Download Part 1: The Pace of Evolutionary Change

Part 1: The Pace of
Evolutionary Change
How fast is Evolution?

Evolution does not always occur at the
same rate all the time.

Evidence: Looking at the fossil record, it
shows some groups of organisms
remaining unchanged for millions of
years, while others are rapidly changing
over time.
Gradualism

The pattern of slow and gradual
evolutionary change over long periods of
time.

Populations slowly diverge from one
another due to different selective
pressure:
 Constant allele frequencies
 Constant environmental conditions
 Few chromosomal mutations
Gradualism

Some examples include:
 Cockroaches
 Sharks
Gradualism

Results from transitional forms that are
seen in the fossil record.
 Example: Trilobites
Punctuated Equilibrium

The pattern of long stable periods in
which species stayed much the same.

Interrupted (punctuated) by short periods
in which there was quick evolution, rapidly
resulting in formations of new species.

Some reasons for it include:
 Mass extinction of many life forms
 Rapidly changing environmental conditions
 Exploitation of new environments
Punctuated Equilibrium

Some examples include:
 Galapagos finches
 Cambrian (time period) explosion of animal
types
Punctuated Equilibrium

The fossil record shows rapid bursts of
evolution following mass extinctions.
 Cretaceous extinction = Lots of mammals
What causes a population to
change?
Part 2: Genetic Variation
1) Mutations constantly
occur

Mutations provide the source of new
alleles, or variation upon which natural
selection can act.

Examples:
 Butterfly colour, giraffe’s long necks, rabbits
long ears
2) Immigration and
Emigration Occur

These both affect allele frequencies and
the gene flow in a population.

Examples:
 Immigrants come to Canada, thus affecting
the gene flow. Immigration affecting
phenotype (animals in the wild)
3) Small Populations

Changes have more of an effect on a
smaller population.

Examples:

Founder effect: when a small # of
individuals from a population wander
away to start (found) their own population.

Bottleneck effect: when a population is
reduced to low number due to disease,
predation, climate change, etc.
Founder Effect

See article on Ellis-van Crevald
Syndrome
4) Non-Random Mating
Occurs

This often produces a change in alleles
within populations.

Examples:
 Animals’ sexual selection is intrasexual
(competition within 1 sex like deer) or
intersexual (completion for a mate like
peacocks).
5. Natural Selection Occurs

Only the most fit survive and pass their
genes on to successive generations.

Therefore, change occurs because the
most fit are always being selected for.

Examples:
 Finches, peppered moths
Hardy-Weinberg Equation

This equation can be used to check if a
population is changing.

If the equation is NOT EQUAL to 1, the
population is CHANGING

If the equation is EQUAL to 1, the
population is in equilibrium
The Hardy-Weinberg law states that the
frequencies of alleles in a population will
remain constant unless acted upon by
outside agents or forces. In other words, the
proportion of dominant to recessive genes
remains the same. The Hardy-Weinberg law
describes the genetics of non-evolving
populations. A non-evolving population is
said to be in Hardy-Weinberg equilibrium.
A population will remain in genetic equilibrium if,
and only if, all of the following conditions are
met:
-No mutations occur.
-Individuals neither enter nor
leave the population through migration.
-The population is large.
-Individuals mate randomly.
-Natural selection does not occur.
If we use A and a as an example of alleles, the
Hardy-Weinberg principle is described by the
mathematical equation:
p represents the frequency of the dominant
allele A
q represents the frequency of the recessive
allele a
p2 represent the frequency of the homozygous
dominant alleles: AA
2pq represent the frequency of the alleles
heterozygous condition: Aa
q2 represent the frequency of the homozygous
recessive pair of alleles: aa