Variation:

● Phenotypic Variation: Differences in features between individuals,

such as height, hair type, intelligence, and blood groups.

● Types of Variation:
o Discontinuous Variation: Clear-cut categories with no in-
betweens (e.g., blood groups: A, B, AB, or O).
o Continuous Variation: Gradual range
of differences (e.g., height, with no fixed
categories).
● Normal Distribution: Continuous variation
often follows a bell-shaped curve, with most
individuals in the middle range and fewer at
the extremes.
● Genetic Variation: Differences between
individuals due to variations in their
genotypes.
● Genes Control Traits: Characteristics like blood groups, hair
colour, eye colour, and height are determined by genes.

Causes of genetic variation:

● Mutation: A sudden change in a gene, creating new alleles and

introducing new traits into the gene pool.
● Chromosomal Mutations: Errors in chromosome separation
during meiosis can lead to conditions like Down’s syndrome (caused
by an extra copy of chromosome 21).
● Factors Increasing Mutation Risk:
o Ionising Radiation: Damages DNA bases, increasing
mutation likelihood, especially in reproductive cells.
o Chemical Exposure: Heavy metals like lead and mercury can
disrupt DNA replication, leading to faulty genetic material.

Adaptive features:

● Adaptive Features: Traits that help organisms survive in their

environment.
● Fitness: A measure of how well-adapted an organism is to its
environment; higher fitness increases chances of survival and
reproduction.

Examples of Adaptive Features

 ● Fish Adaptations:
o Fish have gills to extract oxygen from water.
o Flat, sand-colored fish blend into the seabed for camouflage.
o Predatory fish have streamlined bodies for fast swimming and
sharp teeth for hunting.
o A pygmy seahorse that doesn’t match its surroundings is more
likely to be eaten.

Xerophytes (Plants adapted to live in places with short

supply of water)

● Water Conservation Strategies:

o Closing Stomata: Reduces water loss but also limits
photosynthesis.
o Waxy Cuticle: Makes leaves waterproof to prevent water
loss.
o Hairy Leaves: Trap moist air to reduce evaporation.
o Stomata on Underside: Keeps them cooler to minimize
evaporation.
o Reduced Leaf Surface Area: Less water loss but slower
photosynthesis (e.g., cacti have small leaves or spines).
o Deep or Spreading Roots: Reach deep underground or
spread widely to access water.
o Common Adaptations: Even non-desert plants have some of
these features to survive dry conditions.

Hydrophytes (Water Plant Adaptations)

● No Need to Conserve Water: Unlike desert plants, they don’t

need water-saving adaptations.
● Example - Water Hyacinth:
o Roots float freely in water instead of attaching to the riverbed.
o Hollow stems and leaf stalks filled with air help them float.

Selection:

What is Natural Selection?

Natural selection is the process by which organisms with traits that make
them better adapted to their environment survive and reproduce, passing
those traits to their offspring. Over time, this leads to gradual changes in
populations, making them better suited to their environment.

Darwin’s Theory of Evolution

Proposed by Charles Darwin in his 1859 book On the Origin of Species,
the theory explains how species change over time through natural
selection.

Key Stages of Natural Selection

● Variation: Individuals in a population have slight differences, some

of which provide advantages for survival.
● Over-production: Most organisms produce more offspring than can
survive to adulthood.
● Struggle for Existence: Due to limited resources, competition
occurs among individuals for survival.
● Survival of the Fittest: Only the best-adapted individuals survive
and reproduce.
● Passing on Advantageous Traits: Survivors pass on beneficial
traits to their offspring through their alleles.
● Gradual Change: Over generations, poorly adapted individuals
disappear, and the population becomes better suited to its
environment.
● Genetic Basis of Natural Selection: Selection favours genes that
produce advantageous traits, making them more common in future
generations, while less beneficial genes become less frequent.

This process explains evolution, which is the gradual change in adaptive

features of a population over time due to natural selection.

e.g.
Antibiotic resistance in bacteria:

● Evolution by Natural Selection:

Bacteria can evolve resistance to
antibiotics through natural selection.
● How Resistance Develops:
o Antibiotics, like penicillin, kill
bacteria by preventing cell wall
formation.
o In a large bacterial population, a
rare mutation may occur,
making a bacterium resistant.
o The resistant bacterium survives
while others die and reproduces
rapidly, creating a population of resistant bacteria.
● Impact of Antibiotic Overuse:
o Frequent antibiotic use increases selection pressure, favoring
resistant bacteria.
o Some bacteria, like Staphylococcus aureus, have developed
resistance to multiple antibiotics, leading to difficult-to-treat
infections (e.g., MRSA).
● Solution: Using a variety of antibiotics can help, but overuse may
lead to strains that are resistant to all available treatments.

Stabilising selection:

● No Change in Natural Selection: Natural selection does not

always lead to change; it maintains the best-adapted traits in a
stable environment.
● When Change Occurs: Evolution happens only if the environment
changes or a new beneficial mutation appears.
● Long-Term Stability in Species: Stabilising selection keeps
populations the same across generations.
● So if environment + gene pool stable-> natural selection does not
cause change->populations remain the same between generations-
> can be called stabilising selection
● Examples:
o Peppered Moths: In southwest Britain, the environment has
remained stable, with pale moths staying well-
camouflaged.Dark moths, resulting from mutations, are
disadvantaged and rarely survive.
 o Coelacanths, a deep-sea fish species, have remained
unchanged for 350 million years due to their stable
environment in the Indian Ocean.

Selective breeding:

● What is Artificial Selection?

o Artificial selection is a process where humans selectively

breed organisms with desired traits.
o It works similarly to natural selection, but instead of nature
determining which traits are advantageous, humans make the
selection.
● How It Works:
o Breeders choose individuals with preferred characteristics
(e.g., high milk yield in cattle) and allow only them to
reproduce.
o Repeating this over generations makes the selected traits
more common in the population.
● Shifting Priorities in Farming:
o Some farmers now prioritize traits like resistance to pests and
efficient growth with less fertiliser over maximum yield.
o Older breeds with these traits are being reintroduced for
breeding.
● Limitations of Artificial Selection:
o Traits chosen by humans may not help the organism survive
in the wild.
o Example: Modern cattle, bred for milk or meat production,
would struggle to survive without human care.
● Historical Context:
o Humans have used artificial selection for thousands of years in
domesticating plants and animals.