Evolution PDF

Natural selection To understand natural selection we first need to look at breeding and survival. Under perfect conditions a pair of breeding mice will produce six offspring every two months. After six weeks the offspring mature and produce six offspring every two months as well. ...

Natural selection To understand natural selection we first need to look at breeding and survival. Under perfect conditions a pair of breeding mice will produce six offspring every two months. After six weeks the offspring mature and produce six offspring every two months as well.  Natural selection If all the mice survived we would be overrun but this is not the case because many do not survive.  Natural selection There are selection pressures on the population e.g. food supply and disease Only the mice which are best suited to their environment will survive and produce offspring. These offspring will have a selective advantage because their parents have passed on their advantageous genes.  Natural selection In our mouse example all the mice produced will show variation because they have been produced by sexual reproduction. They will show variation in: – Coat colour/thickness – Speed and reactions – Food finding ability – Resistance to disease – Aggressiveness  Natural selection Mice who inherit the best combination of the genes controlling these characteristics will be more likely to survive. This means they will live to reproduce and pass on these beneficial genes, the other mice will die before they can pass on their genes. This process is known as natural selection.  Natural selection Natural selection is the non-random increase in frequency of DNA sequences that increase survival. Also the non-random decrease in frequency of deleterious DNA sequences.  Natural selection Individuals with favourable alleles survive to reproduce. Favourable alleles are passed onto the next generation. These favourable alleles increase in frequency.  Gene transfer The key process in evolution is inheritance. There are two main ways of transferring genetic material from one organism to another: – Vertical gene transfer – Horizontal gene transfer  Gene transfer Vertical gene transfer refers to the transfer of genetic sequences from parent to offspring. This is a result or either sexual or asexual reproduction.  Gene transfer In sexual reproduction genetically diverse parents produce offspring showing genetic variation. In asexual reproduction a single parent produces offspring with the same genome. Vertical gene transfer occurs in both eukaryotes and prokaryotes.  Gene transfer Vertical gene transfer Vertical gene transfer in yeast (eukaryote) in bacteria (prokaryote)  Gene transfer Horizontal gene transfer occurs in only in prokaryotes. Eukaryotes cannot transfer DNA sequences horizontally. Horizontal gene transfer is different from vertical gene transfer in that the genetic material moves between members of the same population, not from parent to offspring. Gene transfer  Gene transfer Gene transfer between species may confer some advantage. Once a sequence is transferred horizontally between two cells it may be passed vertically to the next generation.  Gene transfer Horizontal gene transfer allows new genetic sequences to be transferred between prokaryotes very quickly. This allows prokaryotes to undergo rapid evolutionary change. This is beneficial to the recipient if the new sequence confers an advantage e.g. bacteria becoming antibiotic resistance.  Selection Selection can affect the characteristics of a population in different ways: – Stabilising selection – Directional selection – Disruptive selection The type of selection depends on which phenotypes in the phenotype range are selected for.  Stabilising selection The average phenotype is selected for, the extremes are selected against. For example a group of birds show feather colour from white through to black. In this type of selection grey birds are selected for and white and black birds are selected against. Stabilising selection  Directional selection One extreme phenotype range is selected for while the average and other extreme phenotype range are selected against. In our example this would mean black birds are selected for and grey and white are selected against. This situation could be caused by a forest fire, blackening the landscape. Directional selection  Disruptive selection Two or more phenotypes are selected for whilst others in the phenotype range are selected against. In our example this would mean black and white birds are selected for and grey are selected against. This situation could be caused by part of the fire blackened landscape being covered with snow. Disruptive selection  Species A species is a group of organisms capable of interbreeding and producing fertile offspring.  Speciation Speciation is the generation of new biological species by evolution. As long as a population has the opportunity to interbreed allowing gene flow, they remain one species. A population of one species can only evolve into more than one species if groups within the population become isolated by barriers that prevent gene flow.  Speciation One species exists which interbreeds and shares the same gene pool.  Speciation The population becomes split by an isolation barrier into two sub-populations. This means gene flow is prevented as the two sub-populations can no longer interbreed.  Speciation Mutations occur at random and are therefore different on each side of the barrier. Selection pressures will be different on either side of the barrier meaning different mutations will be advantageous and selected for.  Speciation Natural selection acts on each sub-group in a different way due to different selection pressures. Selection pressure – Selection pressure – temperature, therefore predation, therefore the hairy form has the blue form has the selective advantage. selective advantage.  Speciation Over a long period of time the gene pools in the two sub populations may have become so altered that the groups cannot interbreed. If the isolation barrier is removed the populations cannot interbreed and produce fertile offspring speciation has occurred.  Speciation There are two types of speciation: – Allopatric speciation – Sympatric speciation The type of speciation depends on the barrier involved.  Speciation Barriers which prevent gene flow between two populations can be: - Geographical - Ecological - Behavioural  Speciation In allopatric speciation the population is split by a geographical barrier Geographical barriers are physical barriers such as rivers, mountains, deserts or the sea.  Speciation In sympatric speciation the two populations live near each other but are separated by behavioural or ecological barriers. Ecological barriers are caused by changes in abiotic factors, while behavioural barriers may be caused by populations become sexually receptive at different times of the year.