18.5 Isolation and speciation

Question 1

What is allele frequency

What is it affected by

Answer 1

The number of times an allele occurs in a gene pool

It is affected by selection which is affected by environmental factors

Environmental factors don’t affect the probability of a particular mutant allele arising, but they do affect the frequency of it in the gene pool

Question 2

What is speciation

What is a species

Answer 2

The evolution of a species from an existing one

A species is a group of individuals that can breed together to make fertile offspring, and that have common ancestry so share the same genes but different alleles

Question 3

What is adaptive radiation

Answer 3

When a species become isolated, so due to different selection pressures, allele frequencies will change so they will develop different phenotypes which contain different combinations of alleles

So each population is now adapted to its environment

And eventually, the populations will become so different that they will no longer be able to interbreed together to produce successful offspring

So they are now different species with different gene pools

Question 4

What is genetic drift
Where does it usually occur

Answer 4

Usually occurs in small populations

The few members of it will possess a smaller variety of alleles than the members of a large population so they have less genetic diversity

As these individuals interbreed, the genetic diversity of the population is restricted to those few alleles
And because the number of alleles is small, the chances of passing them on are not equal

And those that are passed on will quickly affect the whole population as their frequency is high

Any mutation to any of these alleles that are selectively favoured will also quickly affect the whole population because its frequency is high.
So the effect of genetic drift will be greatest in small populations

Question 5

Why is genetic drift less common in large populations

Answer 5

The effect of a mutant allele will be more diluted because it has a lower frequency in the large gene pool

So it will develop into a new species slower

Question 6

What is allopatric speciation

Answer 6

Where the species becomes geographically isolated. So from a physical barrier between the populations to stop them interbreeding. Eg oceans, mountain range, river

. If environmental conditions on either side of the barrier vary, natural selection will influence the two populations differently so both will evolve differently over thousands of years

. And eventually the species will become so different that they will no longer be able to reproduce together to produce fertile offspring

Question 7

What is sympatric speciation

Apple maggot fly

Answer 7

When the population in the same area lead to them being reproductively isolated

In the apple, maggot fly:
Originally this insect only laid eggs inside the fruit of hawthorns, but when apple trees were introduced, the flies would lay their eggs in it
Female flies lay their eggs in it, and male flies look for mates in it, so mating occurs in them.

So flies raised in apples mate together, and flies raised in Hawthornes mate together

Eventually they will develop different mutations so different alleles and phenotypes, so will become reproductively isolated so will become different species

Question 8

Ecological speciation

Answer 8

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

Question 9

Temporal speciation

Answer 9

The breeding seasons are different, so they don’t coincide with each other so they don’t breed together

Question 10

Behavioural speciation

Answer 10

Mating is often preceded by courtship rituals eg the call of a mate
Or the colour or markings of a mate

So any mutations that change these patterns will stop them reproducing together

Question 11

Anatomical speciation

Answer 11

It may be impossible for two mammals to breed together, eg for penis to enter vagina

Question 12

Gametic speciation

Answer 12

The gametes may be prevented from meeting due to genetic incompatability

Eg some pollen grains won’t germinate or grow if on the stigma of a different genetic make up

Question 13

Hybrid speciation

Answer 13

Fusion of gametes from different species will produce sterile offspring

Eg mule from donkey and horse has 63 chromosomes so wont be able to undergo mitosis as they can’t pair up