Download The Gene Pool and Speciation

The Gene Pool and Speciation
- Genetics and Evolution
We have already discussed some of the reasons why the genetics of a species can change
over a long period of time. Charles Darwin's The Origin of Species went into great depth
in this matter, and providing substance into the theory of evolution. The key thing to
remember about evolution is that it favours more preferable genes in the gene pool, and
over time, these preferable characteristics become more exclusive in the gene pool.
This next section rounds up all the factors that can alter the make up of a gene pool:
Natural Selection
Natural selection will favour genes that are more suited to their environment and become
more exclusive in the gene pool over time in such an environment. Different genes will
become more exclusive when the environment changes, or the species migrate.
Mutation
Mutations are random occurrences which change the genome of an organism. They
greatly increase genetic diversity, where advantageous mutations are favoured by natural
selection and disadvantageous ones are phased out.
Gene Migration
Occurs after genetic drift, where two groups of a species become separated and therefore
cannot reproduce. The gene pool of these groups differ over time. If these two groups can
once again meet up and reproduce, their genetic differences can be merged within the
single group and increase genetic diversity.
Non-Random Mating
Can cause in-breeding depression by continuous inbreeding, non-random mating is also
known as selective breeding, where the breakthroughs of Mendelian genetics have
allowed us to predetermine what genes are present in offspring. As advantageous genes
are desired by the breeder, some of the less 'popular' genes are lost due to this random
mating, therefore decreasing genetic diversity.
It is important for a species to have a large gene pool, because in the event of danger,
some alleles will allow the species to survive and reproduce to produce a larger and more
variant gene pool. For example, an extremely contagious disease may threaten 99% of a
species, though the remaining 1% may possess an allele that provides them with
resistance to the disease. If this allele was not present in the population, then chances are
the entire population would be wiped out
Genetic Drift
Sometimes, species can be split into groups, usually as a result of a geographical factor
preventing the two groups from contacting one another. This means that the two groups
are unable to reproduce with one another.
As the two groups now live in different environments, natural selection will favour
slightly different genes in each of the groups that will favour them in their particular
environment. Over time, the difference in gene pool between the two groups can be quite
dramatic.
If these two groups once again become re-united, gene migration occurs (see above). If
they remain seperated, their genetic differences become greater which can result in the
formation of a new species.
Speciation
When the genetic differences become so great, it can come to the stage where the two
groups can no longer reproduce with one another. This results in the formation of a new
species (because organisms who are capable of reproducing but cant reproduce with a
member of the same species are deemed another species).
Long ago, the land of Earth was all on one continent. Over time these continents
separated, with members of the same species drifting away on each continent. Over time,
speciation has occurred. This is evident by looking at the marsupials of Australia, who
have been isolated from other mammals in their ancestral line, and therefore have many
differences to that of mammals on other continents.
Adaptive Radiation
Adaptive radiation is the slow change of genotype and phenotype of a species from its
common ancestor, meaning that species with a common ancestor become more
diversified over time.
When Charles Darwin was in the Galapagos islands, one of the first things he noticed is
the variety of finches that existed on each of the islands. All in all, there were many
different species of finch which differed in beak shape and overall size. This is adaptive
radiation and natural selection at work.
Darwin's Finches
These finches, better known as 'Darwin's Finches' illustrated adaptive radiation. This is
where species all deriving from a common ancestor have over time successfully adapted
to their environment via natural selection.
Previously, the finches occupied the South American mainland, but somehow managed to
occupy the Galapagos islands, over 600 miles away. They occupied an ecological niche
with little competition.
As the population began to flourish in these advantageous conditions, intraspecific
competition became a factor, and resources on the islands were squeezed and could not
sustain the population of the finches for long.
Due to the mechanisms of natural selection, and changes in the gene pool, the finches
became more adapted to the environment, illustrated by the diagram below.
As competition grew, the finches managed to find new ecological niches, that would
present less competition and allow them, and their genome to be continued.
As indicated by the diagram above, the finches adapted to take advantage of the various
food sources available on the island, which were being used by other species. Over the
long term, the original finch species may have disappeared, but by diversifying, would
stand a better chance of survival.
All in all, the finches had adapted to their environment via natural selection, which in
turn, has allowed the species to survive in the longer term, the prime directive of any
species.
The Marsupials
The marsupial mammals occupy Australia, and dither from placental mammals because
they bear their young inside a pouch. Long ago, the land mass of Earth consisted of one
single continent, Pangaea, where all animals existed. When this continent fragmented into
smaller continents, the geographical barrier meant the mammals of the time could no
longer reproduce with one another. From here on in, the gene pool of these groups of
mammals would become increasingly different until they could no longer reproduce with
one another. This occurrence applies to the marsupial family of mammals that occupy
Australia.
All the marsupials in present day Australia would have evolved from one common
ancestor. However, over time and via natural selection, the many marsupial species (i.e.
kangaroo and koala) have occupied their own ecological niche and adapted accordingly.
Kangaroo's have long powerful legs to cover the wide area of land that they occupy while
the koala's smaller structure and more centralised centre of gravity allow them to climb
trees and obtain the eucalyptus that they feed on.
Humans, the most evolved species on the planet, have also underwent many changes over
time from our ancestors. Here are some examples
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Humans are bipedal, meaning we walk on two of our limbs. The other limbs, our
arms and subsequent hands, have adapted to do precision tasks such as typing or
tying shoe laces, common tasks that we perform in our own environment.
The amount of melanin in our skin is representative of the environment we live in,
i.e. dark skinned people occupy hotter climates.
The structure of our body gives an indication of the temperature in our climate
also. Humans who have produced offspring that successfully live in a cold
environment tend to be broader and smaller in stature while hotter environments
are occupied by thinner taller humans.
It is worth noting that if two groups of humans were totally isolated over time, it is
entirely possible that a different species could spawn from own, because the two groups
would be adapting to their own environment over time. This would mean changes in the
gene pool in each of the groups, to the point where the two groups could not reproduce
with another and produce fertile offspring due to these genetic differences. At this point,
with any species, a new one has begun.