11.2 Genetics, Evolution, and Ecosystems
Key Takeaways
- Genes are sections of DNA, alleles are versions of a gene, and inherited traits depend on allele combinations plus environmental factors.
- A Punnett square shows probability for offspring outcomes; it does not guarantee what must happen in a single birth or seed.
- Evolution by natural selection occurs when heritable variation affects survival and reproduction across many generations, shifting allele percentages.
- Food webs and energy pyramids show energy transfer, while decomposers and matter cycles show carbon, nitrogen, and water moving through living and nonliving parts.
- GED ecosystem questions usually require reading arrows, population graphs, or experimental data before choosing a claim.
Inheritance, Populations, and Ecosystems
GED Science treats genetics and ecology as reasoning topics, not memorization marathons. You may see a short passage about inherited traits, a chart of changing populations, or a food web with arrows. The goal is to use evidence to explain patterns in living systems.
Genetics Basics
DNA (deoxyribonucleic acid) carries genetic information. A gene is a section of DNA related to a trait. An allele is a version of a gene. In simple GED examples one allele may be dominant (shown with a capital letter, A) and one recessive (lowercase, a), but real traits can be more complex and shaped by environment.
Key vocabulary the test reuses:
- Genotype is the allele combination (AA, Aa, or aa).
- Phenotype is the observable trait the genotype produces.
- Homozygous means two identical alleles (AA or aa); heterozygous means two different alleles (Aa).
A Punnett square predicts probabilities. If two parents each have genotype Aa, the possible offspring are AA, Aa, Aa, and aa.
| Offspring Genotype | Count Out of 4 | Probability |
|---|---|---|
| AA | 1 | 25% |
| Aa | 2 | 50% |
| aa | 1 | 25% |
If A is dominant, three of the four boxes show the dominant phenotype, a 75% probability for each offspring, not a promise that exactly 3 of 4 children will show the trait. GED items often test this gap between probability and certainty.
Natural Selection From Data
Evolution is a change in the inherited traits of a population over generations. Natural selection happens when individuals with certain heritable traits survive and reproduce more successfully in a particular environment. Consider data from a fictional insect population sprayed with a pesticide.
| Year | Percent With Resistant Allele |
|---|---|
| 1 | 8% |
| 2 | 15% |
| 3 | 29% |
| 4 | 47% |
| 5 | 63% |
The resistant allele becomes more common. A supported explanation: resistant insects survived exposure and passed the allele to offspring. A weak (trap) explanation: the pesticide instantly rewrote every insect's DNA the same way. Selection acts on existing or newly arising variation and shifts population percentages over time; it does not deliberately re-engineer individuals.
Food Webs and Energy Flow
In ecosystem diagrams, arrows show the direction of energy transfer, pointing from the food toward the consumer that eats it.
| Role | Example | What to Track |
|---|---|---|
| Producer | Grass, algae | Captures energy (usually from sunlight) |
| Primary consumer | Rabbit | Eats producers |
| Secondary consumer | Fox | Eats primary consumers |
| Decomposer | Fungus, bacteria | Recycles matter from remains and waste |
Energy decreases at higher levels; roughly only about 10% transfers up each step because organisms use most energy for life processes and lose it as heat. That is why energy pyramids narrow at the top. Matter behaves differently: carbon, nitrogen, and water cycle continuously through organisms and the physical environment and are not used up.
Reading Ecosystem Evidence
If a graph shows rabbit numbers fall after a drought reduces grass, a reasonable conclusion is that reduced food affected the rabbit population. If fox numbers fall later, the data may suggest the foxes had fewer rabbits to eat. Timing matters: a predator decline after a prey decline supports a food-web explanation, while a predator decline before any prey change does not. GED questions may ask you to pick the best-supported claim, identify a missing organism in a web, or predict the effect of removing one population. Follow the arrows, compare before-and-after data, and never add a cause the stimulus does not mention.
Matter Cycles to Know
While energy flows one way and dissipates as heat, matter is recycled. Two cycles appear most often on the GED:
| Cycle | What Moves | Key Players |
|---|---|---|
| Carbon cycle | Carbon dioxide | Photosynthesis removes it; respiration, decay, and burning release it |
| Water cycle | Water | Evaporation, condensation, precipitation, runoff |
A classic item shows that cutting down a forest raises atmospheric carbon dioxide: fewer producers means less photosynthesis to absorb it. Tie the change back to the cycle rather than guessing.
Reading a Population Graph
Predator and prey populations often rise and fall in linked cycles. Suppose a graph shows hawk and mouse numbers over five years.
| Year | Mice | Hawks |
|---|---|---|
| 1 | 500 | 30 |
| 2 | 800 | 45 |
| 3 | 400 | 60 |
| 4 | 250 | 40 |
| 5 | 450 | 25 |
More mice (years 1-2) support more hawks; the hawk peak in year 3 then drives mice down; with less food, hawks decline by year 5, letting mice recover. The supported claim is a predator-prey feedback relationship. A trap answer reverses cause and effect or blames an unmentioned factor like weather.
Common GED Genetics and Ecology Traps
- Reading a Punnett-square probability as a guarantee for one offspring.
- Saying an environment "chooses to mutate" organisms; selection acts on existing variation.
- Reversing food-web arrows (consumers do not feed producers).
- Confusing energy flow (one-directional, decreasing) with matter cycling (recycled).
Two pea plants with genotype Aa are crossed. Allele A is dominant and allele a is recessive. What is the probability that one offspring will have genotype aa?
A pond food web shows algae -> small fish -> large fish -> heron. If a long stretch of cloudy weather reduces algae growth, which prediction is best supported?