Unit 7: Natural Selection

Darwin’s Theory of Natural Selection

• Charles Darwin proposed that natural selection is the process by which individuals with traits better suited to their environment have higher survival and reproductive success. This idea emerged from observations on the HMS Beagle voyage, particularly in the Galápagos Islands, where species showed variations suited to different habitats. Over many generations, these advantageous traits become more common in the population, leading to gradual evolutionary change.
• Natural selection is based on four key observations: variation exists in populations, traits are heritable, organisms produce more offspring than can survive, and those with advantageous traits have a higher probability of survival and reproduction. These principles work together to shift allele frequencies over time, shaping the gene pool in response to environmental pressures. This process is not purposeful but driven by differential reproductive success.
• Darwin’s work integrated ideas from earlier scientists like Lyell (geological change) and Malthus (population limits), showing that environmental challenges play a major role in determining which traits are beneficial. This connection between ecology and evolution highlights that evolutionary change is often context-dependent. A trait that is advantageous in one environment may be neutral or even harmful in another.
• Natural selection acts on phenotypes, observable traits, but its effects are recorded in genotypes through changes in allele frequency. For example, the industrial melanism seen in peppered moths during the Industrial Revolution demonstrates how environmental change can rapidly shift population traits. Such changes can be reversed if selective pressures shift again.
• It is important to recognize that individuals do not evolve; populations evolve over generations. While a single organism’s traits remain fixed during its lifetime, the proportion of individuals with those traits can increase or decrease in the population depending on the selective pressures present. This distinction is critical for understanding the scope and timescale of evolutionary processes.

Evidence for Evolution

• The fossil record provides chronological evidence of past life, showing gradual changes in organisms over millions of years. Transitional fossils bridge the gap between major groups, such as Archaeopteryx linking reptiles and birds. Fossil evidence supports the idea that species are not fixed but have changed over time in response to environmental shifts.
• Comparative anatomy reveals homologous structures, body parts with similar underlying structure but different functions, indicating a common ancestor. For example, the forelimbs of whales, bats, and humans share the same bone arrangement, despite adaptations for swimming, flying, or manipulating objects. This suggests divergent evolution from a shared lineage.
• Embryology shows that early developmental stages are remarkably similar across diverse species, indicating shared genetic instructions inherited from common ancestors. Features like pharyngeal pouches in vertebrate embryos suggest evolutionary relationships even when adult forms differ dramatically. These similarities fade as development progresses, but the early resemblance is significant evidence.
• Molecular biology provides some of the strongest evidence for evolution by comparing DNA and protein sequences. The more similar the sequences between two species, the more recent their common ancestor likely was. This molecular evidence often confirms relationships suggested by morphology and fossils, making it a unifying tool in evolutionary biology.
• Biogeography studies the geographic distribution of species, revealing patterns explained by evolutionary history and plate tectonics. For example, unique species on islands, such as the Galápagos finches, evolved in isolation from mainland relatives, leading to adaptive radiation. This geographic isolation often accelerates the formation of new species.

Population Genetics and Allele Frequencies

• Population genetics focuses on changes in allele frequencies within populations, providing a mathematical framework for studying evolution. Allele frequency refers to how common a particular allele is in a population’s gene pool. Changes in these frequencies over time indicate that evolution is occurring.
• The Hardy–Weinberg equilibrium describes a hypothetical, non-evolving population in which allele and genotype frequencies remain constant. This occurs only if five conditions are met: no mutation, random mating, no natural selection, extremely large population size, and no gene flow. Deviations from this equilibrium reveal that evolutionary forces are acting on the population.
• The Hardy–Weinberg equation \( p^2 + 2pq + q^2 = 1 \) and \( p + q = 1 \) can be used to calculate allele and genotype frequencies. Here, \( p \) represents the frequency of the dominant allele, \( q \) the recessive allele, \( p^2 \) the frequency of homozygous dominant individuals, \( q^2 \) the frequency of homozygous recessives, and \( 2pq \) the frequency of heterozygotes. This mathematical model connects genetic variation to observable traits.
• Microevolutionary changes occur when evolutionary forces, such as natural selection, genetic drift, mutation, and gene flow, alter allele frequencies over generations. These small changes can accumulate to produce significant evolutionary shifts over long timescales. Understanding these mechanisms is essential for predicting how populations might respond to environmental change.
• Population genetics also bridges molecular biology and evolutionary theory by explaining how DNA-level changes influence large-scale patterns in biodiversity. For example, mutations introduce new alleles into a population, while selection determines whether those alleles increase in frequency. This connection makes population genetics a central discipline for both evolutionary biology and conservation science.

Mechanisms of Evolution

• Evolutionary change occurs through several mechanisms, each influencing allele frequencies in distinct ways. Natural selection is the primary driver, favoring traits that improve survival and reproduction. In contrast, genetic drift is a random process that can significantly impact small populations, often reducing genetic variation over time.
• Gene flow, or migration, introduces new alleles into a population or removes existing ones as individuals move between populations. This process can increase genetic diversity but can also homogenize populations, reducing differences between them. It plays a particularly important role in species with large ranges or fragmented habitats.
• Mutation introduces entirely new alleles into the gene pool, providing the raw material for evolution. While most mutations are neutral or harmful, a small proportion can be advantageous and increase in frequency through natural selection. Without mutation, genetic variation would eventually be lost over many generations.
• Non-random mating, such as assortative mating, can alter genotype frequencies without changing allele frequencies directly. This can increase the proportion of homozygotes or heterozygotes depending on the mating pattern, which in turn influences how selection acts on the population. It often works alongside other mechanisms to shape population structure.
• These mechanisms rarely act in isolation; most real populations experience multiple forces at once. For example, a population might be influenced by both natural selection and gene flow simultaneously, leading to complex patterns of genetic change. Understanding how these forces interact is key to predicting evolutionary outcomes.

Types of Selection

• Directional selection favors individuals at one extreme of a phenotypic range, shifting the population’s trait distribution in that direction over time. This often occurs when environmental conditions change, making one trait more advantageous than before. Over many generations, this can lead to significant changes in the population’s mean phenotype.
• Stabilizing selection favors intermediate phenotypes, reducing variation and maintaining traits close to an optimal value. This type of selection is common in stable environments where extreme traits are disadvantageous. As a result, it acts to preserve the existing genetic makeup of the population.
• Disruptive selection favors individuals at both extremes over intermediate phenotypes, increasing variation within the population. This can lead to a bimodal distribution of traits and, in some cases, contribute to the formation of new species. It often occurs when environmental conditions vary within the same habitat.
• Sexual selection is a form of natural selection in which traits that increase mating success become more common, even if they reduce survival. This includes mate choice (intersexual selection) and competition between individuals of the same sex (intrasexual selection). These traits can become exaggerated over time, sometimes to the point of being costly to survival.
• Balancing selection maintains multiple alleles in the population by providing a selective advantage to heterozygotes or to different alleles under different environmental conditions. This can preserve genetic diversity and allow populations to adapt to changing environments more readily. It acts as a counterbalance to the loss of variation from directional or stabilizing selection.

Adaptations and Fitness

• Adaptations are heritable traits that increase an organism’s ability to survive and reproduce in a specific environment. They result from the accumulation of beneficial traits over many generations through the action of natural selection. Adaptations can involve morphology, physiology, or behavior, and often interact with one another to improve overall performance.
• Fitness refers to an individual’s genetic contribution to the next generation, measured by the number of viable offspring produced. It is not about physical strength but about reproductive success relative to others in the population. Higher fitness means an individual’s traits are more likely to be represented in future generations.
• Relative fitness compares an individual’s reproductive success to the most successful individual in the population. This helps biologists measure how advantageous a particular trait is in a given environment. A relative fitness of 1.0 indicates the highest reproductive success in the population.
• Adaptations are context-dependent, meaning a trait that is beneficial in one environment may be neutral or harmful in another. For example, traits that improve cold tolerance may reduce survival in warmer climates. This environmental dependence underscores the dynamic nature of fitness and adaptation.
• It is important to distinguish between adaptations and by-products of evolution. Not every trait is an adaptation — some are neutral features or the result of genetic drift. Misinterpreting these can lead to incorrect assumptions about the evolutionary history of a species.

Speciation

• Speciation is the process by which new species arise from existing ones, typically when populations become reproductively isolated. This isolation can be due to physical barriers (allopatric speciation) or without physical separation (sympatric speciation). Over time, genetic differences accumulate to the point where interbreeding is no longer possible or produces infertile offspring.
• Allopatric speciation occurs when geographic barriers such as mountains, rivers, or oceans separate populations. This prevents gene flow, allowing each population to evolve independently under different selective pressures. Eventually, the genetic divergence can result in the formation of distinct species.
• Sympatric speciation happens within the same geographic area, often due to ecological niches, behavioral changes, or polyploidy in plants. Even without physical separation, reproductive isolation can develop through differences in mating preferences or timing. This type of speciation demonstrates that isolation can be driven by more than just geography.
• Reproductive barriers are categorized as prezygotic or postzygotic. Prezygotic barriers prevent fertilization (e.g., temporal isolation, behavioral isolation, mechanical isolation), while postzygotic barriers occur after fertilization, such as hybrid inviability or infertility. These barriers maintain species boundaries once they are established.
• Adaptive radiation is a rapid form of speciation in which many new species evolve from a common ancestor, often when new habitats or ecological niches become available. This process can produce high biodiversity in a relatively short time, as seen in Darwin’s finches on the Galápagos Islands.

Phylogenetics and Cladistics

• Phylogenetics is the study of evolutionary relationships among species, often represented through phylogenetic trees. These diagrams show hypotheses about how species are related based on shared characteristics and genetic data. Each branch point, or node, represents a common ancestor shared by the lineages that split from it.
• Cladistics is a method of classifying species based on shared derived characteristics (synapomorphies) that can be traced to a common ancestor. The resulting diagram, called a cladogram, emphasizes evolutionary relationships rather than overall similarity. This approach avoids grouping species based on superficial traits that may have evolved independently.
• Monophyletic groups, or clades, include an ancestor and all its descendants. These are the most accurate reflections of evolutionary history and are the goal of modern classification systems. In contrast, paraphyletic or polyphyletic groups omit important evolutionary relationships.
• Molecular data, such as DNA and protein sequences, have greatly improved phylogenetic accuracy. These data allow scientists to identify genetic similarities and differences that may not be visible in physical traits. Such molecular evidence can confirm or challenge traditional classifications based on morphology.
• Understanding phylogenetic relationships helps explain how traits evolved and predict characteristics in related species. For example, identifying a shared ancestor with a beneficial trait can guide research into similar adaptations in other species.

Extinction and Biodiversity Loss

• Extinction is the permanent loss of a species, which reduces global biodiversity. It can occur gradually due to background extinction rates or rapidly during mass extinction events. While extinction is a natural process, human activity has greatly accelerated current extinction rates.
• Mass extinctions are rare events in which a large number of species disappear in a relatively short geological time frame. These events can be triggered by catastrophic environmental changes such as asteroid impacts, volcanic activity, or rapid climate shifts. Each mass extinction has reshaped the diversity of life on Earth.
• Biodiversity loss can weaken ecosystems by reducing genetic variation, species richness, and functional diversity. This loss makes ecosystems more vulnerable to disturbances and less able to adapt to environmental changes. The resulting instability can affect ecological interactions such as pollination, predation, and nutrient cycling.
• Species with small populations or narrow ecological niches are especially at risk of extinction. These species may not be able to adapt quickly to changing conditions or recover from population declines. Their loss can have cascading effects on the broader ecosystem.
• Studying extinction patterns through the fossil record and modern data helps scientists understand long-term trends in biodiversity. These insights can guide predictions about future biodiversity changes and inform conservation priorities.

Common Misconceptions

1. Misconception: “Individuals evolve during their lifetime.” Students often think evolution happens to a single organism because they see animals adapting to their surroundings.
Correction: Evolution occurs in populations over generations through changes in allele frequencies. While individuals can adapt behaviorally or physiologically, these changes are not passed down unless they are genetic.

2. Misconception: “Natural selection gives organisms what they need.” This belief comes from seeing traits that appear ‘designed’ for survival.
Correction: Natural selection does not have foresight or intent. It only favors traits that already exist in some individuals and provide a reproductive advantage under current environmental conditions.

3. Misconception: “The strongest or biggest individuals are always favored by natural selection.” Students may confuse ‘survival of the fittest’ with physical strength alone.
Correction: Fitness in biology refers to reproductive success, not just size or power. Traits that enhance reproductive success, such as camouflage or efficient energy use, may be favored over brute strength.

4. Misconception: “All genetic mutations are harmful.” Many students learn about harmful mutations in the context of diseases and generalize it.
Correction: Mutations can be harmful, neutral, or beneficial depending on the environment. Beneficial mutations may provide an advantage and become more common in the population through natural selection.

5. Misconception: “Humans evolved from modern monkeys.” This comes from misunderstanding evolutionary trees.
Correction: Humans and modern monkeys share a common ancestor that lived millions of years ago. Both lineages evolved separately from that ancestor, meaning modern monkeys are evolutionary cousins, not our ancestors.

6. Misconception: “Once a species evolves, it stops changing.” Students may believe evolution is a one-time event.
Correction: Evolution is an ongoing process. As environments change, selective pressures shift, and populations continue to evolve over time.