Natural selection is a cornerstone concept in evolutionary biology, elucidating how species adapt and evolve over time. It explains the differential survival and reproduction of individuals within a population based on the variation of traits that affect their ability to adapt to specific environmental conditions.
Mechanisms of Natural Selection
Variation
Existence of Variation
Every natural population is characterised by individuals with a diverse range of traits. These traits, ranging from physical characteristics like size and colour to behavioural attributes, are not uniformly distributed within a population.
Genetic Basis of Variation
These variations are often rooted in the genetic makeup of individuals. Genes, the fundamental units of heredity, vary among individuals leading to observable differences in traits. Genetic mutations, recombination, and sexual reproduction contribute to genetic diversity.
Differential Survival and Reproduction
Environmental Interaction
Individuals with traits that confer a survival advantage in a specific environment are more likely to survive to reproductive age and produce offspring. This interaction between traits and environmental conditions is central to natural selection.
Selection Pressure
Environmental factors create a selection pressure that favours certain traits. These factors can be biotic, like predators or competitors, or abiotic, such as temperature or altitude. Traits that enhance survival and reproductive success in response to these pressures become more common over generations.
Adaptation
Increase in Trait Prevalence
Advantageous traits increase in prevalence within the population over successive generations. This change in the frequency of traits is a manifestation of the population adapting to its environment.
Evolutionary Change
As these advantageous traits become predominant, the population evolves. This continuous process of adaptation and change is a driving force behind the biodiversity and complexity of life forms on Earth.
Examples of Natural Selection in Action
Peppered Moth Evolution
Pre-Industrial Revolution
Light-Coloured Moths
Before the Industrial Revolution, the majority of peppered moths in England were light-coloured, providing them with camouflage against the light bark of trees and lichens in their habitat.
Predation
Birds, the primary predators of these moths, found it difficult to spot the light-coloured moths against the similarly coloured background, leading to their survival and proliferation.
Industrial Revolution
Pollution
The onset of the Industrial Revolution brought about significant environmental changes. Soot and pollution darkened the barks of trees, altering the moths’ habitat.
Camouflage Shift
This environmental change favoured the dark-coloured moths, which were now better camouflaged against the darkened trees. Conversely, the light-coloured moths became more visible and susceptible to predation.
Population Shift
As a result, there was a notable increase in the population of dark-coloured moths, exemplifying natural selection in response to environmental change.
Antibiotic Resistance in Bacteria
Initial Exposure
Variation
In any bacterial population, there’s inherent genetic variation. When exposed to antibiotics, most bacteria are eliminated, but a few with natural resistance survive.
Survival
These resistant bacteria are not affected by the antibiotic and continue to thrive, showcasing a direct interaction between a biological trait (antibiotic resistance) and an environmental factor (the antibiotic).
Reproduction and Spread
Reproduction
Resistant bacteria reproduce, passing on the resistance trait to their offspring. Each generation sees an increase in the proportion of antibiotic-resistant individuals.
Population Change
Over time, if the antibiotic pressure continues, the entire bacterial population may become resistant, leading to the ineffectiveness of the antibiotic, a real-world issue that we grapple with in medicine today.
Darwin’s Finches
Environmental Variation
Different Islands
The Galápagos Islands, each with its distinct environment and available food sources, were home to populations of finches with varied beak shapes and sizes.
Beak Variation
Finches with beaks that were well-suited to exploit the specific food sources available on their respective islands were more likely to survive and reproduce.
Adaptation and Speciation
Adaptation
Over generations, through the process of natural selection, distinct beak shapes that were best suited to the available food sources became predominant on each island.
Speciation
This led to the emergence of different species of finches, each adapted to the specific conditions of its environment, a clear demonstration of adaptive radiation following natural selection.
Key Insights
Natural selection is not a linear or uniform process but is dynamic, responding to the ever-changing environmental conditions and pressures. It doesn’t work towards a predetermined goal or perfect organism but is a continuous process of adaptation and change. The examples of natural selection in action, like the peppered moths, antibiotic-resistant bacteria, and Darwin’s finches, provide tangible insights into this process. These examples underscore the adaptative, responsive nature of life, offering foundational perspectives on topics from ecology and genetics to conservation biology. Through a comprehensive understanding of natural selection, students are equipped with the knowledge to explore the intricate, interconnected biological processes that shape life on Earth.
FAQ
Yes, human activities can influence the rate of natural selection. For example, the use of pesticides in agriculture can accelerate natural selection by exerting strong selection pressures on pest populations, leading to the rapid evolution of pesticide resistance. Conversely, conservation efforts that protect endangered species can decelerate natural selection by reducing selection pressures, such as predation or habitat loss. Human-induced changes in the environment, like climate change or habitat modification, can also create new selection pressures, driving rapid evolutionary changes in affected species.
Natural selection leads to adaptation by favouring individuals with traits that enhance their survival and reproductive success in a particular environment. These individuals are more likely to pass on their advantageous traits to their offspring. Over successive generations, these traits become more common within the population. For instance, in a predator-rich environment, prey animals with better camouflage or evasion abilities may survive longer and reproduce more. Over time, these traits would become more prevalent, leading to a population better adapted to evade predators.
No, an individual organism does not adapt during its lifetime due to natural selection. Natural selection acts on populations over generations. It is a process where individuals with favourable traits have a higher chance of survival and reproduction. These traits are then more likely to be passed on to the next generation. Over time, these advantageous traits become more common in the population. However, individual organisms can exhibit phenotypic plasticity, where they can change their behaviour, physiology, or morphology in response to environmental changes during their lifetime, but these changes are not passed on to the next generation.
Natural selection is integral to speciation, the process by which new species form. When populations of a species become isolated, either geographically or otherwise, different environmental pressures act on each isolated population. Natural selection favours different traits in each environment, leading to divergent evolution. Over time, the populations adapt to their specific environments, accumulating distinct genetic and phenotypic differences. If these differences become substantial, members of the two populations can no longer interbreed successfully, leading to the formation of two distinct species. This process is evident in examples like Darwin’s finches, where different environmental conditions on separate islands led to the evolution of distinct finch species.
Genetic variation within a population originates from several sources. Mutations, which are random changes in DNA, introduce new genetic variants. Sexual reproduction also contributes significantly; the combination of genes from two parents ensures offspring have a unique genetic makeup. Additionally, gene flow, the transfer of genes from one population to another due to migration, introduces new genetic variants. This genetic diversity provides the raw material upon which natural selection acts. When environmental conditions change, individuals with favourable traits are more likely to survive and reproduce, leading to an increase in these traits within the population over generations.
Practice Questions
The peppered moth population during the Industrial Revolution is a classic example of natural selection. Initially, light-coloured moths were predominant as they were camouflaged against the light bark of trees, reducing predation by birds. However, industrial pollution darkened the tree barks. Dark-coloured moths, previously easily spotted and predated, now had a survival advantage as they were camouflaged against the soot-covered trees. Consequently, their population increased significantly. This shift in population dynamics, driven by environmental changes and differential survival, exemplifies natural selection in action.
Antibiotic resistance in bacteria is a clear manifestation of natural selection. Bacterial populations contain inherent genetic variation. When exposed to antibiotics, sensitive bacteria are eliminated, while resistant variants survive and reproduce. Over time, if antibiotic exposure continues, the population becomes predominantly resistant, rendering the antibiotic ineffective. This has significant implications for human health, complicating the treatment of bacterial infections, necessitating the development of new antibiotics, and underscoring the importance of prudent antibiotic use to mitigate the evolution and spread of resistance.
