Biology - Paper 2: topic 7/ genetics/ populations, evolution and ecosystems

Definition: Gene

• a section of DNA that codes for amino acids for a specific polypeptide - same genes are located at the same locus on a chromosome

Definition: Allele

• you can have one or more versions of a gene - these different versions are called alleles

• different alleles for the same gene have slightly different bases so code for different characteristics (blue eyes/brown eyes)

Definition: homozygous

• a diploid organism carries two of the same alleles - it is homozygous at that locus

Definition: heterozygous

• a diploid organism carries two different alleles - it is heterozygous at that locus

Definition: genotype

• the genetic constitution of an organisms - all the different alleles an organism has

Definition: Phenotype

• the expression of the genotype, and its interaction with the environment

Definition: Dominant

• dominant allele is always expressed in the phenotype

• written as an uppercase letter

Definition: recessive

• only appear in phenotype in the organism if there are no dominant alleles for that gene

• expressed as a lower case letter

Definition: codominant

• when both dominant alleles for a gene at the same locus are expressed

• Heterozygous dominant

Definition: autosomal linkage

• some characteristics are sex-linked, meaning the alleles taht code for them are located on the sex chromosomes

• most genes are carried on the X chromosome as the Y chromosome is shorter

Definition: diploid cell

• a cell that contains two complete sets of chromosomes

• humans are diploid organisms most cells contain 23 pairs of chromosomes

Monohybrid inheritance (cross)

• inheritance of a single gene

• two alleles of a gene are involved (one maternal/one paternal)

• dominant alleles are expressed if (DD/Dd)

• receive alleles are expressed if (dd)

Dihybrid inheritance

• Dihybrid inheritance = the inheritance of two characteristics controlled by different genes

  • each of the two genes will have different alleles

    • whenever a dihybrid cross with heterozygous parents you get a 9:3:3:1 phenotypic ratio

Definition: phenotypic ratio

• phenotypic ratio = the ratio of different phenotypes in the offspring

  • genetic diagrams let you predict phenotypic ration of F1 and F2 offspring

sex-linkage inheritance

• characteristics are sex-linked to the sex chromosomes

• XX = female

• XY = male

• Y chrosmome is much shorter than the X

• most genes are carried on the X chromosome

• males often only have one allele for sex-linked genes (as they only have one X chromosome)

  • this makes males more likely than females to show recessive phenotypes for genes that are sex-linked

autosomal linkage

• autosome = all chromosomes that are not sex-chromosomes

• autosomal linkage= when two or more genes are located on the same autosomal chromosome

• all linked genes remain together (assuming no crossing over)- so both genes and their alleles are inherited together

  • higher proportion of offspring will have the parents phenotype/genotype

Definition: epistasis

• epistasis occurs when the allele of one gene masks (blocks) the expression of the allele other genes

what are the two types of epistasis

• dominant epistasis -

  • the epistatic allele is dominant

    • so having one copy of the allele will mask the expression of the other gene

• recessive epistasis -

  • if the epistatic allele is recessive

    • two copies pf it will mask the expression of the other gene

the Chi-squared test

• it is a statistical test, to see if the if the results of the experiment support the theory

  • compare how well the observed results match the expected results

• can use chi-squared test to test theories about the inheritance of characteristics

• Method: calculate chi-squared value and then compare it to critical value, to see if observed results and expected results are significant and the null hypothesis can be rejected

• calculating Chi-Squared value:

  X² = (observed results - expected results)²/ expected results

Definition: species

• a group of similar organisms that can breed to produce fertile offspring

Definition: population

• is a group of organisms of the same species living in a particular area at a particular time

Definition: gene pool

• the complete range of alleles present in a population

Definition: allele frequency

• how often an allele occurs in a population

what is the Hardy-Weinberg principle

• a mathematical model that predicts the frequency of alleles in a population won’t change from one generation to the next

what are the 6 criteria for the Hardy-Weinberg principle to be true

• large population

• no immigration / emigration

• no mutations

• no natural selection

• Mating has to be random

• organism has to be diploid

what are the 2 Hardy-Weinberg equations

• used to work out allele frequency

  p+q = 1

  • p = frequency of one allele (dominant)

  • q = frequency of other allele (recessive)

• used when dealing with genotype frequencies

  p² + 2pq + q² = 1

  • p² = frequency of homozygous dominant genotype

  • 2pq = frequency fo heterozygous genotype

  • q² = frequency of homozygous recessive genotype

what causes variation due to genetics

• mutations - changes in number or structure of chromosomes passed onto offspring

• Meiosis - independent assortment/ crossing over: produce new combinations of alleles in the gametes

• random fertilisation of gametes - during sexual reproduction it is random which gametes fuse during fertilisation

what causes variation due to environment

• the conditions in which the organism develops can cause variation

• identical twins have identical genetics for height but can be different hight if they experienced different environments (eg. diet)

Definition: evolution

• frequency of an allele changing over time

• can occur by:

  • genetic shift

  • natural selection

Natural selection - process

Definition: selection pressures

• organism face many pressures that affect their chances of survival, such as predation, disease and competition - known as selection pressures

  • competition can be:

    • interspecific - other species

    • intraspecfic - own species

• process of evolution via natural selection:

  1. there is a variety of phenotypes in a population

  2. an envirmonetal change occurs, changing slection pressures

  3. some individuals posses advantageous alleles to the new selection pressures

  4. those with advantageous alleles have incase chance of survival and reproduction

  5. so more likely top pass on their advantageous alleles to their offspring

  6. over many generations the frequency alleles in a gene pool change

     • advantageous alleles increases

     • non-favours alleles decrease

what increases a populations chances of survival to change in selection pressures

• higher variation in the genotype and phenotype

what are the three types of natural selection

• stabilising selection - environment stays the same and individuals with alleles for characteristics closets toward the middle of the range are more likely to survive and reproduce

  • EG. Birth weight

• directional selection - individuals with allele for a single extreme phenotype are more likely to survive and reproduce

  • can be response to environmental changes

  • EG. smaller fish favoured in hotter sea

• disruptive selection - when individuals with either extreme of the phenotype are more likely to survive and reproduce

  • occurs when environment favours more than one phenotype

  • EG. Beak size large/ small favoured - middle size not favoured

Definition: speciation

• the development of a new species from a pre-existing species

• occurs when populations of the same species become reproductively isolated

  • changes in allele frequency cause changes in phenotype, which mean they can no longer inbreed to produce fertile offspring

what is allopatric speciation

• occurs when a population of the same species becomes geographically isolated - physical barriers to stop inbreeding

• the environmental conditions either side of the barrier vary: so each population will experience different selection pressures

  • overtime allele frequency changes in each population through:

    • natural selection (in each population different alleles are advantageous)

    • mutations occur independently in each population

    • genetic drift may also affect allele frequency

• over time this leads to speciation

what is sympatric speciation

• sympatric speciation can occur when a random mutation prevents an individuals in a population taht have that mutation from breeding with other embers of the population the don’t carry the mutation

  • mutation can reproductive isolation by:

    • change in behaviour (courtship/ sleep, wake cycle etc)

    • autonomical changes (changes to genitalia)

reproductive isolation

• reproductive isolation occurs because of changes in alleles, genotype and phenotypes preventing indiuduals with these changes successfully breeding with individuals that don’t have the same changes

• changes that can lead to reproductive isolation include:

  • seasonal changes - individuals develop different flowering or mating schedules, or become sexually active at different times of the year - so can’t breed together

  • mechanical changes - changes in the size, shape or function of genitalia can prevent successful matting: preventing individuals from breeding

  • behavioural changes - changes in courtship behaviour: cannot attract individuals for matting, preventing individuals from breeding with each other

genetic drift

• evolution can occur by genetic shift

• chance dictates which alleles are passed on to offspring

• how it works:

  • individuals within a population show variation in their genotypes

  • by chance, the allele for one genotype is passed on to more offspring than the others, number of individuals with that allele incase

  • if by chance the same allele is passed on to offspring more often again and again, it can lead to evolution as the allele becomes more common in the population (increase allele frequency in population gene pool)

relationship between genetic drift and population size

• evolution by genetic shift has greater effect in smaller populations

  • as chance has a greater influence

  • in larger populations any chance factor effects are diluted due to large population size

Definition: ecosystem

• an ecosystem is all the organisms within the community and all the abiotic conditions in which they live

Definition: biotic and abiotic conditions

• biotic = living (prey/ predators)

• abiotic = non-living (temperature/ soil pH)

Definition: habitat

• the place in which an organism lives within an ecosystem

Definition: niche

• a niche is the role of a species within its habitat

• the niche a species occupy includes:

  • its biotic interactions - what it eats/ is eaten by

  • its abiotic interactions- temperature an organism lives/ time of day it is active

• each specific niche can only be occupied by one species

Definition: adaption

• an adaption is a feature that members of a species have that increases there chances of survival and reproduction

• these adapted feature can be:

  • anatomical - structural

  • behavioural - defiance behaviours

  • physiological - process in the body

• these adaptions incase survival and reproduction, increasing chances of advantageous allele being passed on

  • adaption becomes more common

  • Natural selection

Definition: population

• a population is all the organisms of one species in a habitat

Definition: carrying capacity

• the maximum stable size of a population that an ecosystem can support

• carrying capacity various due to the biotic and abiotic factors

how do biotic factors effect population size

• the population size of any species varies depending on abiotic factors

  • light/ temp/ space/ water availability

• when abiotic conditions are ideal for a species, organism can grow more quickly and reproduce more succsfully increasing population size

How do biotic factors effect population size

• interspecific competition

  • competition form other species for resources

  • if two species are competing, but one is better adapted to surroundings it will outcompete the other species

• intraspecific competition

  • competition from own species, competing for the same resources

  • follows a cyclical pattern

• predation

  • when an organism kills and eats another organism (prey)

  • population size of predator and prey are linked

Definition: abundance

• abundance is the number of individuals of one species in a particular area

Definition: distribution

• distribution refers to where a particular species is within a area your are investigating

• (describes how individuals are spread out within an area)

what are the two types of sampling (for non-motile species)

• random sampling - using a quadrat

  • reduces bias/ ensures reliability

  • take a few samples, estimate rest of population using the mean data

• systematic sampling - using a transect

  • belt transect - quadrats placed next to each other along transect

  • interrupted belt transect - quadrants placed at regular intervals

the mark-release-recature technique

• used to measure the abundance of motile species

  Total population size=

  No caught in 1st sample x No caught in second sample

  divided by number marked in 2nd sample

• method:

  1. capture a sample of species using an appropriate technique and count them

  2. mark them in a human way

  3. release them back into the habitat

  4. wait a period of time (2 weeks)

  5. take another sample from the population

  6. count how many in the second sample are marked

  7. calculate total estimated population by using this equation:

     Total population size=

     • No caught in 1st sample x No caught in second sample

     • divided by number marked in 2nd sample

what assumption does the accuracy of the Mark-relse-recature method rely on

• there has been no change in population size during the study (birth/death/ migation)

• marking hasn’t effected individuals chances of survival

• marked individuals have been given enough time and opportunity to mix back in with population

• marking remains visible (haunts rubbed off)

Definition: succsession

• succession is the process by which an ecosystem changes over time

• succession occurs in a series of stages

Definition: primary succession

• occurs when newly formed, bare rock or newly barren land - is first colonised by a pioneer species

• new land can form by: volcano erupting and forming new rock/ sea levels falling exposing new land

describe the stages of succession

1. primary succession, when pioneers species first colonies new land

   • that has hostile abiotic conditions that only pioneer species can grow as they are specially adapted

2. pioneer species changes the abiotic conditions

   • they die and microorganism decompose the dead organic matter, which forms a basic soil

3. this makes conditions less hostile, so new organisms with different adaptions can move in a grow

4. the new organisms then die and are decomposed adding more organic material, making the soil deeper and richer in minerals

5. larger plants can start to grow in the deeper soil that retains more water and contains more nutrients

6. some new species may change the environment so that it becomes less suitable for the previous species

later stages of succsession:

• different plants and animal are better adapted for the improved conditions ion each stage

  • so these animals/ plants move in, out-compete the plants/animals taht were all ready there and become the dominant species in the ecosystem

    • the dominant species cause the most change to the abiotic factors

• as succession goes on the ecosystem becomes more complex, more species move in increasing biodiversity

• plants create more habitats for animals/ incase biomass/ more complex food webs

• eventually changes result in a climax community - which remain sin a steady state

secondary succession

• secondary succession happens on land that has been cleared, but where soil remains

  • EG. forest fire/ forest cut down by humans

• this form os succession happens much more quickly than primary succession, due to soil already existing

Definition: conservation

• conservation is the protection an management of species and habitats (ecosystems) in a sustainable way

  • sustainable means that enough resources are taken to meet needs of people today, without reducing the ability fro people in the future to meet their own needs

what are some different conservation methods

• management of succession

  • human activity to prevent an area from reaching its climax community/ or reaching the next stage of succession

  • each stage of succession has different species, that would no longer exist if succession continued

  • increase biodiversity

  • EG. Scottish Moorland - would be a climax community of spruce forrest, which could not support all species, reducing the biodiversity

• seed banks

  • a way of conserving plant species incase of extinction

• captive breeding

  • species can be bread to increase numbers and reduce risk of extinction

• fishing quotas

  • limits to amount of fish of certain species that a fisherman is allowed to catch

• protected areas

  • protect habitats and the species in them