Evolution

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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