Continuing Evolution
AP BiologyΒ· Unit 7, Topic 12: Continuing EvolutionΒ· 12 min read
1. Evolution Does Not Stop At 'Perfect Adaptation'β β ββββ± 10 min
A widespread misconception is that evolution is a process that only happened in the distant past, and that modern species are fully adapted and no longer changing. In reality, all populations accumulate random mutations across generations, and environmental conditions are never permanently static. Even stable ecosystems experience shifts in temperature, predator populations, resource availability, and pathogen exposure that create new selective pressures over time.
Evolution
Formally defined as a change in the allele frequency of a population across successive generations, there is no threshold of change required for evolution to occur
Example:
A 2% increase in the frequency of a drought-resistant plant allele over one generation counts as evolutionary change
A population of wild sunflowers has an allele for drought tolerance at an initial frequency of 0.3. After a 1-year extreme drought, 70% of non-tolerant plants die before reproducing, while 10% of drought-tolerant plants die. Calculate the new allele frequency for drought tolerance in the next generation.
- 1
Assume the population is in Hardy-Weinberg equilibrium before the drought: initial genotype frequencies are AA=0.09, Aa=0.42, aa=0.49, where A = drought tolerance
- 2
Apply survival rates: surviving counts are AA=0.090.9=0.081, Aa=0.420.9=0.378, aa=0.49*0.3=0.147
- 3
Total surviving population = 0.081 + 0.378 + 0.147 = 0.606
- 4
New A allele frequency = (20.081 + 0.378) / (20.606) = 0.54 / 1.212 β 0.446
Confirm your understanding of the core definition of evolution:
Which of the following scenarios counts as evolution?
A single oak tree grows thicker bark to survive an unseasonably cold winter
The frequency of a cold-resistance allele in an oak population rises from 0.1 to 0.15 over 2 generations
A bird learns a new mating call from a neighboring bird
A population of snails all grow larger shells after a year of abundant food
Reveal answer
The frequency of a cold-resistance allele in an oak population rises from 0.1 to 0.15 over 2 generations βOnly shifts in heritable allele frequency across generations meet the formal definition of evolution
2. Antibiotic and Pesticide Resistance As Model Evidenceβ β β βββ± 12 min
The global rise of antibiotic resistant bacterial strains is the most well-documented example of continuing evolution, with measurable change observed in less than 100 years since the widespread introduction of penicillin in the 1940s. Bacteria have extremely short generation times (as little as 20 minutes) and large population sizes, so random mutations that confer resistance arise rapidly, and strong selective pressure from antibiotic use causes those resistance alleles to spread very quickly.
A bacterial population has an initial rifampicin resistance allele frequency of 0.01. The susceptible allele has a relative fitness of 0.1 when exposed to rifampicin, while the resistance allele has a relative fitness of 1. Calculate the resistance allele frequency after 1 generation of full antibiotic exposure.
- 1
Define known values: p_n = 0.01, q_n = 0.99, w_A = 1, w_a = 0.1
- 2
Substitute into the selection formula: p_{n+1} = (0.01 * 1) / ((0.01 * 1) + (0.99 * 0.1))
- 3
Simplify denominator: 0.01 + 0.099 = 0.109
- 4
Final result: p_{n+1} β 0.092, a 9x increase in resistance frequency after one generation
3. Observed Contemporary Speciation Eventsβ β β βββ± 10 min
Speciation, once thought to require millions of years, has been directly observed in multiple species within recorded human history. The most famous example is the apple maggot fly, Rhagoletis pomonella, which originally laid eggs only on hawthorn fruit, but formed a new host population that lays eggs on introduced domestic apples in the northeastern United States in less than 200 years. The two fly populations now have distinct mating seasons and almost no interbreeding, making them nearly reproductively isolated.
Species | Speciation Mechanism | Observed Timescale |
|---|---|---|
Apple maggot fly | Host shift pre-zygotic isolation | ~200 years |
Cichlid fish in Lake Victoria | Sympatric niche partitioning | ~15,000 years |
London Underground mosquito | Habitat separation post-zygotic isolation | ~100 years |
Researchers find that 98% of apple maggot flies emerge from their pupae 3 weeks earlier than hawthorn maggot flies, matching the earlier ripening time of apples vs hawthorns. Explain why this difference is evidence of ongoing speciation.
- 1
Mating for Rhagoletis occurs directly on the fruit they feed on as larvae, so flies that emerge at different times will almost never encounter each other to mate
- 2
This creates a pre-zygotic temporal reproductive barrier between the two populations
- 3
Over time, the lack of gene flow will allow the two populations to accumulate more genetic differences until they are fully distinct species
4. Human Activity As A Major Modern Selective Pressureβ β β β ββ± 11 min
Human activity is now the single largest driver of evolutionary change across the planet. Overfishing has selected for smaller adult body size in many commercial fish species, as larger fish are removed before they can reproduce. Climate change is selecting for earlier flowering times in wild plant populations, and increased pesticide use drives rapid resistance evolution in insect pest species. Even human populations are still evolving, with recent selection for lactose tolerance in cultures that domesticated dairy cattle, and high-altitude oxygen adaptation in Tibetan populations.
A lake is heavily fished, and all fish longer than 30cm are removed before they can spawn. Explain what will happen to the average body size of the fish population over 10 generations.
- 1
Only fish that reach sexual maturity at a size smaller than 30cm will survive to reproduce
- 2
Alleles for slower growth and smaller adult size will be passed on to the next generation at much higher rates than alleles for large adult size
- 3
Over 10 generations, the average body size of the population will decrease significantly, as the large size alleles are almost entirely removed from the gene pool
5. Common Pitfalls
Wrong move:
Claiming 'humans have stopped evolving because we have modern medicine'
Why:
Modern medicine reduces some historical selective pressures but introduces new ones, and human populations still show measurable allele frequency shifts across generations
Correct move:
State that human evolution continues, with new selective drivers including pathogen exposure, dietary changes, and high-altitude migration
Wrong move:
Stating that evolution only happens over millions of years
Why:
Strong selective pressures can drive measurable phenotypic and allelic change in as few as 2-3 generations for fast-reproducing organisms
Correct move:
Cite antibiotic resistance in bacteria as an example of evolution observed in less than 100 years
Wrong move:
Assuming a well-adapted population will remain genetically identical indefinitely
Why:
Environmental conditions are never static, and random mutations continuously generate new genetic variation in all populations
Correct move:
Note that shifting climate, new predators, or new pathogens will always create new selective pressures for even stable populations
Wrong move:
Confusing individual phenotypic plasticity with evolutionary change in a population
Why:
Individual organisms do not evolve; only populations experience shifts in allele frequencies across generations
Correct move:
Distinguish between an individual acclimating to a new environment and a population evolving via differential survival of genetically distinct individuals
Wrong move:
Claiming that 'evolution has finished for humans'
Why:
Genomic studies confirm positive selection for traits like lactose tolerance and high-altitude oxygen adaptation in the last 10,000 years
Correct move:
Reference recent human genomic data to demonstrate ongoing selection in Homo sapiens
6. Quick Reference Cheatsheet
Key Evidence Type | Observed Timescale | Common AP Exam FRQ Point |
|---|---|---|
Antibiotic resistance in bacteria | < 100 years | Connect overuse of antibiotics to increased selective pressure |
Galapagos finch beak size shifts | < 50 years | Link drought conditions to directional selection on beak depth |
Apple maggot fly speciation | ~200 years | Identify host shift as a pre-zygotic reproductive barrier |
Lactose tolerance in humans | ~10,000 years | Correlate dairy agricultural use to positive selection for lactase persistence |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2024 Β· AP Bio FRQ Q3
Analyze antibiotic resistance in E. coli populations
- 2022 Β· AP Bio MCQ Set 4
Evaluate beak size shifts in Galapagos finches
- 2021 Β· AP Bio FRQ Q6
Explain ongoing evolution of viral pathogens
What's Next
Now that you understand that evolution is a continuous, observable process acting on all living populations, you are ready to dive deeper into quantitative analysis of population allele shifts using extended Hardy-Weinberg models for non-equilibrium populations. You will also apply these concepts to evaluate public health policy around antibiotic stewardship, a common real-world FRQ scenario on recent AP Biology exams. Mastery of this sub-topic will help you avoid the most frequently marked-down misconceptions on the Unit 7 natural selection assessment, where 30% of recent exam points relate to evolutionary continuity. These concepts also build the foundation for understanding how small, incremental evolutionary changes accumulate to generate the full diversity of life on Earth.