Factors affecting variation and evolutionary pathways

1. EVOLUTION AND ADAPTATION
Factors Affecting Genetic Variation

2. Factors which increase variation
1: Mutations
• Permanent change to the nucleotide sequence,
genes or chromosomes of an organism
• May be positive, negative or neutral
• Main source of new alleles in a population
• May occur randomly or result from exposure to an
environmental stimuli (mutagen)

3. Factors which increase variation
2: Gene Flow
• Transfer of alleles or genes from one population to another (of the same
species) by migration
• Immigration: Individuals join a new population – adds alleles to the gene
pool
• Emmigration: Individuals leave current population – gene pool loses alleles
• Reduces specialization to the environment
• Prevents divergence of gene pools and speciation

5. Factors which increase variation
3: Recombination
• Crossing Over: During meiosis sections of
homologous chromosomes align and swap
segments of genes
• Meiosis results in the production of gametes that
are genetically different from each other and the
parent

6. Factors which decrease variation
1: Natural Selection
• Survival of the fittest leads to
accumulation of favourable traits
• Fewer non-adaptive alleles remain in
the population

7. Factors which decrease variation
2: Genetic Drift
• Random change in the variation or allele frequency
of a population
• Most common in small populations
A) Bottleneck Effect
• Event drastically reduces population size
• Survivors are not necessarily “fittest” but live by
chance and pass on genes
• Bottleneck event: natural disaster, disease,
habitat destruction etc.

8. Factors which decrease variation
2: Genetic Drift cont’d
B) Founder Effect
• Few individuals leave original population and begin a new
population
• New population contains only alleles carried in by founders

9. Factors which decrease variation
3: Non-Random Mating
• Some individuals are more likely find a successful mating partner, therefore
more of their alleles will be present in subsequent generations
• Proximity – Easier to mate with nearby organisms
• Competition – “Fitter” organisms are more likely to out compete other
possible mating partners and pass on their traits

11. EVOLUTION AND ADAPTATION
Evolutionary Pathways

12. Evolutionary Pathways:
Convergent Evolution
• Occurs when unrelated organisms evolve similar traits in response to
similar selective pressures
• Produces analogous structures – structures with same function from
different evolutionary origins

13. Evolutionary Pathways:
Convergent Evolution
Example:
Dolphin (mammal) Shark (fish) Ichthyosaurus (reptile)
What were the selective pressures acting on each of these organisms that led to the evolution o
such similar body plans?

14. Evolutionary Pathways:
Convergent Evolution
Example:
Hawk (bird) Bat(mammal) Bee (insect)
What were the selective pressures acting on each of these organisms that led to the evolution o
such similar body plans?

15. Evolutionary Pathways:
Divergent Evolution
• Occurs when closely related organisms evolve
very different traits in response to diverse
selective pressures
• May be the result of disruptive selection
• Leads to speciation
• Produces homologous structures – structures
with different functions arising from a common
ancestor

16. Evolutionary Pathways:
Vestigial Structures
• Remnants of a structure that had a function in an ancestor
but no longer does in the evolved organism.
Examples:
• Appendix – used to be for digestion
• Tail bone
• Snake hips – pelvis bone but no legs
• Chickens have gene for teeth but no teeth