MUST EVOLUTION HAPPEN?
The rate of evolution is slow, and sometimes it
Theories of population genetics
- the study of the genetic events in gene
The Hardy- Weinberg Theorem
Godfrey H. Hardy
- it states that the mixing of alleles at
meiosis and their subsequent recombination
do not alter the relative frequencies of the
alleles in future generations, if ccertain
assumption are met.
The asssumption of the Hardy- Weinberg
theorem are as follows:
1.The population size must be large.
2.Sexual reproduction within the population
must be random.
3.Individuals cannot migrate into, or out of,
4.Mutations must not occur.
Population size, Genetic drift, and
- the smaller population, the more
significant may be.
- chance events influencing the
frequencies of genes in populations
- gene frequencies are changing
independently of natural selection
Two special cases of genetic drift have influenced
the genetic makeup of some populations.
- the new colony that emerges from the
founding individuals is likely to have
distinctive genetic makeup with far less
variation than the larger population.
- if population sizes are restored,
they will have only the remnant of the
original gene pool.
-changes in relative allelic frequency from the migration
-changes in the structure of genes and
-origin of all new alleles and a source of
variation that may prove adaptive for an
-measure of the tendency for gene
frequencies to change through mutation.
Natural selection reexamined:
MODES OF SELECTION
Natural selection may affect a range of
phenotypes in tree ways:
1. Directional selection- occurs when
individuals at one phenotypic extreme are at
a disadvantage compared to all other
individuals in the population.
2.Disruptive selection- occurs when natural selection favors both
extremes of continuous variation.
-an intermediate phenotype is selected against.
3. Stabilizing selection- occurs when individuals at both phenotypic
extremes are selected against.
-narrows the phenotypic range.
Both natural selection and neutral evolution
occur, but they may not be equally important in
Balanced Polymorphism and
Balanced polymorphism- occurs when different phenotypes are
maintained at relatively stable frequencies in the population.
Heterozygote superiority- when the heterozygote is more fit than
Species and Speciation
Species – a group of populations in which genes are actually,
or potentially, exchanged through interbreeding.
Reproductive isolation- gene flow among populations or
subpopulations does not occur.
Occurs in different ways:
Premating isolation – prevents mating from taking place.
Postmating isolation – prevents successful fertilization and
development, even though mating may have occured.
-occurs when subpopulations become geographically isolated
from one another.
- occurs in small population called demes
- occurs within a single population
Rates of Evolution
- it is a measurement of the change in an
evolutionary lineage over time
- evolutionary change as occuring over
millions of years
- changes are gradual over long periods
- rapid periods of change interrupt long
periods of stasis
All evolutionary change results from changes in the
base sequences in DNA and amino acids in proteins
- the accidental duplication of a gene on a
chromosome is one way that extra genetic material
- the occurence within a given population of
organisms of different rates of evolutionary change
various body structures aand functions