18.5 - Isolation And Speciation

Question 1

Explain how selection affects allelic frequencies.

Answer 1

• The number of times an allele occurs within the gene pool is referred to as the allelic frequency.
• The allelic frequency is affected by selection and selection is due to environmental factors.
• Environmental changes therefore affect the probability of an allele being passed on in a population and hence the number of times it occurs within the gene pool.
• It must be emphasised that environmental factors do not affect the probability of a particular mutant allele arising, they simply affect the frequency of a mutant allele that is already present in the gene pool.
• Evolution by natural selection is a change in the allelic frequencies within a population.

Question 2

What is speciation

Answer 2

Speciation is the evolution of new species from existing ones.

Question 3

What is a species

Answer 3

• species is a group of individuals that have a common ancestry and so share the same genes but different alleles and are capable of breeding with one another to produce fertile offspring.
• In other words, members of a species are reproductively separated from other species

Question 4

It is through the process of _________ that evolutionary change has taken place over millions of years.

Answer 4

Speciation

Question 5

How are new species formed

Answer 5

reproductive separation followed by genetic change due to natural selection.

Question 6

Explain how new species are formed

Answer 6

• within a species there are multiple populations, all capable of interbreeding
• suppose 1 population becomes isolated, undergoes different mutations, it will be genetically different from the other populations
• each population experiences different selectional pressures because each environment is slightly different for each population
• natural selection leads to changes in alleleic frequencies of each population
• The 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 (known as adaptive radiation leads to changes in allele frequencies of each population = evolution)
• now, each population are no longer able to interbreed and they have now become separate species with their own gene pools

Question 7

Describe genetic drift

Answer 7

• it takes place in small populations.
• because small population = smaller variety of alleles than large population. I.e. their genetic diversity is less
• the few individuals breed = genetic diversity of the population is restricted to the few alleles in the original population
• due to small number of different alleles = not an equal chance of each being passed on.
• Alleles 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.

Question 8

Describe why the effects of genetic drift in a larger population is less

Answer 8

• In large populations the effect of a mutant allele will be diluted because its frequency is far less in the much larger gene pool. - The effects of genetic drift are likely to be less, and development into a new species is likely to be slower.

Question 9

What are the 2 forms of speciation

Answer 9

• allopatric speciation
• sympatric speciation.

Question 10

What is allopatric speciation

Answer 10

• two populations become geographically separated (perhaps by barriers like rivers or mountains)
• These changes may take many hundreds or even thousands of generations, but ultimately may lead to reproductive separation and the formation of separate species

Question 11

Destine sympatric speciation

Answer 11

• form of speciation that results within a population in the same area leading to them becoming reproductively separated.

Question 12

Describe the isolating mechanism that leads to variation: geographical

Answer 12

Populations are isolated by physical barriers such as oceans, mountain ranges, rivers, etc.

Question 13

Describe the isolating mechanism that leads to variation: ecological

Answer 13

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

Question 14

Describe the isolating mechanism that produces variation: Temporal

Answer 14

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

Question 15

Describe the isolating mechanism that leads to variation: Behavioural

Answer 15

• Mating is often preceded by courtship, which is stimulated by the colour or markings of the opposite sex, the call or particular actions of a mate.
• Any mutations which cause variations in these patterns may prevent mating

Question 16

Describe the isolating mechanism that leads to variation: Mechanical

Answer 16

Anatomical differences may prevent mating occurring

Question 17

Describe the isolating mechanism that leads to variation: Gametic

Answer 17

• 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 makeup.

Question 18

Describe how the isolating mechanism leads to variation: hybrid sterility

Answer 18

• Hybrids 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) anda donkey (2n = 62) the resultant mule has 63 chromosomes.
• It is impossible for these chromosomes to pair up appropriately during meiosis and so the gametes formed are not viable and the mule is sterile

Question 19

Define a species

Answer 19

A species is a group of individuals that share similar genes and are capable of breeding with one another to produce fertile offspring. In other words they belong to the same gene pool.

Question 20

Explain the meaning of the term speciation

Answer 20

Speciation is the evolution of new species from existing species.

Question 21

Describe the process of geographical separation.

Answer 21

Geographical isolation occurs when a physical barrier, such as mountains or oceans, prevents two populations from breeding with one another

Question 22

Explain how geographical separation of two populations of a species can result in the accumulation of the differences in their gene pools.

Answer 22

• Geographically isolated populations may experience different environmental conditions.
• In each population, phenotypes that are best suited to the particular environmental conditions are selected.
• The composition of the alleles in each gene pool therefore changes as they pass to subsequent generations.
• The composition of the gene pool of each population becomes increasingly different over time.
• Being geographically isolated, individuals of each population cannot breed with one another and so the two gene pools remain separate and different.

Question 23

Distinguish between allopatric and sympatric speciation.

Answer 23

• Allopatric is speciation as a result of two populations becoming reproductively isolated because they are geographically separated and so unable to interbreed.
• Sympatric is speciation as a result of populations that live together being reproductively isolated for other reasons
• e.g. they have different breeding seasons which do not overlap.