Transcript Lesson 5 Mechanisms of evolution - Blyth-Biology11

Mechanisms of evolution
Lesson 5
Darwin’s Theory
Darwin summarized natural selection in
these words.
“can we doubt (remembering that many
more individuals are born than can
possibly survive) that individuals
having any advantage, however slight,
over others, would have the best
chance of surviving and procreating
their kind?”
Source of Variation
• Darwin admitted that a weakness in his
theory was that he could offer no
explanation for the source of variation.
• Six years after Darwin, Gregor Mendel
presented his work on garden peas.
• Now we understand mutation and
recombination is the source of new
inheritable variations.
Key Terms
• Natural Selection: the way in which nature
favours the reproductive success of some
individuals with in a population over others
• It is the survival of the fittest – the organisms
that are best able to adapt to the environment
will survival and reproduce.
• Artificial Selection – directed breeding of
animals/plants that exhibit a particular trait
Mechanisms of Evolution
• Gene pool is the complete set of all alleles
contained within a species or population
• Not all evolutionary changes are the result of
natural selection
• Evolution can occur due to catastrophic events
• Mutation is the ultimate source of variation in an
individual’s gene pool
Random Change
• Evolution defined in genetic terms as
any change in gene (and allele)
frequencies within a population or
species.
Key factors in a Change to a Gene Pool
1. Genetic Drift; chance fluctuations cause change
2. Non-random mating opportunities; preferred
individuals pass on their alleles in greater numbers
than others
3. Genetic mutation; new alleles are created or alleles
are changed
4. Migration - removes alleles from the population
5. Natural Selection; individuals with certain alleles have
greater reproductive success than others increasing
the relative frequency of their alleles
Genetic Drift
• When populations are small – chance can
play a significant role in altering allele
frequencies
• Change in the genetic makeup of a
population resulting from chance is called
genetic drift
• Genetic drift can lead to a fixation of
alleles
Genetic Drift in Small
Populations
Genetic Drift – Coin Flip analogy
• Take a loonie. And write a capital letter for
a dominant allele and a lower case letter
for the recessive allele
• What should be the ratio of T : t?
• Perform 5 flips and record the results
• Perform 25 flips and record the results
Bottleneck Effect
• When a severe event results in a drastic
reduction in numbers, a population may
experience a bottleneck effect
• A very small sample of alleles survive to
establish a new population
• Relative frequencies may differ from
original population
Bottleneck Effect
Founder Effect
• When a few individuals from a large
population leave to establish a new
population
• The allele frequencies of the new
population will not be the same as the
original population
Polydactyly
Dominant trait
Philadelphia Pennsylvania Amish communities were founded
in in 1700s by few families
http://www.youtube.com/watch?v=9x8lFXgXmZI
Gene Flow
• When organism migrate, leaving one
population and joining another, they alter
the allele frequencies of both
• In this way, genetic information is shared
between populations.
• Unlike genetic drift, gene flow tends to
reduce between populations.
Natural Selection
• Small populations that result from a bottleneck or
founder effect are also subject to the effects of genetic
drift
• This increases the chances that their gene pool will differ
from that of the original population
• Although genetic drift and bottlenecks can be important
in some cases, natural selection is usually the major
driver behind changes that result in the evolution of a
significant adaptation
• Natural selection is the only mechanism known that is
able to shape a species to its environment
Stabilizing Selection
-Favours an ____________ phenotype
- Acts against __________ variants of
the phenotype
The most common phenotype (the
intermediate phenotype) is made
more common in the population by
removing the extreme forms.
This type of selection:
- Reduces ______________
- Improves adaptation of the
population to aspects of the
environment that remain constant
Directional Selection
- Favours the phenotypes at one
________________ over the other
- Common during times of
environmental__________ or when a
population ___________ to a new
habitat that has different
environmental conditions and niches
to exploit
Examples:
- The changes in coloration of
peppered moths
- Antibiotic resistance in infectioncausing bacteria
Disruptive Selection
- Takes place when the __________ of
a range of phenotypes are favoured
over intermediate phenotypes
- Thus, intermediate phenotypes can
be eliminated from the population
Example: The extreme size differences of
mature male coho salmon. The smaller
phenotype averages 500g, and the larger
phenotype averages 4500g. This difference in
size reflects the means by which each
phenotype gains access to females. The
smaller specialize in a “sneaking” technique
to fertilize eggs. The larger specialize in
fighting for access to eggs.
Practice Problem:
Identify the Type of Selection
The graph below shows a representation of the changes
that have occurred in a population of bentgrass over a
period of time.
Use your understanding of the three types of natural
selection (stabilizing, directional, disruptive) to identify
the type of selection that is acting on this bentgrass
population.
Sexual Selection
The different phenotypes of the male coho
salmon are also a specific example of
natural selection referred to as sexual
selection.
In general, sexual selection involves:
- Competition between males through
combat or visual displays
- Females choosing their mates
Sexual Selection
Males and females of many animal species often
have very different __________ characteristics,
such as colourful plumage in male birds and
antlers in male deer.
This difference between males and females is
called ________ _____________
Doesn’t he look fit?
Is mate choice in humans based on
smell?
http://www.youtube.com/watch?v=-drpViV5LSw
http://www.sciencedaily.com/releases/2009/04/090412080748.htm