Populations And Evolution

Question 1

Show the genotypes for homozygous dominant, homozygous recessive and heterozygous dominant?

Answer 1

Homozygous dominant - TT,

Homozygous recessive - tt,

Heterozygous dominant - Tt.

Question 2

Hardy Weinberg principle?

Answer 2

Provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population.

The principle makes assumptions (so that it works):

• no mutations have occurred,
• the population is isolated (so no different alleles have entered gene pool),
• there is no natural selection,
• the population is large,
• mating within a population is completely random.

Question 3

Hardy Weinberg equations?

Answer 3

p + q = 1.0

p2 + 2pq + q2 = 1

p = dominant alleles. 
q = recessive alleles. 

p2 = homozygous dominant genotype (looks at whole).
q2 = homozygous recessive genotype. 
2pq = heterozygous dominant genotype. 

Practice questions for these - best way to learn.

Question 4

How does variation occur due to genetic factors?

Answer 4

• Mutations, (main source of variation).
• Meiosis, (nuclear division which provides new combinations of alleles).
• Random fertilisation of gametes, (fastest sperm wins the race, ahaha. This is random).

These are genetic factors which cause variation. There is also environmental factors which cause variation (next flashcard).

Question 5

How does variation occur due to environmental factors?

Answer 5

• Temperature,
• Rainfall,
• Sunlight,
  (These are all climate conditions).
• Soil conditions,
• pH,
• Food availability.

E.g. a human may have the genes to grow really tall. But there is low food availability and so he is actually small.

Question 6

What are selection pressures?

Answer 6

They determine the frequency of alleles within a gene pool.

They include:

• disease,
• competition,
• predation.

Selection pressures also include environmental factors such as temperature, light, food, space.

Selection pressures cause a favourable phenotype. They’re the reason for natural selection.

What to write (always the same mark scheme) for a natural selection question:

1. Variation caused by mutation.
2. Selection pressure.
3. One organism is better adapted to survive due to favourable phenotype.
4. They can reproduce and pass on advantaged allele onto offspring.
5. That causes an increase in the allele in the population.

Question 7

How does overproduction of offspring affect natural selection?

Answer 7

Charles Darwin realised that populations rarely, if ever increased in size at a high rate.

The death rate of species is always the same as reproductive rate.

When there is too many offspring, there is not enough resources available. This created infraspecific competition for the limited resources.

Therefore, individuals die that can not get the resources. However, this is not totally random because individuals who are best suited to gather food (catch pray, escape predictors, etc) will be better adapted to survive. Therefore, they breed and pass on their advantaged alleles to the next generation, making the alleles more common.

Question 8

Infraspecific competition?

Answer 8

Completion amongst individuals inside a population.

Question 9

Why is genetic variation important, even in populations that are well adapted?

Answer 9

Because of genetic variation does not occur, it makes the individuals vulnerable to diseases and climate changes.

It is also important that populations can adapt to the adaptations and evolution of other species (predators and pray).

The lager the population, the more genetically varied the individuals within it. Therefore, the greater the chances to survive.

Question 10

Three main types of selection?

Answer 10

These affect the characteristics of a population.

1. Stabilising selection.
2. Directional selection.
3. Disruptive selection.

Imagine, with a population, characteristics are on a graph. They should show a ‘normal’ curve. Goes up to peak and then down to peak. These selections change that curve.

Question 11

Stabilising selection?

Answer 11

Eliminates the extremes of the phenotypes.

E.g. on a graph of height, the individuals who are really tall and really small will be eliminated.

This leaves a very, narrow and tall graph curve.

Question 12

Directional selection?

Answer 12

This selection favours a phenotype of one extreme but not the other.

E.g. the individuals in a population who are really tall will survive and reproduce but the really small individuals all die.

This causes the curve on the graph to shift towards one side of the graph.

Question 13

Disruptive selection?

Answer 13

This is the opposite of stabilising selection and favours the extreme phenotypes at the expense of the intermediate ones.

This causes the distribution curve to look like an ‘m’ shape.

E.g. individuals who are really tall and really small will survive and those who are intermediate will die.

Question 14

Selection in the peppered moth?

Answer 14

Polymorphism = when a species of organism have two or more distinct forms but exist within the same interbreeding population (can still reproduce with each-other).

Peppered moth is one of these.

The melanic (black) moth existed in 1819 but died. The natural light form moth was better adapted to rocks and lichen-covered trees.

A Melanic (black) form of the moth was captured in Manchester in 1849. The soot of the industrial revolution covered the walls of buildings. The sulfur dioxide in the smoke emissions killed the lichens that covered the trees.

As a result, the light form was eaten by birds and 95% of Manchester’s moth was the melanic type by 1895.

This is directional selection (favourable extreme).

The moths are still the same species and interbreed but directional selection has caused a favourable phenotype.

Question 15

Allelic frequency?

Answer 15

The number of times an allele occurs within a gene pool.

Allelic frequency can be affected by selection.

Question 16

What is speciation?

Answer 16

The evolution of new species from existing ones.

The members of a species become reproductively separated from other species so that speciation occurs.

Question 17

Adaptive radiation?

Answer 17

When a population (usually after selection pressure has occurred) adapts to its local environment.

It results in changes to the allele frequencies (evolution).

Question 18

Genetic drift?

Answer 18

Something that takes place in small populations (usually).

Small populations usually have a small variety of alleles compared to larger populations. Their genetic diversity is less.

These few individuals breed and the genetic diversity of the population is restricted to the few alleles present in the gene pool. If a mutation occurs, the spread of this new allele moves quickly through the population. This causes a genetic drift, a rapid change.

Question 19

Two types of speciation?

Answer 19

Allopatric speciation and sympatric speciation.

Question 20

Allopatric speciation?

Answer 20

Two populations become geographically separated - e.g. earthquake that physically separated two part of the earth.

Also include oceans, rivers, cliffs.

If environmental conditions either side of the barrier vary, then natural selection will influence the two populations differently and the species will become reproductively separated.

Question 21

Example of allopatric speciation?

Answer 21

Galapagos finch.

A single species of finch is thought to have inhabited the island of Galapagos years ago.

In the absence of competition, the population increases and populations established other habitats on different islands.

Each population became evolved adaptations to suit the new environment (including food).

These adaptations included different beak sizes and lengths so that they could eat and reach different seeds.

The populations of finch are so different now that they cannot reproduce.

Question 22

Sympatric speciation?

Answer 22

Speciation that results within a population in the same area leading to them becoming reproductively separated.

Question 23

Example of sympatric speciation?

Answer 23

The Apple maggot to fly.

Originally, this insect only laid its eggs inside the fruit of hawthorns, which are native to North America.

When apple trees were introduced to North America, the fly started to late it’s eggs in apples also.

Females tend to lay their eggs on the type of fruit in which they developed and males tend to look for mates on the types of fruit in which they developed.

So flies raised in hawthorns usually meet with each other and flies raised in apples time to meet with each other.

Mutations have occurred which has led to the evolution of genetic differences in each population. This could result in them being incapable of successfully breeding with one another and therefore being separate species.

Question 24

Geographical variation?

Answer 24

Populations are isolated by physical barriers such as oceans, mountains, rivers.

Question 25

Ecological variation?

Answer 25

Populations inhabit different habitats within the same area and so individuals rarely meet.

Question 26

Temporal variation?

Answer 26

The breeding seasons of each population do not coincide and so they do not interbreed.

Question 27

Behavioural variation?

Answer 27

Meeting is often preceded by courtship. Courtship involves markings on the opposite sex, the calling or actions of a male.

Any mutations which cause variations in these patterns may prevent meeting.

E.g. male peacocks with the most colourful wings are more likely to mate. If a male peacock has not got colours on their wings, they may not find a female and not reproduce.

Question 28

Mechanical variation?

Answer 28

Anatomical differences may prevent mating occurring, for example, it may be physically impossible for the one is to enter the vagina in mammals.

Question 29

Gametic?

Answer 29

The gametes may be prevented from meeting due to genetic or biochemical incompatibility.

For instance, some pollen grains fail to germinate or grow when they land on a stigma of different genetic make up. Some sperm are destroyed by chemicals in the female reproductive tract.

Question 30

Hybrid sterility?

Answer 30

Hybrid is formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes.

For example, in a cross between a horse (2n=64) and donkey(2n=63), the resultant new has 63 chromosomes. It is impossible for these chromosomes to pair up appropriately during meiosis and so the gametes are formed are not viable and the meal is sterile (cannot reproduce).

Question 31

How can reproductive isolation occur without geographical isolation?

Answer 31

The emerge of polyploidy in a diploid population (as individuals with different numbers of chromosomes cannot reproduce with each other sexually to produce fertile offspring).

It could also result from behavioural, seasonal or mechanical changes that prevent successful reproduction.

Question 32

What is a population?

Answer 32

A group of organisms of one species living in a particular habitat.