paths. The ostrich, rhea, cassowary, and emu are categorized as large flightless birds, while the kiwi is a smaller flightless bird native to New Zealand, and the tinamu comprises several bird species found in South America.

To determine which organism is most closely related to the emu, one must trace the branches of the tree back to the most recent common ancestor. Following this method, it becomes clear that the cassowary is the closest relative to the emu, as they share a more recent node in the tree.

When comparing the tinamu's relationship with the kiwi and the rhea, it is essential to identify the common nodes. The tinamu is more closely related to the kiwi because they share a more recent common ancestor, while the rhea is further back in the evolutionary timeline.

For the ostrich, it is equally related to all other birds on the tree, as it branches out from a node that leads to all the other species, indicating no closer relationship with any specific bird.

To identify which birds share the most traits with the tinamu, one should look for the closest relatives. The emu and cassowary, which branch from the same node as the tinamu, are expected to share many traits due to their recent common ancestor.

When asked to circle all the descendants of the bird that lived at a specific node (node A), one must follow the branches extending from that node. All birds that branch out from node A are considered its descendants, including the emu, cassowary, rhea, and others.

Finally, to find the most recent common ancestor of the cassowary and the kiwi, one should trace their lineages back to the last node where their branches converge. This node represents their most recent common ancestor and is crucial for understanding their evolutionary relationship.

Based on the following simplified tree, which of the following is more closely related to the pear?

• A. Cucumber.
• B. Cotton.
• C. Orange.
• D. California Poppy.

Answer: (A)

Based on the simplified tree below, identify which node represents the most recent common ancestor between pears and cucumbers.

• A. Node A.
• B. Node B.
• C. Node C.
• D. Node D.

Answer: (B)

Which of the following statements is FALSE based on the simplified phylogenetic tree shown?

• A. The cucumber is equally related to the apple and the pear.
• B. The cucumber and the California poppy share a common ancestor at node E.
• C. The most recent common ancestor between the apple and the orange is also the most recent common ancestor of the apple and the California poppy.
• D. The pear and the cucumber are more closely related than the pear and the orange.

Answer: (C)

Here’s what students ask on this topic:

What is the definition of evolution in biology?

In biology, evolution is defined as the gradual change of a population through time. This concept, often summarized by Charles Darwin's phrase 'descent with modification,' means that species undergo changes over generations, leading to differences in their genetic makeup and physical traits. Evolution explains how species adapt to their environments and how new species arise from common ancestors.

How does natural selection drive evolution?

Natural selection is a key mechanism driving evolution. It operates on the principle that traits that enhance an individual's ability to survive and reproduce become more common in a population over time. These advantageous traits are passed on to offspring, leading to gradual changes in the population. This process results in adaptations that make organisms better suited to their environments, ultimately driving evolutionary change.

What is the difference between typological thinking and population thinking in biology?

Typological thinking involves viewing species as having an idealized form, with variations seen as deviations from this ideal. This approach does not account for diversity and assumes species are unchanging. In contrast, population thinking recognizes the importance of variation among individuals within a species. It defines species based on the diversity of individuals and acknowledges that species descriptions can change over time, reflecting the dynamic nature of evolution.

How do phylogenetic trees help us understand evolutionary relationships?

Phylogenetic trees, also known as evolutionary trees, illustrate the relationships between species based on common descent. The branches represent populations evolving over time, while the nodes indicate common ancestors. By examining how branches connect at nodes, we can trace the evolutionary history of species and understand their relatedness. This approach emphasizes the shared evolutionary history and adaptations of species, rather than a hierarchy of complexity.

What is meant by 'survival of the fittest' in the context of natural selection?

'Survival of the fittest' is a phrase that summarizes natural selection. In this context, 'fitness' refers to an organism's ability to survive and reproduce, not physical fitness. Traits that enhance an individual's reproductive success become more common in the population. Thus, the 'fittest' individuals are those whose traits are best suited to their environment, allowing them to pass on their genes to the next generation.