Evolution of Populations: Videos & Practice Problems

Evolution is defined as a change in allele frequency within a population over time, encompassing both macroevolution and microevolution. Microevolution involves small, rapid changes, often driven by mechanisms such as natural selection, genetic drift, and gene flow. Natural selection favors alleles that enhance reproductive success, while genetic drift refers to random changes in allele frequency. Gene flow occurs when alleles move between populations. Understanding these concepts is crucial for grasping the dynamics of genetic variation and the processes that shape biodiversity.

Introduction to Evolution of Populations

Evolution is defined as a change in a population over time, and it can be studied at different scales. Macroevolution refers to large changes that occur over extended periods, such as the evolution of birds from dinosaur ancestors or the divergence of species like tigers and lions from a common ancestor. In contrast, microevolution focuses on small, rapid changes within populations, which is the primary focus of this section.

To illustrate microevolution, consider the example of white and brown rabbits. The variation in their color is linked to specific genes and alleles, represented by the big A allele for brown and the little a allele for white. The key takeaway is that when a predator, like a fox, catches a white rabbit, it removes specific alleles from the population, thereby affecting allele frequencies.

Evolution can be refined to mean a change in allele frequency within a population. Alleles are different variants of a gene, and their frequency can increase or decrease over time, indicating that evolution has occurred. For evolution to take place, genetic variation is essential, which arises from multiple alleles within a population. New alleles can be introduced through mutations, and their prevalence can change due to various evolutionary mechanisms.

Three primary mechanisms drive changes in allele frequency: natural selection, genetic drift, and gene flow. Natural selection occurs when certain alleles enhance an organism's likelihood of survival and reproduction, leading to a higher frequency of those alleles in the population. Genetic drift refers to random changes in allele frequency that can occur due to chance events, which may not always align with expected probabilities. Gene flow involves the movement of alleles between populations, which can alter allele frequencies when individuals migrate and introduce new alleles.

The concepts of evolution were significantly advanced by the modern synthesis, which emerged in the early 1900s. This synthesis integrated Darwinian evolution with Mendelian genetics, revealing that genes and alleles are fundamental to the processes described by Darwin. This integration led to the development of population genetics, a field that studies how genes and alleles change within populations, moving beyond the focus of Mendelian genetics on individual crosses.

Understanding these mechanisms and their implications is crucial for grasping how populations evolve over time, and this section will delve deeper into these concepts through practical examples and applications.

Evolution of Populations Example 1

In the study of evolution, several mechanisms explain how populations change over time. Three key mechanisms are natural selection, gene flow, and genetic drift, each playing a distinct role in the evolutionary process.

Gene flow occurs when individuals from one population migrate to another, introducing new alleles into the gene pool. An example of this is when a camper brings firewood from a different state, which contains a beetle with a variant allele not present in the local population. This introduction of new genetic material exemplifies gene flow, as it enhances genetic diversity within the receiving population.

Genetic drift refers to random changes in allele frequencies within a population, particularly in small populations. For instance, if a population of plants has two variants of a gene coding for a photosynthesis protein, and there is no fitness advantage between the two, over time, one variant may become more common purely by chance. This scenario illustrates genetic drift, as the change in allele frequency is not driven by natural selection but rather by random sampling effects.

Natural selection is the process by which certain traits become more common in a population due to their advantageous effects on survival and reproduction. A clear example is a mutation in an allele that allows eastern screech owls to better camouflage in their environment. Over generations, this trait enhances the owls' fitness, leading to a higher frequency of the camouflage allele in the population, demonstrating natural selection in action.

Lastly, genetic drift can also be observed in situations where a significant event drastically reduces a population's size, such as a forest fire. If the surviving mountain lions happen to have a higher frequency of an allele for brown eyes, this change is not due to any fitness advantage but rather a random occurrence, again illustrating genetic drift.

Understanding these mechanisms is crucial for grasping how populations evolve and adapt over time, highlighting the interplay between chance and selection in shaping biodiversity.

What do natural selection, genetic drift, and gene flow all have in common?

• A. They were introduced as ideas by Charles Darwin in his book On the Origin of Species.
• B. The combination of those three ideas represents what is known as “The Modern Synthesis”.
• C. They all contribute to microevolution within a population.
• D. They are three ways that new alleles may be introduced to a population.

Answer: (C)

Which description below best describes “The Modern Synthesis”?

• A. The combination of modern genetic techniques, such as DNA sequencing, with Darwinian theory.
• B. The combination of Mendel’s and Darwin’s ideas into one theoretical framework.
• C. The combination of micro and macro evolutionary ideas into one unified theory.
• D. The combination of computer modeling and DNA sequencing to investigate evolutionary relationships.

Answer: (B)

Here’s what students ask on this topic:

What is the difference between macroevolution and microevolution?

Macroevolution and microevolution are two scales of evolutionary change. Macroevolution refers to large-scale changes that occur over long periods of time, such as the evolution of new species or major changes in body plans. Examples include the evolution of birds from dinosaur ancestors or the divergence of tigers and lions from a common ancestor. In contrast, microevolution involves small, rapid changes within a population over shorter time frames. These changes are often driven by mechanisms like natural selection, genetic drift, and gene flow. Microevolution focuses on changes in allele frequencies within a population, such as the color variation in rabbit populations due to predation by foxes.

How does natural selection affect allele frequencies in a population?

Natural selection affects allele frequencies by favoring alleles that enhance an organism's reproductive success. Individuals with advantageous alleles are more likely to survive and reproduce, passing these alleles to their offspring. Over time, these beneficial alleles become more common in the population. For example, if a certain allele provides better camouflage, individuals with this allele are less likely to be preyed upon and more likely to reproduce. As a result, the frequency of this allele increases in the population. Natural selection thus leads to a change in allele frequencies, driving the evolution of the population.

What is genetic drift and how does it influence evolution?

Genetic drift is the random change in allele frequencies within a population, often having a more significant impact in small populations. Unlike natural selection, which is driven by differential reproductive success, genetic drift occurs due to random events that cause certain alleles to become more or less common. For example, if a natural disaster randomly kills a large portion of a population, the surviving individuals' alleles may not represent the original population's genetic diversity. Over time, genetic drift can lead to the loss of genetic variation and can cause significant evolutionary changes, especially in small populations.

What is gene flow and how does it contribute to evolution?

Gene flow, also known as gene migration, is the movement of alleles between populations. This occurs when individuals from one population migrate to another and interbreed, introducing new alleles into the gene pool of the recipient population. Gene flow can increase genetic variation within a population and reduce differences between populations, making them more genetically similar. For example, if a few individuals from a population with a unique allele migrate to another population and reproduce, the allele frequency in the recipient population will change. Gene flow thus contributes to evolution by altering allele frequencies and promoting genetic diversity.

What is the modern synthesis in evolutionary biology?

The modern synthesis is a unifying theory in evolutionary biology that combines Darwinian evolution and Mendelian genetics. Developed in the early 20th century, it integrates the concepts of natural selection, genetic variation, and inheritance. The modern synthesis explains how genetic mutations create new alleles, which can be acted upon by natural selection, genetic drift, and gene flow to drive evolutionary change. It emphasizes the role of populations as the units of evolution and uses mathematical models to study the processes affecting allele frequencies. This synthesis laid the foundation for the field of population genetics, which examines how genes and alleles change within populations over time.