Evolution of antibiotic resistance in bacteria

Bacteria in a population are not all genetically identical. Some may carry an allele—a version of a gene—that makes them resistant to a particular antibiotic. This means they are more likely to survive exposure to it than bacteria without the resistance allele.

When the antibiotic is used, susceptible bacteria are more likely to die. Resistant bacteria are more likely to survive and reproduce, passing the resistance allele to their offspring. As these offspring reproduce, the allele is passed on again.

Over many generations, resistant bacteria can make up an increasing proportion of the population. This change can produce an antibiotic-resistant strain. The antibiotic does not make individual bacteria develop resistance because they need it; it favours bacteria that already carry a resistance allele.

Fossil evidence for evolution

A fossil is a preserved remain or trace of an organism that lived in the past. Fossils can include preserved parts of an organism or traces of its activity. They provide evidence of organisms that are no longer alive today.

Fossils found in rocks of different ages show which organisms lived at different times. Comparing fossils from older and younger rocks shows that the forms of life present on Earth have changed over time. Some organisms found as fossils are no longer found alive, while living organisms may differ from organisms preserved in older rocks.

Comparisons between fossils and living organisms can also reveal similarities in their structures. Such similarities are consistent with common ancestry: organisms share features because they descended from an earlier ancestor. Fossils therefore provide evidence that life has changed over time and that groups of organisms can be related through ancestry.

Limits of the fossil record

The fossil record is the collection of fossils that provides evidence of past life. It is incomplete because fossilisation occurs only under particular conditions, so most organisms do not leave fossils. Geological processes can also destroy fossils after they form. As a result, the absence of a fossil does not necessarily mean that an organism never existed.

Despite these gaps, fossils provide evidence of change over time. Alongside other evidence, they support the widely accepted theory of evolution by natural selection.

Genetic inheritance in evolution

Genes are sections of DNA that carry information influencing characteristics. Different versions of a gene are called alleles. Offspring inherit genes from their parents, so alleles can pass from one generation to the next. Differences in inherited alleles can contribute to differences in characteristics between organisms.

Inheritance and natural selection

Some inherited differences affect how likely an organism is to survive and reproduce in its environment. If organisms with a particular inherited characteristic reproduce more successfully, they are more likely to pass on the alleles associated with that characteristic. Over generations, those alleles may become more common in the population. This change in allele frequencies is part of evolution.

For example, suppose an inherited difference in colour makes some organisms harder for predators to see. If those organisms survive and reproduce more successfully, the alleles contributing to that colour may become more common in later generations.

Inheritance therefore provides a mechanism for characteristics involved in natural selection to pass between generations. This supports Darwin’s theory: natural selection can change a population over time because offspring inherit characteristics from their parents.