Evolutionary fitness, as defined in the context of genetics and reproduction, is generally considered a fixed attribute for an individual organism over its lifetime. This concept hinges on the organism's genetic makeup, which determines its inherent potential for survival and reproduction. However, environmental factors can influence the expression of these genetic traits, affecting an organism's realized fitness. For example, an animal with genes conducive to high fitness may find its reproductive success diminished due to environmental changes, such as habitat destruction or food scarcity. Similarly, an organism may achieve greater reproductive success than expected due to favorable environmental conditions. Thus, while the genetic component of fitness remains constant, the actual reproductive success of an organism can vary with environmental circumstances.

A mutation in a gene can have varying impacts on an organism's evolutionary fitness, depending on the nature of the mutation and the environmental context. Mutations are essentially changes in the DNA sequence, which can lead to changes in the structure and function of proteins, thereby affecting an organism's traits. These changes can be beneficial, neutral, or harmful. A beneficial mutation enhances an organism's fitness by improving traits that increase survival or reproductive success. For instance, a mutation that results in better camouflage can increase an animal's survival rate. Neutral mutations do not significantly impact fitness, while harmful mutations can decrease fitness by impairing vital functions or reducing reproductive capability. The impact of a mutation is also influenced by the environment; a trait that is advantageous in one setting might not be in another. Thus, the effect of a mutation on fitness is a complex interplay of genetic changes and environmental factors.

Sexual selection, a form of natural selection, plays a critical role in shaping evolutionary fitness by favoring traits that increase an organism's success in mating. Unlike natural selection, which generally focuses on survival traits, sexual selection is concerned with traits that enhance reproductive success through mate attraction or competition. Traits favored by sexual selection can include physical characteristics like bright plumage in birds or behavioral traits such as elaborate courtship rituals. These traits, while potentially costly in terms of energy or survival risk, provide a significant reproductive advantage by attracting mates or outcompeting rivals. This process leads to an increase in the frequency of such traits in the population, thereby influencing the evolutionary fitness of the species. However, it's important to note that sexual selection can sometimes work against survival fitness, as some traits that are attractive to mates might increase predation risk or other survival challenges.

Evolutionary fitness is applicable to plant species as much as it is to animals. In plants, evolutionary fitness is measured by the ability to survive environmental conditions and successfully reproduce, either sexually or asexually. Reproductive success in plants can be determined by several factors, including the number of seeds produced, the efficiency of seed dispersal, the success rate of seed germination, and the survival of seedlings to reproductive maturity. For example, a plant that produces a large number of seeds with mechanisms for wide dispersal, such as wind or animal transportation, might have higher fitness compared to a plant with limited seed production or dispersal capability. Additionally, adaptations like resistance to drought, disease, or predators (such as herbivores) also contribute to a plant's fitness by enhancing its survival and reproductive potential. The interaction between these traits and environmental conditions determines the evolutionary fitness of plant species.

Environmental changes can significantly impact the evolutionary fitness of a species by altering the survival and reproductive success of individuals with certain traits. As environments change, traits that were once advantageous may become less so, and vice versa. For example, a change in climate might shift vegetation patterns, affecting herbivorous species adapted to specific plants. Individuals within these species that can adapt to new food sources or habitats might have higher fitness under the new conditions. Similarly, the introduction of a new predator or a disease can change the fitness landscape, favoring traits that provide resistance or better evasion strategies. Human-induced changes, such as habitat destruction and pollution, also significantly impact evolutionary fitness by altering the selective pressures on species. These changes can lead to rapid evolutionary responses if the population contains enough genetic variability to adapt. In essence, environmental changes can drive evolutionary processes by changing the criteria for what constitutes fitness in a given context.