Notes on Natural Selection and Evolution (Lecture Transcript) 9/10/25 ecology
Key Concepts and Definitions
Evolution = a change in allele frequencies over time, leading to changes in traits in a population. ext{Evolution}
ightarrow ext{change in allele frequencies over time}Adaptation = a trait that makes an organism a better fit to its environment, often increasing survival or reproductive success.
Fitness = a measure of reproductive success; normally about how many offspring survive to reproduce. W = ext{reproductive success} = ext{number of offspring that survive to reproduce}
Traits that are adaptations often increase fitness by improving survival and/or reproduction.
Reproduction is essential for passing genes to the next generation; survival alone does not change the genetic makeup of a population unless it leads to reproduction.
Evolution is the change observed in populations, not necessarily a change seen in every individual.
Four Pillars of Natural Selection (the four components)
Heritable variation: Variation that is coded for in the DNA and can be inherited from parent to offspring.
Variation can be visible (e.g., color) or internal/physiological; changes in DNA sequences influence phenotypes via the nested DNA→protein path.
Central idea: changes in DNA sequence affect amino acids, which affect protein folding, which determines protein function and ultimately traits.
Recall: DNA sequence → amino acid sequence → protein folding → protein function → phenotype. ext{DNA}
ightarrow ext{amino acids}
ightarrow ext{protein folding}
ightarrow ext{function/phenotype}
Struggle for survival and competition for resources: Not all individuals survive; death occurs due to limited resources, predation, disease, etc.
Example: giraffes with longer necks can reach higher foliage; shorter necks may die from resource limitation.
Differential reproductive success: Some traits confer advantages that lead to more offspring.
Summary: some traits are good (more babies), others are bad (fewer babies).
Change in allele frequencies over time: As a consequence of the above, the frequency of alleles associated with advantageous traits increases in the population across generations.
In natural selection, the environment determines which traits are favored at that moment.
Key Example Illustrations of Natural Selection
Beak size in Galápagos finches during a drought: larger beaks become favored because they crack bigger seeds; the population mean beak size shifts toward the larger extreme after the drought.
Peppered moth (industrial melanism): environmental change (coal pollution) darkened tree bark; darker moths became more common because they were better camouflaged; once pollution decreased, the lighter morphs re-increased, illustrating reversibility when the environment shifts.
Birds predating on caterpillars with color contrast: yellow caterpillars are more visible to predators than brown ones, leading to higher predation on yellow morphs and a relative increase in brown morphs; demonstrates environment-dependent selection.
Texas longhorns: selection for larger horns increases reproductive success in males due to fighting and defense; humans can also drive selection via artificial selection (breeding for specific traits).
Example of ongoing, multi-species size changes: over long time scales, some lineages show a trend toward larger body size (e.g., horse relatives), illustrating directional selection across deep time.
Environment as the Selecting Force
Ecology is the source of selection; the environment (biotic and abiotic) defines which traits are advantageous at any given time.
Selective pressures include: predation, competition for food, parasites, mutualisms, and non-living factors like temperature and resources.
If the environment changes, the direction and strength of selection can change, and previously advantageous traits can become less favorable or vice versa.
Important reminder: selection acts on the organisms in their environment; what is advantageous is context-dependent.
Heredity, Variation, and the History of Ideas
Darwin knew heritability existed (parents resemble offspring from breeding experiments), but he did not know the mechanism (DNA, chromosomes).
Modern understanding: traits that matter for fitness are heritable if they have a genetic basis; life-history changes that occur within an organism’s lifetime are generally not passed on unless changes affect the germ line.
Lamarck's idea (inheritance of acquired traits, e.g., a giraffe stretching its neck and passing that to offspring) is historically important but not supported by evidence; natural selection explains trait changes without acquired characteristics being inherited.
Key point: variation allows evolution to occur; without variation, there is no substrate for selection to act upon. If everyone is identical, evolution cannot proceed.
The role of mutation: new variation arises via mutations; whether they persist depends on whether they are heritable and advantageous in the current environment.
Unit of Selection: Who gets selected?
Population-level evolution requires changes across individuals in the population, but selection acts on individuals.
Common misconception: selection always acts on populations or species; the correct view is that selection acts on individuals, and the differential success of individuals alters allele frequencies in the population over generations.
Clarification through definitions:
An individual is a single organism.
A population is a group of individuals in the same area that interbreed.
A species comprises all individuals capable of interbreeding.
Answer to the unit question: the unit that evolves via natural selection is the population (through differential survival and reproduction of individuals). Selection acts on individuals, and their differential reproduction changes allele frequencies in the population over time.
What If There Is No Variation?
If all individuals have exactly the same traits at a given time, there is no material for selection to act on; evolution cannot occur via natural selection in that moment.
The presence of variation (due to mutation and recombination) is essential for evolution.
Note: other mechanisms of evolution (e.g., genetic drift, mutation, gene flow) can still operate in low-variation contexts, but classic natural selection requires heritable variation and differential reproductive success.
Parental effects and genetic basis of preferences can complicate simple pictures of selection, but many traits with apparent preferences can have a genetic component worth considering in sexual selection (e.g., female preference).
Natural Selection: Types of Selection (three main kinds)
Directional selection: shifts the population distribution toward one extreme; one extreme is favored over the other(s).
Characteristic: the mean trait value changes in a given direction over time.
Snail example: darker or lighter shell color depending on the environment; drought beak-size example for birds; and larger horns in longhorns as a functional advantage.
Example: during drought, bigger beaks are favored because they crack larger seeds, increasing the mean beak size over time.
Stabilizing selection: the intermediate phenotype is favored; extremes are selected against.
Example: birth weight in humans shows high mortality at both very low and very high weights; the distribution centers around an optimal weight (e.g., ~3 kg in this illustration).
Modern changes (e.g., cesarean sections and NICU care) can relax the strength of stabilizing selection, shifting the distribution.
Example: Siberian huskies show an optimal, intermediate weight for sled performance; both very light and very heavy dogs are less favorable.
Disruptive selection: extremes are favored over intermediates; can lead to bimodal distributions and potentially contribute to speciation.
Example: hawthorn flies with timing for emergence and egg-laying; middle timing yields little food; early and late emergers have better access to resources (apples vs hawthorns).
Another classic example: black-bellied seed crackers with two beak sizes suited to two different seed types; intermediate beak sizes are inefficient for both seed types.
Note: disruptions can maintain or increase diversity and potentially promote divergence between populations if conditions persist.
Antibiotic Resistance: An Evolutionary Example (Introductory Overview)
Antibiotic resistance is a well-understood example of evolution by natural selection in bacteria.
Why bacteria are used in these discussions:
They are medically and socially relevant due to antibiotic resistance impacts.
The experiments are straightforward and informative for illustrating selection on a rapid generational timescale.
Core idea (as introduced): bacterial populations harbor genetic variation; exposure to antibiotics imposes strong selective pressure; resistant strains survive and reproduce, increasing their allele frequencies in the population over time.
The lecture indicates that more mechanisms of evolution (beyond natural selection) will be covered on Friday, including genetic drift (random death not tied to traits).
The educational goal is to correct common misconceptions about antibiotic resistance and to help students explain these ideas to others.
Quick Recap: Concepts to Remember
Evolution is a change in allele frequencies over time, leading to trait changes in populations. ext{Evolution}
ightarrow ext{allele frequency changes in a population over time}Adaptations are traits that increase fitness in a given environment; fitness measures reproductive success.
Natural selection requires heritable variation, struggle for survival, differential reproductive success, and results in changing allele frequencies.
The environment (ecology) is the selecting force; selection direction can reverse as the environment changes.
The unit of selection is the individual, even though evolution is observed at the population level across generations. However, selection acts on individuals, altering allele frequencies in populations over time.
Without variation, natural selection cannot drive evolution. Mutations provide new variation; not all mutations are advantageous, and many are neutral or deleterious.
Lamarckian ideas (inheritance of acquired traits) are historically important but not supported by current evidence; natural selection provides a robust framework for understanding trait changes over generations.
On Friday, other mechanisms of evolution (e.g., genetic drift, gene flow, mutation, non-adaptive processes) will be explored to contrast with natural selection.
Key Figures and Takeaways
Natural selection is the mechanism by which adaptation arises and trait frequencies shift in response to environmental pressures.
Examples across species (finches, moths, huskies, hawthorn flies, seed crackers) illustrate how selection pressures can produce different evolutionary outcomes (directional, stabilizing, disruptive).
Antibiotic resistance demonstrates rapid evolutionary change and highlights the relevance of understanding selection to public health and everyday life.
Understanding the basics of heredity, variation, and selection provides a foundation for recognizing how evolution shapes biodiversity and how human actions can influence evolutionary trajectories.