Chapter 15: The Theory of Evolution
Charles Darwin (1809-1882)-founder of modern evolutionary theory
Fossils have been used since the 18th century to form basic
concepts of evolution
Darwin began in 1831 (age 21) as a naturalist on HMS Beagle- 5
year collecting and mapping expedition to South America and
South Pacific- studied and collected specimens from every
port along route
Galapagos Islands- 1000 km off west coast of South America
many species that are unique but similar to species elsewhere
first indication that species may change over time
Spent the next 22 years doing experiments working out the details
Species produce more offspring than there is food to support
Therefore, individuals struggle to survive
Individuals vary, some variations inherited
Bred pigeons with desirable variations to get offspring with
those variations- artificial selection (selective breeding)
Natural Selection
method for change within populations
Individuals with favorable variations survive, reproduce, and pass on trait, while individuals without those variations don't survive and reproduce.
On the Origin of the Species by Natural Selection (1859)
Natural Selection and Adaptations-Evidence
Adaptation- any variation that enhances an org. chance of survival
Structural Adaptation- changes in the structure of body parts
Mole-rats- blind, live underground
large teeth and claws, went blind b/c vision did not benefit
Mimicry- one species resembles another
mantispid fly looks like a wasp to evade predators
Camoflage-enables species to blend with surroundings
Physiological adaptations- changes in organisms metabolic
process
antibiotic resistance in bacteria, pests and insecticides
More Evidence
Fossils- record of early life and evolutionary history
incomplete
Homologous structure- structural features with common
evolutionary origin
similar in arrangement and/or function
forelimbs of whales, crocodiles, birds
Analogous structure- similar in function but not structure- different
evolutionary lineage
compare butterfly and bird wings
Vestigial structure- body structure that has no function in organism
but was valuable to ancestors
eyes of the mole-rat, wings on flightless birds
Embryological development- embryos from different species
VERY similar, and some have unnecessary structures
(mammal embryos have gill slits)
Biochemistry- comparisons of DNA and RNA can be used to track evolutionary relationships
Population Genetics and Evolution
populations, not individuals, evolve
gene pool- all of the alleles of the pop. genes
allelic frequency- percentage of any specific allele in the gene pool
genetic equilibrium- frequency of alleles does not change from
generation to generation
Evolution can occur only if genetic equilibrium is disturbed
1. Mutation- disrupts equilibrium by introducing new genetic
material to the gene pool (mutation can be harmful,
helpful, or neither)
2. Genetic Drift- alteration of genetic equilibrium by chance events
A. usually in small populations (Amish)
B. gene flow- movement of individuals in and out of a
population
3. Natural Selection- causes the greatest change in gene pools
Stabilizing selection favors average indiv. in pop.
Spider size
Directional selection extreme form of trait favored
Woodpecker beaks
Disruptive selection individuals with both extreme forms of
trait are at selective advantage
White, tan, dark limpets
Speciation
evolution of a new species
occurs when members of similar pops no longer interbreed to produce fertile offspring within natural environment (prevented somehow)
1. Geographic isolation-physical barrier separates pop. into groups (natural selection would act on each group separately)
2. Reproductive Isolation- former interbreeding org. are prevented from producing fertile offspring
A. genetic material becomes so diff.- fertilization impossible
B. behavior- different mating seasons
3. Polyploidy- results in immediate reproductive isolation
Speciation can occur quickly or slowly
1. gradualism- evolution occurs at a slow gradual rate as a result of buildup of new adaptations (camels)
2. punctuated equilibrium- Gould and Eldridge (1971)- evolution occurs in rapid bursts with long periods of genetic equilibrium between. Environmental changes, introduction of competitive species lead to rapid changes in gene pool (takes less than 10 000 yr) (elephants)
Patterns of Evolution
1. Divergent Evolution species that once were similar to an ancestral species diverge, or become increasingly distinct
Adaptive Radiation- one species evolves into array of species
to fit diverse habitats- Hawaiian honeycreeper
2. Convergent Evolution- distantly related organisms evolve similar traits (often because similar environments exert similar selective pressures)
