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Natural selection is constrained by several factors. It can only act on existing heritable genetic variation within a population at a given time. Additionally, organisms inherit developmental features from distant ancestors that limit evolutionary possibilities. For example, birds evolved wings from existing limbs rather than developing them from scratch, constraining their flight capabilities.

Trade-offs occur because natural selection acts on an organism's overall phenotype, making traits interconnected. An adaptation beneficial for one function may be harmful for another. The ostrich's large body suits its environment but prevents flight. Similarly, an allele for red tusks might deter poachers but make tusks brittle, reducing their usefulness for fighting and foraging.

Natural selection can only increase the frequency of traits if heritable genetic variation already exists in the population. A population cannot evolve a trait unless alleles for that trait are present. For instance, a bird population cannot evolve faster flight without existing heritable variation in flight performance among individuals.

Neighboring genes on the same chromosome are often inherited together as a unit. If a beneficial allele is linked to a harmful allele, both are passed together to offspring. An allele for red tusks inherited with an allele causing infertility would reduce in frequency because the combined effect harms the organism more than the individual allele benefits it.

Intermediate phenotypes can be less favorable than either the original or target trait. An elephant population with traditional tusks, red tusks, and intermediate rose tusks might find rose tusks both brittle and coveted by poachers. This harmfulness of the intermediate form restricts the population's ability to transition from traditional to red tusks.

Gene flow and genetic drift can introduce harmful alleles or eliminate helpful ones, opposing natural selection's effects. Migration introduces new alleles into populations, while chance events like natural disasters cause random changes in allele frequency. Evolution results from combined pressures on populations, not a movement toward perfection.

Organisms inherit developmental plans from distant ancestors that restrict evolutionary possibilities. Birds evolved from nonflying vertebrates with four limbs, inheriting a body plan that made wings unlikely to form independently. Instead, repeated modifications to existing limbs allowed powered flight to evolve, demonstrating how ancestral constraints shape evolutionary outcomes.