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Chapter 16. GENES AND VARIATION. 16.1 - Variation of Traits in a Population (gene pool). Population: all the members of the same species that live in a particular location at the same time and have the potential to interbreed.

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chapter 16

Chapter 16


16 1 variation of traits in a population gene pool
16.1 - Variation of Traits in a Population (gene pool)
  • Population: all the members of the same species that live in a particular location at the same time and have the potential to interbreed.
Members of a population, even though of the same species, are somewhat different genetically.
  • Why?

mutations in DNA

  • recombination of genes during meiosis
  • random fusion of gametes
  • A difference in genotype usually
  • results in a difference in phenotype
Often, variation in traits are due also to environmental factors as well
  • Example:  Why are some of the perch in a pond bigger/longer than others? Why do snowshoe hares change fur color with the seasons? (environmental triggers)
allele frequencies and genetic equilibrium population genetics
Allele Frequencies and Genetic Equilibrium: Population Genetics

Variations in genotype arise throughmutation,

recombination and


causes of variation
Causes of variation
  • Mutation – change in DNA sequence, if change in gene then there may be a change in protein (phenotype)
  • Recombination – due to meiosis and and fertilization (it’s how you get two siblings that are not anything alike)
  • Crossing –over - happens in meiosis, pieces of chromosomes switch place
gene shuffling
Gene shuffling
  • Mark in notes that recombination and crossing-over are considered – GENE SHUFFLING
some important words
Some important words
  • Gene Pool: the collection of genes for all the traits in a population (contains all the alleles for all the genes)
  • Allele Frequency: the percentage of a specific allele of a gene in the gene pool
  • A population in which allele frequencies do not change from one generation to the next is said to be in Genetic Equilibrium
16 2 the hardy weinberg principle
16.2 - The Hardy-Weinberg Principle

-no mutations-no migrations-large populations-random mating-no selection of alleles

disruption of genetic equilibrium
  • When gene frequencies change over time (Hardy-Weinberg does NOT hold true) then EVOLUTION happens.
factors that cause change in gene pools
Factors that cause change in gene pools:
  • Mutation
  • Migration
  • Nonrandom mating
  • Genetic Drift: allele frequencies in a population change as a result of random events or chance
  • Selection: some alleles are more favorable to have than others
types of natural selection
  • Stabilizing Selection
  • Directional Selection
  • Disruptive Selection
  • Sexual Selection: mate preferences based on traits, usually some physical feature
stabilizing selection read this
Stabilizing Selection (read this)
  • Stabilizing selection favors the norm, the common, average traits in a population. Look at the Siberian Husky, a dog bred for working in the snow. The Siberian Husky is a medium dog, males weighing 16-27kg (35-60lbs). These dogs have strong pectoral and leg muscles, allowing it to move through dense snow. The Siberian Husky is well designed for working in the snow. If the Siberian Husky had heavier muscles, it would sink deeper into the snow, so they would move slower or would sink and get stuck in the snow. Yet if the Siberian Husky had lighter muscles, it would not be strong enough to pull sleds and equipment, so the dog would have little value as a working dog. So stabilizing selection has chosen a norm for the size of the Siberian Husky.
directional selection
Directional selection

favors those individuals who have extreme variations in traits within a population. A useful example can be found in the breeding of the greyhound dog. Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 64km/h (40mph).

disruptive selection
Disruptive Selection
  • Disruptive selection, like directional selection, favors the extremes traits in a population. Disruptive selection differs in that sudden changes in the environment creates a sudden forces favoring that extreme. Think about the changes in the environment when that meteor crashed into Earth 65mya. A sudden decrease in light levels as the dust rose over large portions of the Earth. Extremely large tidal waves washing miles over the land. Increased seismic activity. The sudden lost of food along the coast, possible plague due to the high initial death rate, dust filling the lungs of animals would have been the most stressful on larger animals. Large animals need a large oxygen supply to supply energy to their muscles. They also need a large, constant supply of food. The sudden drop of oxygen due the dust, and the drop in fresh food, large animals would be stressed. If a plague started by the high death rate also hit these stressed animals, they would have been sorely pushed to survive. Evidence shows that they did not. So disruptive selection occurs quickly, selecting for those extreme traits that help organisms survive in the new environmental conditions.
apply your knowledge
  • Look at the graphs in your notes and decide which type of selection they are showing
genetic drift disruption by chance
GENETIC DRIFT – Disruption by Chance
  • Founder Effect – allele frequency changes due to the migration of a small sub group
  • EX. Amish community from the 32 original Swedish descendents
genetic drift continued
Genetic drift continued
  • Bottleneck effect – elimination of alleles in a population due to a disaster
  • EX - the elephant seal population was hunted down to just 20 individuals. Then the population rebounded to 30,000 BUT little genetic variation between animals.
16 3 formation of species how do species form
16-3 FORMATION OF SPECIESHow do species form?
  • A "species" is a group of like organisms that are capable of reproducing viable offspring in nature.
  • Morphological species:
    • based on similarities/differences in structure; easy to observe
  • Biological species:
    • can interbreed; are chromosomally and genetically similar enough to be considered the same.
formation of species
  • "Speciation"
  • Isolated Populations:   become two (or more) separate gene pools, where different selection and mutations (DNA alterations) make them very different over different that they will no longer interbreed. 
  • Two main causes: geographical isolation or reproduction isolation
1 geographic isolation pg 405
1.  Geographic Isolation (pg 405)
  • "Allopatric speciation", also known as geographic isolation , occurs when populations physically isolated by a physical barrier
  • These populations evolve genetic reproductive isolation such that if the barrier between the populations breaks down, individuals of the two populations can no longer interbreed.
ex grand canyon squirrels
ex:  Grand canyon squirrels 

The Abert squirrel inhabits the Coconino Plateau, just to the south of the Grand Canyon, and what is called the Kaibab squirrel inhabits the Kaibab Plateau, just to the north of the Grand Canyon.  The Grand Canyon, 200 miles long, 5,000 feet deep, and 12 to 15 miles across, with the Colorado River running through it, acts as a barrier to terrestrial animal movement.

2 reproductive isolation
2.  Reproductive Isolation
  • "Parapatric speciation":  no specific extrinsic barrier to gene flow. The population is continuous, but nonetheless, the population does not mate randomly. Two (or more) separate gene pools form, and eventually these diverge into different species .
prezygotic mechanisms – before zygote formation (no fertilization occurs)
    • EX 1 – BEHAVIORAL ISOLATION – different mating calls (songs in birds, chirps in crickets) or different rituals such as dances, etc (pg 404)
    • EX 2 – TEMPORAL ISOLATION – reproduction at different times, frogs lay eggs at different time of yr, pollen released diff. time of yr, etc (405)
postzygotic mechanisms – fertilization occurs but zygote dies or is sterile
    • Ex - mule
  • Polyploidy – DON’T NEED TO KNOW!
17 4 rates of speciation
17-4 Rates of Speciation
  • How quickly do new species form?  Is it a long, slow, even process over time ("Gradualism")
  • or does it happens in spurts ("Punctuated Equilibrium") of rapid genetic change?
  • Extinction – more than 99% of all species that have ever lived are now extinct, many mass extinctions have occurred
topics from chapter 17 4 that should be included now
TOPICS FROM CHAPTER 17.4 that should be included NOW!
  • Coevolution: the change in two or more species that are closely associated with one another
  • Camouflage
  • Predator/prey
In this snowy environment, the polar bear is white to avoid being noticed as it approaches the seal, and the seal pup is white to avoid being noticed by the bear.
Despite their very different evolutionary paths – one as a mammal, the other as a bird – both have evolved and adapted to the aquatic environment they now inhabit.
Divergrent Evolution: two or more closely related organisms become more and more dissimilar over time

2 important types

1) adaptive radiation

2) artificial selection