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Speciation and Evolution.
How does speciation occur?
For speciation to occur there must be reproductive isolation. Geographical isolation and natural
selection lead to reproductive isolation. Random mutations cause variation between members of the
same species, and through natural selection this can cause speciation. Geographical isolation occurs
when two populations are divided by a physical barrier, so different pressures in each area, and
causing independent genetic drift. The allele frequencies in each population will change and
mutations will take place independently in each population, both leading to changes in phenotype
frequencies reproductive isolation occurs in two forms, when fertilisation is prevented, prezygotic,
or when the zygote fails or is unable to breed, postzygotic.
Prezygotic Reproductive barriers.
Temporal isolation, mating periods do not overlap.
Mechanical isolation, physical barriers, e.g. organs do not fit together.
Behavioural isolation, male fails to attract female.
Gametic isolation, female gametes do not attract male gametes or male gamete cannot
penetrate female gamete.
Postzygotic Reproductive Barriers.
Low hybrid zygote vigour, zygotes fail to develop properly and die in embryonic
Low hybrid adult viability, offspring are born but fail to thrive and grow properly.
Hybrid infertility, offspring are healthy but infertile, e.g. mules.
It is hard to identify using phenotype as external conditions can cause individuals of the same species
to look very different, so DNA profiling is used, looking at the non-coding parts of the genetic code
to identify patterns. The more closely a species is related to another, the more recently they
diverged. DNA profiling supports evolution.
Proteomics is the study of proteins, of the size, shape and amino acid sequence. Related organisms
have similar DNA sequences, and so similar amino acid sequences in their proteins, so organisms that
diverged from each other more recently should have more similar proteins, as less time has passed
for changes to occur. Findings have supported this.
The scientific community validates evidence about evolution. First, the scientist collects data to test
theories and ideas, and the data either supports or doesn't support the theory, e.g. DNA and
proteomic data has been collected which supports the theory of evolution. Then, the scientific
community around the world share and discuss the evidence on the theory to make sure it is valid and
reliable. The scientific community can share and discuss their work in three ways:
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Scientific journals, as they allow sharing of ideas and experiments, which other scientists can
repeat to see if they get the same results using the same methods, and the more they do,
the more reliable the evidence becomes.
2. Peer review, before publishing, other scientist who work in the same are read and review
the work to check that it is valid and supports the conclusions. This helps ensure that
experiments are carried out to the highest possible standards.