topic 7 - genetics, populations, evolution & ecosystems

What is succession?

The process by which an ecosystem changes over time.

The biotic conditions change as the abiotic conditions change causing one community of organisms to be succeeded by another.

How does succession happen?

It happens in a series of stages where at each stage,the plant & animal communities in a habitat/area slowly change the environmental conditions making them less hostile and more suitable for other species with different adaptation.

What does succession occur as a result of?

Changes to the environment (abiotic factors) causing the plant and animal species to change.

What are the two types of succession?

1. Primary succession.

2. Secondary succession.

Where does primary succession occur?

On an area of land that has been newly formed or exposed. There is no soil and no inital life.

What are come conditions/environments where primary succession occurs?

When:

• Volcanoes erupt, depositing lava - when lava cools and solidifies, igneous rock is created.

• Sand is blown by the wind or deposited by the sea to create new sand dunes.

• Slit and mud are deposited at river estuaries.

• Glaciers retreat depositing rubble and exposing rock.

Overall hostile aboitic conditions

Where does secondary succession occur?

On areas of land where soil is present, but it contains no plant or animal species. The established community of species is detroyed but with little disturbance to the soil.

It can occur at any stage after the pioneer stage.

What are the differences between primary & secondary succession?

Primary 

• No soil to begin with

• No living species to begin with

• Succession starts from pioneer species - species specifically adpated to very harsh conditions

• Takes a very long time to reach climax community

Secondary

• Soil is present from the start

• No plant & animal species to beign with

• Succession starts from pioneer species but at a later stage & the species are larger plants e.g. shrubs

• Doesn’t take as long of time 

How does primary succession begin?

By the colonisation of an inhospitable environment, by organisms known as pioneer species which are adapted specifically to the hostile conditions.

How does the pioneer species arrive?

As spores or seeds carried by the wind from nearby land masses or sometimes by the droppings of birds or animals passing through.

What are the adaptations of pioneer species that allow them to colonise the bare environment?

• The ability to produce large quantities of seeds of spores, which are blown by the wind and deposited on the ‘new land‘.

• Seeds that germinate rapidly.

• The ability to photosynthesise to produce their own energy as light, rainfall and air are often the only abiotic factors present.

• Tolerance to extreme environments.

• The ability to fix nitrogen from the atmosphere, so adding to the mineral content of the soil.

What are some example of pioneers species?

• Maram grass - can grow on sand dunes near the sea as they have deep roots to get to water & can tolerate the salty environment

• Lichens - able to live in rocky conditions as they can secrete acids which erode the rock releasing minerals

What happens after the introduction of the pioneer species?

They change to abiotic conditions to be less hostile :

• The pioneer species die and the basic organic material is decomposed by microorganisms 

• This forms a basic soil allowing new organisms with different adapations to move in 

• These tend to be ferns/ taller plants than the pioneers which only need a small amount of soil & water but are fast growing

What happens in primary succession after a basic soil has been formed?

• The organisms that arrive once the basic soil formed die and are decomposed

• This adds more organic material making the soil deeper & richer in minerals e.g. nitrates

• Nitrogen-fixing bacteria convert the nitrates to ammonia which is can be used by plants

• This means larger plants such as shrubs can start to grow in the deeper soil which retains more minerals & water

How does the decomposition of organisms affect succession?

They contribute to a deeper, more nutrient rich soil, which retains more water and makes the abiotic conditions more favourable.

What happens through out the stages of primary succession?

The abiotic factors become less hostile which means there are new species with different adaptations.

This increases the biodiversity of the community but also means that new species which better adapted to the improved conditions out-compete the species there previously. They can also change the conditons so that they’re no longer suitable to the previous species.

However the biodiversity will still remain high as long as the previous species aren’t completely wiped out which they rarely are.

What are dominant species?

Ones which cause the most change to the abiotic environment making it more suitable for other species.

How does a community in primary succession go from larger plants e.g. shrubs to a climax community?

The shrubs and other larger plants attract more animals etc overall causing the ecosystem to become more complex. New species move in alongside existing species - increases biodiversity. Plants create more habitats for animals - abiotic conditions become less hostile & the amount of biomass increases.

As this continues over time these changes result in a climax community.

What is a climax community?

The ecosystem supporting the largest most complex community of plants and animals it can with no human intervention. It is always climate specific.

What are the characteristics of a climax community?

• Stable - shows little change over time.

• A few dominant animal and plant species.

Which stage of succession has the most biodiversity?

Biodiversity tends to reach a peak in mid-succession.

It then tends to decrease due to the dominant species outcompeting pioneer and other species, resulting in their elimination. The more successful the dominant species the less the biodiversity in a given ecosystem.

Alongside the succession of plant species, what do animals undergo?

Similar progression.

What animals are first to colonize an area, and what must they do?

Primary consumers, and they must move in from neighbouring areas so animal succession is usually much slower than plant succession.

When do secondary consumers arrive in animal succession?

Once a suitable food source has been established and the existing plant cover will provide them with suitable habitats. Again, these species must move in from neighbouring areas. Eventually, larger organisms such as mammals and reptiles will colonise the areas when the biotic conditions are favourable.

What is deflected succession?

When the natural flow of succession is halted, often by human activities.

When succession is stopped artificially, what is the final stage that is formed known as?

Plagioclimax.

What is one of the main reasons for deflection?

Agriculture.

How is agriculture one of the main reasons for deflected succession?

• Grazing and trampling of vegetation by domesticated animals - large areas remain as grassland.

• Removing existing vegetation to plant crops - the crops become the final community.

• Burning trees as a means of clearance - often leads to an decrease in biodiversity.

What is a gene?

A section of DNA which code for a protein or functional RNA

What is an allele?

A type/variation of a gene.

What is the locus?

The position of a gene on a chromosome.

What is the genotype?

The genetic makeup of an organsim.

What is the phenotype?

The physical appearance of a trait/genotype and the environment.

What does it mean when a genotype is homozygous?

The two alleles for a trait are the same.

This can be homozygous recessive - two recessive alleles or homozygous dominant - two dominant alleles.

What does it mean when a genotype is heterozygous?

The two alleles for a trait are different to each other - one reccessive & one dominant.

What is a dominant allele?

Allele which is always expressed in the phenotype even when theres only one copy of it/ in a heterozygous genotype.

What are recessive alleles?

Allees that only appear in the phenotype if 2 copies are present/homozygous recessive

What are codominant alleles?

Alleles that are both expressed in the phenotype in a heterozygous genotype because they are equally dominant

What does it mean when there are multiple alleles?

There are more than two alleles of which only two copies are present e.g. black, brown, blond alleles but the genotype can only be black & blond etc.

What is the F₁ generation vs the F₂ generation?

F₁ generation - grandparents

F₂ generation - parents 

What is mono-hybrid inheritance?

Inheritance of a characteristic controlled by a single gene

What type of offspring would a monohybrid cross with 2 homozygous parents produce?

Will always produce all heterozygous offspring

What is a phenotypic ratio?

The ratio of different phenotypes in the offspring

What is the usual phenotypic ratio you would get if you did a monohybrid cross with two heterozygous parents?

3:1 ratio of dominant:recessive characteristics

What is dihybrid inheritance?

The inheritance of 2 characteristics/phenotypes which are controlled by different two genes on two different chromosomes. E.g. fur colour & eye colour.

What phenotypic ratio would you expect to see if you did a dihybrid cross with 2 heterozygous Parents?

9 : 3 : 3 : 1

dominant both : dominant first, recessive second : recessive first, dominant second : recessive both

What does sex linked mean?

The alleles that code for a gene are located on a sex chromosome

Which sex chromosome carries most genes?

X chromosome

Which gender is more likely to to express the characteristic of sex linked genes? Why?

Males- they have one X chromosome so they only have one copy of the allele, they express it even if it's recessive

Why can't males be carriers of X-linked disorders?

Because they only have one copy of the X chromosome so if they have the allele they have the disease whether is recessive or not

What is an autosome?

All other non-sex chromosomes - anything which isn’t X or Y.

What are genes on the same autosome said to be? Why?

Linked - they'll stay together during the independent segregation of chromosomes in anaphase 1 and the allele of each gene will be passed on to their offspring together

How does autosomal linkage affect the combinations of alleles in genetics etc?

Instead of getting 4 different gametes (for a dihybrid cross) you can only get 2 e.g. AB, Ab, aB, ab vs AB, ab.

This is because the alleles are on the same autosome so independent segregation doesn’t effect the combinations of alleles - only crossing over does. 

If two genes are autosomally linked, what ratio would you expect to see in the F₂ generation and why?

3 : 1 for a dihybrid cross because the 2 autosomally linked alleles are inherited together

What is epistasis?

When the allele of one gene, can mask or block the expression of alleles from other gene.

This means the outcome & ratios of genetic crosses is different to the expected outcomes & ratios if ther wasn’t epistatis.

What is the Hardy-Weinberg principle?

A mathematical model that predicts that the frequencies of alleles in a population won’t change from one generation to the next.

This however only works under assumptions that are unlikely/near impossible in real life populations of organisms.

What can the Hardy-Weinberg principle be used for?

To calculate allele & genotype frequencies.

Predict how these frequencies will change across generations.

If doing Harvey-Weinberg equations doesn’t result in a correct we can say that the population doesn’t have the assumptions etc for the Harvey-Weinberg principle to work so selection etc must have occured.

What are the conditions/ assumptions needed for the Harvey-Weinberg principle?

• Diploid organisms

• Organisms only reproduce sexually

• There is no overlap between generations - parents don’t mate with offspring

• Mating is random

• The population is large

• There is no migration, mutation or selection

• Allele frequencies are equal in each set

Why are there two Hardy-Weinberg equations?

One is used to calculate allele frequency.

One is usually used when dealing with genotype and phenotype frequencies.

What is the Hardy-Weinberg equation for allele frequency?

p + q = 1

• Where p = the frequency of one allele (usually the dominant one)

• Where q = the frequency of one allele (usually the recessive one)

What does it mean for a Hardy-Weinberg equation for allele frequency to equal 1 and not equal 1?

The total frequency of all the possible alleles for a characteristic in a certain population is 1.0 or 100% so the frequencies of the individual alleles must add to 1.

If the equation gives an answer less than one there are more than 2 alleles for the gene.

If the equation gives an answer more than one they’re not alleles for the same gene.

What is the Hardy-Weinberg equation for genotype frequency?

p² + 2pq + q² = 1

• Assuming p is dominant & q is recessive 

• p² = frequency of homozygous dominant genotype

• 2pq = frequency of heterozygous genotype 

• q² = frequency of homozygous recessive genotype

Why are the Hardy-Weinberg equations for allele vs genotype freqency different?

In the allele one there are only 2 alleles for each gene as per our assumptions.

In the genotype one there are 3 genotypes/combination of alleles - homozygous dominant, heterozygous & homozygous recessive.

The genotype equation can also be used for phenotype frequencies but the allele equation can’t be used for this.

In what conditions do the Hardy-Weinberg equations also work in?

2 Co-Dominant alleles

If we don’t know which is recessive or dominant - p & q just have to continously represent one.

What is a community?

All the populations of different species living in the same place (habitat) at the same time

What is an ecosystem?

A community and the non-living (abiotic) components of its environment

What is a niche?

The specific role of a species within its habitat e.g. what it eats, where and when it feeds governed by its adaptation to both abiotic and biotic conditions

What is an abiotic factor?

The non-living features of an ecosystem, such as the temperature and soil.

What is a biotic factor?

The living features of an ecosystem, for example, the presence of predators or food.

What are the advantages of species occupying different niches?

Less competition for food / resources

If two species tried to occupy the same niche, one species would outcompete the other

What is carrying capacity?

The maximum (stable) population size of a species that an ecosystem can support

What are the two factors that affect carrying capacity?

The affect of abiotic factors

Interactions between organisms - interspecific and
intraspecific competition and predation.

What are some abiotic factors that affect carrying capacity/population size?

• Light intensity

• Temperature

• Soil pH

• Mineral content

• Humidity

How might abiotic factors affect animal population size / carrying capacity?

If conditions are favourable, organisms more likely to survive and reproduce -> increasing carrying capacity

• E.g. increasing light intensity increases rate of photosynthesis, increasing nitrates increases protein production and increasing phosphates increases phospholipid production in plants

This increases carrying capacity of a variety of plant species so increases the number and variety of habitats, niches and food sources for animals
Overall increasing carrying capacity of a variety of animal species.

What is interspecific competition?

When organisms of different species compete with each other for the same resources.

This can mean that the resources available to both populations are reduced, e.g. if they share the same source of food, there will be less available to both of them.

What is the effect of interspecific competition?

Both populations' will be limited by a lower amount of resources e.g. food. They’ll have less energy for growth and reproduction, so both species' population sizes will be lower.

If two species are competing but one is better adapted to its surroundings than the other, the less well-adapted species is likely to be out-competed — it won’t be able to exist alongside the better-adapted species.

What is intraspecific competition?

When organisms of the same species compete with each other for the same resources.

It can cause a cyclical change in population size around the ecosystem’s carrying capacity — where the population grows, shrinks, grows, and so on.

What is the effect of intraspecific competition?

Cyclical change in population size around the ecosystem’s carrying capacity.

This is because the population of a species increases when resources are plentiful. As the population increases, more organisms compete for the same amount of space and food. Eventually, these resources become limiting.

If the population grows beyond the carrying capacity, there won’t be enough resources for all the organisms and the population will begin to decline.

A smaller population means there’s less competition for space and food which is better for growth and reproduction —the population starts to grow again.

How does predation affect population size/carrying capacity?

Prey and predator populations fluctuate in cycles, with the predator population peaking after the prey population (lag time).

Why do prey-predator populations fluctuate in cycles?

• Prey population increases so predators have more food - so more predators survive and reproduce

• Predator population increases so more prey killed and eaten -so less prey survive and reproduce

• Prey population decreases so predators have less food -so less predators survive and reproduce

• Predator population decreases so less prey killed and eaten - so more prey survive and reproduce (cycle repeats)

What are the different ways you can estimate the size of a population and when can you use them?

• Random sampling - randomly placed quadrats, or quadrats along a belt transect

  • For slow-moving or non-motile organisms

• Mark-release-recapture method

  • For motile organisms

How do we carry out random sampling?

• Divide area into a grid / square e.g. place 2 tape measures at right angles

• Generate a pair of coordinates using a random number generator

• Place a quadrat at the coordinate generated and count the frequency of species you are interested in

• Repeat a large number of times (20 or more) and calculate a mean per quadrat

• Population size = (total area of habitat / quadrat area) x mean per quadrat

How do we carry out mark-release-reacapture?

• Capture sample of species, mark and release using safe & appropriate techniques

• Ensure marking is not harmful & does not affect survival

• Allow time e.g. 1 week for organisms to randomly distribute in habitat

• Collect second sample and count how many are marked

• Population = (number in sample 1 x number in sample 2) / number marked in sample 2

What assumptions does the mark release recapture method make?

• Sufficient time for marked individuals to mix / distribute evenly within the population

• Marking not removed so marked organisms are recaptured OR marking is not toxic so doesn’t affect chances of survival OR marking not visible (to predators) so doesn’t affect predation

• Limited / no immigration / emigration

• No / few births / deaths / breeding / change in population size

Why can the mark-release-recapture method produce unreliable results in very large areas?

• Unlikely that organisms will distribute randomly / evenly

• Lower chance of recapturing organisms (that were marked initially)

What are the reasons for conservation?

• personal

• ethical

• economic (medical resources, food, tourism)

• cultural + aesthetic

What is conservation?

The protection and management of species and habitats in a sustainable way.

What does sustainable mean?

That enough resources are taken to meet the needs of people today without reducing the ability of people in the future to meet their own needs.

What does it it mean for conservation to be an active and dynamic process?

• active measures - we have to do something

• dynamic - methods must change alongside changes in the ecosystem

What are the 5 main conservation methods?

• Management of succession

• Seed banks

• Captive breeding

• Fishing quotas

• Protected areas

How can conservation involve management of succession?

• Further succession can be prevented to stop a climax community forming by removing or preventing growth of species associated with later stages e.g. by allowing grazing

• This preserves an ecosystem at a certain point / in its current stage of succession

• So early species are not outcompeted by later species and habitats / niches are not lost

How do seed banks act as a method of conservation?

Seed banks store seeds from lots of different plant species and act as a backup for the conservation of plant species in the wild in case a species gaes extinct etc.

What are the advantages and disadvantages of seed banks?

Advantages :

• Large numbers of seeds can be stored in a small area

• Seeds can be stored anywhere for long periods of time under cool & dry conditions

Disadvantages :

• The seeds have to regularly tested to make sure they’re still viable which can be expensive & time-consuming

How does captive breeding act as a method of conservation?

Captive breeding programmes involve breeding animals in controlled environments in order to increase the numbers of endangered or already extinct in the wild species.

The animals can then be reintroduced into the wild increasing their numbers in the wild and so conserve their numbers.

What are the issues with captive breeding?

Animals can have problems with breeding outside their natural habitat which can be hard to recreate in a zoo.

Reintroduction of animals into the wild could also lead to issues e.g. new diseases to habitats.

How do fishing quotas act as a method of conservation?

Fishing quotas a limits to the amount of certain fish species that fishermen can catch so help to conserve fish species by reducing the numbers that are caught and killed - prevention of extinction.

What are the issues with fishing quotas?

• Unpopular with fishermen as it limits their income

• There can be problems with discards - when too many fish are caught so some have to be thrown back in to not exceed the quota - the fish are often dead already

• International cooperation is required for quotas etc that cover international fishing areas

How do protected areas act as a method of conservation?

Protected areas protect habitats and species by restricting urban development, industrial development and farming and they can be managed to conserve them.

What are the issues with protected areas?

Some are used as tourist destinations which can cause conflict in the need for conservation and the need to allow people to visit and use them

What are the all the different statistical tests?

• Spearman’s Rank

• Chi Squared

• Students T test