Unit 18 - Variation & Selection
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Variation
The differences between individuals of the same species.
Phenotypic Variation
The difference in features between individuals of the same species.
What causes variation?
Genes and the environment
What are the two types of phenotypic variation?
Continuous variation, Discontinuous variation
Continuous Variation
When there are many small degrees of difference for a particular characteristic. (Can usually be measured)
Discontinuous Variation
When there are distinct differences for a characteristic.
Continuous Variation Graph
Smooth bell curves.
Discontinuous Variation Graph
Step-shaped.
What causes phenotypic variation?
Genetic and Environmental
Which type of variation is affected by both genes and the environment?
Continuous Variation
Which type of variation is only affected genetically?
Discontinuous Variation
Mutation
Genetic changes.
Mutations happen continuously and spontaneously, but their frequency can be increased by…
• Gamma rays • X-rays • Ultraviolet rays • Certain types of chemicals.
What happens to cells with increased rates of mutation?
They become cancerous.
Sources of Genetic Variation in Populations
• Mutations • Meiosis • Random mating • Random fertilization
Adaptive Features
The inherited functional features of an organism that increases its fitness.
Fitness
The probability of an organism surviving and reproducing in the environment in which it’s found.
Hydrophytes
Plants adapted to live in extremely wet conditions.
What are the adaptations of hydrophytes?
• Large air spaces in leaves to keep them close to the water surface (where there’s more light for photosynthesis). • Small roots for extracting nutrients from surrounding water. • Stomata concentrated on the upper epidermis for easier gas exchange.
Xerophytes
Plants adapted to live in extremely dry conditions.
What are the adaptations of xerophytes?
• Thick waxy cuticle to cut down water loss. • Small leaves that reduce surface area and hence evaporating area. • Extensive shallow roots for quick absorption of lots of water when it rains.
What causes natural selection?
How, in any environment, individuals that have the best adaptive features are the most likely to survive and reproduce.
Natural Selection
Survival of the fittest.
When some individuals develop a phenotype (or mutation) they may get a survival advantage and will therefore…
• Live longer • Breed more • Be more likely to pass on their genes