) Explain How Natural Selction May Bring About Changes In A Population And What Conditions May Be Necessary To Bri About Speciation

1823 words - 7 pages

a)} How may Variation Arise in Natural populations

b) Explain how natural selction may bring about changes in a
population and what conditions may be necessary to bri about
speciation

A species is a group of organisms that look alike and can reproduce
successfully to produce fertile offspring. Within a species there is
variation which can be caused my meiosis, random fertilisation,
mutations and the natural environment. Genetic variation increases the
chance for survival of a population if the environment changes. For
example the introduction of a new virus in to the new environment may
kill some organisms but others may be resistant to it and survive. If
all the organisms had been genetically identical they may have all
been killed. The species that survive are likely to live longer and
pass on their useful genes to future generations. The change in a
species over time is called evolution. There are two types of
variation; continuous and discontinuous variation. Discontinuous
variation is when there is a complete range of measurements from one
extreme to the other for a normal characteristic, such as human
height. Discontinuous variation is where characteristics are fit into
separate categories with no intermediates such as human blood groups.

During the process of meiosis each homologous pair of chromosomes
aligns independently along the equator and whether the maternal or
paternal chromosome faces a particular pole is quite random. This is
known as independent assortment and is one of the causes of variation
in the gametes. Unlinked genes are transmitted independently of each
other, leading to random distribution of genes. Alleles can also be
separated and reformed if crossing over occurs. This is where the
paternal and maternal chromatids of chromosomes intertwine and cross
over each other at points called chiasmata. The chromatids break at
these points and rejoin so that the alleles from one chromatid join
alleles on another. An exchange of chromatids takes place between
chromatids.

Although sexual reproduction produces variation it does not actually
introduce new alleles. These are brought in by mutations. A mutation
is a change in the structure or amount of DNA in the amino acids
sequence produced. A mutation in a gamete will be inherited, whereas a
mutation in other body cells will not be passed to offspring. Gene
mutation is an alteration in the base sequence of one or two base
pairs by addition, deletion or substitution. This occurs at a single
gene locus on a chromosome altering the DNA sequence of bases and the
amino acid sequence being produced. The resulting protein shape may be
significantly altered and then does not function properly. This is
important if the altered protein is an enzyme which if changed, may no
longer match the shape of its...

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