Learning Objectives
By the end of this lab, you should be able to:
- Explain why genetic variability within a population is important for survival in a changing environment.
- Define selective pressure and distinguish between biotic and abiotic selective pressures.
- Describe the three modes of natural selection (stabilizing, directional, and disruptive) and predict their outcomes.
- Explain how mutations, differential survival, and reproduction drive evolutionary change over time.
- Connect phenotypic traits to fitness in a specific environment.
- Predict how geographic isolation and new selective pressures can lead to divergent evolution.
Background Reading
Read the following sections before beginning the simulation. These concepts will be directly tested in the lab questions.
Genetic Variation: The Raw Material of Evolution
Within any population of organisms, individuals differ from one another. These differences — in body size, color, behavior, metabolism, and countless other traits — arise primarily from genetic variation. Variation enters a population through two main routes: mutations (heritable changes in DNA sequence) and sexual recombination (shuffling of alleles during meiosis and fertilization).
Not all variation is adaptive. Much of it is neutral — neither helpful nor harmful under the current environment. However, when the environment changes, previously neutral or even detrimental alleles can suddenly become advantageous. This is why genetic diversity in a population is so critical: it provides the "library" of options from which natural selection can draw.
Populations with very low genetic variation (due to small size, inbreeding, or a recent bottleneck) have fewer options when challenged by a new selective pressure. They are at higher risk of extinction because no individuals may carry traits suited to the new conditions.
Selective Pressures
A selective pressure is any environmental factor that affects whether an individual survives and reproduces. Selective pressures fall into two broad categories:
Biotic Selective Pressures
Biotic pressures arise from interactions with other living organisms.
- Predation — predators exert pressure on prey to be faster, better camouflaged, or more toxic.
- Competition — individuals compete with others (same or different species) for food, mates, territory, or light.
- Parasitism & Disease — pathogens select for immune resistance.
- Mutualism — partners select for compatible traits (e.g., flower shape and pollinator tongue length).
A hawk population selects for better-camouflaged mice — dark mice on dark soil survive and reproduce more than light mice.
Abiotic Selective Pressures
Abiotic pressures arise from the non-living physical environment.
- Temperature — cold climates favor insulating fat layers or dense fur.
- Precipitation / Humidity — desert organisms evolve water-conservation traits.
- Light availability — plants in low-light environments evolve large leaf surfaces.
- Soil chemistry — plants on serpentine soils evolve tolerance to heavy metals.
- Salinity — coastal organisms evolve osmoregulation strategies.
Drought conditions select for plants with deep root systems or thicker waxy cuticles.
Natural Selection and Fitness
Natural selection occurs when individuals with certain heritable traits survive and reproduce at higher rates than others. Over generations, alleles associated with those beneficial traits become more frequent in the population. This is NOT a goal-directed process — organisms do not "try" to evolve. Rather, individuals with certain traits simply leave more surviving offspring.
- Fitness
- The measure of an individual's reproductive success — specifically, the number of viable offspring it contributes to the next generation, relative to other individuals.
- Fitness is always defined relative to a specific environment. A trait that is highly fit in one environment may be a liability in another.
Three Modes of Natural Selection
Selective pressures can act on a population's trait distribution in three distinct ways. Imagine a population in which a given trait (like body size) shows a normal bell-curve distribution:
Table 1: Three Modes of Natural Selection
| Mode | What is selected for? | Long-term outcome |
|---|---|---|
| Stabilizing Selection | Intermediate (average) phenotypes survive best; both extremes are selected against. | Reduced variation; population converges on the mean. Example: human birth weight. |
| Directional Selection | One extreme phenotype has higher fitness; selection pushes the population in one direction. | Population mean shifts toward the favored extreme. Example: antibiotic resistance, beak size in Darwin's finches. |
| Disruptive Selection | Both extremes have higher fitness than the intermediate phenotype. | Population splits into two subgroups; can lead to speciation. Example: bill size in the black-bellied seedcracker. |
Mutation, Variation, and Long-Term Change
In the short term, selection acts on existing genetic variation. Over longer timescales, new mutations continuously arise and are tested by selection. A mutation that is deleterious under one set of conditions may become beneficial if the environment changes. This is particularly important when a population colonizes a new habitat with novel selective pressures.
When populations become geographically isolated (e.g., after being blown to a new island by a storm), they experience different selective pressures. Over thousands of generations, each isolated population accumulates different mutations and responds to its own unique environment. Given sufficient time, the two populations may diverge enough that they can no longer interbreed — this is the process of speciation.