Individual organisms do not evolve in the strict biological sense; evolution is defined as changes in gene (allele) frequencies across generations in a population, not within a single lifetime. However, individual organisms do change in many ways (development, learning, immune adaptation), which can feel like ā€œevolvingā€ but are classified differently in biology.

What ā€œevolveā€ means in biology

  • In modern evolutionary biology, evolution is usually defined as a change in allele frequencies in a population from one generation to the next.
  • Once an individual is born (or formed, in microbes), its genetic makeup is essentially fixed, so its own allele frequencies do not change over its lifetime.

ā€œPopulations evolve, not individualsā€ is shorthand for this population-level definition of evolution.

Why individuals donā€™t ā€œevolveā€

  • Evolution by natural selection needs:
    1. Heritable variation.
    2. Differential survival and reproduction.
    3. Transmission of successful variants to the next generation.
  • A single organism can have mutations in some cells, but those changes normally do not spread through a population unless they are passed on via reproduction, which a lone individual cannot do by itself in a sexual species.

In other words, an individual can carry evolutionary changes (mutations), but the process of evolution is about how those changes spread or disappear in populations over generations.

But individuals do change

Biology recognizes several kinds of within-lifetime change that are not evolution:

  • Development: Growth from embryo to adult, including changes in form and physiology.
  • Learning and behavior: Individuals develop stable behavioral differences (ā€œpersonalitiesā€) that can have evolutionary explanations, but the behavior change itself in one lifetime is not evolution.
  • Immune adaptation: Your immune system ā€œadaptsā€ to pathogens by selecting among immune cells, a rapid Darwin-like process within the body, yet these changes are mostly not inherited by offspring.

These processes can influence which individuals survive and reproduce, thereby feeding into evolution at the population level, but they are conceptually distinct from evolution itself.

Edge cases: microbes, cancer, and cell lineages

Some borderline cases show why the phrase can be confusing:

  • Asexual microbes: Bacteria in a clonal line accumulate mutations over many generations; each cell is an individual, but evolution is still described at the level of the lineage or population of cells, not one cell changing its genes over its own lifetime.
  • Cancer cells: Cancer cell populations within one body mutate and undergo selection, so the cancer line ā€œevolves,ā€ but the organism as a whole is not said to be evolving; it is being harmed by an evolving subpopulation of its own cells.

These examples show Darwinian processes can occur at multiple levels (cell lineages, organisms, species), yet the standard use of ā€œevolutionā€ in textbooks refers to populations across generations.

Forum-style takeaway (for ā€œcan individual organisms evolveā€)

If the question is ā€œCan I, as one organism, evolve during my lifetime?ā€ the scientific answer is no: you can develop, learn, and adapt physiologically, but your genetic identity stays basically the same.

Evolution is the story of how many such genetically varying individuals live, die, and reproduce so that, over many generations, a populationā€™s genetic makeup shifts.

TL;DR: Individual organisms change, adapt, and develop, but in evolutionary biology ā€œevolveā€ is reserved for genetic change in populations over generations, not for changes within a single lifetime.

Information gathered from public forums or data available on the internet and portrayed here.