Factors affecting variation and evolutionary pathways

EVOLUTION AND ADAPTATION

Factors Affecting Genetic Variation

Factors which increase variation1: 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)

Factors which increase variation2: 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

Factors which increase variation3: 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

Factors which decrease variation1: Natural Selection• Survival of the fittest leads to

accumulation of favourable traits• Fewer non-adaptive alleles

remain in the population

Factors which decrease variation2: 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.

Factors which decrease variation2: Genetic Drift cont’dB) Founder Effect• Few individuals leave original population and begin a new

population• New population contains only alleles carried in by

founders

Factors which decrease variation3: 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

EVOLUTION AND ADAPTATION

Evolutionary Pathways

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


Example:Dolphin (mammal) Shark (fish) Ichthyosaurus (reptile)

What were the selective pressures acting on each of these organisms that led to the evolution of such similar body plans?


Example:Hawk (bird) Bat(mammal) Bee (insect)

What were the selective pressures acting on each of these organisms that led to the evolution of such similar body plans?

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

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

Factors affecting variation and evolutionary pathways

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