Notes
Natural selection
Natural selection needs variation in heritable traits, more offspring than can survive, and differential reproductive success. Individuals with traits better suited to the environment leave more offspring, so those alleles become more common.
- Fitness means reproductive success, not strength.
- Individuals don't evolve; populations do.
- Types: directional (one extreme favored), stabilizing (the middle favored), disruptive (both extremes favored). Sexual selection favors traits that improve mating success.
- Artificial selection is humans choosing which organisms breed.
Other ways evolution happens
- Genetic drift: random changes in allele frequency, strongest in small populations. Bottleneck effect (a disaster shrinks the population) and founder effect (a few individuals start a new population).
- Gene flow: migration moves alleles between populations, making them more similar.
- Mutation is the original source of new alleles.
Hardy-Weinberg equilibrium
$p + q = 1$ and $p^2 + 2pq + q^2 = 1$, where $p$ and $q$ are allele frequencies and $p^2$, $2pq$, $q^2$ are genotype frequencies.
- Equilibrium requires: no mutation, random mating, no natural selection, a very large population (no drift), and no gene flow. If allele frequencies change, the population is evolving.
Evidence and relationships
- Evidence: fossils, homologous structures (same origin, different function, like a human arm and a bat wing), vestigial structures, embryology, and shared DNA and protein sequences.
- Analogous structures (bird and insect wings) come from convergent evolution, not common ancestry.
- Phylogenetic trees show relationships; a node is a common ancestor. Species that share a more recent common ancestor are more closely related.
- Speciation: allopatric (geographic isolation) or sympatric (in the same area, often from polyploidy in plants). Reproductive barriers can be prezygotic or postzygotic.
Topics in this unit
Each topic has its own short notes and practice.
Practice
$q^2 = 0.16$, so $q = 0.4$.
$p = 0.6$, so $2pq = 2(0.6)(0.4) = 0.48$.
Selection favors the intermediate phenotype.
A random, drastic reduction in population size is drift (bottleneck).
Same underlying structure inherited from a common ancestor.
Equilibrium needs no forces changing allele frequencies, including nonrandom mating.
Geographic isolation stops gene flow.
Selection acts on existing variation; mutations are random, not caused by need.
Free response
Write your answer first, then open the scoring guide.
Free-response question
In a population of 1,000 beetles, black color ($B$) is dominant over brown ($b$). There are 360 brown beetles.
(a) Calculate the frequency of the $b$ allele and the number of heterozygous beetles, assuming Hardy-Weinberg equilibrium.
Show the answer and scoring guide
$q^2 = 0.36$, so $q = 0.6$ and $p = 0.4$. Heterozygotes: $2pq = 0.48$, which is 480 beetles.
Check off the points you earned:
(b) Birds start to eat more brown beetles. Predict the change in $q$ over several generations and explain.
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$q$ decreases. Brown ($bb$) beetles are less likely to survive and reproduce, so fewer $b$ alleles are passed on.
Check off the points you earned:
(c) Explain why the $b$ allele is unlikely to disappear completely, even with strong selection.
Show the answer and scoring guide
Many $b$ alleles are hidden in black heterozygotes, which aren't selected against, so they keep passing $b$ on.
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