Unit 4 Biology

Notes from the AQA A2 Biology texbook

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Population definitions

Sere - the name given to stage in succession 

Net productivity - The energy available for the next tropic level after respiration 

Climax community - The end stage in succession 

Population - All the organisms of the same species living together in a particular habitat 

Community - All the populations that live in a particular place at the same time 

Abiotic factors - Physical and chemical features of the enviroment that impact organisms

Gross productivity - The total energy fixed during photosynthesis 

Trophic level - the position a organism occupies in a food chain 

Biotic factors - biological featurs of the enviroment that impacts organisms 

Food chain - linked series of living organisms showing energy flow through ecosystem 

Niche - The role an organims has in an ecosystem, Where it lives and what it eats

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Population definitions con

Habitat - The place within an ecosystem where a population lives 

Biomass - The dry mass of organisms 

Ecosystem - The community of living organisms and the biotic factors that affect them 

Succession - The changes that occcur in an ecosystem over time

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1.1

Population - All the organisms of same species living together in a particular habitat

Ecology - Study of interelationships between organisms and their enviorment

Abiotic - non living Biotic - living Biosphere - life supporting

Community - all the populations that live in a particular place at the same time

Habitat - The place within or ecosystem where a population lives.

Niche - role of organism in ecosystem where it lives and eats.

Number of individuals of species in a given space - abundence. Rpresentative of habitat valid - random sampling, systematic

Qudrats size: depends on size of animals and distribution. Evenly distributed large number of small animals. Position? Statistically significant results random sampling must be used. Number of sample qudrats? Larger number more reliable but time consuming.

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1.1

Random sampling: Avoids bias, ensures valid results.

Measuring abundence - abundence number of individuals of species within a given space. Frequency - likelihood of species occuring in a qudrat - useful when hard to count. - quick idea - no information on density and detialed distribution of species.

Percentage cover - estimate of the area within qudrat that species covers - difficult to count - collect rapidly - less useful occur in overlapping layers eg grass.

Random sampling methods: 1) Lay out two long tape measures at right angles, along two sides of the study area. 2) obtain a series of coordinates by usiing random numbers taken from table or computer. 3) place quadrat at intersection of each pair of coodinates and record species in it.

Systematic sampling along transcets stages zonation. Line transcect any organism over which line passes is recorded - belt transect is a strip, metre wie marker by putting second line parallel to first.

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1.1

Postive correlation - increase in values of one vairable accompanied by an increase in the value of other variable

negative correlation - increase in value of one variable is accompanied by decrease in the value of other variable

Test strength of data and direction - spearmans rank

Estimate abundence estimated pop size: total number 1st sample x second number/ number marked 2nd

No immigration or emigration, few births and deaths, marking not toxic nor liable to predation, marking not lost or rubbed off.

Quantitative - table or graph, easy to compare by statiscial analysis spread of mean - standard deviation. Many factors contribute to differences chance show if differences due to factor or chance.

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1.2, 1.3

Number of individuals in population is pop size growth 3 phases:

1) period slow growth initally small number indiviudals reproduce to slowly build up their numbers.

2) Rapid growth ever increasing number of individuals contiune to reproduce. Doubles during each interval time, increasely steep.

3) Declines, size stable - decline due to food supply or increase predation. Cyclic fluctations due to variations in factors. 

Population is a group interbreeding individuals of same species in a habitat.

Biotic and abiotic influence rate of growth and size of population always limiting factor.

Cold blooded - enzymes work more slowly so metabolic rate reduced. Above less effientively as undergo denaturation. 

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1.3

Warm blooded can maintain body temperature regardless of external temperature. Further from temperature of opitiuim more energy expend to maintain body temperature therefore less for growth mature slowly, reproductive rate slows, population smaller. 

Light in ecosystems - rate of photosynthesis increases as light intensity increases 

-Faster plant grows more seeds produced 

- Population increased 

-Population of animals increase as food source more readily available. 

PH affects action of enzymes optiumim. PH a population of organisms is larger where appropriate PH exists. 

Water and humdity - water scarce only adapted to survive. Humdity affects transpiration rates in plants and evaporation of water. 

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1.4

Competition between same species - Intraspecific competition 

Different species - Interspecific competition 

Availbility of resources determines size of population. 

Interspecific - where population occupy same niche, one will outcompete other, lead to removal of species if condiitons remain same. This is known as competitive exclusion principle. 

States that one that uses resources most effectively will elimate the other. No two speicies can occupy same niche. 

- To show how factor influences size of population it is nessary to link birth rate and death rate of individuals in a population 

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1.5

Predator feed on another organisms known as prey 

Predators evolved better adapting to capturing prey: faster, movement, camflague, detecting prey

Prey adapt to avoid predators: better camoflage, protective features e.g spines, concealment behaviour. 

Predation occurs when 1 organism consumed by another. 

Labatory - limited travel, more potential refuges, not reliable not possible to count all only sampling and survey. 

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1.5

Predator-prey relationship on population size 

- Predators eat prey, reduce population of prey

- Fewer prey, predators greater competition with leftover prey

- Predator population reduced as some unable to obtain prey

- Fewer predators, fewer prey eaten

- Prey population increases 

- More prey for food. Predator population increases 

- But range normally eaten, so less severe 

Other factors such as diease, climatic factors play a part create selection pressure only individuals escaping predators and suriving diease will reproduce therefore evolves to be better adapted to conditions 

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1.6

Explosion of human population: agriculture, manafacturing and trade created industrial revolution. 

Exponential phase in which population grows rapidly. 

Factors affecting growth and size of human population birth rate and death rate. 

Immigration - Joins population 

Emmigration - Leaves population 

Population growth = (births + immigration) - (deaths + emmigrations)

Percentage population growth :

Population change during/ Start period =_______ x 100 

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1.6

Affecting birth rate: cultural, religion, birth control, social pressures

Affecting death: war, medicial care, drinking water, life expentency, age profile. 

Change in societies from life expectancy short, birth rates high to those life expectancy long and birth rates low called demographic transition. 

Future size depends on number of female child bearing age displayed in age population pyramids 

Types of population:

Stable population - birth rate and death balance no decrease 

Increasing popultion - high birth rate, fewer elderly, developing 

Decreasing - lower birth rate, lower mortality, more elderly, developed 

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1.6, 2.1

Surivial curve plots number of people alive as a function of time. 

Typically plots % of population still alive following event such as diease. 

Average life expectancy age which 50% of population still alive can be caluclated by survival curve. 

Plants use solar energy to combine water and c02 into complex organic molecules by process of photosynthesis. Organic molecules break down to make ATP, which is used as energy source to carry out processes.

Energy has a variety of forms: light, heat, sound etc 

- Changed from one to another 

- Cannot be created or destoryed 

-Measured in joules 

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2.1

Energy needed for: metabolism, maintence, movement, active transport, repair and division, production substances, maintance body temperature. 

Energy and metabolism 

1) Light energy from sun converted by plants into chemical during photosynthesis 

2) Chemical energy from photosynthesis in organic molecule converted ATP during respiration 

3) ATP used by cells to perfom useful work 

How ATP stores energy 

- 3 phosphate groups

- Bonds between unstable low activation energy, easily broken 

- Release energy when broken 

ATP + H20 > ADP + Pi + E

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2.1

Water used to convert ATP to ADP known as hydrolysis. ATP to ADP reversible. 

Synthesis ATP from ADP addition phosphate occurs 3 ways: 

- Photophosphorylation: chlorophyll containing plants during photosynthesis 

- Oxidative phosphorylation: Occurs in mitochondria of plant and animal cells during process of election transport 

- Substrate level phosphorylation: Occurs in plant and animal when phosphate transferred from donor to ADP to ATP e.g pyrvate formation 

1st two causes ATP by energy released during transfer electrons during chain of electon carrier molecules.     ATP - release of energy, Good donor 

- Unstability (not good as long term store), immediate energy source, rapidly reformed, less energy, manageable, single reaction not long series. 

- Made in mitochondria cells such as muscle fibres epithelium. Needed for metabolic processes, DNA, movement, active transport, secretion, activation of molecules 

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3.1

Energy flow by light energy transformed chemical energy use to produce ATP during respiration. Leaf sire of photosynthesis < chloroplasts 

Adaptions: Large surface area, arrangment, thin diffusion short, transparent, stomata for gas exchange 

Photosynthesis: 6C02 + 6H20 > C6H1206 + 602

3 main stages of photosynthesis 

10 Capture light by chloroplast pigments such as chlorophyll 

2) Light dependent reaction - light converted into chemical, water split photolysis into protons, electons and oxygen. Products reduced NADP, ATP and oxygen

Chloroplast surronded by double membrane inside: grana stacks disc structures called thylakoids where light dependent takes place. 

Stroma - fluid matrix, light independent within stroma, Starch grains 

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3.2

Light in light dependent used for:

pi + ADP = ATP

Splitting water into H+ and OH ions, caused by light known as photolysis

Oxidation - loss of elections or loss of hydrogen or gain of oxygen.

 Reduction - gain of electons, or gain of hydrogen or less of oxygen 

H+ taken by electron carrier NADP, NADP reduced 

Enter light indepedent along electrons from chlorophyll molecules. The oxygen by product from the photolysis of water is either used in respiration or diffuses out of the leaf as waste product of photosynthesis.

Site of light dependent reaction: on thylakoids of chloroplasts stacked in groups called grana. Adaption: large surface area for attachment of chlorophyll, electron carriers and enzymes that carry out light depedent reaction. Network of proteins in grana hold chlorophyll max absorption light. DNA and ribosomes quickly make proteins.  

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3.2

Making ATP

- Chlorophyll molecule absorbs light energy boosts energy of a pair of electrons within chlorophyll molecule raising them to higher energy level. 

- Electrons leave chlorophyll by electron carrier chlorophyll oxidised, electron carrier reduced 

- Electrons passed in series oxidation reduction reactions from electron transport chain located in membranes of thylakoids 

- Electrons lose energy each stage 

- Energy used to combine pi with ADP = ATP. Loss of electrons when light strikes chlorophyll molecule leaves it start of electrons. To contiune absorbing light electrons must be replaced. Provided by water spent by light energy 

2H20 > 4H+ + 4e- + 402

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3.3

Light independent 

Does not require light but requires products from light dependent so rapidly ceases when light absent. Takes place in stroma, calvin cycle. 

1) C02 diffuses into leaf by stomata dissolves in water around walls of mesophyll. Diffuses through plasma membrane, cytoplasm + chloroplast into stroma of chloroplast

2) In stroma c02 combines RuBp using rubico enzyme

3) C02 + RuBp = 2x glycerate-3-Phosphate

4) Atp and reduced NADP from light dependent reaction used to reduce activated glycerate-3-phospate to triose phosphate.

5)NADP re-formed goes back to light dependent reaction to be reduced again by accepting more hydrogen

6) Triose phosphate converted into glucose

7) Triose phosphate to regernate RuBp using ATP from light dependent   

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3.3

Site of light independent: Stroma 

Adaption: 

Stroma contains all enzymes to carry out light independent reaction 

Stroma fluid surronds grana so products of light dependent can readily diffuse into stroma 

Contains DNA and ribosomes so quickly and easily manfacture protein for light independent 

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3.4

Factors affecting photosynthesis - Rate of the process at any given moment is not affected by all the factor, rather by the one whose level least favourable - limiting factor. - limits rate of processes.

C02, light intensity, PH

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4.1

Aerobic respiration: requires 02 and produces 02, water and ATP

Anaerobic respiration; Absence of 02 produces lactate ( animals) ethanol and c02 (plants) little ATP 

Glycolysis - cytoplasm 

Stages of respiration; Glycolysis - cytoplasm 

1) Activation of glucose by phosphylation phosphate molecules from hydrolysis of ATP to ADP. Provides energy to activate glucose (lowers activation energy) 

2) Splitting phospharylated glucose 

3) Oxidation triose phosphate 

4) Production ATP, 2 ATP regernated ADP glucose oxidsed to pyruvate 

-Net gain 2 ATP, 2 reduced NAD, 2 pyruvate 

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4.2

Link reaction - matric of mitochondrion, pyruvate oxidation by removing H 

H accepting by NAD = reduced NAD = acetyl produced

- Acetyl combines C0A = acetyl COA 

- CO2 formed each pyrvate 

- Pyruvate + NAD + COA = actyl COA 

Krebs - matricc mitrochondrion 

1) actyl COA conbines 4c molecule = 6 c molecule 

2) 6c molecule loses C02, hydrogens = 4c, ATP produced by substrate level phosphorlation 

3) 4c molecule combine new actyl CoA 

Produces (single pyruvate): reduced NAD,FAD, ATP, 3c02, occurs twice due to 2 actyl COA. NAD works with dehydogenase enzymes that catalyse the removal of H from substrates and transfer to other molecueles such as NAD involved in oxidative phosphylat

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4.2, 4.3

Signiifance of krebs - breaks down molecules 

Produce H carried by NAD to electron transport for oxidtive phosphylation = production ATP 

- Regenates 4c molecuke 

- source of intermediate compounds for making amino acids, fatty acids 

Election transport chain - cristae 

Energy of electrons in H converted into ATP. On cristae are enzymes and proteins involved in ETC e.g ATP synthase. Mitochondria occur great numbers in metabolic active cells. Densely packed criaste greater surface area synthesis of ATP. 

Synthesis of ATP - Reduced NAD and FAD donate e- of H to 1st molecule in ETC. Releases protons from H, actively transported across inner mito membrane. Electrons pass along ETC series of oxidation-reduction reactions.

Some energy used to ADP + pi= ATP. Remaining released as heat 

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4.3, 4.4

Protons accumlate in space between 2 mitochondrial membrane before diffuse back into matrix through protein channel ATP synthase. 

Electrons combine with protons and 02 to form water. 02 final acceptor of electrons

Importance of 02; 

Removing H without H and e backup along chain and respiration comes to a halt. 

Occurs with cynide, noncompetive inhibitor of final enzyme

Anaerobic respiration - only glycolysis. Products pyrvate + H removed

Energy occurs in 2 ways: Substate level phosphorylation: glycolysis + krebs pi to ADP= ATP

Oxidative phosphorylation in electron transport indirect linking pi and ADP = ATP. Use of H from glycolysis and krebs carried on NAD and FAD most ATP produced this way. 

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4.4

Pyrvate and reduced NAD > latate + NAD 

Occurs in animals when storage of 02, surivial value e.g baby mammal after birth and animals living in water

-Latate production occurs in muscles as result of strenous exercise. 02 used up more rapidly than supplied. 

- In absence of 02, glycolysis would normally cease due to accomiation, NADH 

- Pyrvuate takes up 2 H atoms from reduced NAD to form latate 

- At some point lactate needs to be oxidised back to pyrvate occurs when 02 available again. If not lactate cause cramp and muscle fatigue - lactate however removed by blood and taken to liver to convert to glycogen 

Energy occurs in 2 ways: Substrate level phosphorylation: glycolysis, krebs. pi to adp = atp. Oxidative phosphorylation in electron transport indirect linking pi+adp=atp. Use of H from glycolysis, krebs carried on NAD and FAD most ATP produced this way. 

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5.1

In food chain + Food web there are producers, consumers, decomposers. Each stage in chain is referred as tropic level. Producers are photosynthetic manufacture organic structures using light. 

6C02 + 6H20+light >glucose and oxygen 

Directly eating producer called primary consumers

Decomposers break down dead matter 

-Most animals do not rely upon single food source in single habitat - many food chains linked together to form a food web.

Problems with food webs - relationships not fixed depend on time of year, age, pop size. Help understand populations. 

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5.2

How is energy lost? - Cloud reflect suns energy back into space or absorbed by atmosphere. Not all wavelengths of light are absorbed and used ofr photosynthesis. 

Light may not fall on chlorophyll molecule - limiting factor, limit rate of photosynthesis 

Total quantity of energy called gross production. Rate at which they store energy is called net production. Net production = gross - repiratory losses 

Low percentage of energy transferred to each stage 

- Same not eaten 

- Cannot digested lost in faces 

- Lost as heat, energy needed to maintain body temp when heat constantly lost to enviroment 

Inefficiently of energy transfer. Only 4 or 5 trophic levels because insufficent energy to support large enough breeding. Total mass of organisms in particular place is less at trophic levels. Total amount energy less at each level as one moves up food chain. Energy transfer= energy after transfer/before transfer x 100 

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5.3

Food chains + webs useful to show eating and energy flows but do not provide quantitative useful to know number, mass or amount at each trophic level to construct. 

Ecological pyramids 

Drawbacks to pyramids numbers 

- Size not taken into account, not pyramid at all

- Number so great impossible to represent accurately on same scale

Pyramids biomass 

- Move reliable quantitve. Biomass - total mass of plants and/or animals in a particular place. 

Fresh mass easy assess but varying amounts of water makes it unreliable. Dry mass overcomes problem but: killed organism, small sample, not representative, measured grams per square metre, only particular time. 

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5.3, 5.4

Pyramid of energy 

Difficult + complex collecting data 

Most accurate 

Set period taken 

More reliable as two organisms could store dry mass differently 

Agricultural ecosystems made up domesticated animals + plants to produce food for mankind. Bad effect on other organisms in food web 

Productively rate something is produced. Rate at which plants assimilate energy called gross productivity KHm-2year-1 

-Energy used, remainder known as net productively, available to next orgainsm.

Net productively = gross productivity - respiratory losses 

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5.4

Net productivity affected by: Efficiency of crop at carrying out photosynthesis. This is improved if all necessary conditions for photosynthesis are supplied. Area of ground covered by the leaves of the crop. 

Most land in Britiain would be covered by forest if left to develop naturally. This is known as climax community. 

To maintain agricultural, prevent climax community by excluding most species in that community, leaving only the crop to grow. 

- To remove max growth requires energy plough, sow

Energy comes in 2 forms: food - farmer work on farms expend energy as work. Energy comes food they eat. Fossil fuels - fuel to plough, harvest, transport. 

Productively, low in natural additional energy increases productivity reducing limiting factors exclude other species, reduced competition. Fertilisers for ions etc. 

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5.5

Pesticides kill pests. Effective pesticide should;

- Be specific, only toxic to organisms 

- Biodegrade, break down into harmless but needs to be chemically stable for shelf life. 

- Be cost-effective as pesticides remain useful only for limited time as develop genetic resistance. 

Chemical pesticides 

- some effect on non-target species 

- must be reapplied - expensive 

-genetic resistance so new pesticides have to be developed 

Integrated pest control: All forms of pest control, minimum impact on enviroment, maintainign crp, animal or plant. 

One type of plant - monoculture 

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5.5

- Not accumulate - does not build up or pass along food chains 

pest-organism compete with humans fo food or space or could be a danger to health 

Biological control 

- do not act quickly, interval of time between introducing and reduction 

- control organism can be pest

- may alter to eating crops

-reproduces itself 

-specific 

-no resistant in pests

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5.6

Intenstive rearing - produce max yeild at lowest possible costs 

Converting smallest possible amount of food energy into the greatesr quantity of animal mass.

Movement restricted less energy used in muscle contraction 

enviorment warm to reduce heat loss 

feedin controlled, optiumim and type of food maximum growth = no wastage 

predators excluded no loss at other organisms in food web 

Other ways of improving energy conversion rate: selective breeding of animals to produce varietes more effiecent at converting the food they eat int body mass, using hormones to increase growth rates, regularly given antibodies, stress, harm to other debeaked, reduce genetic diversity, use fossil fuels 

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6.1

Human actitivies influencing c02, carbon cycle

- Combustion of fossil duels - release c02 from burning coal, oil, peat. 

- Deforestion - removing natural sinks by removing photosynthesising biomass so less c02 is removed from atmosphere 

c02 is a greenhouse pas - contributing to global warming. 

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6.3

Nitrogen cycle 

- Plants absorb nitrogen in form of nitrate ions, use active transport by root hairs 

- Nitrate ions souble, easily leach through soil 

4 stages of nitrogen cycle;

Ammonification - production ammonia by decomposers(death by animals) and nitrogen fixing bacteria (N from atmosphere).

Nitrification- ammonia>nitrite>nitrate N02-N03 

Oxidation reaction, releases energy. Occurs by nitrifing bacteria 

Nitrogen fixation: nitrogen gas > nitrogen compounds. 

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6.3, 6.4

Nitrogen fixing bacteria use to manufacture amino acids released when die. Mutulistic nitrogen fixing, live in nodules, legume plants, obtain, carbs from plants. Plant acquies amino acids. 

Denitification: anaerbic, nitrates>nitrogen hass. Reduces aviliablity of nitrogen containing compounds for plants. Occurs when storage of 02 so crops kept aerated to prevent build up of dennitifying bacteria. 

Minerals removed, need nitrogen to grow replaced by fertilisers to prevent being a limting factor. Natural fertisliers: dead, decaying. Artifical: deposits blend together, nitrogen present. Nitrogen needed protein and DNA increase photosynth, crop productivity. 

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6.5

Nitrogen fertilisers effects: 

Reduces species diversity - favour grasses, nettles and rapid growing species, outcompete other species.

Leaching - pollution

Eutrophication, caused by leaching 

Leaching 

Nutrients removed soil

Rain dissolve soluble nitrates carry to soils and plant roots find way to water courses 

Result eutrophication 

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7.1

Succession term to describe changes over time in the species that occupy a particular area.

First stage colonisation by pioneer species, features to suit. 

-Vast production

-Rapid germination no period dormancy 

-Fix nitrogen from atmosphere for nutrients 

-Tolerence to extreme conditions 

Other speicies then outcompete species in existing community. Change abiotic enviroment. Ultimate community likely to be deciduous oak woodland. Stable state comrises balance equilbrium, climax community. Animals undergone similar series determined by plant types, food, habitats. 

In climax, dominant plant and animal species 

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7.1

During succession number of features emerage. 

- Non living enviroment less hostile, soil nutrients leads to

greater number and variety habitats produce:

Increased biodiveristy ,

More complex food webs - increased biomass 

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7.2

Conservation management of earths natural resources in such a way that max use of them can not made in future 

- maintain ecosystem 

-biodiversity 

Reasons:

- ethical, respect, occupied longer

-economic

-gene pool, valuable future long term productivity 

- cultural and aesthic, interests, inspires 

Conservation of succession 

Manages species prevents change to next stage 

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8.1

Genotype - genetic makeup 

Alleles - different verison of a gene 

Changes to DNA called mutation and it is inherited if occurs in formation of gametes 

Phenotype - observable characteristics enviroment can alter phenotype called modificaiton 

Gene- section of DNA sequence of nucleotide bases codes for particular polypeptides

Position of a gene on chromosome - locus 

Pairs of chromosomes - homologus chromosomes 

Alleles the same homozygous 

Alleles different hetrozygeous 

Expressed pheno - dominant, Not expressed - recessive 

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8.1, 8.2

Two alleles contribute to pheno - co-dominant. Blend of features 

Gene with more than 2 allelic forms multiple alleles e.g blood grouping 

Pure breeding homozygous, two alleles are same for particular gene. 1st offspring of pure breeding called F1 generatio. Basic law of genetics (law of segregation):

In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete. 

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8.3, 8.4

Y= smaller in size, shaped differently. Sex determined by chromosomes not genes 

Characterstics controlled by recessive alleles on non homologyous X chromosome appear frequently in the male. 

This is because no homologyous on Y that might have dominant allele.

X carries many genes 

Haemophilla - blood clots slowly 

Cause - recessive allele with altered DNA nucleotides that therefore do not code for required protein. Result not able to produce protein for clotting process. 

Co-dominance - both alleles equally dominant. Multiple alleles - more than two alleles, of which only two may be present at the loci of an individuals homologous chromosomes. 

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8.5

All alleles of all genes in a population at any one time called gene pool.

Number of times an allele occurs within a gene pool is reffered to as the allelic frequency 

Hardy weinberg principle - calculate frequencies of the alleles of a particular gene in a population. Predicts the proportion of dominant and recessive alleles of any gene in a population. 

Remains the same from 1 gen to next. No mutations, isolation, no selection, large population, mating random. 

p+q= 1,     2pq = px q x 2

frequency p- dominant 

frequency q- recessive 

percentage p2- pheno

percentage q2 - pheno

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8.6

Difference between reproductive success of individuals affects allele frequency process works like this: 

organisms produce more offspring than can be supported

remain constant size

compeition to be ones surivie 

in population, gene pool variety alleles 

better able to survive in competition: obtain resources, grow rapid and live longer, successful  breeding and mroe offspring 

pass on alleles to next gen, advantage passed on 

 over generations, advantageous increase. Non advantageous decrease. Advantageous depends upon enviromental conditions at any one time. Subjected to selection based on suitability and for suriving. 

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8.6, 8.7

- favour one direction from mean, favour extreme - directional 

- favour average, stablising, select against extremes, stablising 

Plot variation on graph to get normal distribution curve 

Speciation evoluation new species from existing species. A species group of individuals share similar genes and capable of breeding with another and produces fertile offspring - same gene pool. 

Allopatic specication - physical barrier prevents 2 populations breeding. Selected better for conditions genes so great, different species. No longer interbreed. 

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