Unit 7 – Natural Selection
7.1 – Introduction to Natural Selection
• Natural Selection is the major mechanism of evolution
• Charles Darwin’s Theory of Natural Selection, competition for
limited resources results in differential survival. Individuals with
more favorable phenotypes are more likely to survive and
produce more offspring, thus passing traits to subsequent
generations.
• Evolutionary fitness is measured by reproductive success.
• Biotic and abiotic environments can be more or less
stable/fluctuating, and this effects the rate and direction of
evolution; different genetic variations can be selected in each
generations.
Natural
Selection
7.2 – Natural Selection
• Natural selection acts on phenotypic variations in populations
• Environments change and apply selective pressures to
populations
• Some phenotypic variations significantly increase or decrease
fitness of the organism in particular environments.
7.3 Artificial Selection
• Through artificial selection, humans affect variation in other
species.
• Convergent evolution occurs when similar selective pressures
results in similar phenotypic adaptations in different populations
or species.
Artificial
Selection
Evolution Types
7.4 Population Genetics
• Reduction of genetic variation within a given population can increase
the differences between populations of the same species
• Mutation results in genetic variation, which provides phenotypes on
which natural selection acts.
• Evolution is also driven by random occurrences
• Mutation is a random process that contributes to evolution
• Migration/gene flow can drive evolution
• Genetic drift is a nonselective process occurring in small populations
• Bottleneck effect– a sudden reduction in population size due to a catastrophic event
(natural disaster)
• Founder effect - a type of genetic drift where a small group of individuals from a larger
population establishes a new, isolated population
Bottleneck
Evolution
Founder
Effect
7.5 Hardy – Weinburg Equilibrium
• Hardy – Weinburg is a model for describing and predicting allele
frequencies in a nonevolving population. Conditions for a
population or an allele to be in Hardy-Weinburg Equilibrium are
• 1. A large population size
• 2. Absence of migration (no movement)
• 3. No net mutations
• 4. Random mating
• 5. Absence of natural selection
7.5 Hardy – Weinburg Equilibrium
• Changes in allele frequencies provide evidence for the
occurrence of evolution in a population
• Small populations are more susceptible to random
environmental impact than large populations
7.6 Evidence of Evolution
• Evolution is supported by scientific evidence from many
disciplines (geographical, geological, physical, biochemical and
mathematical)
• A comparison of DNA nucleotide sequences and/or protein
amnio acid sequences provides evidence for evolution and
common ancestry
• Fossils can be dated a variety of ways –
• The age of the rocks where the fossils are formed
• The rate of decay of isotopes
• Geographical data
7.9 Phylogeny (Cladograms)
• Phylogenetic trees or cladograms show evolutionary
relationships among lineages
• Phylogenetic trees and cladograms both show relationships
between lineage
• Traits that are either gained or lost during evolution can be used
to construct phylogenetic trees or cladograms.
• Shared traits are present in more than one lineage
• Shared, derived traits/characteristics indicate common ancestry
7.10 – Speciation
• Speciation may occur when two populations become
reproductively isolated from each other allowing new species to
form
• Punctuated equilibrium is when evolution occurs rapidly after
long periods of time
• Gradualism is when evolution occurs slowly over hundreds of
years
• Speciation results in diversity of life forms
7.12 Variations in Populations
• The level of variation in a population affects population genetics
• Population ability to respond to change in the environment is influenced
by genetic diversity. Species and populations with little genetic diversity
are at a risk for extinction
• Genetically diverse populations are more resilient to environment
survival because they are more likely to contain individuals who can
withstand the environmental pressures.
• Alleles (traits) that are adaptative in one environmental condition may
be deleted in another because of selective pressures.