Populations and Evolution

Question 1

Species

Answer 1

Group of similar organisms that can reproduce to give fertile offspring

Question 2

Population

Answer 2

• Group of organisms of the same species living in a particular area at a particular time-so they have the potential to interbreed
• Species can exist as one or more populations

Question 3

Gene Pool

Answer 3

• Complete range of alleles present in a population
• How often an allele occurs in a population is called the allele frequency (usually given as percentage of total population)

Question 4

Genetic variation

Answer 4

• Mutations= Sudden changes to genes and chromosomes may, or may not, be passed on to the next generation. Mutations are a main source of variation.
• Meiosis= This special form of nuclear division produces new combinations of alleles before they are passed into the gametes, all of which are therefore different.
• Random fertilisation of gametes= In sexual reproduction this 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 2 parents can produce.

Question 5

Variation due to environment

Answer 5

• Affects how genes are expressed
• Include climatic conditions (temperature, rainfall, sunlight), soil conditions, pH and food availability
• Characteristics that are controlled by many genes (polygenes) show a type of variation where some characteristics of organisms grade into one another, forming a continuum e.g. height and mass
• Heights of people will show a normal distribution curve (bell-shaped)

Question 6

Variation

Answer 6

• Differences that exist between individuals
• Variation within a species (intraspecific) means that individuals in a population can show a wide range of different phenotypes
• Variation can be caused by genetic/environmental factors
• Although individuals of the same species have the same genes, they have different alleles-this causes genetic variation within a species
• Main source of genetic variation is mutation
• Genetic variation also through crossing over of chromatids and independent segregation (and random fertilisation of gametes)
• Variation within a species can also be caused by differences in the environment, like food,climate, or lifestyle
• Most variation within a species is caused by a combination of genetic and environmental factors, but only genetic variation results in evolution

Question 7

Evolution

Answer 7

• The frequency of an allele in a population changes over time (evolution)
• Evolution can occur by genetic drift or natural selection

Question 8

Natural Selection

Answer 8

• Organisms face many pressures that affect their chances of surviving, such as predation, disease and competition (these are called selection pressures)
• Selection pressures create a struggle for survival
• As members of the same species have different alleles, there is variation between individuals, meaning that some are better adapted to the selection pressures than others
• This means there are differential levels of survival and reproductive success in a population
• Individuals with a phenotype that increases their chance of survival are more likely to survive, reproduce and pass on their genes (including the beneficial alleles that determine their phenotype), than individuals with a different phenotype
• This means that a greater proportion of the next generation inherit the beneficial alleles
• They in turn, are more likely to survive, reproduce and pass on their genes
• So the frequency of the beneficial alleles in the gene pool increases from generation to generation

Question 9

Types of natural selection

Answer 9

• Effect of natural selection on allele frequencies depends on the selection pressures acting on the population
• Stabilising, directional and disruptive selection

Question 10

Stabilising selection

Answer 10

• This is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
• Occurs when the environment isn’t changing, and it reduces the range of possible phenotypes
• Mean stays in middle

Question 11

Directional selection

Answer 11

• This is where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
• This could be in response to an environmental change
• Mean moves in one direction or the other

Question 12

Disruptive selection

Answer 12

• This is where individuals with alleles for extreme phenotypes are more likely to survive and reproduce
• It’s the opposite of stabilising selection because characteristics towards the middle of the range are lost
• Occurs when the environment favours more than one phenotype

Question 13

What does natural selection depend on?

Answer 13

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

Question 14

What is speciation?

Answer 14

• Development of a new species from an existing species (results in diversity)
• It occurs when populations of the same species become reproductively isolated- changes in allele frequency cause changes in phenotype, which mean they can no longer interbreed to produce fertile offspring
• Reproductive isolation can occur when a physical barrier, e.g. flood or earthquake, divides a population of a species, causing some individuals to become separated from the main population
• Known as geographical isolation
• There is no gene flow (transfer of genes) between the 2 populations, which can lead to allopatric speciation
• Speciation can also occur when a population becomes reproductively isolated without any physical separation (sympatric speciation)
• Environmental changes affect probability of an allele being passed on in a population and hence the number of times it occurs in a gene pool (affect frequency of mutant allele already present)

Question 15

Allopatric speciation

Answer 15

• Populations that are geographically separated will experience slightly different conditions
• Populations will experience different selection pressures and so different changes in allele frequencies could occur
• Different alleles will be more advantageous in the different populations, so natural selection occurs e.g. if geographical separation places one population in a colder climate than before, longer fur length will be beneficial