1. Evolution

2. Evolution Evolution refers to changes in gene frequencies over time
In Darwin's time, it was thought to involve the gradual change in organisms over a long period
Now the theory looks at changes in gene pools and genetic mechanisms

3. Genotypes and gene pools
The allele combination possessed by an individual is referred to as genotype
The allele frequencies present in a population is referred to as the gene pool

4. Natural selection Variation Overpopulation ‘struggle for existence’
‘survival of the fittest’
Inheritance of successful variations
Change in gene frequencies

5. Variation Inheritable differences due to different allele combinations
These are due to
Mutations changes in DNA
Mixing of alleles during sexual reproduction meiosis and fertilisation

6. Mutations Changes in DNA These can be Beneficial eg disease resistance
Harmful eg haemophilia
Neutral eg tongue rolling

7. Sexual reproduction
Crossing over – swapping of genetic material between homologous chromosomes
Random segregation during meiosis – its random which combination of alleles ends up in each gamete
Random combination of gametes at fertilisation – its random which gametes end up together

8. Struggle for existence
More offspring are produced than can survive so most will die or be killed
This leads to winners - survivors with more offspring and losers – those with less offspring
Winners pass on their genes to their offspring, so more of the next generation will have the successful allele combinations

9. Selection pressures
Anything that can affect survival or reproduction. They include:
Competition eg for food, water, shelter, mates, nesting sites, etc
Predation – eg being eaten, also disease or parasites
Environmental forces – eg heat, cold, drought, fire, flood, snow, high UV radiation, etc

10. Examples of natural selection
Insecticide resistance in insects
Variation exists in the population – so some are more resistant to chemicals than others
When they are sprayed, the resistant forms are more likely to survive
These will breed and pass on their resistant alleles to their offspring
After several generations, the population will consist of mainly resistant insects
Antibiotic resistance in bacteria
Variation exists in the population – so some are more resistant to antibiotics than others
When they are treated, the resistant forms are more likely to survive
After several generations, the population will consist of mainly resistant bacteria
Peppered moths
2 forms exist – dark and light
Dark forms are more easily caught and eaten by their predators (birds) if they live on light coloured trees or walls, light forms are more likely to be eaten if their surroundings are dark
In unpolluted areas, trees are pale and the dark forms decrease
In polluted areas, trees are darkened with pollution and the pale forms decrease
In this case the selection pressure (predation) is affected by the colour of their surroundings

11. Artificial selection
This is – where humans provide the selection pressures and breed for certain characteristics
Examples include – domestic animals eg dogs, cats, pigeons, sheep, cattle, etc and crops eg wheat, rice, fruit, vegetables, etc

12. Random genetic drift
This is change in gene frequencies due to chance processes, not selection pressures. The alleles affected are usually survival neutral.
It occurs most often in smaller populations because variation is usually lower to start with, and with fewer individuals, the loss of one or two can have large effects on the alleles remaining in the population
Founder effect refers to random genetic drift occurring in small isolated populations eg Dunkers in USA

13. Speciation
Speciation refers to development of new species – population is so different to the original that they no longer can interbreed
A species is a group of organisms that, under natural conditions, can breed to produce fertile offspring
Races or subspecies are smaller breeding groups within a species that exhibit differences from other breeding groups. If separated they may develop into new species
Problems with the species concept include fossils or extinct species – can’t breed these, ring species – breed with intermediate populations, but those at the ends won’t breed with each other

14. Isolation
Isolation prevents gene flow, thus reducing the inflow of alleles selected against – this intensifies the effect of natural selection
Barriers include:
Geographical – rivers, oceans, mountains, deserts, etc
Reproductive – features that prevent interbreeding
Mechanical – physical or anatomical differences that prevent mating eg different copulatory organs
Behavioural – different mating calls or dances
Ecological – different breeding locations, mating seasons
Hybrid sterility – either no offspring are produced, or offspring are sterile

15. Migration Migration includes immigration coming in to a population
emigration leaving a population
Migration into new areas allows organisms to be subjected to a new set of selection pressures, and hence a new round of evolution
In some cases, organisms unsuited to an environment can emigrate, thus increasing the effects of natural selection
In other cases, organisms unsuited to an environment can immigrate, thus decreasing the effects of natural selection

16. Extinction Extinction is the disappearance of a species
Organisms become extinct when the selection pressures become too great for species survival
Examples of extinction events and their causes include catastrophes eg volcanoes & meteor hits; climate change; sea level change; human activity
Examples of organisms made extinct due to human activity include: dodo & Tasmanian tiger – hunted to extinction
Examples of organisms at risk of extinction due to human activity include: rhino, giant panda, elephant, etc – includes hunting, habitat loss, effects of pollution