Biology: Cells to AP Biology · Module 7 of 15
Evolution
The evidence for evolution, how natural selection works, and how new species form.
Evidence for Evolution
- The fossil record shows gradual changes in organisms and transitional forms over time.
- Homologous structures (similar structure, common ancestor, e.g. human arm/whale flipper) support shared ancestry; analogous structures (similar function, different origin, e.g. bird wing/insect wing) result from convergent evolution.
- Species with more similar DNA and protein sequences are more closely related — molecular evidence for common ancestry.
A human arm, a bat wing, and a whale flipper have very different functions but similar underlying bone structure. What does this suggest?
Solution. These are homologous structures, suggesting all three species share a common ancestor that had this same basic bone structure, later modified for different functions.
Natural Selection & Adaptation
- Natural selection requires variation within a population, heritability of traits, and differential survival or reproduction based on those traits.
- Individuals with traits better suited to their environment are more likely to survive and reproduce, passing those traits to offspring.
- Over many generations, helpful traits become more common in the population.
Why did peppered moth populations shift from mostly light-colored to mostly dark-colored during industrial pollution in England?
Solution. Dark moths were better camouflaged against soot-darkened trees, so they were less likely to be eaten by predators and more likely to survive and reproduce, making the dark allele more common over generations.
Common mistake: Organisms don't "try" to adapt or evolve on purpose — natural selection acts on existing variation through differential survival and reproduction.
Speciation & Genetic Drift
- Speciation is the formation of new species, often when populations become reproductively isolated (e.g. by a geographic barrier) and accumulate enough genetic differences over time.
- Genetic drift is a random change in allele frequencies, especially significant in small populations, such as after a population "bottleneck.".
A river splits a squirrel population into two groups that can no longer interbreed. Over thousands of years, they become different enough that they can no longer produce fertile offspring, even if reunited. What has occurred?
Solution. Speciation — reproductive isolation (the river) allowed the two populations to evolve independently until they became separate species.
Phylogeny & Classification
- Phylogenetic trees (cladograms) show hypothesized evolutionary relationships among species, often based on DNA comparisons.
- Modern classification groups organisms into a hierarchy — domain, kingdom, phylum, class, order, family, genus, species — from broadest to most specific.
On a phylogenetic tree, two species share a more recent common ancestor than either does with a third species. What does that tell you about their relationship?
Solution. The two species that share the more recent common ancestor are more closely related to each other than either is to the third species.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What type of structures are a human arm and a whale flipper, and what do they suggest?
2. What are the three requirements for natural selection to occur?