Chapter 12 Processof Evolution
- Biological Evolution is genetic change in a line of descent (lineages)
through successive generations.
- Members of a population generally have the same number and kinds of
genes, which give rise to the same assortment of traits.
- Each gene may exist in two or more slightly different forms (alleles)
- Different individuals don’t necessarily inherit the same alleles so they may
differ in the details of their traits.
- Natural selection is simply the result of a difference in survival and
reproduction among individuals who differ in one or more traits
- A species consists of one or more populations of individuals that can
interbreed under natural conditions and produce fertile offspring, and that
are reproductively isolated from other such populations.
There are two types of reproductive isolation mechanism, pre-mating and post-
- Pre-mating reproductive isolating mechanisms act to prevent mating,
these include temporal isolation, ecological isolation, behavioural isolation
and mechanical isolation
- Post-mating reproductive isolation mechanisms act after mating has
occurred, this includes gamete mortality, and post zygote mechanisms (as is
found with the product of the union of a female horse and a male donkey, the
mule that results is sterile)
- New Species evolve from variant individuals of existing species. Therefore,
all species that have ever lived on Earth are related
- However, it is estimated that nearly 99.9% of all species that have existed
since the beginning of life on the Earth are no longer alive.
- Most scientists have concluded that extinction is the ultimate fate of most, if
not all, species.
*****Be sure to understand what causes evolution?
Have a clear understanding of natural selection and how it operates.
You should know the kinds of anatomical, biochemical and genetic
evidence in support of evolution.
You should be able to distinguish between the different kinds of
reproductive isolating mechanisms
Section 12.7 in NOT on exam.
1 12.2 Making Waves in the Gene Pool page 213
population – a group of organisms of the same species that live in a specific location
and breed with one another more often than they breed with members of other
allele frequency – abundance of a particular allele among members of a population
gene pool – all the alleles of all the genes in a population; a pool of genetic
lethal mutation – mutation that alters phenotype so drastically that it causes death
microevolution – change in an allele’s frequency in a population or species
genetic equilibrium – allele frequencies of a population do not change ie. The
populating is not evolving. Genetic equilibrium can occur if everyone of the
following five conditions are met:
a. mutations never occur
b. the population is infinitely large
c. the population is isolated from all other populations of the species
d. mating is random
e. all individuals survive and produce the same number of offspring
*** All five conditions can never be met in nature so natural populations are
never in equilibrium***
1. The individuals of a population share physical, behavioural and
2. Alleles which arise by mutation are the basis of differences in the forms of
1. All alleles of all genes in a population form a pool of genetic resources called
a gene pool
2. Mutation may be lethal, neutral or adaptive
3. Microevolution or changes in allele frequency within a population’s gene
pool occurs consistently in natural populations by process of mutation,
natural selection and genetic drift.
4. We use deviations from a theoretical genetic equilibrium to study how
2 Take home message
What is microevolution?
1. Individuals of a natural population share morphological, physiological and
behavioural traits characteristic of the species. Alleles are the basis of
differences in the details of those shared traits.
2. All alleles of an individuals in a population make up the population’s gene
pool . An allele’s abundance in the gene pool is called the allele frequency
3. Microevolution is change in allele frequency. It is always occurring in
natural populations because processes that drive it are always operating.
***Microevolution is always occurring in natural populations because
processes that drive it – mutation, natural selection and genetic drift- are
always operating. Even though we recognize evolution none of it is
Sources of Variation in Traits Among individuals of a species: table 12.1 pg213
Mutation- source of new alleles
Crossing over at - introduces new combinations of alleles into chromosomes
Independent- mixes maternal and paternal chromosomes
At Meiosis 1
Fertilization- combines alleles from 2 parents
Changes in- often dramatic changes in structure and function
Number or structure
3 12.3 Modes of Natural Selection page 215
directional selection –mode of natural selection in which a phenotype at one end
of a range of variation is favoured
stabilizing selection – mode of natural selection in which an intermediate form of
a trait is favoured over extreme forms
disruptive selection – mode of natural selection that favours forms of a trait at the
extremes of a range of variation; intermediate forms are selected against
1. Natural selection, the process in which environmental pressures result in
the differential survival and reproduction of individuals of a population,
occurs in patterns.
2. With directional selection, a phenotype at one end of a range of variation is
3. Stabilizing selection acts on extreme forms of a trait, so intermediate forms
4. With disruptive selection, intermediate forms of a trait are selected against,
and forms at the extremes are favoured.
Take Home Message
How does natural selection drive Evolution?
1. Natural selection occurs in different patterns depending on the species
involved and the pressures in their particular environment
2. With directional selection, a phenotype at one end of a range of variation
is favoured ***Example: populations of rock pocket mice: mice with dark
fur are more common in dark areas and mice with light fur are more
common in light areas because mice with coat colors that do not match their
surroundings are more easily seen by predators so they are preferentially
eliminated from the populations. Pg 216
3. With Stabilizing selection, an intermediate phenotype is favoured and
extreme forms are selected against ***Example: pg. 217 Social Wavers with
an intermediate body mass of 29gr are more likely to survive breeding
4. With disruptive selection an