Chapter 8 Flashcards
Gene pool
- frequency of each allele within a population
- evolution is the change in the gene pool of a population over time
Define Hardy-Weinberg principle + 5 conditions
- in large populations in which only random chance is at work, allele frequencies are expected to remain constant from generation to generation
1. no natural selection
2. population must be very large
3. no mutations
4. population must be isolated from other populations (no immigration or emigration)
5. random mating and no horizontal gene transfer
Selective pressures
- may result from any number of biotic or abiotic factors
- can result in different patterns of natural selection
Directional selection
- a selection that favors individuals with a more extreme variation of a trait
- results in a shift away from the average
- common in artificial selection
Stabilizing selection
- occurs when the average phenotype within a population is favored
- selection against extreme traits
Disruptive selection
- favours individuals with variations at opposite extremes of a trait over the average phenotype; favours more than one phenotype
Sexual selection
- favours any traits that specifically enhance the mating success of an individual
- leads to males and females of a species evolving appearances and behaviours that differ from each other (sexual dimorphism)
Genetic Drift
changes to allele frequency as a result of chance; effects small population more than large population
Bottleneck Effect
- occurs when a population undergoes a drastic reduction in size
- caused by chance events (volcanic eruption, hurricane)
- bad luck, not bad genes
- can cause big losses of genetic variation in small populations
Example of Bottleneck Effect
- Northern elephant seals have reduced genetic variation
- hunting reduce their population size to 20 individuals at the end of the 19th century
- have since rebounded but have less genetic variation than southern elephant seals
Founder effect
- results when a small number of individuals separate from their original population and establish a new population
- this small population size means that the new population may have…
- reduced genetic variation from original population
- non-random sample of the genes in original population
Example of Founder Effect
- Afrikaner population of dutch settlers in south africa descended from a few colonists
- Today, Afrikaner population have a high frequency of Huntington’s disease because the original settlers carried the gene for Huntington’s disease
Gene flow + 2 types
occurs when individuals migrate to/from one population to/from another and interbreed
immigration - individuals enter a population
- possible addition of new genetic material to the gene pool of a population
emigration - individuals leave a popluation
- possible removal of genetic material from the gene pool of a population
Mutations
random genetic mutations can introduce new alleles into a gene pool
Microevolution
changes in gene (allele) frequencies and phenotypic traits within a population over a relatively short period of time
Speciation
the formation of a new species
Macroevolution
- major evolutionary change
- applies to the evolution of whole taxonomic groups over long periods of time
Biological species concept
- species are groups of individual that can mate and produce fertile offsprings under normal circumstances
- members of different species are reproductively isolated from one another
- popluations have no gene flow between them
Reproductive isolating mechanism
- any behavioral structural or biochemical trait that prevents individuals of different species from reproducing together
Prezygotic mechanisms
- prevents interspecies mating and fertilization
Post-zygotic mechanisms
- prevents maturation and reproduction in offspring from interspecies reproduction (hybrid)
Reproductive isolating mechanism
- any behavioral structural or biochemical trait that prevents individuals of different species from reproducing together
1. Prezygotic mechanisms
- prevents interspecies mating and fertilization
2. Post-zygotic mechanisms
- prevents maturation and reproduction in offspring from interspecies reproduction (hybrid)
Prezygotic mechanism - Behavioural isolation
- different species use different courtship and other mating clues to find and attract a mate
- example: male frogs of different species have unique calls that attract females of only their own species
Prezygotic mechanism - Temporal isolation
- different species breed at different times of the year
- example: frogs in the same pond may reproduce at different times of the year
Prezygotic mechanism - Ecological (Habitat) Isolation
- very similar species may occupy different habitats within a region
- example: two closely related antelope squirrels are on opposite sides of the Grand Canyon
Prezygotic mechanism - mechanical isolation
- differences in morphological features may make two species incompatible
- example: pollen sacs in orchids becomes attached to specific insects and not others
Prezygotic mechanism - gametic isolation
- males gametes may not be able to recognize and fertilize an egg of a different species
- example: many marine animals (ie. coral) release their sperm into open water. The sperm recognize eggs of their own species through chemical markers on the surface of the eggs
Postzygtoc mechanism - zygotic mortality
- mating and fertilization are possible. zygotes formed will not survive, embryos will not develop to maturity
- example: some species of sheep and goat are able to mate, but the zygote is not viable
Postzygotic mechanism - hybrid inviability
- a hybrid individual develops but either dies before birth or, if born alive, cannot survive to maturity
- example: when tigers and leopards are crossed, the zygote begins to develop but the pregnancy ends in a miscarriage or still born offspring
Postzygotic mechanisms - hybrid infertility
- hybrid offspring remain healthy and viable but are sterile
- example: mules are the sterile (63 chromosomes) hybrids offspring of a horse-donkey cross
Types of Speciation
allopatric speciation
- the formation of a new species as a result of evolutionary changes following geographic isolation
sympatric speciation
- the evolution of populations within the same geographic area into separate species
Adaptive Radiation
- The relatively rapid evolution of a single species into many new species, filling a variety of formerly empty ecological niches
- Occurs when a variety of new resources become available that are not being used by other species
- example: Darwin’s finches
Divergent evolution
- the large-scale evolution from a common ancestor into many different forms
- The pattern of speciation (allopatric, sympatric, adaptive radiation) follow this pattern
Convergent evolution
the evolution of similar traits in very distantly related species
examples :
- Birds and insects both have wings yet they are evolved from different ancestors
- Sharks and dolphins have similar body shapes
Coevolution
- a process in which one species evolves in response to the
evolution of another species - Include, predator-prey, plant-pollinator and parasite - hosts relationships
