Chapter 22 and 23 Review
Descent with Modification
The phrase descent with modification summarized Darwin’s perception of the unity of life
The phrase refers to the view that all organisms are related through descent from an ancestor that lived
in the remote past
In the Darwinian view, the history of life is like a tree with branches representing life’s diversity
Darwin made two major points in his book:
Many current species are descendants of ancestral species
Natural selection is a mechanism for this evolutionary process
Darwin developed two main ideas:
Evolution explains life’s unity and diversity
Natural selection is a cause of adaptive evolution
Natural selection
The major microevolutionary process that results in differential survival and reproduction
There is variation among individuals
More are born than can survive
There is competition for resources
Those individuals that are most fit for their environment survive, reproduce and pass on their alleles.
Summary of Natural Selection
Natural selection is differential success in reproduction from interaction between individuals that vary in
heritable traits and their environment
Natural selection produces an increase over time in adaptation of organisms to their environment
If an environment changes over time, natural selection may result in adaptation to these new conditions
Some similar mammals that have adapted to similar environments have evolved independently from
different ancestors
Individuals don’t evolve—populations do
The gene pool: all of the genes in a population
Each gene exists in two or more forms called alleles
Variation in a species results from one or more of the following: mutations, crossing over during meiosis
1, independent assortment of alleles, fertilization, changes in chromosome structure or number
Only mutation creates new alleles
Microevolution
Change in relative allele frequency over time; if allele frequency changes evolution occurs
Causes of microevolution
Genetic drift: change in a small gene pool due to chance
Bottleneck event: population size is drastically reduced, leaving only the alleles of the survivors in the
gene pool
Founder effect: the small group starting a new colony contribute only their alleles to the new population
Gene flow: gain or loss of alleles through immigration or emigration
Non-random mating: organisms tend to mate with neighbors although they are capable of mating with
any member of their species anywhere on earth
Elephant seals
One adult male seal has enough blubber to make 25 gallons of oil
Only 20-100 animals existed in the late 1800’s
1922 they become a protected species
Today’s population: 150,000-200,000
Hardy-Weinberg
Their formulas are used to establish allele frequencies at genetic equilibrium (no evolution is occurring)
The following conditions must all be fulfilled
The population is very, very large
There is no migration of individuals
No mutations
Mating is completely random
All members survive and reproduce successfully
The formulas
Allele frequency: fraction of that particular allele in the population
The sum of all the allele frequencies = 1
p = frequency of the dominant allele
q = frequency of the recessive allele
p+q=1
To figure the frequency of each genotype use: p2 + 2pq + q2 = 1