Evolution is not just a historical event recorded in fossils; it is an ongoing biological process. Populations can change genetically within a few generations, and those changes can be detected with careful observation and data.

What it means for evolution to be “ongoing”

Evolution describes change in a population across generations, so it can only be assessed by tracking heritable variation over time, not by looking at a single individual’s lifetime.

To say evolution is ongoing is to say:

• Every living species is part of a continuing lineage that has changed in the past and is still capable of changing now.

• Evolution can occur on multiple time scales:

  • Short-term: detectable across years or decades (especially in fast-reproducing organisms).

  • Long-term: accumulated differences across thousands to millions of years.

Key idea: populations evolve, not individuals

Natural selection and other evolutionary forces can change which inherited variants are more common in the next generation, but an individual organism’s genetic makeup is largely fixed at fertilisation.

These three panels show how different modes of natural selection reshape a population’s trait distribution across generations. The dashed curve represents the original population distribution, while the solid curve represents the distribution after selection removes certain phenotypes. Directional, stabilizing, and disruptive selection each predict distinct, measurable shifts that can occur on short time scales. Source

• Individuals can acclimate (non-heritable physiological adjustment), but acclimation is not evolution.

• Evolution requires inheritance: traits must be passed from parents to offspring.

Why evolution does not “stop”

As long as organisms reproduce and pass genetic information to offspring, evolution remains possible. Evolution continues because the conditions that would prevent change indefinitely are rarely met in real ecosystems.

Ongoing sources of change

Even without detailing every mechanism, AP Biology expects you to recognise that real populations constantly experience conditions that promote evolutionary change:

• Genetic variation is continually present within and among populations.

• Reproduction creates new combinations of existing variants each generation.

• Environmental conditions (biotic and abiotic) are not perfectly constant, so which variants are favoured can shift over time.

• Chance events can alter which individuals reproduce, especially when population sizes fluctuate.

Stable appearance does not mean no evolution

A species may look “unchanged” over long periods, yet still be evolving at the genetic level.

• Selection can favour the same general phenotype while allele frequencies change in the background.

• Different traits can experience different pressures simultaneously, producing little obvious net change in outward form.

• Many traits are controlled by multiple genes and influenced by the environment, so genetic change may not immediately create a visible shift.

How scientists recognise evolution happening now

Because evolution is defined as heritable change across generations, evidence for ongoing evolution comes from measurable, repeatable patterns observed in living populations.

What can be measured in real time

Scientists can detect ongoing evolution by tracking:

This diagram illustrates how allele frequencies ($p$ and $q$) in a population predict genotype frequencies ($p^2$, $2pq$, $q^2$) under Hardy–Weinberg conditions. By comparing observed genotype or allele frequencies to these expectations across generations, biologists can test whether evolutionary forces (like selection or drift) are changing the gene pool. It makes the core idea “evolution = change in allele frequencies” visually and quantitatively explicit. Source

• Shifts in trait distributions across generations (for traits with a genetic basis).

• Changes in genetic variant frequencies using DNA sequence data.

• Differences in reproductive output associated with inherited traits (e.g., some inherited variants consistently leave more offspring).

What counts as strong evidence

Ongoing evolution is most convincing when studies show all three of the following:

• The trait or variant is heritable (offspring resemble parents more than unrelated individuals).

• The environment or interactions create consistent differences in reproductive success among variants.

• The population shows a generational shift in the frequency of those variants or associated traits.

Common misconceptions to avoid

• “Evolution is goal-directed.” Evolution has no foresight; changes reflect which heritable variants happen to leave more descendants under current conditions.

• “Individuals evolve because they need to.” Need does not create heritable variants; evolution reflects differential reproduction among existing variants.

• “If a species is well-adapted, it won’t evolve.” Well-adapted populations can still evolve because environments, interactions, and chance events change through time.

• “Evolution always makes organisms more complex.” Evolution only favours higher reproductive success in a given context, not complexity or “progress.”

Why “ongoing evolution” matters in biology

Recognising evolution as continuous helps connect many observations in modern biology:

• Populations can respond (or fail to respond) to changing conditions, influencing persistence across generations.

• Inheritance and reproduction mean that small shifts can accumulate, shaping future variation and outcomes.

• Evolutionary thinking provides a framework for interpreting patterns in biodiversity as dynamic rather than fixed.