Overview
This lecture explains natural selection as a main driver of evolution, using examples like peppered moths, finches, giraffes, human birth weight, and peacocks. It also introduces modes of selection, sexual selection, artificial selection, and Darwin’s life and family as related to inheritance.
Natural Selection and Evolution
- Natural selection: certain inherited traits help individuals survive and reproduce, changing population genetics over time.
- Evolutionary change also involves mutation, migration, and genetic drift, but natural selection is most powerful.
- Individuals do not evolve; populations evolve as allele frequencies change.
- Evolution happens through interactions between individuals’ traits and their environment.
Peppered Moth Example
- Before industrialization, light-colored moths blended with light lichens and bark, avoiding bird predation.
- Soot from coal factories darkened trees, making light moths visible and dark moths better camouflaged.
- Initially, dark moths were about 2% of the population; by 1895 they were about 95%.
- Dark coloration was a heritable variation that increased survival and reproduction under polluted conditions.
Darwin’s Observations and Adaptation
- Darwin developed natural selection theory from extensive observation of nature and specimens.
- During the voyage on the H.M.S. Beagle (1830s), he observed organisms worldwide and kept detailed journals.
- Adaptations: traits that increase survival and reproduction in specific environments.
- Galapagos finches showed different beak shapes and sizes adapted to available food on each island:
- Thick beaks for hard seeds.
- Skinny, pointed beaks for insects.
- Sharp beaks for cactus fruit.
Fitness
- Fitness: relative ability of an organism to survive and produce offspring in its environment.
- Adaptations that improve fitness become more common over generations.
Four Principles of Natural Selection
- Darwin’s observations summarized into four main principles that describe how natural selection works.
| Principle | Description | Illustrative Points |
|---|
| 1. Variation | Members of a population differ in many characteristics (phenotypes). | Traits include body size, hair color, blood type, facial markings, metabolism, reflexes. |
| 2. Heritability | Many variations are heritable and passed to offspring. | Favorable traits affect future generations only if they can be inherited. |
| 3. Overproduction & Struggle | Populations produce more offspring than resources can support. | Leads to competition for food, water, etc.; called “the struggle for existence.” |
| 4. Differential Survival & Reproduction | Heritable traits affecting fitness cause differences in survival and reproduction. | Favorable traits become more common; unfavorable traits become rarer. |
- Inspired by Thomas Malthus, Darwin applied the idea of population pressure and limited resources to all organisms.
- For natural selection to occur:
- Population must show variation.
- Some variations must be heritable.
- Heritable variations that increase competitiveness are selected.
Phenotype, Genotype, and Alleles
- Phenotype: observable characteristics like beak shape, coloration, birth weight.
- Genotype: genetic makeup underlying phenotypes.
- Alleles: different versions of genes that produce heritable variation.
- As organisms with certain alleles survive longer and reproduce more, those alleles increase in frequency in the population.
Modes of Natural Selection
Directional Selection
- Favors one extreme phenotype at one end of a trait range.
- Over time, the population’s trait distribution shifts toward that extreme.
| Example | Trait Under Selection | Selection Direction | Outcome |
|---|
| Peppered moths | Coloration from white to black | Toward darker moths in polluted environment | Mostly dark moths replace mostly light moths. |
| Giraffe necks | Neck length | Toward longer necks | Short-neck giraffes selected against; neck length increased over time. |
| Peacock tails (sexual) | Tail size and ornamentation in males | Toward more impressive tails | Attractive males mate more; plain tails decline. |
Stabilizing Selection
- Selects against extreme phenotypes and favors intermediate variants.
- Example: human birth weight.
- Very small babies: higher risk from infections and difficulty staying warm.
- Very large babies: difficult or impossible natural delivery.
- Historically, mid-range birth weight had higher survival, stabilizing average weight.
- Widespread Cesarean sections modify this pattern in modern populations.
Disruptive Selection
- Favors extreme phenotypes at both ends, selects against intermediate forms.
- Example (Daphnia with yeast parasite, 2008 study):
- One group: less susceptible to parasite, smaller, fewer offspring.
- Other group: more susceptible, larger, more offspring while alive.
- Two extremes selected: high resistance with low reproduction, or high fecundity with low resistance.
- If an individual had one extreme trait, it did not have the other.
Sexual Selection
- A form of selection driven by mating success rather than direct survival.
- Defined by Darwin as a struggle between individuals of one sex (often males) for access to the other sex.
Strategies in Sexual Selection
- Strategy 1: Attracting the opposite sex.
- Example: peacock tail.
- Large, ornamented tail increases chance of finding a mate.
- Tail is not good camouflage but improves reproductive success.
- Directional sexual selection increases elaborate tails; plain tails decline.
- Strategy 2: Competing with same-sex rivals.
- Involves fighting or intimidating rivals.
- Favors individuals that are bigger, stronger, or more threatening in appearance.
Artificial Selection
- Artificial selection: selection caused by human choices, not by natural environmental pressures.
- Humans encourage some traits in plants and animals and discourage others.
| Process | Agents of Selection | Examples | Key Points |
|---|
| Natural selection | Environment (predators, food, parasites, climate) | Peppered moths, finch beaks, giraffe necks | Traits affect survival and reproduction without human intent. |
| Sexual selection | Mates and same-sex rivals | Peacock tails, fighting males | Traits improve mating success, not necessarily survival. |
| Artificial selection | Humans (breeders, farmers) | Pigeons, cattle, crop plants, dogs | Humans choose breeding stock for desired traits. |
- Long history: humans have bred plants and animals for thousands of years.
- Examples:
- Crop plants selected for higher nutrient yield.
- Gray wolf artificially selected into diverse dog breeds.
- Very different breeds (e.g., small and large) are still one species.
- Different dog breeds can interbreed and produce viable offspring.
Darwin’s Family and Inbreeding
- Darwin requested a census question about cousin marriage and offspring health; request was denied.
- Reason: he was married to his first cousin, Emma Wedgwood.
- Emma’s grandfather, Josiah Wedgwood, was also Darwin’s grandfather.
- Many Wedgwood–Darwin cousin marriages occurred in the family.
- Resulting family tree had high levels of shared genetic material.
Darwin’s Research on Inbreeding
- Darwin studied crossbreeding and inbreeding in plants and animals.
- He found that consanguineous (related) pairs often produced weaker, sicklier descendants.
- His own family showed similar patterns:
- 10 children total; 3 died in childhood from infectious diseases.
- Infectious disease more likely in highly inbred individuals.
- Remaining 7 children:
- No deformities, but described as “not very robust.”
- 3 were unable to have children, likely due to inbreeding effects.
Key Terms & Definitions
- Natural selection: process where certain inherited traits increase survival and reproduction, changing population genetics over time.
- Adaptation: inherited trait that enhances an organism’s survival and reproductive success in a specific environment.
- Fitness (evolutionary): relative ability to survive and produce offspring in a given environment.
- Phenotype: observable characteristics of an organism (e.g., coloration, beak shape, birth weight).
- Genotype: genetic makeup of an organism that determines its phenotype.
- Allele: different form of a gene that contributes to variation in traits.
- Genetic drift: random changes in allele frequencies in a population across generations.
- Directional selection: selection that favors one extreme phenotype, shifting trait distribution in one direction.
- Stabilizing selection: selection that favors intermediate phenotypes and acts against extremes.
- Disruptive selection: selection that favors extreme phenotypes at both ends of a trait range and disfavors the average.
- Sexual selection: selection based on traits affecting mating success rather than survival.
- Artificial selection: human-directed breeding that increases desired traits and decreases undesired ones.
- Consanguineous: relating to individuals who are closely related by blood (e.g., cousins marrying).
Action Items / Next Steps
- Review the four principles of natural selection and relate each to the peppered moth and finch examples.
- Practice explaining differences between directional, stabilizing, and disruptive selection using original examples.
- Distinguish clearly between natural selection, sexual selection, and artificial selection in written summaries.
- Prepare for the next topic: how one species can change into another (speciation).