Download Variation and Evolution notes

Role of Variation in Evolution
4/28 Meiosis and Variation
•Evolution can only take place if there is
variability in a population.
•There are two ways this can happen.
1 – Mutations:
• Are a direct source of variation.
• Are rare:
• Relatively small amount of mutations
• Cause physical change to the gene
• Change in genotype
• Possible change in phenotype
• Excessive mutations have a negative impact on continuity.
• Otherwise, it is a critical process for generating
diversity in organisms.
• Environmental factors can affect mutations.
• Only mutations in gametes are passed on to
offspring.
2) Recombination
• This is a new combination of alleles during meiosis
where most genetic variation occurs
• Three processes:
1) Segregation of alleles and Independent
Assortment
• Recombination can happen during Prophase I when
alleles separate from each other.
• Members of a chromosome pair move to different of
the cells independently of each other. Independent
Assortment
3) Crossing Over
• In Prophase I, homologous pairs of chromatids (one
from mother and one from father) line up.
• The arms of the pairs intertwine (cross-over) .
• Pieces of chromosome partners exchange places and
each has a new combination of genetic material.
Meiosis is not always perfect
• Can lead to fatal mistakes
• Non-disjunction
• Chromosome pairs do not separate
• Half the cells ends up with more chromosomes and
half have nothing.
• Happens with first and second divisions