7.8Natural Selection
Continuing Evolution — AP BIOLOGY Revision Notes & Topical Questions
5 min read 8 parts Lesson 9 of 13 1 quiz
Lesson summary
7.8 Continuing Evolution — Notes — AP Biology notes: Explain how evolution is an ongoing process in all living organisms.Continuing change — all species have evolved and continue to evolve as environments change; populations evolve today.Evidence — genomic changes over time, cont…
- Antibiotic resistance
- Horizontal gene transfer
- Emergent disease
- Generation time
- Reassortment
- Coevolution
Section 1 of 8Evolution is ongoing
1
Continuing change — all species have evolved and continue to evolve as environments change; populations evolve today.
2
Evidence — genomic changes over time, continuous change in the fossil record, evolution of resistance, and pathogens causing emergent diseases.
3
Genomes — mutations and genetic drift drive divergence, and greater differences indicate a more distant common ancestor.
4
Rate — depends on generation time, population size, mutation rate and strength of selection, so short-generation organisms evolve observably fast.
5
Humans — also continue to evolve, e.g. lactase persistence in some populations and sickle-cell allele frequency in malarial regions.
Section 2 of 8Resistance as natural selection
1
Antibiotic resistance — random mutations produce some resistant bacteria;
- when an antibiotic is used, susceptible bacteria die while resistant ones survive and reproduce,
- so resistance alleles increase.
- Antibiotics select for resistance; they do not cause the mutation.
- Selective pressure comes from both antibiotic use and the immune system.
2
Bacteria can also gain resistance genes by horizontal gene transfer (conjugation, transformation, transduction), speeding evolution.
3
MRSA demonstrates natural selection in action.
4
Pesticide and herbicide resistance — insects and weeds (e.g. resistance to DDT) evolve the same way under strong selection.
5
Chemotherapy resistance — cancer cells show increasing resistance through selection of resistant cell lines.
6
Full explanation — random mutation, selective pressure, greater survival and reproduction of resistant cells, and a rising frequency of resistance.
Section 3 of 8Why prokaryote evolution is so fast
1
Generation time — as short as 20 minutes, so many generations occur in a day.
2
Population size — enormous, so even rare mutations occur somewhere each day.
3
Haploid genomes — a new allele is expressed immediately with no dominant allele to mask it.
4
Gene transfer — a useful gene can move between unrelated species.
5
Extreme environments — adaptation to hot springs, salt lakes and deep-sea vents is far faster than in larger organisms.
Section 4 of 8Emerging pathogens and viruses
1
Viruses — high mutation rates and short generation times produce new variants that may evade immunity; flu and HIV genomes (often RNA) are copied with little error checking and huge numbers of particles are produced.
2
Selection — the host immune system and drugs favour variants that escape them, so new strains appear.
3
Seasonal flu — mutations alter antigens each year and make current vaccinations ineffective, so a new vaccine is needed each year.
4
Viral populations — one infected person holds many slightly different genomes,
- and if a drug removes most of them any surviving variant becomes the ancestor of the next population.
5
Combination drug therapy — HIV treatment uses several drugs because a virus needs several separate mutations at once to survive, which is unlikely.
6
Reassortment — two influenza strains infecting the same cell can swap RNA segments, creating a combination to which people have little immunity.
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Viruses evolve only as they replicate inside host cells, so widely spreading outbreaks give more chances for new variants (SARS-CoV-2 variants).
8
Coevolution — two species evolve in response to each other.
Section 5 of 8Evolution happening now
| Example | Selective pressure | Result | Time scale | Response |
|---|---|---|---|---|
| Antibiotic-resistant bacteria | Antibiotics | MRSA | Years | Careful antibiotic use |
| Flu virus | Immunity | New strains | Yearly | New vaccine each year |
| HIV | Drugs, immune system | Drug resistance | Months | Combination therapy |
| Pesticide-resistant insects | Pesticides | Resistant pests | Years | Integrated pest management |
| Galápagos finches | Drought | Beak size changes | One generation | — |
Section 6 of 8More from the specification
1Evolutionary cladistics and phylogenetic trees
- Evolutionary cladistics and phylogenetic trees aligning homologous DNA and protein sequences allows evolutionary biologists to construct rooted phylogenetic trees depicting shared ancestry.
2Habitat
- The place where an organism lives.
3Protein
- A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
4Genetic diversity
- The range of alleles within a population.
5Phylogenetic tree
- Diagram showing evolutionary relationships between species.
6Allopatric speciation
- Speciation caused by geographical isolation.
7Punctuated equilibrium
- Speciation in rapid bursts between long stable periods.
8Geographical barrier
- A physical feature, such as a mountain or sea, separating populations.
9Sympatric speciation
- Speciation in the same area without a geographical barrier.
10Speciation
- The formation of new species.
11Extinction
- Loss of every member of a species.
12Isolation
- Separation of populations so that they cannot interbreed.
13Barrier
- A structure that prevents pathogens from entering.
14Cladogram
- Diagram showing branching pattern of relationships with no timescale.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Continuing evolution | ongoing change in populations over generations |
| Antibiotic resistance | bacterial ability to survive an antibiotic |
| MRSA | methicillin-resistant Staphylococcus aureus |
| Horizontal gene transfer | gene exchange between organisms other than by reproduction |
| Emergent disease | disease newly appearing through pathogen evolution |
| Generation time | time between one generation and the next |
| Reassortment | swapping of RNA segments between virus strains |
| Coevolution | two species evolving in response to each other |
Section 8 of 8Exam technique
1
Saying antibiotics or drugs cause the resistance mutation — resistance arises randomly and they select existing resistant individuals.
2
Thinking evolution stopped in the past — populations evolve today.
3
Saying individuals evolve resistance during their lifetime — populations change over generations.
4
Treating viruses as cells that evolve alone — they evolve only as they replicate inside hosts.
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