Transcript 05 Evolutionary Mechanisms

Hardy-Weinberg
Principle
SBI3U
Evolutionary Change
Gene pool: the genetic information for an
entire population
 Allele frequency: the proportion of gene
copies of a given allele in a population.

Evolution

Evolution occurs when the allele frequencies
(gene pool) of a population change over time.
Hardy –Weinberg Equilibrium

In large populations where only random
chance is at work, allele frequencies are
expected to remain constant from
generation to generation.
Remaining constant = EQUILIBRIUM

However, there are ‘disturbing factors’ that
cause allele frequencies to change, which
lead to evolutionary change
EVOLUTIONARY
MECHANISMS
AKA
“Disturbing Factors”
1. Small vs. Large Populations

Genetic Drift: When a population is small,
chance fluctuations can cause changes in allele
frequencies.

Could become the dominant trait of the
species.

Could be wiped out.
1. Small vs. Large Populations

Bottleneck effect: drastic reduction in
numbers, small sample of alleles survives, new
generation different from original

Northern elephant seal (reduced to 20
individuals in 1890s…)
1. Small vs. Large Populations

Founder effect: a few individuals leave to
establish a new population
 Ex. Seeds/insects carried by wind, birds to
another island
More Founder Effect Examples

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Self-pollinating plants
Amish community in Pennsylania (30 emigrants from
Switzerland, 1720)
Ground finches in the Galapagos (1982-1983, a new
population developed – discovered by scientists at
Queen’s)
2. Random vs. Non-random
Mating

When mating opportunities are non-random,
individuals that are preferred as mates will pass on
their alleles in greater numbers than less preferred
mates.
3. Genetic Mutations

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Genetic mutations create new alleles or
change an existing one into another, thereby
changing the frequency of both alleles.
Gene duplications are the main source of new
genetic material, as extra copies they are free
to mutate with less likelihood of causing harm.
Mutations occur as 1 in 10000 in a small
genome (bacteria) to about 1 or more per
gamete in larger genome.
4. Migration

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
When individuals migrate, this alters the allele
frequency of both the population it left and the one it
joined.
Gene flow: movement of alleles from one population
to another.
Tends to reduce differences between populations
5. Natural Selection

When natural selection occurs, individuals with
certain alleles have greater reproductive success
than others do, thereby increasing the relative
frequency of their alleles in the next generation.

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Harmful genes selected against
Useful genes accumulate
In Nature

In reality, the conditions for Hardy- Weinberg
equilibrium (no disturbing factors, therefore only
random chance) are NEVER met

There are ALWAYS disturbing factors

HW can be approximated in the lab

It has usefulness as a model for studying real
populations