GED · Science · 5 min read
Natural Selection and Ecosystem Interactions
- Explain how natural selection drives the adaptation and evolution of biological populations over generations.
- Trace how energy flows and chemical matter cycles through food webs and trophic levels in an ecosystem.
- Identify types of symbiotic relationships and analyze the factors that govern population growth and carrying capacity.
The Principles of Natural Selection and Adaptation
Within any biological population, genetic variation naturally exists among individuals. Inherited traits are passed down from parents to offspring through genetic material. In nature, organisms typically produce more offspring than the environment can support. This overproduction leads to competition for limited resources such as food, water, and shelter.
Natural selection is the process by which individuals with favorable traits survive and reproduce at higher rates than others. Scientists describe this reproductive success using the concept of biological fitness, which measures an organism's ability to survive and successfully produce fertile offspring. Rather than acting directly on DNA sequences, natural selection acts on expressed traits, including physical, physiological, and behavioral traits. Over generations, heritable traits that improve reproductive success tend to increase in frequency within a population, while heritable traits that reduce reproductive success tend to decrease in frequency.
Environmental pressures such as climate changes, predators, and pathogens drive the direction of natural selection. Under these pressures, populations develop adaptations, which are inherited characteristics that enhance an organism's survival and reproduction in a specific habitat. A central rule of biology is that biological populations evolve over generations, but individual organisms do not evolve during their lifetimes.
Everyday observations illustrate how quickly natural selection can operate. For example, bacterial resistance to antibiotics is an observable example of natural selection occurring in real time. Similarly, pest resistance to chemical pesticides is an example of natural selection driven by human activity. When chemical pesticides are applied, individual pests with heritable traits that allow them to survive reproduce, passing those traits to their offspring until the resistance trait becomes common throughout the population.
Common mistakes
- Assuming individual organisms evolve to survive environmental changes, whereas natural selection changes the frequency of inherited traits in populations over generations.
- Interpreting food web arrows as pointing to the food being eaten, rather than pointing from the organism being consumed to the consumer receiving the energy.
- Believing that energy cycles endlessly through an ecosystem like matter does, when energy actually flows in a one-way direction and is steadily lost as metabolic heat.
- Confusing density-dependent and density-independent factors by assuming all limiting factors worsen with crowding, forgetting that density-independent factors like droughts affect populations regardless of density.
Key points
- Natural selection occurs when individuals with favorable inherited traits survive and reproduce at higher rates, altering the frequency of expressed traits in a population over generations.
- Adaptations are inherited characteristics shaped by environmental pressures such as climate changes, predators, and pathogens, as seen in bacterial antibiotic resistance and pest pesticide resistance.
- Food webs show interconnected feeding relationships among producers and consumers; decomposers break down dead material from organisms at every trophic level and return nutrients to the environment.
- Energy transfers are inefficient: on average, about 10 percent of the energy at one trophic level reaches the next, while much of the remainder dissipates as heat and some remains in uneaten or undigested material.
- Energy flows in a one-way direction through an ecosystem, while chemical matter cycles continuously.
- Symbiosis includes mutualism (both benefit), commensalism (one benefits, one unharmed), and parasitism (one benefits at the expense of a host).
- Carrying capacity is the maximum sustainable population size, where logistic growth forms an S-shaped curve governed by biotic, abiotic, density-dependent, and density-independent limiting factors.
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