How do new species form, and how does natural selection cause antibiotic and pesticide resistance?
Analyze and interpret data to explain patterns in biodiversity that result from speciation, and develop a model to explain how natural selection causes biological resistance such as pesticide and antibiotic resistance (GSE SB6.b, SB6.e).
A Georgia Milestones Biology EOC answer on speciation and resistance: how reproductive isolation (often a geographic barrier) splits one species into two, how speciation builds biodiversity, and how natural selection produces antibiotic and pesticide resistance, a fast, real-world example of evolution.
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What this topic is asking
Standards SB6.b and SB6.e ask you to explain how speciation produces patterns in biodiversity, and to model how natural selection causes biological resistance (pesticide, antibiotic, and vaccine resistance). For the Georgia Milestones Biology EOC that means understanding how reproductive isolation can split one species into two, how repeated speciation builds the diversity of life, and how resistance is a fast, observable example of natural selection. Items often describe a population being split, or a drug or pesticide being applied repeatedly, and ask you to predict or explain the outcome.
Speciation: how new species form
Once two groups are isolated, they no longer exchange genes. Each group experiences its own mutations and its own selection pressures, so over many generations they accumulate different heritable changes. If the differences grow large enough that the two groups could no longer interbreed and produce fertile offspring, even if the barrier were removed, they have become separate species. The EOC scenario is usually a population split by a barrier, asking whether and how a new species could form.
Speciation builds biodiversity
So patterns in biodiversity (for example, many related species on a chain of islands, each suited to its island) are read as the result of repeated speciation from a shared ancestor as populations were isolated and diverged.
Resistance: natural selection you can watch
Biological resistance is the clearest everyday example of natural selection, and SB6.e asks you to model it. The same four conditions apply, but the timescale is short because bacteria and insects reproduce quickly:
- Variation. Within the population, a few individuals already carry a heritable resistance trait (usually from a mutation), while most do not.
- Selection pressure. Applying an antibiotic (to bacteria) or a pesticide (to insects) kills the susceptible individuals.
- Differential survival and reproduction. The resistant individuals survive and reproduce, passing the resistance allele to their offspring.
- Inheritance and frequency change. Over a few generations, the resistant type comes to dominate, so the allele's frequency in the population rises sharply.
The drug or pesticide does not create resistance; it selects for resistant variants that were already present. Overusing antibiotics or pesticides, or stopping a course of antibiotics early, speeds this up by repeatedly favoring the resistant survivors.
Why this matters in the real world
Antibiotic resistance is a major public-health problem precisely because evolution is real and fast: bacteria evolve resistance to drugs faster than new drugs are developed. The same logic explains why pests evolve resistance to pesticides and why flu vaccines must be updated as influenza viruses change. The EOC uses these cases to show that evolution is not only ancient history but an ongoing, testable process.
Try this
Q1. Explain how a geographic barrier can lead to the formation of two species from one. [3 points]
- Cue. The barrier reproductively isolates two groups so they cannot interbreed; each accumulates different mutations and faces different selection over generations; if they become unable to interbreed even if reunited, they are separate species.
Q2. Explain why finishing a full course of antibiotics helps reduce the spread of resistance. [2 points]
- Cue. Finishing the course kills more of the bacteria, including the hardiest partly resistant cells, before they can survive and reproduce; stopping early leaves resistant survivors to multiply and spread the resistance allele.
Exam-style practice questions
Practice questions written in the style of GaDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
GA Biology EOC (style)1 marksA river permanently splits one population of frogs into two groups that can no longer interbreed. Over a long time the groups become so different that they could not reproduce together even if reunited. This process is: (A) extinction. (B) speciation. (C) cloning. (D) artificial selection.Show worked answer →
A 1-point selected-response item on speciation.
The correct answer is B. When a geographic barrier reproductively isolates two groups, each accumulates different changes through mutation and selection; if they become unable to interbreed and produce fertile offspring, a new species has formed, which is speciation. Extinction (A) is the loss of a species, and cloning and artificial selection (C, D) are not described.
GA Biology EOC (style)2 marksA patient stops taking an antibiotic early, and the infection returns, now resistant. (a) Explain how natural selection produced the resistant bacteria. (b) State why stopping the antibiotic early makes resistance more likely.Show worked answer →
A 2-point item on antibiotic resistance as natural selection.
(a) 1 point: the bacterial population had heritable variation (a few cells carried a resistance allele, usually from mutation); the antibiotic killed the non-resistant cells but the resistant ones survived and reproduced, passing on resistance, so the population became mostly resistant (a change in allele frequency by natural selection).
(b) 1 point: stopping early leaves behind the hardiest, partly resistant cells that were not yet killed; they survive and reproduce, so resistance spreads, whereas finishing the course kills more of them before they can multiply.
Markers reward identifying heritable variation, the antibiotic as the selection pressure, and survival and reproduction of resistant cells.
Related dot points
- Use mathematical and conceptual models to explain how natural selection acts on heritable variation to change the traits of a population over generations (GSE SB6.d).
A Georgia Milestones Biology EOC answer on natural selection: the four conditions (variation, overproduction, differential survival and reproduction, inheritance), what fitness really means, how selection produces adaptation, and the key idea that populations evolve while individuals do not.
- Develop and use mathematical models to support explanations of how undirected genetic changes, including genetic drift and gene flow, alongside natural selection, lead to changes in populations of organisms (GSE SB6.d).
A Georgia Milestones Biology EOC answer on the mechanisms that change allele frequencies: mutation as the source of new alleles, genetic drift (including bottleneck and founder effects), gene flow, and natural selection, plus the Hardy-Weinberg idea of a non-evolving population for comparison.
- Construct an argument using valid and reliable sources to support the claim that evidence from comparative morphology (analogous vs. homologous structures), embryology, biochemistry, and genetics supports common descent (GSE SB6.c).
A Georgia Milestones Biology EOC answer on the evidence for evolution: the fossil record, homologous, analogous, and vestigial structures, embryological similarities, and molecular evidence from DNA and proteins, and what each line shows about common descent.
- Evaluate the factors that affect biodiversity and the stability of ecosystems, including keystone species, the effects of removing species, and symbiotic relationships (GSE SB5.c).
A Georgia Milestones Biology EOC answer on biodiversity and stability: why diverse ecosystems are more stable, the role of keystone species, the effects of removing a species, and the three types of symbiosis (mutualism, commensalism, parasitism).
- Analyze and interpret cladograms and phylogenetic trees based on shared derived characteristics and common ancestry to determine relationships among groups of organisms (GSE SB4.b).
A Georgia Milestones Biology EOC answer on cladograms and phylogenetic trees: how to read branch points (common ancestors) and shared derived characters, determine which organisms are most closely related, and use the diagrams as models of evolutionary relationships.
Sources & how we know this
- Biology Georgia Standards of Excellence (GSE) — Georgia Department of Education (2024)
- Georgia Milestones Biology EOC Assessment Guide — Georgia Department of Education (2024)