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Natural Selection• Developed by Charles Darwin in
1859• Mechanism by which better
adapted organisms survive to produce a greater number of viable offspring
• Improve chances of survival• Variation
– Individuals in a population vary in phenotype which also means their genotype
– Some variations are better suited for survival and are inherited
• Overproduction – Populations tend to produce
more offspring than are needed
Neo-Darwinism• Refined version Darwin’s
theory• Combines Mendel’s
genetics • Evolution is driven by
chance• 2 ways– Small scale mutations
(single nucleotide polymorphisms)
– Large scale mutations (recombination)
• Creates new DNA by lucky accidents
Gene Pools • Total genetic information (alleles) in the gametes
of all individuals in any given population
Population• Group of organism’s of one species that
interbreed and live in the same place at the same time
Factors that Affect Gene Pools • Mutation • Emigration • Geographical Barriers • Non-Random Mating • Genetic Drift – Bottleneck effect,
founder effect • Gene Migration • Speciation • Selection Pressure • Adaptive Radiation
Factors Contribute to Fitness of Individual • Fitness– Measure of how well suited
an organism is to survive in its habitat and its ability to maximize the numbers of offspring surviving to reproductive age
– How successful an organism is at passing on its genes
– Natural selection tends to lean towards individuals who have specific traits that favour them in the environment
Allele Frequency (Gene Frequency) • Proportion of a particular allele (variant of a gene) among all
allele copies in a specific gene pool • Hardy-Weinberg principle is used to determine allele
frequency: • p² + 2pq + q² = 1– “p” and “q” represent frequency of alleles – “p” added to “q” always equals one (100%)
Mutation• Random occurrences which change the genome of the
organism• Increase genetic diversity • Advantageous mutations are favoured by natural selection• Disadvantageous mutations are phased out
Geographical Barriers• Isolates the gene pool and
prevents regular gene flow between populations
Non-Random Mating• Members of gene pool
seek out particular phenotypes increasing frequency of particular alleles
• Decreases genetic diversity • Also known as selective
breeding – Humans breed livestock
and plants for particular traits (favourable)
– Can lead to in-breeding depression caused by deleterious recessive alleles (cause abnormalities or death)
Genetic Drift • Species of same population split
into groups• Geographical barriers do not
allow members of same gene pool to reproduce with one another increasing genetic diversity
• Founder Effect– New population is started by few
members of original population – Contains reduced genetic
variation – Non-random samples of genes
• Bottleneck Effect – Population size is reduced for at
least one generation – Reduces genetic variation
Gene Migration• Immigration
– Populations gain alleles from other gene pools
– Dependent on difference in allele frequencies between gene pools
• Gene Flow– Members from one gene
pool mate with members of another gene pool leading to alteration of allele frequencies
• Emigration – Population loses alleles
from gene pool
Speciation• Genetic variation among population is so different
that members can no longer reproduce with one another
• New species is formed
Selective Pressure • Environmental factors reducing reproductive
success among members of a population • Contribute to evolutionary change or extinction
through process of natural selection • Include– Competition, predation, disease, parasitism, land
clearance, climate change, pollutants
Antibiotic Resistant Bacteria• Microorganisms show resistance
to an antimicrobial drug that was originally effective for treatment
• Conjugation – Transfer of genetic material
between bacterial cells– Creates genetic diversity– Antibiotic resistance
Adaptive Radiation • Species from a common ancestor have successfully
adapted to their environment via natural selection • Less competition in population • New ecological niches established (organisms
function in environment)• Darwin’s finches
Types of Natural Selection• Occur with or without
environmental change• Effect a population• Stabilizing Selection– Constant environment– Maintain status quo
• Directional Selection – New variation arises in
constant environment • Disruptive Selection– Changing environment– Variations that result in
better fitness in environment are favoured
Stabilizing Selection• Favours the average
individuals in a gene pool• Selects against extreme
phenotypes of gene pool• Favours majority of
population in gene pool• Diversity is decreased • Human birth weight• Infants with average birth
weight have increased chance of survival
Directional Selection• Favours one extreme phenotype
over another extreme phenotype• Phenomena is observed in
environments that have changed over time (climate change, food availability)
• Population bell curve shifts to left or right
• Fewer average individuals when compared to stabilizing selection
• Beak length of Galapagos finches • Influenced by human interaction
(hunting)
• Average individual in a population is not favoured
• Extreme phenotypes are observed
• Lead to speciation (new species)
• Diversity increased• Influence by human
interaction (environmental pollution)
• London’s peppered moths
Disruptive Selection
Sexual Selection• Special case of natural
selection• An organism’s ability
to successfully copulate with a mate
• Usually female chooses among males
• 2 ways – Female choice– Male competition
Female Choice• Intersexual selection between sexes • Females choose males based on specific
characteristics or behaviours
Male Competition• Intrasexual selection• Males compete against other males for territory
or mating rights with females • Lead to intense battles
Evidence of Evolution• Fossil record of change
in earlier species• DNA/protein
homologies• Geographic
distribution of related species
• Recorded genetic changes in living organisms over many generations
DNA/Protein Homologies• All living things on earth
share the ability to create complex molecules out of carbon and a few other elements
• All plants and animals inherit their specific characteristics from a combination of genes
• Protein is made from DNA made up mostly of only 20 kinds of amino acids