Lesson 7 - Speciation
Starter
Part 1
A severe cold spell in 1996 killed over 50% of swallows living off cliffs in Nebraska.
Biologists collected nearly 2000 dead swallows from beneath the cliffs and
captured around 1000 living ones. They found that birds with a larger than average
body mass survived the cold spell. State, giving reasons, which type of selection
was taking place.
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Part 2
Look at picture below – what biological factors do you think the picture is trying to
illustrate?
Speciation
- Geographic separation of populations of a species can result in the accumulation of difference in the gene pools.
- The importance of geographic isolation in the formation of new species.
Speciation
This refers to the development of new species from a common ancestor.
Remember that organisms of the same species:
• have similar DNA and occupy the same ecological niche.
• can breed amongst themselves to produce fertile offspring.
Speciation occurs when gene flow stops between two populations where it previously
existed. Speciation results from natural selection affecting a population to such an
extent that the varieties produced can no longer interbreed to produce fertile
offspring.
The most important factor initiating speciation is isolation especially geographical
isolation.
***Main processes leading to speciation***
 Geographical isolation e.g. different islands or separated by barriers such as
mountains or rivers.
 Variation of phenotypes (characteristics) will already be present in the populations
as a result of random mutations.
 The different populations are exposed to different selection pressures (biotic e.g.
predation, disease, competition and/or abiotic (environmental) factors e.g.
temperature).
 Different (advantageous) alleles may be selected for in each different environment.
 This alters the frequency of alleles within the different populations.
 The gene pools remain separate with no interbreeding between members of the
different populations.
 This occurs over a long period of time and eventually members of the different
populations are unable to interbreed to produce fertile offspring, i.e. they have
become separate species.
Geographical Isolation
1. Start with an interbreeding population of one
species.
2. The population becomes divided by a physical
barrier such as water, mountains, desert, or
just a large distance. This can happen when
some of the population migrates or is
dispersed, or when the geography changes
catastrophically (e.g. earthquakes, volcanoes,
floods) or gradually (erosion, continental drift).
3. If the two environments (abiotic or biotic) are
different (and they almost certainly will be),
then the two populations will experience
different selection pressures and will evolve
separately. Even if the environments are
similar, the populations may change by random
genetic drift, especially if the population is
small.
4. Even if the barrier is removed and the two
populations meet again, they are now so
different that they can no longer interbreed.
They are therefore reproductively isolated
and are two distinct species. They may both be
different from the original species, if it still
exists elsewhere.
Task 1 – Construct – Peer Assessment
Annotate the same picture from the starter but this time with key aspects that you
didn’t think about earlier.
Task 2 – Application – Island Isolation – Self Assessment
Task 3 – Application – Exam Questions
Madagascar
1.
The island of Madagascar has been described as the laboratory of evolution. It broke away from mainland Africa at
least 120 million years ago and, following this, many new species developed. Estimates of the number of plant
species on the island vary from 7 370 to 12 000, making it botanically one of the richest areas in the world. Of 400
flowering plant families found worldwide, 200 grow only in Madagascar. Among animals, true lemurs are found
nowhere else, and 95 per cent of the country’s 235 known species of reptiles evolved on the island. Over the past 25
years the human population has doubled. Land shortage is leading to clearing of the forest, and Madagascar is now
facing deforestation on a massive scale. Scientists have estimated that even if the forest could recover, regeneration
could take up to a hundred years.
(Reproduced with permission from New Scientist magazine  RBI Ltd)
(a) Explain what is meant by the term species.
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(2)
(b) (i) Explain the processes which might have led to the evolution of new species on the island.
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(ii) Suggest and explain how the number of animal species on the island may have changed as it became ‘botanically one
of the richest areas in the world’.
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(2)
Task 4 – Extension Question
2.
The apple maggot fly (Rhagoletis pomonella) was found originally only on hawthorn trees in North America. In the
19th century, it spread as a pest to apple trees introduced into North America by farmers. The flies attacking the
apple trees are now genetically different from the flies that attack hawthorn trees. It is thought that a new species of
fly may be evolving.
(a) Suggest how the flies that feed on apple trees could evolve from those that fed on hawthorn trees.
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(3)
(b) Suggest one way in which scientists could find out whether the flies from apple trees were a different species from
those found on hawthorn trees.
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(Total 5 marks)
Hints for Application Tasks
Application – Island Isolation
1. Offspring?
2. Think about the type of prey the ‘dragons’ would be trying to capture. What
type of selection would be involved?
Examination Questions
1. (a) (b) (i) Remember the steps in speciation from earlier.
(b) (ii) Botanically one of the richest in the World – large number of different
plant species – what do we call animals that eat plants?
2. (a) Think about what starts speciation in the first place. They do you think
some of the flies could eat on the apple trees and not others? Was this
was advantage to those flies?
(b) -
Mark Scheme for Application Exam Questions
Application – Island Isolation
1. You could check whether the finches could interbreed to produce fertile
offspring
2. On the island there were no other predators and plenty of prey. The prey
included quite large animals, such as pigs, so the largest ‘dragons’ could be
the most successful at capturing these. Directional Selection. They would
then pass on alleles for large size to their offspring.
Examination Questions
1. (a)
- similar characteristics / physically similar / DNA similar;
- breed among themselves;
- to produce fertile offspring;
- do not share same ecological niche with any other species;
(b) (i)
- isolation;
- no gene flow between populations;
- variation;
- different environmental factors;
- natural selection / selection for specific alleles / characteristics;
- change in allele / phenotype frequency;
- changes over a long period of time;
(b) (ii)
- more habitats / niches; more / greater range of food for herbivores;
- more / greater range of food for carnivores / predators; more detritus;
2. (a)
- isolated population/group of flies/no gene flow;
- variation/mutation (in population);
- some able to use new food;
- so less competition; (survive and) reproduce to give new population.
(b)
- take flies from each population and interbreed;
- if no fertile offspring, then different species/ if present, same species.
Plenary
- to explain what is meant by the terms species and speciation.
- to describe how speciation involves the splitting and isolation of a population.
- to recall all the main steps leading to speciation and explain the significance of
the main steps.
- to apply knowledge of speciation to unfamiliar contexts.
From this lesson how many of the outcomes do you think you have achieved?
- Write on one yellow post-it note something from this lesson you knew something
about before today.
- Write on one or two green post-it notes the objective(s) you have achieved and
prove it!
- Write on one pink post-it the objective you want to work on more.