Transcript Evolutionary Change without Selection

Evolutionary Change
without Selection
Natural Selection
• The way nature selects for some traits over
others
• Selection is NOT random
• BUT evolutionary change doesn’t always happen
this way
Change without Selection
• Changes within a
gene pool can occur
without the
influence of
successful traits
• These changes tend
to reduce genetic
diversity within a
population
Change without Selection
• Change without selection occurs through
genetic drift, the bottleneck effect, and the
founder effect.
• Allele: a variation of a gene
Genetic Drift
• Genetic drift: The random shifting of the
genetic make-up between generations
– This shifting is entirely due to chance
– So some generations may see more of a particular
feature, and others may see less of that feature
Genetic Drift and Population
• Population size has a major effect on genetic
drift
• Large populations tend to produce a
stabilizing effect on alleles
Genetic Drift and Large
Populations
• So in large populations,
we expect to see a lot of
genetic variability
• One trait may be more
common in a generation,
and less common in
another generation
• Overall though, genetic
variability remains fairly
constant
Genetic Drift and Population
• A particular trait is more likely to become very
common or to disappear in a small population
Genetic Drift and Small
Populations
• So small populations
tend to have less
genetic variation
• Certain features
become more
common within that
population, and
certain features are
lost
Variety and Large
Populations
• So in order to maintain a high level of genetic
variability, a large population is best
• Features remain in the population at a fairly
constant level
Genetic Bottlenecks
• Bottleneck: A dramatic
reduction in population
size, usually resulting in
significant genetic drift,
due to catastrophe
• A population of 10000
that suddenly drops to
50 will experience a
significant genetic
bottleneck
How Does it Happen?
• A population reduction can occur in any
number of ways:
– Overhunting
– Habitat loss
– Natural disaster
How Does it Happen?
• The surviving
population is likely to
have a smaller gene
pool than the initial
population
• These particular traits
will be passed along
• In this way, certain
traits become
favoured, while certain
alleles disappear
As the Population Recovers
• If the population is allowed to recover, that
population will be limited to those few traits
left as a result of the bottleneck
Results of a Bottleneck
• Overall, a bottleneck reduces the size of the gene pool, and
genetic variety
• This is generally harmful to the remaining population
– Increased vulnerability to disease
– Low reproductive success
– Increased infant mortality
Cheetahs
• The cheetah species
suffered a severe
bottleneck about
10,000 years ago
• As few as 7
cheetahs survived,
and all cheetahs
today are
descendents of
these
Elephant Seals
• Population
reduced to as few
as 20 in the 1890s
• Today over
125,000 exist, but
all are genetically
very similar
The Founder Effect
• The Founder Effect:
occurs when a small
number of
individuals separate
from their original
population and
establish a new
population
The Founder Effect
• The founding population would have a unique mix of
alleles different than the larger original population
• By chance, some alleles that are less common in the
larger population will be more common in the
founding population
Frequency Differences
• Perhaps a rare variant of a gene occurs once in
every 10000 people in the large population
• But in the founding population, that trait occurs
in 1 of the 20 founders
• Original Population = 1/10000 (0.1%)
• Founding Population = 1/20 (5%)
• This represents an 50x increase in the frequency
for that variant
Reduced Frequencies
• The founder effect leads to a population with
a new, smaller, gene pool
• We can expect less variation within the new
population
Founder Effect
Founder Effect
Founder Effect
Founder Effect
Founder Effect
The Galapagos Tortoise
• The entire population
is the result of as few
as one pregnant
female tortoise from
South America
• Her genes, and those
of her offspring,
formed the entire
gene pool for the new
Galapagos Tortoise
population
The Amish Community
• A closed-off religious
community that
shuns the modern
world
• Isolation has created
a smaller gene pool
within the Amish
community
Bottlenecks vs. Founder
Effect
Bottleneck
Founder Effect
Effect on Gene Pool
Smaller
Smaller
Results in Genetic
Drift?
Yes
Yes
How does it
Happen?
Catastrophe
Some individuals
leave the population
to start their own
Examples
Cheetahs, Elephant
Seals
Galapagos Tortoise,
Mennonites
A Mathematical Model for
Evolution
• We can think of evolution as a change in the
genetic makeup of a population
• We can also think about evolution from a
math perspective:
– The frequency of each allele in the population
• Can we predict the relationship between allele
frequency and the chances of those
frequencies remaining constant?
The Hardy-Weinberg
Principle
• The Hardy-Weinberg Principle: states that in
large populations where only random chance
is at work, alleles should stay constant
between generations
The Hardy-Weinberg
Principle
• Other factors will result in evolution, as these will lead to
changes in variant frequencies
–
–
–
–
–
Natural selection
Small population size
Mutation
Immigration or Emigration
Transfer of new alleles from a different species
Predicting Change
• By considering
these factors, we
can predict which
populations are
most likely to
undergo the most
evolutionary
change
Applying the Principle to
Different Species
• Some species will undergo evolutionary
change faster than others
• A species with
– High genetic diversity
– Fast reproduction
will respond to natural selection more quickly
• This is why bacteria develop resistance to
antibiotics so quickly
Homework Questions
• Page 335
• # 9, 10, 13