01 Evolution and Natural Selection
Learn how inherited variation changes populations, how natural selection shapes adaptation, and how reproductive isolation can lead to new species.
in Populations
describes change in a population’s inherited characteristics across generations. The key distinction is that individuals develop during their lifetimes, while populations evolve as the frequencies of inherited variants change.
has no predetermined goal. A trait that is useful in one environment may be neutral or harmful in another. Several processes can alter variant frequencies: , genetic drift (chance changes in frequencies), and gene flow (the movement of variants between populations).
How Changes Populations
occurs when three conditions are present:
Individuals in a population vary in traits.
Some of that variation is inherited.
Individuals with certain inherited traits leave more surviving offspring, on average, than others.
Selection acts on individuals’ traits, but the population evolves. When inherited traits contribute to reproductive success, the associated variants tend to become more common over generations. The environment does not create a useful mutation because it is needed; selection changes the frequency of variants that already exist or arise by chance.
Example: antibiotic resistance. Some bacteria may already carry inherited variants that confer resistance to an antibiotic. When the antibiotic is used, susceptible bacteria are more likely to die, while resistant bacteria are more likely to reproduce. Resistant variants can therefore become more common in later generations. The antibiotic selects among variants; it does not direct bacteria to develop resistance.
Takeaway: depends on inherited variation and differences in reproductive success; it is not a purposeful change within an individual.
and Environmental Conditions
An is an inherited feature that contributes to survival or reproductive success in a particular environment. Adaptations may be structural, physiological, or behavioral. can increase the frequency of such traits, making a population better suited to current conditions—but not necessarily to future ones.
Example: finch beaks. Galápagos ground finches vary in beak size. If environmental conditions change the seeds available, birds with beak sizes better suited to the available food may leave more offspring. Over generations, the population’s average beak size can shift. Individual birds do not change their beaks in response to need; the change occurs across the population.
Takeaway: An is defined in relation to an environment, and its advantage may change when conditions change.
From Diverging Populations to New Species
is the formation of new species as populations diverge. Under the biological species concept, a species consists of populations whose members can potentially interbreed and produce viable, fertile offspring. This concept is most useful for sexually reproducing organisms and does not apply equally well to every form of life.
generally involves : barriers reduce or prevent gene flow between populations. With less gene flow, populations can diverge through mutation, , and genetic drift.
occurs when a geographic barrier separates populations. The separated populations may diverge over time.
occurs without geographic separation. For example, changes in chromosome number may prevent successful breeding with the original population.
Barriers to interbreeding may include different mating behaviors, breeding seasons, or incompatible reproductive structures. is often gradual, so the boundary between populations and distinct species can sometimes be difficult to identify.
Takeaway: Reduced gene flow allows populations to diverge, and accumulated differences can eventually prevent successful interbreeding.