Getting Started
This chapter explores evolution at the population level, the smallest scale at which it can occur. We will examine the core concept of a population's gene pool—the collection of all its genetic information—and the mechanisms that cause it to change over time. The central process is the fluctuation of allele frequencies, which can be driven not only by adaptive pressures but also by purely random events.
What You Should Be able to Do
After completing this section, you will be able to:
Explain how chance events like genetic drift alter a population's genetic composition.
Describe how mutation, gene flow, and genetic drift contribute to the evolutionary path of a population.
Use changes in allele frequencies as evidence to explain that a population is evolving.
Compare and contrast the bottleneck effect and the founder effect as mechanisms of genetic drift.
Key Concepts & Mechanisms
Evolution is fundamentally a change in the genetic makeup of a population over generations. While natural selection is a major driver of this change, it is not the only one. Random, non-adaptive processes also play a crucial role in shaping the evolutionary trajectory of a population.
Baseline Condition: The Gene Pool
To understand how a population evolves, we must first establish a baseline. This baseline is the population's gene pool, which is the complete set of all alleles for every gene present in all interbreeding members of a population (a localized group of individuals of the same species). The genetic makeup of this gene pool can be quantified by measuring the allele frequency for various genes—that is, how common each allele is within the population. In a theoretical, non-evolving population, these allele frequencies would remain constant from one generation to the next. This stable state serves as a null hypothesis against which we can measure real-world change.
Key Changes: Random Drivers of Evolution
Any deviation from stable allele frequencies indicates that evolution is occurring. The following random processes are significant drivers of such change.
Mutation
Mutation is a random change in an organism's DNA sequence. It is the ultimate source of all new genetic variation, creating new alleles upon which other evolutionary forces can act.
Mechanism: Mutations can arise from errors in DNA replication or from damage by environmental factors like UV radiation. They are not goal-oriented; they do not occur because an organism "needs" a new trait.
Effect: By introducing a new allele, a mutation directly changes the gene pool, albeit at a very low rate. While most mutations are neutral or harmful, a rare beneficial mutation can be favored by natural selection. However, its initial appearance in the population is a purely random event.
Genetic Drift
Genetic drift refers to changes in allele frequencies due to random chance events from one generation to the next. Imagine flipping a coin 1000 times versus 10 times; you are far more likely to get a significant deviation from a 50/50 split with the smaller sample size. Similarly, genetic drift has its most profound effects on small populations.
- Mechanism: In each generation, not all individuals will reproduce, and of those that do, their gametes represent a random sample of the parent generation's alleles. In a small population, this random sampling can easily lead to the loss of some alleles and the "fixation" (a frequency of 100%) of others, purely by chance.
There are two primary scenarios where genetic drift can have a major impact:
The Bottleneck Effect: This occurs when a population's size is drastically reduced by a non-selective event, such as a natural disaster, famine, or disease. The surviving individuals may have a gene pool that, by chance, is not representative of the original population. Certain alleles may be overrepresented, underrepresented, or lost entirely.
The Founder Effect: This occurs when a few individuals become isolated from a larger population and establish a new one. The gene pool of this new "founder" population is limited to the alleles of its founders. If the founders, by chance, had different allele frequencies than the source population, the new population will also have different frequencies.
Gene Flow
Gene flow, also known as migration, is the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.
Mechanism: An individual from one population may mate with an individual from another, introducing alleles from its source population into the new one. Pollen or seeds can also be transported over long distances, facilitating gene flow between plant populations.
Effect: Gene flow tends to reduce the genetic differences between populations. If extensive enough, it can merge neighboring populations into a single one with a common gene pool. It can also introduce new alleles to a population, increasing its genetic variation. Thus, gene flow often acts to counteract the divergent effects of genetic drift.
Key Continuities: The Persistence of Populations
Despite these changes, there are continuities. The population, though genetically altered, persists as a group of the same species. The fundamental laws of inheritance (meiosis and fertilization) that shuffle existing alleles and pass them to the next generation remain constant. Even as populations diverge due to drift or mutation, they retain a vast majority of their genetic information, reflecting their shared ancestry.
Key Models & Diagrams
The following table summarizes the random processes that drive changes in a population's genetic makeup.
| Mechanism | Description | Effect on Genetic Variation | Key Factor |
|---|---|---|---|
| Mutation | A random change in the DNA sequence, creating a new allele. | Increases genetic variation by introducing new alleles. | Randomness of DNA replication/repair. |
| Genetic Drift (General) | Random fluctuations in allele frequencies from one generation to the next. | Tends to reduce genetic variation over time as alleles are lost or fixed. | Small population size. |
| Bottleneck Effect | A drastic reduction in population size due to a random event. | Reduces genetic variation; the surviving gene pool may not reflect the original. | Severity of the population crash. |
| Founder Effect | Establishment of a new population by a small number of individuals. | Reduces genetic variation; the new gene pool reflects the alleles of the founders. | The specific alleles of the founders. |
| Gene Flow | The transfer of alleles between populations. | Can increase variation within a population but reduces differences between populations. | Rate of migration. |
Key Components & Evidence
Allele Frequency: The fundamental metric of a population's genetic makeup. A documented change in allele frequency over time is direct evidence of evolution.
Gene Pool: The total collection of genes and alleles in a population. Random events act upon this pool to change its composition.
Mutation: The ultimate source of novelty. The existence of multiple alleles for a single gene is evidence of past mutation events.
Genetic Drift: A powerful evolutionary force in small populations. Its effects are visible in the reduced genetic diversity of species that have undergone bottlenecks, such as the African cheetah.
Bottleneck Effect: A specific mechanism of drift. The low genetic variation in the northern elephant seal population is evidence of a severe bottleneck caused by overhunting in the 1890s.
Founder Effect: Another specific mechanism of drift. The high frequency of certain rare genetic disorders (e.g., Ellis-van Creveld syndrome) in isolated human populations like the Amish of Pennsylvania is a classic example.
Gene Flow: A homogenizing force. The spread of pesticide-resistance alleles from one insect population to another is a modern example of gene flow with significant consequences.
Skill Snapshots
Causation
A volcanic eruption that wipes out 90% of a plant population (cause) can lead to a bottleneck effect, drastically altering the population's allele frequencies (effect).
The migration of a few birds to a new island (cause) results in a founder effect, creating a new population with a gene pool that differs from the source population (effect).
The movement of pollen by wind between two previously isolated plant populations (cause) leads to gene flow, making the two populations more genetically similar (effect).
Comparison
Genetic drift causes random, often non-adaptive, changes in allele frequencies, whereas gene flow introduces or removes alleles in a directed way based on migration patterns.
The bottleneck effect stems from a drastic reduction in the size of an existing population, while the founder effect involves the establishment of a new population by a small number of individuals.
Mutation is the only mechanism that creates entirely new alleles, while genetic drift and gene flow primarily act by changing the frequencies of existing alleles.
Change and Continuity Over Time
Baseline: A large, genetically diverse mainland population of lizards has stable frequencies for color-pattern alleles.
Change 1: A few lizards are swept away on a log to a new island (founder effect). This new, small population has, by chance, a much higher frequency of a rare recessive allele for albinism.
Change 2: Over many generations on the isolated island, genetic drift causes the albino allele to become fixed (100% frequency), while other color alleles are lost. The island population now looks distinctly different from the mainland population.
Continuity: Despite the divergence in allele frequencies for coloration, both populations are still the same species, sharing the vast majority of their genes and a recent common ancestor.
Common Misconceptions & Clarifications
Misconception: Evolution is a process of "improvement" that always leads to better-adapted organisms.
- Clarification: Genetic drift can cause the frequency of beneficial alleles to decrease and harmful alleles to increase or even become fixed, purely by chance. Evolution is simply change over time, and that change is not always adaptive.
Misconception: An individual organism can evolve.
- Clarification: Individuals cannot evolve. Evolution is defined as the change in allele frequencies in a population over generations. An individual is born with and dies with the same set of genes.
Misconception: Genetic drift is the same as natural selection.
- Clarification: Genetic drift is change due to random chance (e.g., "bad luck"), while natural selection is change due to differential survival and reproduction based on heritable traits (e.g., "fitter" traits). In drift, the change in allele frequencies is not related to the allele's influence on fitness.
Misconception: Mutations arise to meet the needs of a population.
- Clarification: Mutations are entirely random with respect to fitness. A mutation for antibiotic resistance in a bacterium does not arise because antibiotics are present; the mutation may already exist randomly in the population, and its frequency will only increase when the antibiotic acts as a selective pressure.
One-Paragraph Summary
The genetic makeup of a population is not static; it is constantly changing, and this change is the essence of evolution. Beyond the well-known mechanism of natural selection, evolution is powerfully driven by random processes. Mutation serves as the ultimate source of new genetic material by creating novel alleles. Genetic drift, which encompasses the bottleneck and founder effects, causes random fluctuations in allele frequencies that are especially significant in small populations, often leading to a loss of genetic diversity and divergence from other populations. In contrast, gene flow (migration) involves the transfer of alleles between populations, which tends to reduce genetic differences and can introduce new variation. The observation of changing allele frequencies over time provides direct and compelling evidence that a population is evolving.