General Biology: Evolution and Ecology

Evolution and ecology are the two organizing frameworks that make biology intelligible. Evolution explains how biological diversity arises and changes over time. Ecology explains how organisms interact with one another and with their physical environment. Together, they connect processes that happen across very different scales, from genetic variation within a population to nutrient cycling across entire ecosystems.

Understanding these ideas is not only central to general biology. It is also practical. The same principles help explain why antibiotic resistance spreads, why invasive species can transform habitats, and why predator removal can destabilize food webs.

Evolutionary mechanisms: how populations change

Evolution is change in the heritable characteristics of populations over generations. It does not describe individuals “adapting” in a lifetime; instead, it tracks how gene variants become more or less common. The core mechanisms are natural selection, genetic drift, gene flow, and mutation. These are not competing explanations. They often operate simultaneously.

Natural selection and adaptation

Natural selection occurs when individuals differ in traits that affect survival or reproduction, and those trait differences are heritable. Over time, alleles associated with higher reproductive success tend to increase in frequency.

Selection is easiest to see when the environment imposes a strong filter. In a population of bacteria exposed to an antibiotic, some cells may already carry resistance alleles. Those cells leave more descendants under antibiotic pressure, so resistance becomes common. The key point is that selection does not create the needed variation on demand. It sorts existing heritable variation, while new variation is introduced by mutation.

Selection can take different forms:

• Directional selection favors one extreme of a trait distribution, shifting the population mean.
• Stabilizing selection favors intermediate values, reducing variation.
• Disruptive selection favors both extremes, potentially increasing variation and setting the stage for divergence.

Adaptations are traits shaped by selection because they increased fitness in a particular environment. Fitness is best understood as reproductive success relative to others in the population, not as strength or health in a general sense.

Mutation, gene flow, and genetic drift

• Mutation is the ultimate source of new genetic variation. Most mutations are neutral or harmful; a small fraction can be beneficial depending on context.
• Gene flow moves alleles between populations through migration and reproduction. It can introduce new variation and can also prevent populations from diverging if exchange is frequent.
• Genetic drift is random change in allele frequencies, strongest in small populations. Drift can fix or eliminate alleles regardless of their effects on fitness.

Two classic drift scenarios are the bottleneck effect, where a population is sharply reduced and loses variation, and the founder effect, where a new population is established by a small number of individuals. Both can leave lasting genetic signatures.

Speciation: how new species arise

Speciation is the process by which populations evolve reproductive isolation. Once gene flow is sufficiently reduced, populations can diverge through selection, drift, or both.

A useful way to frame speciation is through reproductive barriers:

• Prezygotic barriers prevent mating or fertilization (differences in habitat use, mating signals, timing, or mechanical compatibility).
• Postzygotic barriers occur after fertilization (reduced hybrid viability or fertility).

Speciation often begins with separation. In allopatric speciation, a geographic barrier limits gene flow, allowing populations to diverge. In some cases, speciation can occur with less obvious physical separation when strong selection and assortative mating reduce gene exchange. Regardless of the pathway, the outcome is the same: lineages become independently evolving.

Population dynamics: ecology at the scale of populations

Ecology asks how organisms survive, reproduce, and persist in real environments with limited resources. Population dynamics focuses on changes in population size and structure over time and the factors that regulate those changes.

Growth, limits, and carrying capacity

When resources are abundant, populations can grow rapidly. A simplified model of unrestricted growth is exponential growth:

$$\frac{dN}{dt}=rN$$

where $N$ is population size and $r$ is the intrinsic rate of increase.

In natural systems, growth is constrained by resources, space, predation, disease, and other factors. As density increases, competition intensifies and growth slows, often approximated by logistic growth:

$$\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right)$$

Here, $K$ is the carrying capacity, the population size the environment can sustain over the long term under given conditions. Carrying capacity is not a fixed property forever. It can shift with climate, habitat change, nutrient inputs, and human land use.

Interactions that shape populations

Population size and stability are influenced by interactions such as:

• Competition, when organisms use the same limited resource. Competition can be within a species or between species and can drive niche differentiation.
• Predation and herbivory, which can regulate prey populations and select for defenses like camouflage, toxins, or behavioral avoidance.
• Parasitism and disease, which can spread faster in dense populations and alter survival and reproduction.
• Mutualism, where both partners benefit, as in pollination systems or some microbiome relationships.

These interactions link ecology back to evolution. Predators can drive selection for faster escape behaviors, while parasites can select for immune traits. Over time, interacting species can coevolve, each shaping the other’s evolutionary trajectory.

Ecosystems: energy flow and nutrient cycling

An ecosystem includes living communities plus the physical environment, connected by energy flow and the cycling of matter. This broader level explains why changes in one species can ripple through entire landscapes.

Food webs and trophic levels

Energy enters most ecosystems through photosynthesis, captured by primary producers such as plants and algae. That energy is transferred through trophic levels as organisms eat and are eaten. Because energy is lost as heat during metabolism at every transfer, higher trophic levels typically support less biomass.

Food webs are more realistic than simple food chains. Many species feed at multiple trophic levels, and the strength of feeding links varies. This complexity can buffer ecosystems against some disturbances, but it can also transmit shocks, especially when key species are removed.

Top-down and bottom-up controls

Ecosystem structure can be shaped by:

• Bottom-up control, where productivity and nutrient availability determine population sizes higher in the web.
• Top-down control, where predators regulate herbivores and indirectly affect plants.

A well-known ecological pattern is the trophic cascade: removing or adding predators can cause shifts that propagate down the food web, altering vegetation and habitat structure.

Nutrient cycling and ecological stability

While energy flows in one direction, matter cycles. Nutrients such as carbon, nitrogen, and phosphorus move between organisms, soil, water, and the atmosphere. Decomposers play an essential role by breaking down dead organic material and returning nutrients to forms that producers can reuse.

Ecosystem stability depends on both biological diversity and environmental conditions. Diversity can contribute to resilience, the ability to recover after disturbance, because different species may respond differently to stress. However, resilience is not guaranteed. Rapid habitat loss, pollution, and climate shifts can push ecosystems beyond thresholds where recovery is slow or trajectories change.

How evolution and ecology reinforce each other

Evolutionary change can happen on ecological timescales. A population facing a new predator, pollutant, or pathogen can evolve measurable trait shifts in a relatively small number of generations. Those evolutionary changes can then alter ecological relationships, such as competitive balance or disease dynamics.

Conversely, ecological conditions shape selection pressures. Resource scarcity can favor efficiency, while variable environments can favor flexibility. Even population size, an ecological variable, affects evolution by influencing the relative importance of genetic drift and selection.

Practical significance in biology and society

The value of evolutionary and ecological thinking is that it helps predict outcomes:

• In medicine and agriculture, understanding selection and population dynamics guides strategies to slow resistance evolution, such as careful antibiotic use or integrated pest management.
• In conservation, recognizing gene flow, drift, and small-population effects informs decisions about habitat corridors, protected area size, and genetic rescue.
• In environmental management, ecosystem concepts clarify why protecting a single species may be insufficient if energy flow, nutrient cycling, or food web structure is disrupted.

General biology becomes coherent when evolution explains the origins of traits and diversity, and ecology explains how those traits play out in the real world. Together they provide a disciplined way to interpret life as a system, shaped by history and constrained by environments that are always changing.