Download Grade 11 University Biology – Unit 3 Evolution

Grade 11 University Biology – Unit 3 Evolution
Mechanisms of Evolution
Section 9.1 Pages 350-359
Mechanism of Evolution really means “How do we get variation?” Evolution is responsible for both
the remarkable similarities we see across all life and the amazing diversity of that life. Yet, how exactly does
it work?
Evolution that occurs WITHIN A POPULATION is microevolution. Fundamental to that process is
genetic variation, and the mechanisms that selective forces act so that evolution occurs.
 We (…and Darwin…) have defined evolution as DESCENT WITH MODIFICATION from
a common ancestor, but exactly what has been modified? For starters, evolution only
occurs when there is a change in GENE FREQUENCY within a population over a long
period of time. These genetic differences are heritable such that they can be passed to
future generations. Moreover, Variation WITHIN A SPECIES is due to the variety and
combination of alleles possessed by individuals. Now, what has been modified?
An Example – Environment vs Genes
We have two populations of beetles that eat the same plant. Imagine a
year or two of drought in which there are fewer plants.
All the beetles have the same chances of
survival and reproduction, but because of food
restrictions, the beetles in the population are a little
smaller than the preceding generation of beetles.
The difference in weight is NOT evolution. It was due to environmental
influences (i.e., low food supply). It was not due to a change in the frequency of genes. The
smaller body size was not genetically determined, and consequently, this generation of smallbodied beetles will produce beetles that will grow to normal size if they have a normal food
supply.
Most of the beetles in the population (say 90%) have the genes for green
colouration and a few of them (10%) have a gene
that makes them brown. Some number of
generations later, the frequency has
changed: brown beetles are more common
than they used to be and make up 70% of
the population. The changing colour is
evolution. Why? These two generations of the
same population are genetically different.
The more genetic variation there is in a population, the greater the diversity of the population
AND the greater the change of a selective advantage to some individuals in a changing
environment.
There are five sources of genetic variation.
1. Mutations
 Mutation is a change in the DNA of an individual, and that a heritable mutation
has the potential to impact an entire gene pool.
 It could be harmful, beneficial or neutral
 A single mutation can have a very large and significant effect. NOTE:
Evolutionary change is usually based on the accumulation of many mutations
 The mutations that matter to “large-scale” evolution are the mutations that can be
passed to offspring. These are GERM LINE MUTATIONS because they occur in
reproductive cells
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Mutations can occur in many ways including during gene replication, Crossing
Over (e.g., Duplication, Translocation), Non-disjunction, or due to external factors
such as radiation.
Mutations are RANDOM
Not all mutations matter to evolution. Since all cells in our body contain DNA,
there are lots of places for mutations to occur. If the mutation occurs in a nonreproductive somatic chromosome, the mutation will not be passed to offspring.
2. Genetic Drift
 The change in allele frequency in a gene pool due to chance.
 In small population, drift can result in significant change.
 It contradicts Darwin’s “survival of the fittest”
 Bottleneck Effect
 Changes in gene
distribution that result in a
rapid decrease in
population size
 Results in loss of genetic
diversity since variation is
lost at the time of the
bottleneck
 Founder Effect
 A change in the gene pool
that occurs when a few
individuals start a new
isolated population
 A Founder Effect occurs
when a new colony is
started by a few members
of the original population.
This small population size
means that the colony
may have: (1) reduced
genetic variation from the
original population and (2) a non-random sample of the genes in the original
population.
 This could explain Darwin’s finches on the Galapagos Islands
3. Gene Flow
 The movement of alleles
from one population to
another due to the migration
of individuals.
 It is also called MIGRATION
 Individuals carry and
introduce genes to a
different population where
those genes may not have
previously existed. As a
result, genetic diversity in
the population may increase.
 Gene flow can be a chance event (i.e., unrelated to the genotype)
 If gene flow stops, isolated populations may change enough so they can no
longer mate with members of the other population

Examples include pollen from plants blowing into a new field OR people moving
to a new country
4. Non-Radom Mating
 Mating among individuals on the basis
of mate selection for a particular
genotype or due to inbreeding
 Preferred Phenotypes - In wild animal
populations, individuals may choose
mates based on their physical and
behavioural traits (i.e., their
phenotypes)
 Inbreeding – In wild or controlled
environments, inbreeding occurs when
closely related individuals breed.
Since parents have the same
genotypes, the frequency of
homozygous genotypes is
increased...which negatively increasese the frequency of expression of harmful
recessives alleles (e.g., purebred pets have higher incidence of health problems
and often earlier death).
5. Natural Selection
 Selective forces (e.g., predation, competition) mean some individuals are more
likely to survive and reproduce. If having a single allele gives a slight and
consistent selective advantage, the frequency of the allele in the population
increases over generations, and moreover, a greater likelihood of those favoured
individuals surviving, reproducing and passing the allele to their offspring. Thus,
natural selection changes the allele frequency of a population. The outcome is
evolutionary change.
 There are three types of selection.
 Stabilizing Selection – Favours an intermediate phenotype and acts
against extreme variants. It reduces variation in a population and
improves adaptation in a non-changing environment
 Directional Selection – Favours the phenotypes at one extreme over
another, resulting in the distribution of phenotypes shifting toward the
extreme. It is common during environmental change or population
migration when new environmental conditions are forming
 Disruptive Selection – Favours the
extremes of a range of phenotypes
and eliminates the intermediate
phenotypes. The diagram represents
disruptive selection. Over time, the
intermediate phenotypes (middle part
of the curve) disappear from the
population.
Sexual Selection
 Natural Selection for mating based on competition
between males and choices by female.
HOMEWORK
 Page 352, Questions 1-4
 Page 356, Questions 8-12
 Page 359, Questions 2, 6-9