Chapter 8 Biology Notes: Evolution and Natural Selection
Population: group of organisms of the same species living in a particular geographic region
Evolution: genetic change in the population
Charles Darwin
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grew up in the 19th century in a biblical world
influenced by many people
went on a round-the-world voyage
1. noticed the resemblance of island finch species to mainland finch species
2. resemblance of extinct species to living species in the same area
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first published his ideas in 1858
worldview after 1859: organisms change over time; some organisms have gone extinct; earth is
more than 6,000 years old; geology of earth is not constant but always changing
Evolution
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evolution occurs when the allele frequencies in a population change; the altering of the
population amongst organisms of different traits
even a 1% change over time will make up a larger proportion of the population after multiple
generations
Causes of Evolution: Mutation, Genetic Drift, Migration, Natural Selection
1. Mutation: a direct change in the DNA of an individual which may change an allele's property; ultimate
source of variation; causes can be due from complexities within cell division and the duplication of 3
billion base pairs, induced by environmental phenomena (mutagens), ultraviolet radiation; mutations
are random because the environment does not control which genes are mutated; the change in the
base-pair sequence of a person's DNA may cause the production of a gene product that has never
existed before; mutations are fairly rare because our bodies are built to protect our sperm or eggproducing DNA with a variety of error correction mechanisms
2. Genetic Drift: random change in allele frequencies in a population; genetic drift's change in allele
frequency does not affect the allele's influence on reproductive success (unlike natural selection);
impact of genetic drift is much greater in small populations than large populations; fixation from genetic
drift when an allele's frequency in a population reaches 100% and causes no more variability in the
population for this gene; founder effect is when a small group of individuals leave a population and
become founding members of a new isolated population, if the new population has different allele
frequencies, there has been evolution because the new population will be dominated by the genetic
features that happened to be in the founding population (Amish population and polydactyly);
population bottlenecks may occur during a great death of large proportions, remaining members are
essentially a random, small sample of the original population, new population will be dominated by
genetic features present in the surviving members (no genetic variation in the cheetahs)
3. Migration (gene flow): movement of some individuals of a species from one population to another;
movement from population to population within a species distinguishes migration from the founder
effect in which individuals migrate to a new habitat previously unpopulated by that species; if migrating
individuals can survive and reproduce in the new population and carry a different proportion of alleles
than the individuals in their new home, then the recipient population experiences a change in allele
frequencies and experiences evolution; because alleles are lost from the population that the migrants
left, that population will experiences a change in its allele frequencies as well; influenced by the mobility
of the organisms and other factors such as barriers and human activity
4. Natural Selection: three conditions are necessary and when these three are satisfied, evolution by
natural selection is occurring
1. There must be variation for the particular trait within a population: not limited to physical
features; can vary in physiological and biochemical ways too
2. Variation must be inheritable (it must be capable of being passed on from parents to their
offspring): offspring must inherit the trait from their parents for natural selection to work;
transmission of traits from parents to their children through genetic information inheritance or
heritability
3. 3 Individuals with one version of the trait must produce more offspring than those with a
different version of the trait; deduced from more organisms are born than can survive,
organisms are continually struggling for existence, some organisms are more likely to win this
struggle and survive and reproduce; differential reproductive success led Darwin to this third
condition
 natural selection can be turned on its head and viewed as the elimination of some heritable
traits from a population
 a mechanism of evolution that occurs when there is heritable variation for a trait and individuals
with one version of the trait have greater reproductive success than do individuals with different
versions of that treat; can be thought from the losing side as well
Fitness: Measure of the relative amount of reproduction of an individual with a particular phenotype as
compared with the reproductive output of individuals with alternative phenotypes; an organism's fitness
depends on the environment in which it lives in; has everything to do with reproductive success
1. An individual's fitness is measured relative to other genotypes or phenotypes in the population
2. Fitness depends on the specific environment in which the organism lives
3. Fitness depends on an organism's reproductive success compared with other organisms in the
population
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organisms that possess traits that allow them to better exploit the environment in which they
live will tend to produce more offspring than the organisms with alternative traits
a population will be made up of more fitter organisms as the generations pass
Adaptation: refers to both the process by which organisms become better matched to their
environment & to specific features that make an organism more fit; bats have an extremely
accurate type of hearing for navigating/finding food in complete darkness
Natural selection does not lead to perfect organisms; Galapagos finches' beak sizes changes
along with the average rainfall and there is no definition of a "fit" size and suggests that
adaptation does not simply march toward some optimal endpoint; evolution/natural selection
do not guide organisms toward perfection, it is a process by which the alleles that cause
organisms to have the traits that make them most fit in that environment to increase in
frequency
Perfection is not achieved because:
1. Environments change quickly; natural selection may be too slow to adapt the organisms
2. Variation is needed as the raw material of selection; if the "perfect" gene never occurs, individuals
within a population will never be perfectly adapted
3. There may be multiple different alleles for a trait that each causes an individual to have the same
fitness; there might be an equally fit "solution" to the environmental challenges
Artificial Selection
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farmers bred crops for max yield; dog, horse, and pigeon fanciers selectively bred the animals
with their favourite traits
is also considered natural selection because all three conditions are satisfied; differential
reproductive success is determined by humans, making it artificial
Changing traits in a population
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traits are subject to natural selection and can be changed in any of several ways
Directional Selection: individuals with one extreme from the range of variation in the population have
higher fitness; ex. milk production; a lot of variation in milk production from cow to cow and farmers
select breeding cows who have the highest milk production (milk production in cows grew by 50% from
1920 to 1945); variation for the trait decreases a bit because only one extreme is selected; turkeys with
larger breast muscles and more meat (but need artificial insemination because of it)
Stabilizing Selection: said to occur when individuals with intermediate phenotypes are the most fit; baby
size; outcomes have kept the average weight of a baby constant over many generations
Disruptive Selection: individuals with extreme phenotypes experience the highest fitness and those with
intermediate phenotypes have the lowest; rare in nature; some species of fish only have the largest
males acquire good territories (Coho salmon), intermediate-size fish get run out of good territories and
tiny males are able to sneak in and fertilize eggs before the owner detects presence; increase in the
frequency of small and large fish with a reduction in the frequency of medium-size fish
Evolution of Complex Traits and Behaviours
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natural selection is able to cause evolution of complex traits and behaviours; ex. improve mazerunning ability in rats
1954 William Thompson trained a group of rats to run through a maze for a food reward; after
six generations, maze-dull rats made twice as many errors as the maze-bright rats before
mastering the maze
natural selection can also produce complex traits in unexpected ways; complex organ such as a
fly wing when 1-2% of a wing doesn`t help an insect fly; often structures are enhanced or
elaborated on by natural selection because they enhance fitness by serving some other purpose;
incomplete wings within flies allows the organisms to control their body temperature;
Evidence For Evolution
1. Fossil Record: physical evidence of organisms that lived in the past; very incomplete record;
radiometric dating helps telling the absolute age; only those organisms that happened to live in
a particular area and also had physical structures could leave fossils (very incomplete); evolution
of horses are most well preserved; missing links are frequent amongst fossil records (fish with
fingers & fins)
2. Biogeography: distribution patterns of living organisms around the world; species often more
closely resemble other species that live less than a hundred miles away but in radically different
habitats than they do species that live thousands of miles away in nearly identical habitats;
nearly any bird in Hawaii is some sort of honeycreeper; species were not designed from scratch
to fill a particular niche; isolated habitats also have interesting biogeographic patterns unique to
any other places
3. Comparative anatomy and embryology: a little gill pouch and bony tail on the sides of the neck
in vertebrate embryos; gill pouches disappears for all but fish; reveals the ghost of evolution and
can only be explained through evolutionary relatedness; same bones for various species
modified extensively called homologous structures (human, horse, bat, porpoise); evolutionary
leftovers called vestiigial structures exist because they had value ancestrally (molars, eye
sockets without eyes, pelvic bones in whales, appendix for humans); convergent evolution uses
different starting materials available and modifies them until they serve similar purposes
(marsupial and placental mammals)
4. Common genetic sequences link all life forms: similarity of DNA among related individuals within
a species; codes for the amino acids used to build hemoglobin molecule in rhesus monkeys and
humans are nearly identical; as time elapsed since common ancestors, the greater the genetic
differences
5. Laboratory and fiend experiments enable us to watch evolution in progress: a single species of
grass is planted on a golf course; putting greens are cut very frequently, fairways are cut
occasionally and roughs are almost never cut; sexual menstruation and seed production for
these grasses go from rapid/high to slowest/lowest descending from putting to rough; evolution
of antibiotic resistance is an example as well; penicillin was once effective in killing certain
bacterium but natural selection has led to an increase in antibiotic-resistant alleles; increasing
risks from untreatable conditions such as staphylococcus infections