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A species is defined as a group of organisms that can interbreed to produce fertile offspring.
Expanding on this definition, a species must be able to reproduce successfully (either sexually or
asexually) in order to be deemed a species. Speciation occurs when two or more separate species
are formed by a biological evolutionary process.
Mutations can increase the gene pool of a population by increasing the number of different
alleles available. If a gene pool is changing, natural selection is acting as a force for change. So if a
mutation results in an advantageous feature then the allele frequency (the relative frequency of a
certain allele in a population) in the population will be selected for and so
increase in frequency. When there is competition for resources, natural
selection acts and only the most successful will survive and pass on their
genes to their offspring. Changes in allele frequency due to natural selection
can lead to the evolution of new species.
The allele frequency can also be influenced by the size of the
population, or if the individuals move in or out of the population. If a
population is small, there is a greater probability that the genes of one
generation won't be accurately represented in the next. Genetic drift can
result, which refers to a change in allele frequency caused by chance. When
some individuals move to a new environment and begin a population the
founder effect can be observed. If new individuals breed, gene flow may
New species can also be formed either through allopatric or
sympatric speciation. Allopatric speciation refers to the situation in which a
population becomes geographically isolated into two separate populations.
Some species evolve when populations become geographically isolated
due to the formation of physical barriers such as mountain ranges or rivers.
Different geographic regions are likely to have different selection
pressures, e.g. climate, predators and competitors. Exposed to different
environments, the two separated populations will evolve differently to
each other in order to adapt to their new environment. These populations
are prevented from interbreeding and over time can become two separate
An example of this is the type of house mice (Mus musculus domesticus) that were found on
the island of Madeira after a study conducted in 2000. The karyotypes of 143 house mice from
various locations along the coast were examined. It was discovered that there were six distinct
populations, each with a unique karyotype. The different populations were allopatric as they had
been isolated in different valleys leading down towards the sea. It is likely that the distinct
karyotypes found in each population arose from genetic drift rather than natural selection. The six
populations don't have the opportunity of interbreeding. While these mice could form hybrids, as no
pre-zygotic isolating mechanisms have an effect, these would probably be infertile.
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Sympatric speciation is when different species arise within the same overlapping geographic
area. As they are not geographically isolated, other factors will prevent interbreeding. When it
comes to speciation, reproductive isolation is very important and it can occur when fertilisation is
prevented. Pre-zygotic isolating mechanisms prevent the formation of hybrid zygotes. Two
populations may live alongside each other, but may have different breeding seasons of different
mating calls.…read more
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