Changes in gene pools

Bio3B

Genotypes and gene pools

• The allele combination possessed by an individual is referred to as genotype

• The allele frequencies present in a population is referred to as the gene pool

Natural selection

• Variation• Overpopulation • ‘struggle for existence’ • Selection pressures• ‘survival of the fittest’ • Inheritance of successful

variations • Change in gene

frequencies

Random genetic drift

This is change in gene frequencies due to chance processes, not selection pressures. The alleles affected are usually survival neutral.

It occurs most often in smaller populations because variation is usually lower to start with, and with fewer individuals, the loss of one or two can have large effects on the alleles remaining in the population

Founder effect refers to random genetic drift occurring in small isolated populations eg Dunkers in USA

Selection vs random genetic drift

Forces act to increase the frequency of one characteristic compared to another

Frequencies change due to random mating and production of offspring

Artificial selection

This is – where humans provide the selection pressures and breed for certain characteristics

Examples include – domestic animals eg dogs, cats, pigeons, sheep, cattle, etc and crops eg wheat, rice, fruit, vegetables, etc

Sexual selection

Selection forces are due to mating selection –

This can be females choosing males of a particular appearance eg peacocks

This can also be due to battle for the right to mate eg horns on many species of goats and deer

Experiments with selection 1Guppy spots are largely genetically

controlled. Spots that help the guppy blend in with its

surroundings protect it from predation - but spots that make it stand out help it attract mates.

Endler set up similar populations of guppies in artificial ponds in the laboratory.

Ponds varied in the coarseness of gravel on the bottom and all ponds had predators.

Opposite is a simplified representation of Endler’s experiment.

After fewer than 15 generations of selection, the markings of guppies in different ponds had substantially diverged as a result of natural selection.

In the presence of predators, guppies evolved to blend in with their background.

Experiments with selection 2

Endler then performed another experiment, with the same pond set-ups but without predators.

Without predators, there was sexual selection for male guppies that stood out from their background and attracted the attention of the females.

Speciation

• A species is a group of organisms that, under natural conditions, can breed to produce fertile offspring

• Speciation refers to development of new species – the population is so different to the original that they no longer can interbreed

Subspecies

• Races or subspecies are smaller breeding groups within a species that exhibit differences from other breeding groups. If separated they may develop into new species

• Problems with the species concept include fossils or extinct species – can’t breed these, ring species – breed with intermediate populations, but those at the ends won’t breed with each other

Extended answer 2002 37(b)Ichthyosaurs are extinct marine animals that appear remarkably similar to a modem marine

animal, the dolphin. However, ichthyosaurs were reptiles, related to snakes and lizards while dolphins are mammals related to dogs, cats and humans. Explain how two animals from such very different groups could have evolved to look so similar.

Extended answer 2002 37(b)Ichthyosaurs are extinct marine animals that appear remarkably similar to a modem marine

animal, the dolphin. However, ichthyosaurs were reptiles, related to snakes and lizards while dolphins are mammals related to dogs, cats and humans. Explain how two animals from such very different groups could have evolved to look so similar.

Similarities arise by convergent evolution• where different / unrelated taxa evolve to look similar• in response to similar selection pressures.• Example other than dolphins & ichthyosaurs• Natural selection works in populations showing variation• causing differential survival rates• and differential reproduction rates.• Results in changes to gene frequencies

Sources of variation:• Crossing over• Mutation• Independent assortment• Random recombination / fertilization

• Selection pressure caused by aquatic environment / predatory nature results in• streamlining• fins• rapid locomotion• large mouth for prey capture

c) Name and describe two events that result in every gamete containing a different combination of maternal and paternal DNA

e) Describe how the occurrence of a favourable DNA mutation might affect the process of speciation in an isolated population of birds

Biology 1999 WATP Q33

c) Name and describe two events that result in every gamete containing a different combination of maternal and paternal DNA

Crossing over/chiasmata- Exchange of DNA between chromatids during meiosisIndependent assortment/segregation- Each homologous pair separates independently & randomly to the othersMutations - Change in DNA due to mistakes during meiosis

e) Describe how the occurrence of a favourable DNA mutation might affect the process of speciation in an isolated population of birds (remember VOSSSIC)

Isolated population has different conditions to original populationMutation may give adaptive advantage More likely to survive or breed More offspring More offspring have mutation Gene frequency changes Over a long time speciation occurs – the new population is no longer able to

breed with the original population

Biology 1999 WATP Q33

Graph analysis

a) Describe one conclusion that can be made from these data. (2 marks)

b) Suggest an explanation for the changes seen in these data. (2 marks)

Annual area of land clearing and vegetation protected under covenant in Western Australia

Graph analysis

a) Describe one conclusion that can be made from these data. (2 marks)Clearing has decreased, protection has increasedb) Suggest an explanation for the changes seen in these data. (2 marks)reduced population growth means less land needs clearing / environmental

concerns have led to reduced clearing/increased protection

Annual area of land clearing and vegetation protected under covenant in Western Australia