Evolution
Dec 18, 2015
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 insectsVariation exists in the population – so some are more resistant to chemicals than
othersWhen they are sprayed, the resistant forms are more likely to surviveThese will breed and pass on their resistant alleles to their offspringAfter several generations, the population will consist of mainly resistant insects• Antibiotic resistance in bacteriaVariation exists in the population – so some are more resistant to antibiotics than
othersWhen they are treated, the resistant forms are more likely to surviveThese will breed and pass on their resistant alleles to their offspringAfter several generations, the population will consist of mainly resistant bacteria• Peppered moths2 forms exist – dark and lightDark 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 decreaseIn polluted areas, trees are darkened with pollution and the pale forms decreaseIn 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 driftThis 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
IsolationIsolation 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,
etcReproductive – 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
ExtinctionExtinction 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