Download 11. The roles of genes and environment in evolution

Title:
The roles of genes and environment in evolution
12th February 2014
Learning question: What factors affect evolution?
Homework: Module test next week – study this unit
Starter: what do these two organisms have in common?
Learning Outcomes
• Explain, with examples, how environmental
factors can act as stabilising or evolutionary
forces of natural selection.
• Explain how genetic drift can cause large
changes in small populations.
Variation and Natural Selection
• The set of alleles in a population is it’s gene pool
• Each individual can have any combination of alleles in
the gene pool
–
–
–
–
producing variation
Some individuals more likely to survive
They reproduce and pass genes on to offspring
Advantageous alleles become more frequent in the
population
Environmental Resistance
• Environmental factors that limit the growth of a population
offer environmental resistance
• These factors can be biotic (living factors) or abiotic (nonliving factors)
Selection pressures
• An environmental factor that “selects” for some members of a
population over others
• Offers an advantage onto certain individuals
Discuss!
• Explain to your neighbour what environmental
resistance and selection pressures are.
• If the person speaking is incorrect, the partner
must explain the correct term and give an
example.
Stabilising Selection
• If the environment stays stable
• The same alleles will be selected for in successive
generations
• Nothing changes, this is called stabilising selection
Stabilising Selection
Stabilising Selection
Directional Selection
• Change in the environment resulting in a change in the
selection pressures on the population
• Previously disadvantageous alleles maybe selected for
• Change in the genetically determined characteristics of
subsequent generations of the species
• A.k.a. evolution
Directional Selection
Directional Selection
Discuss!
• Explain to your neighbour what stabilising
selection and directional selection are.
• If the person speaking is incorrect, the partner
must explain the correct term and give an
example.
Genetic Drift
• A change in the gene pool and characteristics within the
population.
• This change has occurred by chance rather than as the result
of natural selection.
Genetic Drift and Islands
• Genetic drift is thought to happen relatively
frequently in populations on islands.
– Small populations
– Geographically separated from other members of
their species
• Evidence
– Many isolated islands have their own endemic
species of plants and animals
Genetic Drift
• Reduces genetic variation
• Reduce the ability of the population to survive in
a new environment
• May contribute to the extinction of a population
or species
• Could lead to the production of a new species
Genetic Drift – Frog Hoppers
• The colours of the common
frog-hopper are determined
by seven different alleles of
a single gene.
• The range of colours and
their frequencies, on
different islands in the Isles
of Scilly, are very variable,
• There are different selection
pressures on the different
islands
Genetic Drift – Frog Hoppers
The answers
Your task
• Complete questions 1-4 from page 141 of your
textbook.
• Correct your work using the mark scheme at
the back of the book.
Natural selection video