populations and evolution- chapter 18

Question 1

what is a population

Answer 1

a group of organisms of the same species that occupies a particular space at a particular space at a particular time and that can potentially interbreed

Question 2

what is a gene pool

Answer 2

All the alleles of all the genes of all the individuals in a population at a given time

Question 3

what is the allelic frequency

Answer 3

The number of times an allele occurs within the gene pool

Question 4

what is the Hardy-Weinberg principle and what assumption does it make

Answer 4

• 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 the assumption that the proportion of dominant and recessive alleles of a gene in a population remains the same from one generation to the next

Question 5

what 5 conditions have to be met for Hardy-Weinberg principle

Answer 5

• No mutations arise
• The population is isolated (no flow of alleles into or out of the population)
• There is no selection (all alleles are equally likely to be passed to the next generation)
• The population is large
• Mating within the population is random

Question 6

why do individuals within a population of a species show a range of variation within their phenotypes

Answer 6

due to both genetic and environmental factors

Question 7

what are 3 reasons for genetic variation arising

Answer 7

1. Mutations- sudden changes to gene and chromosomes may, or may not, be passed onto the next generation.
2. Meiosis- produces new combinations of alleles before they are passed into the gametes, all of which are therefore different.
3. Random fertilisation of gametes- produces new combinations of alleles and the offspring are therefore different from parents. Which gamete fuses with which at fertilisation is a random process further adding to the variety of offspring two parents can produce.

Question 8

what environmental influences are there

Answer 8

climatic conditions (temperature, rainfall and sunlight), soil conditions, pH and food availability

Question 9

what are polygenes

Answer 9

• Some characteristics of organisms grade into one another, forming a continuum
• Characters that display this type of variation are not controlled by a single gene, but by many genes called polygenes

Question 10

what are selection pressures

Answer 10

• The environmental factors that limit the population of a species
• These vary and determine the frequency of all alleles within the gene pool
  -These include predation, disease and competition

Question 11

what factors affect the process of evolution by the means of natural selection

Answer 11

• Organisms produce more offspring than can be supported by the available supply of food, light, space, etc
• There is genetic variety within the population of all species
• A variety of phenotypes that selection operates against

Question 12

what is the role of overproduction of offspring in natural selection

Answer 12

• All species have the potential to increase in numbers exponentially although this rarely happens.
• High reproductive rates have evolved in many species to ensure a sufficiently large population survives to breed and produce the next generation. This compensates for high death rates from predation, competition for food and water, extremes of temperatures, natural disasters and diseases.
• Some species have evolved lower reproductive rates along with a high degree of parental care.
• When there are too many offspring for the available resources, there is competition amongst individuals (intraspecific competition) for the limited resources available.
• Those individuals in a population best suited to prevailing conditions will be more likely to survive than those less well adapted, they will be more likely to breed and so pass on their more favourable allele combinations to the next generation.

Question 13

what is the role of variation in natural selection

Answer 13

• Conditions change over time, so having a wide range of genetically different individuals within a population means that some will have the combination of genes needed to survive in almost any new set of circumstances.
• Populations showing little individual genetic variation are often more vulnerable to new diseases and climate changes. It is also important that a species is capable of adapting to changes resulting from the evolution of other species.
• The larger a population is, and the more genetically varied the individuals within it, the greater the chance that one or more individuals will have the combination of alleles that lead to a phenotype which is advantageous in the struggle for survival.
• Variation therefore provides the potential for a population to evolve and adapt to a new circumstances.

Question 14

what do environmental factors help create in a population

Answer 14

variation

Question 15

what are the 3 main types of selection that affect the characteristics of a population

Answer 15

• Stabilising Selection- preserves the average phenotype of a population by favouring average individuals, in other words, selection against the extreme phenotypes
• Directional selection- changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population, in other words, selection for one extreme phenotype.
• Disruptive selection- favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population.

Question 16

explain stabilising selection

Answer 16

• Tends to eliminate the extremes of the phenotype range within a population and with it the capacity for evolutionary change.
• It tends to occur where the environmental conditions are constant over long periods of time.
• However, if the temperature is constantly 10C, then individuals at the extremes will never be at an advantage, so they are selected against in favour of those with average fur length. The mean remains the same but fewer individuals at either extremes

Question 17

explain directional selection

Answer 17

• Within a population there will be a range of genetically different individuals in respect of any one phenotype. The continuous variation forms a normal distribution curve.
• This curve has a mean that represents the optimum value for the phenotypic character under the existing conditions.
• If the environmental conditions change, so will the optimum value for survival.
• Some individuals, either to the left or the right of the mean, will possess a combination of alleles with the new optimum for the phenotypic character.
• As a result there will be a selection pressure favouring the combination of alleles that results in the mean moving to either the left or the right of its original position.
• Directional selection therefore results in one extreme of a range of variation being selected against in favour of the other extreme or even the average.

Question 18

explain disruptive selection

Answer 18

• The opposite of stabilising selection.
• Favours the extreme phenotypes at the expense of the intermediate phenotypes
• Although the least common form of selection, it is the most important in bringing about evolutionary change.
• Disruptive selection occurs when an environmental factor, such as temperature, takes two or more distinct forms.

Question 19

what is polymorphism

Answer 19

Some species of organisms have two or more distinct forms. They are different forms are genetically distinct but exist within the same interbreeding population

Question 20

how is the allelic frequency affected by selection

Answer 20

• is due to environmental factors
• Environmental changes affect the probability of an allele being passed on in a population and hence the number of times it occurs within a gene pool.
• Environmental factors do not affect the probability of a mutant allele arising, they simply affect the frequency of a mutant allele that is already present in a gene pool.

Question 21

what is speciation

Answer 21

• Is the evolution of new species from existing ones.
• A species is a group of individuals that have common ancestry and so share the same genes but different alleles and are capable of breeding with one another to produce fertile offspring.

Question 22

how are new species formed (adaptive radiation)

Answer 22

-individuals tend to only breed within their population they are capable of breeding with the other.
- These populations get separated and undergo different mutation becoming genetically different.
- Each of the populations will experience different selection pressure because the environment of each will be slightly different.
- Natural selection leads to change in the allelic frequency of each population.
- Different phenotypes each combination of alleles produces will be subject to selection pressure that will lead to each population becoming adapted to its local environment. This is called adaptive radiation, and results in evolution.
- Each population is now a different species with its own gene pool

Question 23

how are new species formed (genetic drift)

Answer 23

• Something that can take place in small populations because relatively few members posses a smaller variety of alleles so their genetic diversity is less.
• As these few individuals breed, the genetic diversity of the population is restricted to those few alleles in the original population. As there are only a small number of different alleles there is not an equal chance of being passed on.
• Those that are passed on will quickly affect the whole population as their frequency is high.
• Any mutation to one of these alleles that is selectively favoured will also more quickly affect the whole population because its frequency will be high.
• The effects of genetic drift will be greater and the population will change relatively rapidly, making it more likely to develop into a separate species.
• In large populations the effect of mutant allele will be diluted because its frequency is much less in the much larger gene pool.

Question 24

what is allopatric speciation

Answer 24

• Where two populations become geographically separated.
• This may be the result of a physical barrier between two populations which prevents them interbreeding.
• The barriers will be different depending on the species.
• If environmental conditions either side of the barrier vary, then natural selection will influence the two populations different and each will evolve leading to adaptations to their local conditions.

Question 25

what is sympatric speciation

Answer 25

Results within a population in the same area leading to the becoming reproductively separated

Question 26

what are 7 example of isolating mechanisms

Answer 26

1. Geographical- isolated by physical barriers 2. Ecological- inhabit different habitats 3. Temporal- breeding seasons of each population do not coincide, 4. Behavioural- mating and courtship. Mutations which causes variations in affecting mating call or appearance. 5. Mechanical-anatomical difference 6. Gametic- Gametes prevented from meeting due to genetic or biochemical incompatibility 7. Hybrid sterility- hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes.