POPULATIONS AND EVOLUTION

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HARDY WEINBERG PRINCIPLE

SPECIES - defined as a group of similar organisms that can reproduce to give fertile offspring

POPULATION - group of organisms of the same species living in a particular area at a particular time - potential to interbreed. Species can exsist as one or more population e.g. groups can exsist in different parts of the country.

GENE POOL - the complete range of alleles present in a population.

ALLELE FREQ - how often an allele occurs in a population. Often given as a percentage of the total population e.g. 35& or 0.35

HARDY WEINBERG PRINCIPLE - predicts the freqeuencies of alleles in a population wont change from one generation to the next. However this is only true under certain conditions, there has to be a large population with no immigration, emmigration, mutations or natural selection and there has to be random fertilisation - all possible geneotypes breed with all others. The equation can be used to calculate the frequency of particular alleles, genotypes and phenotypes within populations. They can also be used to test whether or not the principle applies to particular alleles in participating populations e.g. to test whether selection/any other factors are influencing allele frequencies - if frequencies do change then theyre are influencing factors.

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HARDY WEINBERG PRINCIPLE

Allele frequency - p + q = 1. P = frequency of dominant allele. Q = frequnecy of recessive allele.

Genotype and phenotype frequency - p2 + 2pq + q2 = 1. P2 = homozygous dominant genotype. 2PQ = heterozygous genotype. Q2 = homozygous recessive genotype.  Genotype frequencies can be used to predict phenotype frequencies e.g. P2 + 2PQ = dominant phenotype, Q2 = recessive phenotype.

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VARIATION AND SELECTION

VARIATION - the differences that exsist between individuals.. Within a species = individuals in a population can show a wide range of different phenotypes.

Although individuals of the same species have the same genes, the have different alleles - causing variation within a species.

Main source of genetic variation = mutations (changes in the base seqeunce lead to new alleles being produced) Genetic variation is also introduced during meiosis (crossing over of chromatids & independent segregation of chromosomes & randome fertilisation of gametes) 

Variation within a species can also be caused by environmental factors e.g. climate, food, lifestyle. 

Most variation within a species = a combination of genetic and environmental factors. Only genetic variation = evolution. 

EVOLUTION - change in allele frequencies over time.

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VARIATION AND SELECTION

Natural selection is one method by which evolution occurs;

  • indidviduals vary in a species because they have different alleles
  • predation, disease and competiton (selection pressures) create a survival struggle.
  • some are better adapted to the selection pressures than others
  • there are differential levels of survival and reproductive success in a population.
  • those with phenotypes than increase survival chances = more likely to survive, reproduce and pass on their genes.
  • a greater proportion of the next gen inherit the beneficial alleles. They are more liekly to survive, reproduce and pass on their genes.
  • frequency of beneficial alleles in the gene pool increases from gen to gen. 
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VARIATION AND SELECTION

Different types of natural selection lead to different frequency patterns;

STABILISING SELECTION - individuals with characteristic towards the middle of the range are more likely to survive and reproduce. Occurs when envitonment isnt changing and reduces the range of possible phenotypes. e.g. animals with average fur length rather than very short = too cold or very long = too hot

DIRECTIONAL SELECTION - individuals with alleles for a single extreme phenotype are more likely to survive and reproduce. Could be in response to an environmental change. e.g. being the fastest cheetah = able to catch more prey

DISRUPTIVE SELECTION - individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce. Characteristic towards the middle of the range are lost. Occurs when the environment favous more than one phenotype. e.g. bird seeds are either very small or very big, corresponding beak sizes are favourable. 

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SPECIATION AND GENETIC DRIFT

SPECIATION - the development of a new species from an exsisting species 

It occurs when populations of the same species become reproductively isolated - changes in allele frequency cause changes in phenotype which mean they can no longer interbreed to produce fertile offspring. Can occur when a physical barrier (flood, earthquake) divides a population of a species, some are seperated - geographical isolation = allopatric speciation. Can also occur when a population becomes reproductively isolated without physical seperated - sympatric selection.

Allopatric speciation requires geographical isolation. Seperated populations will experience different conditions e.g. different climate. The populations will experience different selection pressures & different changes in allele freq could occur; different allleles will be more advantageous in different populations. Mutations will occur independently in each population. Genetic drift may also affect allele freq in one or both populations. Changes in allele freq = differences in the gene pools, causing changes in phenotype freq. Eventually, the different populations wont be able to breed and produce fertile offspring = reproductively isolated. The two groups have becomes seperate species. 

Sympatric selection doesnt require geographical isolation. Random mutations occur within a population, preventing members of the population from breeding. e.g. if a polyploid emerges (more chromosomes than diploid) they cant produce fertile offspring when breeding. If they emerge in a diploid population, theyll be reproductively isolated. If they reproduce asexually (without breeding) a new species could develop. Polyploidy only leads to speciation if its not fatal to the organism & more can be produced. Its more common in plants than animals. 

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SPECIATION AND GENETIC DRIFT

Reproductive isolation occurs in many ways;

  • Seasonal - individuals from the same population develop different flowering or mating seasons/become sexually active at different times of the year.
  • Mechanical - changes in genitalia prevent succesful mating,
  • Behavioural - a group of individuals develop courtship rituals that arent attractive to the main population.

Genetic drift can lead to speciation. Different selection pressures can change the allele freq in two geographically isolated species. This is evolution by natural selection. Evolution can also occur by GENETIC DRIFT - when chance, rather than environmental factors, dictates which indivduals survive, breed and pass on their alleles;

  • individuals within a population show variation in their genotypes.
  • by chance, the allele for one genotype is passed on to the offspring more often than others.
  • so the number of individuals with the allele increases.
  • changes in allele frequency in two isolated populations could eventually lead to reproductive isolation and speciation. 
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SPECIATION AND GENETIC DRIFT

Natural selection and genetic drift work alongside each other to drive evolution but one process can drive evolution more than the other depending on the population size. Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence. In larger populations any chance variations in allele freq tend to even out across the whole population. 

Evolutionary change has resulted in a great diversity of organisms. Diversity of life on earth is the result of speciation and evolutionary change over million of years. There was one population of organisms, the population was divided and the new populations evolved into seperate species. The new species were then divided again and the new populations evolved into more seperate species. This process has been repeated over a long period of time to create millions of species. 

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ECOSYSTEMS

HABITAT - the place where an organism lives ( a rocky shore or a field)

POPULATION - all the organisms of one species in a habitat.

COMMUNITY - populations of different species in a habitat make up a community. 

ECOSYSTEM - a community, plus all the non-living (abiotic) conditions in the area in which it lives. Ecosystems can be small e.g. a pond, or large e.g. an entire ocean. 

ABIOTIC CONDITIONS - non living features of the ecosystem e.g. temp and availability of water. 

BIOTIC CONDITIONS - the living features of the ecosystem e.g. the presence of predators or food. 

NICHE - the role of a species within its habitat e.g. what its eats, where and when it feeds. 

ADAPTATION - a feature that members of a species have that increases their chance of survival & reproduction.

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ECOSYSTEMS

The niche a species occupies within its habitat includes its biotic interactions e.g. the organisms it eats and its predators & its abiotic interactions e.g. the oxygen an organism breathes in, and the carbon dioxide it breathes out. 

Every species has its own unique niche - a niche can only be occupied by one species.If two species try to occupy the same niche, theyll compete with each other with one being more successful.

Adaptations can be physiological (processes inside the body), behavioural (the way an organism acts) or anatomical (structural features of the body). Organisms with better adaptations are more liekly to survive, reproduce and pass on their alleles to their offspring, the adaptations then become more common in the population - NATURAL SELECTION.

Every species is adapted to use an ecosystem ina way that no other species can - has its own unique niche.

Organisms are adapted to both the abiotic conditions (how much water is available) e.g. otters have webbed paws so they can walk on land and water and the biotic conditions (predators) in their ecosystem. e.g. sea otters use rocks to smash open shellfish and clams, increasing their chance of survival as they have access to more food.

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VARIATION IN POPULATION SIZE

POPULATION SIZE - total number of organisms of one species in a habitat.

CARRYING CAPACITY - the maximum stable population size of a species that an ecosystem can support.

Abiotic factors include amount of light, water or space available, the temp or chemical composition of the surroundings. When these are ideal for a species organisms can grow fast and reproduce successfully. When these arent ideal for a species they cant grow as fast or reproduce as successfully. 

Population size also varies because of biotic factors; interspecific and intraspecific competition & predation.

INTERSPECIFIC - when organisms of different species compete with each other for the same resources. This means that resources available are reduced, both populations are limited. This means both populations will have less energy for growth and reproduction, the pop, size will be smaller for both. If one species is better adapted they will out-compete the other. 

INTRASPECIFIC - organisms of the same species compete with each other for the same resources. Populations increase when resources are plentiful, as the pop. grows there will be more organisms competeing for the same resources. These resources then beocme limiting - isnt enough for all organisms & population declines. A smaller population = less competition for space and food, which is better fro growth and reproduction - so the population starts to grow again. 

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VARIATION IN POPULATION SIZE

PREDATION - where an organism (predator) kills and eats another organism (prey). 

Population sizes of predators and prey are interlinked. As prey pop. increases theres more food for predators so predator population grows. As the predator population increases there is more prey being consumed so prey populations decline. This means theres less food for the predators so they starve and die, decreasing the pop. size. There are other factors involved in predator prey relationships (availability of food for the prey) 

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INVESTIGATING POPULATIONS

When choosing an area to sample, it must be randomly chosen as this avoids bias.

Quadrats and transcets are used to investigate non-motile species.

  • Quadrats are square frames, divided into 100 smaller squares. Theyre placed on the ground at different points within the sample area. Species frequency is recorded in each quadrat. Percentage cover is also measured (1 sqaure = 1% - has to cover more than half of the square)
  • Transcets are lines to see how plants are distributed across an area. Belt transcets involve quadrats being placed next to each other along the transcet to calculate species freq. & percentage cover along the transcet. To cover a larger distance, quadrats are placed at intervals (interrupted belt transect)

Mark- Release - Recapture is used to investigate more motile organisms

  • capture a sample of a species 
  • mark them in a harmless way
  • release back into their habitat
  • wait 7 days, then collect another sample from the same population
  • count how many are marked in the second sample
  • no. caught in 1st sample X no. caught in 2nd / no. marked in 2nd sample. - estimates total pop. size. 
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INVESTIGATING POPULATIONS

You can investigate ennvironmental factors and species distribution. 

Distribution of a species changed within a particular area. e.g. you may find more shade loving plants at the edge of a field where they are sheltered by a tree than in the centre where theyre exposed to full sunlight. 

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SUCCESSION

SUCCESSION - process by which an ecosystem changes over time. The biotic conditions change as the abiotic conditions change. Two types of succession; 

PRIMARY - happens on land thats been newly formed or exposed e.g. where a volcano has erupted to form a new rock surface. Theres no soil or organic material.

SECONDARY - happens on land thats been cleared of all the plants but the soil remains e.g. after a forest fire.

CONSERVATION - the protection and management of species and habitats in a sustainable way.

Human activities can prevent succession, stopping a climax community from developing. When succession is stopped atrifically the climax community = a plagioclimax. e.g. a regularly mown lawn wont develop shrubs and trees. 

Conservation often involves preventing succession in order to preserve an ecosystem in its current state of succession;

  • animals are allowed to graze on land - similar to mowing as animals eat growing points of the shrubs and trees stopping them from establishing and helping to keep vegitation low.
  • managed fires are lit, after these secondary succession will occur - pioneer species are being conserved. Larger species will take longer to grow back and will be removed again with the next fire. 
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SUCCESSION

SUCCESSION

  • primary succession starts when a new species colonise a new land surface. Seeds and spores are blown in by the wind and begin to grow. The first to colonise is the pioneer species.
  • abiotic conditions are hostile. Only pioneer species grow because theyre specially adapted to cope with harsh conditions. e.g. maram grass.
  • pioneer species change the abiotic conditions - they die and microorganisms decompose the dead organic material (humus) This forms a basic soil.
  • conditions are now less hostile, new organisms with different adaptations can move in and grow. These then die and are decomposed adding more organic material, making soil deeper and richer in minerals. Larger plants (shrubs) can start to grow helping the soil to retain more water
  • secondary starts in the same way - but as there is already a soil layer succession starts at a later stage. The pioneer species is normally larger plants e.g shrubs.
  • at each stage, different plants and animals that are better adapted for the improved conditions move in, out-compete exsisting species and become the dominant species in the ecosystem. 
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SUCCESSION

  • as succession goes on, the ecosystem becomes more complex. New species move in alongside exsistin species, increasing biodiversity.
  • the final stage is the climax community - the ecosystem is supporting the largest and most complex community of plants and animals possible. Its a stable environment. 

Different ecosystems have different climax communities e.g. in a temperate climate theres ample water, mild temps and not much change between the seasons. The climax community will contain large trees as they can grow in these conditions once deep soils have developed. However in a polar community theres not much water, temps are low and theres huge change between the seasons. Large trees cant grow in these conditions, the climax community only contains herbs/shrubs. 

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CONSERVATION

Theres often conflict between human needs and conservation. e.g. in kenya people rely on livestock for earning a living but overgrazing can destroy grassland for wildlife. 

There are many different methods of conservation. Some focus on conserving a particular species while others protect the habitat for all the species that live there;

  • plants can be conserved using seedbanks (stores of lots of seeds from many different plant species.) If the plant species becomes extinct in the wild, the seeds can be used to grow new plants.
  • endangered species can be bred in captivity to increase numbers, then returned into the wild
  • protected areas (national parks, nature reserves) protect habitats by restricting urban development, industrial development and farming. 
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