Evolution

Quick Check: A population of beetles lives on green leaves. Most are green, but some are brown. Birds eat beetles. Explain, using natural selection, how this population could become mostly brown if the leaves dried out and turned brown.

Initially most beetles are green — they are well camouflaged and survive easily. Brown beetles are poorly camouflaged against green leaves and are more likely to be eaten by birds. When the leaves turn brown, the selection pressure reverses: brown beetles are now better camouflaged and more likely to survive and reproduce. Brown beetles pass on the allele for brown colouration to their offspring. Over many generations, the proportion of brown beetles in the population increases (the frequency of the brown allele rises) while the proportion of green beetles decreases. Eventually the population is mostly brown.

Quick Check: A student says that a giraffe stretched its neck during its lifetime and passed on a longer neck to its offspring. Identify whose theory this represents and explain the scientific evidence that disproves it.

This represents Lamarck's theory of inheritance of acquired characteristics. It is disproved by our understanding of genetics: characteristics are determined by alleles in DNA. Physical changes to an organism's body during its lifetime (such as muscle growth through exercise, or stretching) do not alter the DNA in gametes. Therefore, acquired characteristics cannot be inherited. Darwin's theory of natural selection — random variation already present in the population, acted on by selection — is supported by the fossil record, observed evolution of antibiotic resistance, and DNA evidence.

Quick Check: Explain why a single mutation that gives one bacterium antibiotic resistance can lead to the entire bacterial population becoming resistant within a short period of time.

The bacterium with the resistance mutation is better adapted to survive in the presence of the antibiotic. When the antibiotic is used, it kills all non-resistant bacteria, leaving only the resistant bacterium alive. This bacterium reproduces rapidly (bacteria can divide every 20 minutes) and all its offspring inherit the resistance allele. With no competition from non-resistant bacteria (which have been killed), the resistant population grows exponentially. Within hours to days, the entire population can consist of resistant bacteria. This is an example of natural selection operating over an extremely short timescale due to the short generation time of bacteria.