Understanding Variation and Selection

 

What is Variation?

• Variation refers to the differences that exist between individuals of the same species.
• It's like how people in a class might have different heights, hair colors, or blood types.
• Phenotypic variation refers to the differences in features between individuals of the same species.

Types of Variation

1. Continuous Variation

   • Definition: Traits that show a range of values between two extremes. This variation can usually be arranged in order from one extreme to the other.
   • Examples:
     • Height in humans.
     • Body mass in animals.
   • Characteristics:
     • Affected by genes and environment.
     • Plotted as a smooth curve (e.g., a bell curve).

2. Discontinuous Variation

   • Definition: Traits with distinct categories and no intermediates. These traits are either/or and cannot be arranged in order.
   • Examples:
     • Blood groups (A, B, AB, O).
     • Seed color in peas (green or yellow).
   • Characteristics:
     • Determined by genes only.
     • No overlap between categories.

 

Continuous vs. Discontinuous Variation

Feature	Continuous Variation	Discontinuous Variation
Definition	Range of values between extremes	Limited categories, no intermediates
Examples	Height, body mass	Blood groups, seed color
Causes	Genes and environment	Genes only
Graph Shape	Bell curve	Bar chart

 

What Causes Variation?

1. Genetic Causes:

   • Different genes (alleles) lead to different traits.
   • Mutation: A genetic change that creates new alleles.

2. Environmental Causes:

   • Conditions like diet, sunlight, or exercise affect traits like body weight or skin color. These can cause individuals to have different body structures and look different despite being identical genetically i.e twins.

3. Combination:

   • Traits like height are influenced by both genes (e.g., tall parents) and environment (e.g., nutrition).

 

Understanding Mutation

What is Mutation?

• A random change in the DNA sequence.
• It can happen naturally or due to external factors.
• Most mutations don't cause an effect on the phenotype as a protein that a mutated gene produces can be as effective as one that hasn't mutated
• At times the mutation can cause negative changes that could be dangerous and maybe fatal. One example is sickle cell anaemia
• It is rare for a mutation to cause a new allele and therefore a new phenotype to be formed
• Should a mutation happen they are usually small overall affects on the organism

Key Points About Mutation:

1. Mutations have the ability to create new alleles (e.g., different eye colors).
2. Mutations are usually random, but they can happen more often if exposed to certain things for example:
   • Ionizing radiation (e.g., X-rays).
   • Chemicals (e.g., tobacco smoke).

Types of Mutation:

Type	Description	Example
Gene Mutation	A random change in the DNA base sequence	Sickle cell anemia

 

How Does Variation Arise in Populations?

1. Mutation: New alleles arise.
2. Meiosis: Shuffles genes during gamete formation.
3. Random Mating: Each parent contributes different genes.
4. Random Fertilization: Combines random gametes (e.g., sperm + egg).