11.2 Genetics, Evolution, and Ecosystems
Key Takeaways
- Genes are sections of DNA, alleles are versions of genes, and inherited traits depend on allele combinations and environmental factors.
- Punnett squares show probability for offspring outcomes; they do not guarantee what must happen in a single birth or seed.
- Evolution by natural selection occurs when inherited variation affects survival and reproduction across many generations.
- Food webs and energy pyramids show energy transfer, while decomposers and cycles show matter moving through living and nonliving parts of ecosystems.
- 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. You may see a short passage about inherited traits, a chart of changing populations, or a food web with arrows. The goal is not to memorize every species or gene. The goal is to use evidence to explain patterns in living systems.
Genetics Basics
DNA carries genetic information. A gene is a section of DNA related to a trait. An allele is a version of a gene. In many simple GED examples, one allele may be dominant and one may be recessive, but real traits can be more complex and can also be affected by environment.
A Punnett square predicts probabilities. If two parents each have genotype Aa, the possible offspring combinations 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 trait. That means a 75% probability for each offspring, not a promise that exactly 3 of 4 children will show the trait. GED questions often test this difference between probability and certainty.
Natural Selection From Data
Evolution is a change in inherited traits in a population over generations. Natural selection happens when individuals with certain inherited traits survive and reproduce more successfully in a particular environment.
Consider this data from a fictional insect population exposed to 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 is that insects with resistance were more likely to survive pesticide exposure and pass the allele to offspring. A weak explanation would be that the pesticide instantly changed every insect's DNA in the same way. Selection acts on existing or newly occurring variation and changes population percentages over time.
Food Webs and Energy Flow
In ecosystems, arrows usually show the direction of energy transfer, often from food to consumer. Producers, such as plants and algae, capture energy. Primary consumers eat producers. Secondary consumers eat primary consumers. Decomposers break down dead matter and wastes, returning materials to the environment.
| Role | Example | What to Track |
|---|---|---|
| Producer | Grass | Source of food energy for consumers |
| Primary consumer | Rabbit | Eats producers |
| Secondary consumer | Fox | Eats primary consumers |
| Decomposer | Fungus | Recycles matter from remains and waste |
Energy decreases at higher levels because organisms use much of the energy they take in for life processes. Matter is different. Carbon, nitrogen, and water cycle through organisms and the physical environment.
Reading Ecosystem Evidence
If a graph shows that rabbit numbers fall after a drought reduces grass, a reasonable conclusion is that food availability affected the rabbit population. If fox numbers also fall later, the data may suggest that the foxes had fewer rabbits to eat. The timing matters. A predator decrease after a prey decrease can support a food-web explanation.
GED questions may ask you to identify the best-supported claim, the missing organism in a food web, or the effect of a change in one population. Follow the arrows, compare before-and-after data, and avoid adding causes that the stimulus does not mention.
Two pea plants with genotype Aa are crossed. The allele A is dominant and the 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 period of cloudy weather reduces algae growth, which prediction is best supported?