3.7: Genetics, Populations, Evolutions and Ecosystems

Define genotype

The genetic constitution of an organism

Define phenotype

the expression of this genetic constitution and its interaction with the environment

Describe monohybrid inheritance

One allele in each gamete, basic punnet squares; think Mendel crossing basic tall and short pea plants

What do we label the first and second generations of offspring?

F1 and F2

What is a genotypic ratio?

A ratio of the combination of alleles

How do you perform a test cross?

Pair the genotype in question with a homozygous recessive. If the questionable genotype is AA, then ALL offspring will show dominant trait. If it's a 1:1 ratio, then it's Aa.

What is the homogametic sex?

female: XX (both the same)

What is sex linkage?

the inheritance of characteristics controlled by alleles carried on sex chromosomes

What is autosomal linkage?

alleles carrier on the same pair of homologous chromosomes, which are inherited together

What happens as a pair of linked genes are further apart on a chromosome pair?

they are more likely to be separated by crossing over

What are autosomes?

Any chromosome that is not a sex chromosome; chromosomes 1-22

What is epistasis?

arises when the allele of one gene affects or masks the expression of another in the phenotype

Define population

a group of organsisms of the same species occupying a particular space at a particular time that can potentially interbreed

Define gene pool

all the genes and alleles in a population

Define allelic frequency

The number of times an allele occurs within the gene pool

What is the Hardy-Weinberg principle?

A mathematical model which predicts that allelic frequencies will not change from one generation to another if certain conditions are met

What are the 5 conditions that need to be met for the Hardy Weinberg principle to be true?

no mutations arise

the population is isolated (no emigration or immigration)

no selection (all alleles are equally likely to be passed down)

the population is large

mating within the population is random

What are the two important ideas set out by the Hardy Weinberg principle?

- no change in the allelic frequencies between generations of the conditions are met

- allows us to calculate allelic frequencies in a population starting from the simple observation of the number of recessive phenotypes

What is the equation for Hardy Weinberg?

p^2+2pq+qp^2=1

where p = P(A) and q = P(a)

AA + Aa + aA + aa = 1

Define evolution.

the change in allele frequency from one generation to another

What are the four sources of variation?

- random mutations

- random fertilisation

- crossing over

- independent segregation

Describe natural selection.

random mutation -> new allele

sometimes harmful but can be advantageous to possessor, leading to increased reproductive success

allele inherited by offspring

over several generations, allele increases in frequency in population

can led to a new species arising (speciation)

When does speciation occur?

a new species arises from an existing species when the population becomes unable to interbreed. This is because they become reproductively isolated

What does prezygotic mean?

before fertilization

Give examples of prezygotic isolating mechanisms.

- temporal isolation (seasonal, e.g. rainbow and brown trout reproduce in different seasons)

- gamete isolation

- behavioural isolation

- mechanical isolation

Give examples of post zygotic isolating mechanisms?

- hybrid sterility

- hybrid inviability

- hybrid breakdown

What is a niche?

the role played by a species in their ecosystem

What are the two types of isolating mechanisms in speciation?

- allopatric speciation

- sympatric speciation

What is allopatric speciation?

geographic isolation which results in speciation, a physical separation

What is sympatric speciation?

- occurs when populations become reproductively issolated despite living in the same area; occurs when two populations occupy different niches in the same habitat.

How can geographical isolation occur?

- mountain range

- flood

- desert

- plate tectonics

- ice sheet

- volcanic eruptions

Describe Darwin's finches.

there are 13 different species of finch found on the Galapagos islands, which look similar to each other which could be explained if they evolved from a common ancestor from the mainland. As they spread to different islands, they experienced different size and shapes of beaks. Eventually, they became reproductively isolated and new species developed.

What is stabilising selection?

Selection favouring the intermediate phenotype, selecting against the extremes.

e.g. robins lay four eggs (larger clutches result in malnourished chicks, while smaller clutches may result in no viable offspring)

When does stabilising selection occur?

when environmental conditions are constant over long periods of time

What is directional selection?

selection favouring one extreme and against another extreme. Over time, the mean will move in the direction of the favoured extreme.

When does directional selection occur?

when environmental conditions change

What is disruptive selection?

Opposite extreme traits are favoured, selecting against the intermediate trait. Can easily lead to speciation.

When does disruptive selection occur?

when an environmental factor takes two or more distinct forms

What is genetic drift?

The random change in allele frequencies in populations which are unrelated to natural selection.

Why is the effect of genetic drift more pronounced in small populations?

there is a smaller sized gene pool, so genetic drift can lead to rapid loss or fixation of alleles

What is the founder effect?

If a small number of individuals from a large population becomes isolated from their original parent population, their sample of alleles is unlikely to represent the allele proportions of the parent population.

What is a genetic bottleneck?

When a sudden environmental change drastically reduces the size of the population. The population may recover, but the genetic diversity often does not.

What is allopatric specication?

genetic divergence of a population after it becomes subdivided and isolated.

Define population.

A group of species occupying the same area at the same time.

Define gene pool.

All the genes and alleles in a population are known as the gene pool of that population.

Define community.

All the populations of different species form a community.

Define ecosystem.

A community and the non-living components of its environment together form an ecosystem.

What is an ecological niche?

The role played by an organism in an ecosystem.

What happens when two species try to occupy the same niche?

One will outcompete the other; eventually, the less successful competitor disappears from the habitat.

Give examples of ecosystems.

they can vary from very small to very large; e.g. a garden pond or a tropical rainforest

What are the two types of competition?

Intraspecific and interspecific.

Intraspecific is between members of the same species. Interspecific is between members of different species.

What is a carrying capacity?

the maximum size that a population can remain sustainable in an ecosystem.

Give examples of biotic factors.

-food availability

-predation

-parasitism

-competition

Define succession.

the change from simple open unstable and low species diversity ecosystem to a complex closed and high species diversity ecosystem.

How does succession occur?

ecological communities may change the physical conditions enabling other plants and animals to survive as the habitat

Describe succession on a bare rock.

The pioneer community such as lichen establish themselves on the rock. THey are tolerant of being exposed to wide temperature fluctuations and arid conditions. They die and are decomposed and form a thin layer of humus.

Mosses are then able to grow, especially in crevices, and sheltered areas and again cause an increase in hte amount of organic matter.

The increased organic matter enables herbaceous plants to colonise the area as the soil increases and leads to weathering of rock, more niches and greater variety of plant establishing the area.

The climax community consists of deep-rooted shrubs and trees and is dependent on the type of soil.

succession takes thousands of years.

Describe succession on a sand dune.

The onshore wind moves the constantly shifting sand to form embryo dunes, which pioneer plants can inhabit. Water and nutrients drain straight through the sand due to the large sized particles and the constantly moving sand makes it difficult for the roots to anchor the plant. Pioneer species, like sea rocket and marram grass have adaptations to establish themselves as they are xerophytes. They have dense root systems which hold the water and humus particles and stabilises the movement of sand, making the environment less hostile. As the marram grass dies, humus forms in the sand and enables other species to inhabit the area. So, species diversity increases and, as these conditions improve, the marram grass is outcompeted and dies out.

How can humans conserve ecosystems?

TO MANAGE SUCCESSION:

- seed banks

- managed fires being lit (so secondary succession can occur)

- let animals graze

ALSO

- seed banks to conserve plant species

- captive breeding

- fishing quotas

- protected areas

Define conservation.

Maintaining ecosystems and the biotic component of them. It frequently involve the management of succession.

Why should conservation take place?

- personal: maintain organisms on our planet to protect biodiversity

- ethical: all species should be allowed to coexist with us

- economic: organisms have a large gene pool which could be potential medical supplies in the future

- cultural and aesthetic: green spaces improve our wellbeing and habitats and organisms enrich our lives

What are SSSIs?

Sites of Special Scientific Interest

Why is managing succession good?

enable species to survive by preventing succession to the next stage; many species who lived in earlier stages of succession are no longer present in the climax community as their habitats have dissappeared or they have been outcompeted by other species.

What is the name for a stage of succession?

sere

How is moorland managed to prevent a climax community being reached?

- heather is burnt so succession can re-occur

- grazing of sheep

- so young saplings are destroyed

- prevents a deciduous woodland forming

Which organisms prevent a climax community from forming in chalk downland?

sheep and rabbits

Explain the decrease in productivity as woodland matures?

more competition for light leads to reduced photosynthesis

Why is there an increase in biomass during the first 20 years of succession?

pioneer species adjusts abiotic components to make the habitat less hostile

allows more species to survive so there is increased species diversity

What is dihybrid inheritance?

The inheritance of two characteristics which are controlled by different genes, so 2 alleles in each gamete