Investigating Selection (AQA A-Level Biology): Revision Notes
Investigating Selection
Natural selection affects different populations in various ways, and understanding these patterns helps us investigate how organisms adapt to environmental pressures. This topic explores how you can apply selection principles to real organisms like bacteria and study these effects in laboratory conditions.
Types of natural selection and frequency patterns
Natural selection modifies allele frequency within populations over time. The type of selection that occurs depends on which characteristics provide survival advantages in specific environments.
Directional selection
Directional selection occurs when individuals possessing alleles for extreme characteristics have improved survival and reproductive success. This selection type favours one end of the characteristic range, shifting the population towards that extreme.
Worked Example: Antibiotic Resistance in Bacteria
Antibiotic resistance provides a clear example of directional selection in action:
- Within a bacterial population, some individuals carry alleles conferring resistance to antibiotics
- When exposed to antibiotic treatment, bacteria lacking resistance die
- Resistant bacteria survive and reproduce without competition, passing resistance alleles to offspring
- Over time, most population members carry the antibiotic resistance allele
The frequency distribution shifts from a normal curve towards the extreme (high resistance), demonstrating how environmental pressure creates directional change.
Stabilising selection
Stabilising selection favours individuals with characteristics near the population average. This occurs in stable environments where extreme variations reduce survival chances, leading to a narrower range of characteristics.
Worked Example: Human Birth Weight
Human birth weight demonstrates stabilising selection effectively:
- Humans show variation in birth weight across a normal distribution
- Very small babies struggle with temperature regulation and survival
- Very large babies create delivery difficulties, reducing survival prospects
- Medium-sized babies experience optimal survival conditions
Environmental conditions favour average birth weights, causing the population to maintain this middle range while reducing extreme variations.
Interpreting selection data
When examining unfamiliar species data, you need to identify selection patterns and suggest underlying causes.
Worked Example: Rabbit Fur Length Analysis
Consider data showing rabbit fur length changes over six years during cold winters:
- Average fur length gradually increased from 21mm to 24mm
- This pattern suggests directional selection favouring longer fur
- Longer fur provides better insulation during cold periods
- Rabbits with shorter fur were less likely to survive harsh winters
- Over successive generations, longer fur alleles became more frequent in the population
When interpreting selection data, look for:
- Direction of change (towards extremes or middle values)
- Environmental factors that might influence survival
- How the characteristic affects organism survival or reproduction
Investigating antibiotic effects on bacterial growth
You can investigate how antimicrobial substances affect bacterial growth using controlled laboratory methods.
Testing antibiotics using agar plates
The standard method involves comparing bacterial growth in the presence and absence of different antibiotics:
- Grow bacteria in liquid broth containing distilled water, bacterial culture, and nutrients
- Transfer bacteria to agar plates (Petri dishes with agar jelly) using sterile equipment
- Place paper discs soaked with different antibiotics at separate locations on the plate
- Include a negative control disc soaked only in sterile water
- Incubate the sealed plate at 25°C for 48 hours
- Observe results - bacterial growth appears as a visible lawn, while inhibition zones appear as clear patches around effective antibiotics
The inhibition zone size indicates antibiotic effectiveness - larger zones show stronger bacterial inhibition.
Aseptic techniques for contamination prevention
Aseptic techniques prevent contamination by unwanted microorganisms that could affect experimental results or pose health risks.
Essential aseptic practices include:
- Disinfect work surfaces regularly and avoid placing utensils on contaminated areas
- Use sterile equipment and dispose of materials safely after use
- Sterilise glassware using autoclave (high pressure steam) or other approved methods
- Work near a Bunsen burner flame to create sterile air currents that draw contaminants away from cultures
- Minimise exposure time by keeping agar plate lids closed when possible
- Flame sterilise glass container necks when opening and closing broth cultures to prevent airborne contamination
These techniques ensure reliable results by maintaining pure bacterial cultures throughout the investigation.
Data analysis and evaluation
When investigating selection, you must interpret graphical data and suggest explanations for observed patterns. Look for:
- Changes in characteristic frequency over time
- Correlation between environmental conditions and trait variation
- Evidence supporting specific selection types
- Possible alternative explanations for observed changes
Practice identifying whether data shows directional, stabilising, or other selection patterns, then justify your conclusions using biological principles.
Key Points to Remember:
- Directional selection shifts populations towards extreme characteristics when environmental pressures favour one end of the range
- Stabilising selection maintains average characteristics in stable environments by selecting against extremes
- Antibiotic resistance demonstrates directional selection as resistant bacteria gain survival advantages
- Aseptic techniques prevent contamination and ensure valid results in microbial investigations
- Always interpret selection data by considering environmental factors that influence survival and reproduction