Speciation and Genetic Drift

Speciation is the development of a new species Speciation is the development of a new species from an existing species. Speciation occurs when populations of the same species become reproductively isolated - changes in the allele frequency cause changes in phenotype, which means they can no longer interbreed to produce fertile offspring. This can happen when a physical barrier, e.g. a flood or an earthquake, divides a population of a species, causing some individuals to become separated from the main population. This is known as geographic isolation and leads to allopatric speciation. Alternatively, speciation can also occur when a population become reproductively isolated without any physical separation. This is known as sympatric speciation.

Allopatric speciation requires geographical isolation1. Populations that are geographically separated will experience slightly different conditions, for example there might be a different climate on each side of a physical barrier.2. This means that the populations will experience different selection pressures and so different changes in allele frequencies could occur: Different alleles will be more advantageous in the different populations. For example, if geographical separation places on population in a colder climate than before, longer fur length will be beneficial. Directional selection will then act on the alleles for fur length in this population, increasing the frequency of the allele for longer fur length. Allele frequencies will also change as mutations occur independently in each population. Genetic drift may also affect the allele frequencies in one or both populations. 3. The changes in allele frequency will lead to differences accumulating in the gene pools of the separated populations, causing changes in phenotype frequencies. 4. Eventually, individuals from the different populations will have changed so much that they won't be able to breed with one another to produce fertile offspring - they'll have become reproductively isolated. 5. The two groups will have become separate species.

Sympatric speciation doesn't require geographical isolationA population doesn't have to become geographically isolated to become reproductively isolated. Random mutations could occur within a population, preventing members of that population breeding with other members of the species. EXAMPLE Most eukaryotic organisms are diploid - they have two sets of homologous chromosomes ion their cells. Sometimes, mutations can occur that increase the number of chromosomes. This is known as polyploidy. Individuals with different numbers of chromosomes can't reproduce sexually to give fertile offspring - so if a polyploid organisms emerges in a diploid populations, the polyploid organism will be reproductively isolated from the diploid organisms. If the polyploid organims then reproduces asexually, a new species could develop. Polyploidy can only lead to speciation if it doesn't prove fatal to the organism and more polyploid organisms can be produced. It's more common in plants than animals.

Reproductive isolation occurs in many waysReproductive isolation occurs because changes in alleles and phenotypes in some individuals prevent them from breeding successfully with individuals without these changes. These changes include: Seasonal - individuals from the same population develop different flowering or mating seasons, or becomes sexually active at different times of the year. Mechanical - changes in genitalia prevent successful mating. Behavioural - a group of individuals develop courtship rituals that aren't attractive to the main population.

Genetic drift can lead to speciation Different selection pressures can change the allele frequencies in two geographically isolated species. This is evolution by natural selection. But evolution can also occur by genetic drift. This is when chance, rather than environmental factors, dictates which individuals survive, breed and pass on their alleles: Individuals within a population show variation in their geneotypes (e.g. A and B) By chance, the allele for one geneotype (B) is passed on to the offspring more than others. So the number of individuals with the allele increases. Changes in allele frequency in two isolated populations could eventually lead to reproductive isolation and speciation. Natural selection and genetic drift work alongside each other to drive evolution, but one process can drive evolution more than the other depending on population size. Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence. In larger populations, any chance variations in allele frequency tend to even out across the whole population.

Evolutionary change has resulted in a great diversity of organisms The diversity of life on Earth today is the result of speciation and evolutionary change over millions of years. To start with there was one population of organisms. The population was divided and the new populations evolved into separate species. The new species were then divided again and the new populations evolved into more separate species. This process has been repeated over a long period of time to create millions of new species.