Stage 2 Biology – Ecosystems Notes
E8. Natural
selection acts on variation in a population
·
Gene pool = sum of all of the alleles of all the
individuals in a population
·
Changes in the gene pool
o
Mutations
§
Changes in DNA and can give rise to new allelic
forms of genes
§
Gene mutations are any locus are rare – but they
are the only source of new alleles
o
Natural selection
§
Most powerful force likely to bring about
changes in the gene pool
o
Gene flow
§
Exchange of alleles between populations
§
Can occur by movement of organisms with
different genotypes in and out of populations
§
Consequent change in frequency of genes in the
gene pool
§
If flow of genes is reduced = greater diversity
between the two populations
§
Observed differences between three sub-species
can be due to little flow of genes between the populations, meanwhile they have
evolved and developed in separate environments
·
Genetic variability in populations
o
Variation is essential for evolution and makes
it possible for populations to change with environmental changes
o
Variation is the ‘raw material’ on which natural
selection can act
o
Extent of variation depends on whether a species
reproduces sexually or asexually
§
In asexual species, mutations are the only
source of variation
§
In sexual species, mutations are the only way
new alleles can arise, but as a result of meiosis and fertilisation, new
combinations are found in the offspring compared to parents
o
Effects of the environment can cause some
variation, but these will not be inherited
o
In humans, phenotypic differences may be those
characteristics that are distinguishable (ie. hair/skin), but behaviour and
intelligence have an inherited component; genetic diseases
·
Dominant and recessive characteristics
o
Colour blindness is simple and due to one gene pair
o
Height cannot be modelled using a single pair of
genes
§
Polygenic
§
Two or more separate genes in the genome have an
additive effect to determine a single characteristic
·
Genetic variation may not be apparent
o
Some are bigger than others
o
Some have slightly different colours, shape and
therefore resistance to environmental factors
·
Variation exists between individuals in nearly
all populations (structural, functional or behavioural)
Theory of evolution
·
The oldest life form on Earth existed over 4.5
billion years ago
·
All organisms have evolved from a simple common ancestor
·
Species change through time
·
Evolution can be recognised by the changing
frequencies of genes in populations
·
Charles Darwin
o
Hypothesised that for all populations there were
more individuals born than ever survive to reproduce
o
Factors in the environment act to hold all
populations in check
o
Suggested natural selection – nature select
those individuals who were best adapted – fittest
o
Variation between individuals gave some
organisms a reproductive advantage which would enable them to leave more
offspring surviving in the next generation
o
Proposed that life evolved from a common
ancestor and that due to natural selection, populations were modified over time
according to the different selective pressures
o
He did not have any knowledge about the gene as
the unit of inheritance
·
Gene frequency
o
Characteristics are a result of expression of
one or more genes
o
When nature selects characteristics, certain
gene types are being favoured or selected
o
Leads to changes in frequency of particular
alleles in the gene pool
o
Therefore changes in the gene pool are evidence
of evolution occurring
o
Natural selection acts on characteristics
expressed in the phenotype
o
Selection can be by biotic or abiotic factors
§
Predators, disease-causing organisms, organisms
that co-exist
§
Rainfall, temperature, nutrient levels, soil
characteristics, light availability, wind
o
Evolution is extremely slow, although can be
observed over shorter periods if selective pressures are strong (example of the
moth Biston betularia)
§
Most individuals were light coloured and found
on lichen-covered trees/rocks – well camouflaged
§
There was a black mutant form that was rare
§
A change occurred due to the production of soot
from industry and many lichens had died out so the tree trunks were darker –
then the black moth camouflaged better
§
These changes occurred over 80-100 years
(quickly)
o
Rabbit population and myxoma virus
§
Rabbits were introduced – became a major pest
destroying pastureland
§
Virus was introduced to control numbers – spread
by a mosquito host
§
Virus initially killed rabbits in 6-10 days
after infection, but started taking 3-4 weeks
§
Natural selection
·
Milder form lived in rabbits for longer so
spread more easily
·
Rabbits that were naturally resistant were surviving
an dpassing on genes
§
Now only has 25% kill rate
·
Bacterial resistance
o
Natural selection by an abiotic factor
o
Some variation between bacterial populations
o
Variation arisen by mutations in DNA or transfer
of genes by plasmids
§
Sections of DNA in bacteria and yeast
§
Replicate independently of the main DNA
§
Contain genes that confer resistance to
antibiotics
§
Can transfer these sections to other bacteria in
a process called conjugation
o
Antibiotics kill or inhibit growth of
microorganisms – natural antibiotics produced by fungi provide protection
against other microbes that may be competing for resources
§
Interfere with cell wall production; block
protein synthesis; affect function of cell membranes
o
Penicillin
§
Was effective against a range of bacteria
§
With continued use, individual bacteria with
resistance have survived and reproduced and passed on genes
§
Over generations there have been changes in the
gene pool – increase in frequency of genes coding for resistance
o
Other antibiotics have been developed, but
resistant strains continue to appear
o
Overuse and misuse – leads to increased
resistance
§
Taking antibiotics when they are not required
§
Not finishing a dose
§
Taking more than one antibiotic for a single
infection
§
Pressure from patients to get doctors to
prescribe antibiotics when the infection is viral (antibiotics won’t work)
§
Taking excess can lead to increase in resistant
strains in the intestine – could spread to the bloodstream
§
Antibiotics used in Australia with livestock to
be used as growth promoters
§
Ingestion of antibiotics from eating
meat/fruit/vegetables treated in this manner could increase risk of spreading
resistant strains
·
It is the population that evolves, not a single
individual
