- Define and explain Natural Selection in your own words
- Use key terms such as species accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
All life on Earth — from bacteria to blue whales — descends from common ancestors through billions of years of change. In this chapter you will master the mechanism of natural selection, weigh the evidence that convinced the scientific world, see how new species form, and use classification to read the tree of life.
This lesson focuses on Natural Selection: explain Darwin's theory step by step, with real examples like antibiotic resistance.
Explain Darwin's theory step by step, with real examples like antibiotic resistance.
Key ideas
Natural selection
Natural selection needs four ingredients: variation within a population, competition for limited resources, survival of the best-adapted individuals (survival of the fittest), and inheritance of the advantageous alleles by offspring. Over generations, the advantageous allele becomes more common. Antibiotic resistance shows it in action: a rare resistant bacterium survives treatment, reproduces, and its descendants dominate the population.
Speciation and classification
Speciation often begins with geographical isolation: a barrier splits a population so the groups cannot interbreed. Different mutations and selection pressures accumulate in each group until they can no longer produce fertile offspring together — they are separate species. Classification reflects these relationships: organisms are grouped into kingdom, phylum, class, order, family, genus and species, and modern phylogeny uses DNA evidence to build evolutionary trees showing who diverged from whom.
Key term — species: A group of organisms that can interbreed to produce fertile offspring.
A bacterial infection is treated with antibiotics, but some bacteria survive and the infection returns, now resistant. Explain this using natural selection.
Within the large bacterial population there is genetic variation; a few bacteria carry a mutation giving antibiotic resistance. The antibiotic kills the non-resistant bacteria — this is the selection pressure. The resistant survivors reproduce, passing the resistance allele to their offspring. Each generation, the proportion of resistant bacteria increases until the population is dominated by them.
Answer: The antibiotic did not create resistance; it selected for resistant variants that were already present. This is natural selection driving evolution in real time.
- Saying organisms evolve because they need to Individuals do not choose to evolve. Mutations create variation randomly; selection then favours the variants that happen to survive and reproduce better.
- Treating 'theory' as a guess In science, a theory is a well-tested explanation supported by extensive evidence. Evolutionary theory is supported by fossils, anatomy, embryology and DNA.
Practice
Antibiotics kill non-resistant bacteria, leaving resistant ones alive to reproduce. Their offspring inherit resistance, so the resistant allele spreads through the population.
Variation within the population → competition for resources → survival and reproduction of the best adapted → inheritance of advantageous alleles by offspring.
A barrier (e.g. a river or mountain range) splits a population so the groups cannot interbreed. Different mutations and selection pressures build up in each group until they can no longer produce fertile offspring together, forming two species.
Structures with the same basic bone arrangement but different functions, e.g. a human arm, bat wing and whale flipper. They suggest the species evolved from a common ancestor.
Quick check
Which of these best defines "species"?
Put these taxonomic groups in order from largest to smallest: genus, kingdom, species, class, phylum, family, order.
Give two pieces of evidence for evolution besides fossils.
- Natural Selection: explain Darwin's theory step by step, with real examples like antibiotic resistance.
- Natural selection: Natural selection needs four ingredients: variation within a population, competition for limited resources, survival of the best-adapted individuals (survival of the fittest), and inheritance of the advantageous alleles by offspring.
- fossil: The preserved remains or traces of an organism that lived long ago, providing evidence of how life has changed.
- Watch out for: saying organisms evolve because they need to