7 Mechanisms and Evidence of Evolution
Trace how evolutionary mechanisms change populations, how new species form, and how independent evidence reveals relationships among organisms.
How populations evolve
is change in the heritable characteristics of populations across generations. It helps explain both the diversity of life and shared features among organisms. The key unit of evolutionary change is the population: individuals vary, but populations change over generations.
occurs when inherited differences affect survival or reproduction in a particular environment. Individuals with certain variants may leave more offspring, so those variants can become more common over time.
For example, some bacteria may already carry an inherited variant that provides antibiotic resistance. When exposed to the antibiotic, resistant bacteria are more likely to reproduce, and resistance can spread through the population. The bacteria do not deliberately change, and the antibiotic does not create the specific useful variation because the bacteria need it.
Other mechanisms, including mutation, genetic drift, and gene flow, can also change populations. is important, but it is not the only mechanism of .
Takeaway: describes population-level change across generations; is one process that can drive that change.
and environment
An is an inherited trait that improves reproductive success in a particular environment and has become common through evolutionary processes. Adaptations are not changes an individual consciously acquires during its lifetime. They arise as inherited variants become more common in populations over generations.
Whether a trait is advantageous depends on the environment. A trait that improves reproductive success in one setting may have little effect or may be harmful in another. The bacterial resistance example illustrates this dependence: resistance can be advantageous when bacteria encounter the antibiotic that it protects against.
Takeaway: An is a population-level evolutionary outcome, and its benefit depends on the environment.
From isolation to new species
is the formation of new species from existing populations. It commonly begins when populations become isolated and exchange fewer genes. Isolation may result from a physical barrier, such as a river or mountain range, or develop without a physical barrier through differences in habitat, behavior, or mating time.
As isolated populations accumulate genetic differences, may develop. These barriers can prevent populations from mating, fertilizing eggs, or producing fertile offspring. When populations become sufficiently reproductively isolated, they are generally considered separate species under the biological species concept.
is often gradual. Species definitions can also be more complex for organisms that reproduce asexually or hybridize.
Takeaway: Isolation can reduce gene exchange, allowing populations to diverge until distinguish separate species.
and evidence
means that different species descend from ancestral populations they share. Evolutionary history is often represented as a branching tree: lineages split and diversify, and some eventually go extinct. Species that are closely related share a more recent common ancestor than species that are more distantly related.
Humans did not descend from modern chimpanzees. Humans and chimpanzees are separate lineages that share an extinct common ancestor.
Several independent lines of evidence support and :
Fossils occur in a time-ordered record in rock layers. They document extinct organisms, changes through time, and combinations of traits that can help reveal transitions between groups.
reveals shared underlying structures among related organisms, even when those structures have different functions. The similar arrangement of bones in human, bat, and whale forelimbs is consistent with inheritance from a common ancestor.
DNA and proteins provide molecular comparisons. Species with more similar sequences generally share a more recent common ancestor, and genetic comparisons can help test and refine evolutionary family trees.
examines where organisms live. Geographic patterns can fit evolutionary relationships and the history of continents, islands, and barriers to dispersal.
Observed includes documented population changes, such as the spread of antibiotic resistance in bacteria. Such observations show evolutionary change occurring now, while fossils and other evidence reveal change over much longer periods.
Scientists assess these lines of evidence together. Their agreement helps explain both the shared features of life and the differences among species.
Takeaway: Branching evolutionary relationships are supported by multiple independent kinds of evidence, from fossils and anatomy to molecular comparisons and geographic patterns.