Selection and Speciation

•Write a sentence to describe what the phrase ‘survival of the fittest’ means to you

Species• Organisms that belong to the samespecies are able to breed together to produce fertile offspring

Natural Selection• Hardy-Weinberg principle cannot be true if individuals of

a particular genotype are –• More likely to die before reproducing• Unable to grow sufficiently well to reproduce successfully• Unable to attract a mate

In all cases, organisms with one particular genotype will reproduce less successfully than others in the population, and leave fewer, if no, offspring. There is a differential reproductive success between the genotypes in the population

Natural Selection1. Within a population, a characteristic has more

than one phenotype. These phenotypes result from genetic variation in the genotypes controlling this characteristic

Natural Selection2. There is different reproductive success between different phenotypes

Natural Selection• Organisms with a greater reproductive success leave more

offspring than those with less reproductive success

Natural Selection• Organisms with greater reproductive success will

pass their favourable allele to their offspring. As a result, the frequency of this allele will increase in the population, ie natural selection has occurred.

Natural Selection (Summarised)

• Frequency of an allele within a population changes.• Not all individuals are as likely to reproduce. • More advantageous alleles, increase likelihood of

survival there more chance of these being passed on. • Greater proportion of next generation inherit

beneficial allele.• They in turn are more likely to survive.• Frequency of allele increases• NATURAL SELECTION!

Natural Selection

Directional Selection• Acts against one of the extremes in a range of

phenotypes. As a result one phenotype becomes rare, and an alternative phenotype becomes more common.

Directional Selection• The upper graph represents in the

frequency of phenotypes in a population. The graph has a normal distribution with a fairly large standard deviation, this is before natural selection has occurred.

• The second graph is a frequency distribution of the same population after natural selection has occurred. The standard deviation of this curve is less than the upper curve and its mode has shifted to the right on the x-axis.

Natural selection has caused a genetic change in this population, favouring organisms with a characteristic towards the upper range of the frequency distribution

Stabilising Selection• Stabilising Selection acts against both extremes in a

range of phenotypes. As a result the variation about the mode is reduced.

• After selection, the mode is in the same position: This is the most advantageous phenotype. Stabilising selection has reduced the variation about this modal value.

Speciation• Formation of a new species by natural selection is

known as SPECIATION

• Geographic Isolation:

• This can happen when a species are physically separated into two. • It could be due to a mainland population being

separated from a population on an island• A creation of a volcano and therefore the two

populations are unable to reproduceThis prevents gene flow within the popuation

Drosophila • There are many different species of fruit fly. The

differences between them are so small, you may not be able to tell one species from another. But the flies can!

• Before mating, male and females undergo a courtship ritual

• The whole sequence, including the response from the female is controlled by genes and is species-specific

• http://www.youtube.com/watch?v=SVV-Oo1QA8M

• If the male does not have the correct courtship dance, the female does not allow him to mate with her

• If the fruit fly is homozygous recessive for wing length, it will not kill him, however his courtship dance will not attract a female, and he will be unable to pass on his alleles of the gene for wing length.

• His mutated gene for wing length will not be passed on. However, suppose these genetic changes had occurred not in an individual but in an entire population that was isolated from another population of the same species – This will be conditions in which speciation can occur

Geographic Isolation• Geographic isolation will lead to a population of a

species being isolated, and as a result there will be no GENE FLOW to neighbouring populations

• Gene mutations occur at a constant and low rate, some are beneficial and result in increasing the organisms reproductive success. This mutation will therefore be passed on.

• An accumulation of mutations can occur, which could mean that if the population was reintroduced to the original population, that it would be unable to reproduce. This has resulted in SPECIATION

Speciation• This can occur if two populationshave been separated

The conditions for each populationwill now be slightly different

As a result selection may occur in differentdirections, some alleles maybe favouredby one population but selected against in another.

Speciation• An experiment was conducted with fruit

flies. One population was split into two and each was fed on different food. After many generations the two populations were placed together and it was observed that they were unable to breed together.

1. What evidence shows speciation has occurred?(1)2. Explain why the experiment resulted in speciation.

(3)3. Suggest two possible reasons why members of the

two populations were not able to breed together (2)

Answers

1. The new species were unable to breed with each other (1)2. Populations were isolated and fed on different foods (1)

This caused changes to allele frequencies between the populations (1)Which made them reproductively isolated and eventually resulted in speciation (1)

3. Seasonal changes (1)Mechanical changes (1)Behavioural changes (1)