What are the causes of variation?
- random assortment
- crossing over
- non-disjunction
- random fertilisation
- mutations
What is random assortment?
Random assortment of chromosome during meiosis results in gametes that have a large number of possible combinations of the chromosomes that originally came from the male and female parent.
What is crossing over?
Crossing over of chromatids during meiosis may result in pieces of chromatic being broken off and attaching to a different chromatid. This results in a changed sequence, or recombination, of the alleles along the resulting chromosome
What is non-disjunction?
Where one or more members of a chromosome lair fail to separate during meiosis. This results in gametes that have more or less than the correct number of chromosomes. If such gametes are involved in fertilisation, the resulting embryo will have the incorrect number of chromosomes.
What is random fertilisation?
Because each person will produce a huge number of different sperm or eggs with respect to the alleles each contain, and because any speed can fertilise any egg, there is an almost infinite number of possible combinations of the alleles in the offspring.
What are mutations?
Permanent changes in the DNA of a chromosome and may result in totally new characteristics in an individual. If the mutation occurs in a gamete, it can be passed on from generation to generation.
What is natural selection?
A process by which a species becomes better adapted to its environment; those individuals with favourable characteristics have a survival advantage and so pass those characteristics on to subsequent generations.
What are geographical barriers to gene flow?
A feature of the landscape that prevents populations from interbreeding.
What are some examples of geographical barriers?
- oceans
- mountain ranges
- large lake systems
- deserts
- expansive ice sheets
What are sociocultural barriers to gene flow?
Barriers to interbreeding that are due to social and cultural factors.
What are some examples of sociocultural barriers?
- religion
- economic status
- educational background
- social status
What were Darwin’s 3 observations?
- variation
- birth rate
- nature’s balance
Darwin’ observations: Variation
- all members of a species vary
- these variations were passed on from one generation to the next, characteristics being displayed by parents passed on to their children
Darwin’ observations: Birth rate
- all living organisms reproduce at a rate far greater than that at which their food supply and other resources increase
- this would normally result in overcrowding
Darwin’ observations: Nature’s balance
- although the birth rate of organisms was high, each species’ numbers tended to remain at a relatively constant level.
What is speciation?
The process of new species being developed
What are the steps of speciation?
- variation
- isolations
- selection
- speciation
Speciation: isolation
- a population exists on an island
- a range of variations exists within the population, which shares a common gene pool
Speciation: isolation
- the species is divided into two populations
- a barrier has formed, dividing the population into two
- no interbreeding occurs between the two populations
- each population has a separate gene pool
Speciation: selection
- two subspecies begin to form
- different selection pressures act on each of the two populations over a number of generations
- this brings about a change in the gene frequencies of each gene pool
- such changes lead to the evolution of seperate subspecies
Speciation: speciation
- two species now exist
- over a long period of time the changes in the gene frequencies may be great enough to prevent the production of fertile offspring by interbreeding between the two populations from ever occurring again
- when this happens, two species exist
What is an example of natural selection in humans?
Body stature
How does evolution by natural selection occur?
- Variations in a species
- More offspring than can survive to maturity
- Excessive birth rate + limited resources = struggle for existence
- Favourable characteristics, more likely to survive and reproduce = survival of the fittest
- Favourable characteristics passed on
- Over time proportion of alleles that produce favourable characteristics increases
What is gene flow?
The transfer of alleles from one population to another through migration
