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Unit 7
13–20% of examThis unit is about evolution: how and why populations change over generations. You'll study natural selection and the random forces that also change allele frequencies, test whether a population is evolving with Hardy–Weinberg math, read the evidence for common ancestry, and see how new species form. It's the most heavily weighted unit on the exam.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Natural selection is a major way evolution happens: individuals compete for limited resources, and those with traits that help them survive and reproduce leave more offspring. Fitness means reproductive success, and because environments change, the traits that are favored can change too.
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Selection only works because individuals differ. Changing environments put pressure on populations, and differences in traits, even in molecules inside cells, can raise or lower fitness in a particular setting, as with insects that survive the pesticide DDT.
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When humans decide which plants or animals get to breed, people become the selecting force. Selective breeding produced dog breeds and turned one wild plant species into broccoli, cabbage, kale and cauliflower, showing how quickly selection can change a population.
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Evolution isn't only selection; chance changes allele frequencies too. Mutation adds new alleles, genetic drift causes random shifts that matter most in small populations (as in bottleneck and founder effects), and gene flow moves alleles between populations, keeping them alike.
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The Hardy–Weinberg equations (p + q = 1 and p² + 2pq + q² = 1) describe a population that isn't evolving, which needs a large population, no migration, no new mutations, random mating and no natural selection. Real populations never meet all five, so the model works as a null hypothesis you compare real data against.
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Many kinds of evidence show that life has changed over time. Fossils (dated by rock layers and radioactive decay such as carbon-14), shared body structures including vestigial ones, and similar DNA and protein sequences all point to common ancestry.
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All eukaryotes, from yeast to oak trees to you, share membrane-bound organelles, linear chromosomes and genes with introns. Features shared that widely are strong evidence that eukaryotes descend from one common ancestor.
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Evolution is still happening. You can see it in genomes changing over time, in the fossil record, in bacteria evolving resistance to antibiotics and pests to pesticides, and in new diseases that appear as pathogens evolve.
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Cladograms and phylogenetic trees are testable hypotheses about how groups are related, built from shared derived characters and, more reliably, from DNA and protein data. Each node marks a common ancestor, the outgroup is the least closely related lineage, and only phylogenetic trees show time or amount of change.
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A species is a group that can interbreed and produce fertile offspring (the biological species concept), so new species form when populations become reproductively isolated, either with a geographic barrier (allopatric) or without one (sympatric). Speciation can be slow and steady (gradualism) or come in quick bursts after long stable periods (punctuated equilibrium), and it speeds up when open habitats let one group branch into many (adaptive radiation).
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Genetic diversity helps a population survive change: a diverse population is more likely to include individuals that can handle a new disease or environment. Populations with little variation, like the California condor, are at higher risk of decline or extinction, and an allele that helps in one environment can hurt in another.
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Rocks and fossils give a timeline: Earth is about 4.6 billion years old, it was likely too harsh for life until roughly 3.9 billion years ago, and the oldest fossils of living things are about 3.5 billion years old. The RNA world hypothesis suggests RNA, which can both store information and speed up reactions, was the first genetic material, before DNA and proteins took over those jobs.
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