Welcome to Genetic Diversity and Adaptation!
Hi there! In this chapter, we are going to explore why living things aren't all exactly the same and how those differences help them survive in a changing world. This is the heart of evolution—understanding how "nature" picks the best traits to keep a species going. Don't worry if it sounds a bit "big" at first; we’ll break it down into simple steps!
1. What is Genetic Diversity?
Before we look at how animals and plants change, we need to understand what makes them different in the first place. This is called genetic diversity.
Definition: Genetic diversity is the number of different alleles of genes in a population.
A Quick Refresh on Terms:
- A gene is a section of DNA that codes for a characteristic (like eye color).
- An allele is a different version of that gene (like blue eyes vs. brown eyes).
- A population is a group of organisms of the same species living in the same place.
The Analogy: Imagine a deck of playing cards. If every card in the deck was the Ace of Spades, there is zero diversity. If you have a full deck with different numbers and suits, you have high "card diversity." Genetic diversity is just like having a "full deck" of different alleles in a population.
Why is it important?
If a population has high genetic diversity, it is more likely that at least some individuals will have the right "tools" (alleles) to survive if the environment changes (e.g., a new disease or a colder winter). Genetic diversity is the factor that enables natural selection to occur.
Quick Review Box:
- More alleles = Higher genetic diversity.
- Higher diversity = Better chance of the species surviving environmental change.
Key Takeaway: Genetic diversity is the variety of alleles in a group. Without this variety, evolution couldn't happen!
2. The Principles of Natural Selection
Natural selection is the process by which species become better adapted to their environment. It doesn't happen because an animal "tries" to change; it happens because of a specific chain of events.
Step-by-Step: How Natural Selection Works
- Random Mutation: A random change in the DNA base sequence occurs, which can result in new alleles of a gene.
- Benefit or Harm: Many mutations are harmful, but in certain environments, a new allele might actually benefit its possessor (e.g., better camouflage).
- Increased Reproductive Success: The individuals with the advantageous allele are more likely to survive and reproduce successfully.
- Inheritance: These survivors pass on their advantageous allele to the next generation.
- Increased Frequency: Over many generations, the frequency of this "good" allele increases in the population.
Memory Aid: Think of the mnemonic "M-S-R-P":
- Mutation (creates new allele)
- Survival (of the fittest)
- Reproduction (passing it on)
- Passed on (allele frequency increases)
Did you know? Mutations are completely random! An organism can't "choose" to mutate to survive a drought; the mutation either exists by chance or it doesn't.
Key Takeaway: Natural selection is a cycle where mutations lead to better survival, which leads to more babies with the same "good" genes.
3. Types of Selection
Not all selection looks the same. The AQA syllabus focuses on two main types: Directional and Stabilising. Don't worry if the graphs seem tricky; focus on what is happening to the "average" individual.
Directional Selection
This happens when the environment changes and the "extreme" phenotype is favored. The whole population "moves" in one direction.
Example: Antibiotic resistance in bacteria.
1. Most bacteria are killed by penicillin.
2. A random mutation gives one bacterium an allele to resist penicillin.
3. When penicillin is used, the normal ones die, but the resistant one survives and multiplies.
4. Soon, the "average" bacterium in that population is resistant. The graph has shifted toward the resistance extreme.
Stabilising Selection
This happens when the environment is stable. It favors the "average" individuals and selects against the extremes.
Example: Human birth weights.
- Very small babies are less likely to survive (higher risk of infection/cold).
- Very large babies are less likely to survive (difficulties during birth).
- "Middle-sized" babies have the best chance. Over time, most babies are born within a narrow weight range. The graph gets taller and thinner in the middle.
Common Mistake to Avoid: Don't say "the animal adapts." Individual animals do not adapt; populations adapt over many generations through selection.
Key Takeaway: Directional selection changes the average (shifts the graph). Stabilising selection keeps the average (narrows the graph).
4. Adaptations
As a result of natural selection, organisms develop adaptations. These fall into three categories:
- Anatomical: Physical features of the body (e.g., a giraffe’s long neck or a cactus’s spikes).
- Physiological: Internal processes or chemical changes (e.g., a desert rat producing very concentrated urine to save water, or bacteria producing enzymes to break down antibiotics).
- Behavioural: The way an organism acts (e.g., birds migrating south for winter or a possum "playing dead" to avoid predators).
Quick Review Box:
- Anatomical = Look/Shape
- Physiological = Inside/Chemical
- Behavioural = Actions
Key Takeaway: Adaptations can be physical, chemical, or behavioral—all designed to help the organism survive and reproduce.
5. Required Practical 6: Testing Antimicrobial Substances
In this section of the course, you need to understand how we test how well "antimicrobials" (like antibiotics or disinfectants) work. This links to Directional Selection because if a substance doesn't kill the bacteria, it might be because they have evolved resistance.
Key Concept: Aseptic Technique
When working with bacteria, we use aseptic techniques. This just means "sterile" methods to prevent contamination of your plate or yourself.
Steps for Success:
1. Disinfect the work surface.
2. Work near a Bunsen burner flame (the hot air rises, pushing microbes away).
3. Flame the neck of the glass bottles and the metal loops used to move bacteria.
4. Only open the Petri dish lid slightly (the "clam-shell" method).
5. Sterilise everything after use.
Key Takeaway: Aseptic technique is all about keeping your experiment clean and safe so you know exactly which bacteria you are growing.
Final Summary Checklist
- Can you define genetic diversity? (Variety of alleles in a population).
- Can you explain natural selection? (Mutation -> Advantage -> Survival -> Reproduction -> Allele frequency increases).
- Do you know the difference between Directional and Stabilising selection? (Directional moves the average; Stabilising keeps it).
- Can you name the three types of adaptations? (Anatomical, Physiological, Behavioural).
- Do you know why we use aseptic technique? (To prevent contamination).
You've got this! Keep reviewing these steps, and the patterns of nature will start to make perfect sense.