D4: Natural Selection, Ecological Stability, and Climate Change (IB Biology)

Natural selection is a fundamental mechanism of evolution, frequently examined in Paper 2 assessments. It is crucial to understand that variation arises within a population primarily through random mutations; individuals do not evolve or adapt during their lifetimes. Instead, evolution is driven by a process where organisms produce more offspring than the environment can sustain, resulting in a struggle for existence.

Among these offspring, individuals possessing variations that confer a survival advantage—traits that are better suited to the environment—have increased fitness. This greater fitness enhances their chances of surviving to reproductive maturity and successfully passing their advantageous genes to the next generation, a concept popularly encapsulated in the phrase "survival of the fittest." Over generations, features that enhance survival and reproductive success are disproportionately inherited, leading to an increase in the frequency of these favorable traits within the gene pool.

Evolution itself is defined as the change in the heritable characteristics of a population across generations, which ultimately results in the incredible biodiversity observed over billions of years.

Historical Context and Genetic Basis

Before Charles Darwin solidified the theory of natural selection, Lamarckism was the dominant paradigm, suggesting that individuals could pass on traits acquired throughout their lives. Darwin's theory marked a pivotal change in our understanding of biological evolution. To fully grasp the concept of natural selection, a basic understanding of genetics is necessary. Alleles, which are different forms of a gene, may vary even by a single base pair, while gametes (the reproductive cells, such as egg and sperm) carry half of the genetic information from each parent.
While mutation serves as the ultimate source of genetic variation, meiosis and sexual reproduction contribute additional variation by generating new combinations of alleles. In contrast, asexual reproduction mainly relies on mutations, often proving insufficient for generating the necessary diversity to cope with rapid environmental changes.

Population Dynamics and Selection Pressures

Environmental capacity, or the carrying capacity ($K$), represents the maximum population size that an environment can support. This capacity is often illustrated by a sigmoidal growth curve, where various limiting factors determine the plateau phase of growth.

Selection pressures can be classified into biotic factors (living components such as food availability, mate competition, predation, and interspecies rivalry) and abiotic factors (non-living elements such as space, temperature, altitude, and sunlight). Limiting factors that influence population dynamics can be further divided into density-dependent factors, where pressure rises with increased population density—examples include predation, disease, parasites, and the accumulation of waste—and density-independent factors that impact populations regardless of their density, such as natural events like floods or fires.

Competition occurring within a single species is referred to as intraspecific competition, driving natural selection.

Reproductive Fitness and Sexual Selection

Fitness not only pertains to survival but also encompasses the ability to produce viable offspring. Misconceptions frequently arise around the inheritance of acquired characteristics; however, only heritable mutations in the gametic DNA (eggs or sperm) can be passed to offspring.
Sexual selection plays a significant role in evolution as individuals assess the reproductive fitness of potential mates. Courtship rituals, which could include physical dominance demonstrations, anatomical displays such as the elaborate tail feathers of peacocks, or complex behaviors displayed by birds of paradise, signal the overall strength and viability of individuals as mates. This sexual selection process also promotes intraspecific mating, thus minimizing the chances of interspecific hybridization, which often leads to sterile offspring.

Case Study: John Endler and Trinidadian Guppies

John Endler's studies of guppies in Trinidad and Tobago provide a compelling illustration of natural selection in action. Male guppies adorned with more spots are generally favored by females, although this attractive characteristic increases their visibility to predators within shallow waters. Over various months, male guppy populations in Pond Q (with no predators) and Pond R (with weak predators) exhibited an increase in spot frequency due to female mate preference. Conversely, in Pond S, where predation pressure was high, the number of spots dramatically decreased; here, the disadvantages presented by heightened predation outweighed the reproductive benefits associated with their coloration.