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GCSE level exam revision notes on Evolution:
3.
Modern developments in
evolution theory in biology - scientific data from genetics, the role of genes
in explaining genetic variation & resulting adaptations and natural selection
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Sub-index of biology notes on all aspects of EVOLUTION
(3)
Further developments in
evolution theory in biology - genetics, variation and adaptation
On this page I am assuming you have studied
some genetics - the science of inheritance, and are familiar with the
concepts of genes, mutations etc.
What is evolution and central ideas of the theory developed from the work of
Darwin and later scientists?
-
Evolution is a slow and continuous change of
the inherited characteristics of a organisms passed on from one
generation to another.
-
All species of living things have evolved
from simple life forms - we now know simple single-cell organisms
first developed more than three billion years ago.
-
Individuals in any population show genetic
variation due to differences in their alleles (variations of a gene)
and new alleles are created by mutations.
-
Factors such as competition for resources
(food, water, mates, domain), susceptibility to disease and
predation create selection pressures to affect the chance of any
organism surviving and reproducing.
-
Those individual organisms with
characteristics best suited (adapted) to the 'local' conditions are
more likely to survive and breed in their environment.
-
Genetically, we can now say that the alleles
responsible for the useful survival characteristics are more likely
to be passed on via reproduction to the next generation.
-
Those individuals less well adapted are less
able to compete against the selection pressures and therefore less
likely to survive and reproduce - the less effective alleles are
lost from the gene pool.
-
The better adapted characteristics will tend
to become more common in the population - the more beneficial
alleles are more likely to be passed on from one generation to
another.
Know and understand that there are not only differences between different species of plants and animals
but also between individuals of the
same species.
There are clearly major
differences between plants and animals, but there can be even significant
differences between members of the same animal/plant species or closely
related species e.g.
in the human population there are
differences in hair colour, skin colour, eye colour, facial features
etc.
Differences between members of
the same species is called variation.
Even within a family group you will see
differences in hair colour, pattern of colours of hair (e.g. cats
and dogs), facial shape, height etc.
All of these are examples of variation
within a species.
These differences are due partly to the information in the cells
they have inherited from their parents
(the DNA) and partly to the different environments in which the individuals live and grow.
Variation can be described in several
ways
You can have genetic variation and
environmental variation,
and within the term 'variation' you can have continuous variation and
discontinuous variation.
Evolution and adaptations
Adaptation can be described as the process,
resulting from natural selection, by which populations become more
suited to their environment over many generations
Following on from that, you can think of evolution as the change in adaptive features of a population
over time as the result of natural selection make them more
successful in utilising resources and reproduction.
See
detailed notes on adaptations
Genetic
variation
Genetic variation only results from changes in the DNA -
mutations.
Phenotypic variation results from both genetic and
environmental factors.
Different species of plants or animals
have different genes.
Individual species look different from each
other because they have different combinations of genes.
The gene differences within a species are very
similar, BUT, not absolutely identical, just look around and see
the variation in the human species!
You see differences in height, eye colour, hair
colour, height, skin colour, all of which are characteristic
features within the same species.
Characteristic features can be inherited from
your parents via the gene combination when an egg is fertilised
(genetic factors), though some characteristic features
can develop due to your surroundings and lifestyle (environmental
factors).
So, all plants and animals have similar
characteristic to their parents - but NOT an exact match.
This is a direct consequence of the genes
inherited by an organism from its parents.
Reminders:
The genome is the complete genetic DNA
code of an organism - arranged in chromosomes in the nucleus.
On the chromosomes are the shorter lengths
of DNA called genes which code
for protein production;
The proteins and control the
characteristics of an organism and how it develops.
A genetic change is a mutation and how new alleles are
formed.
Genetic variants are
caused by alterations in the common nucleotide sequences in the DNA of genes
(mutations).
The term variant can be used to describe an alteration that may be benign
(harmless), pathogenic (harmful), or of unknown significance.
The term
variant
is increasingly being used in place of the term mutation.
Variants are key to successful evolution because genotype changes (usually of
the smaller type) can lead to changes in phenotype.
Human genetic
variation is
the genetic differences
both within and among populations.
There may be multiple variants of
any given gene in
the human population, that is
two different alleles.
The
different alleles, different versions of a same gene, can lead
to difference in phenotypes - the characteristics an
organism displays.
A
mutation
may defined as
any change in a DNA compared to normal that results in a rare and abnormal variant.
The diagram below summarises possible 'chains of
events' for human organisms.
Genetic variation in a species is created by
organisms having different alleles which lead to differences in
phenotypes - the observed characteristics.
An organisms genes are inherited and passed on
by parents to the next generation.
These genes are passed in by the gametes
(haploid sex cells) from which offspring develop after
fertilisation.
Genetic variation can be caused by (1)
new alleles due to random changes in the DNA sequences known
as occurrence of mutations.
However, most mutations have little effect
on the phenotype, some mutations can have a minor effect, but it
is very rare for a single mutation to have a significant effect
on a phenotype.
In most animals, and many plants, the
offspring get genes from both parents, so sexual reproduction is
also a cause of genetic variation (see diagram for human
reproduction below).
The combination of genes from a 'mother' and
'father' causes genetic variation (2) because DNA sections get 'shuffled'
around at random - alleles combine in different arrangements,
albeit, to a small extent.
See
sexual reproduction - cell division
by meiosis. from which the two diagrams above were
copied.
AND, (3),
new combinations of alleles may also interact with each other
to produce new phenotypes.
So there are at least three causes of
genetic variation.
As a consequence of these genetic variations, no two individuals in a species
can be genetically identical (apart from identical twins), and
this produces genetic variation and observed in differences
in phenotype details (the results of gene expression).
Apart from 'identical twins', no two
animals of the same species look exactly the same, there will
always be differences in their visible characteristics.
(Note that recent detailed inspection of the genome of
twins has revealed that they are not absolutely identical at the
molecular level - but its hard for us to tell them apart because
the DNA of them is so similar!)
Many characteristics are determined
by
genes alone:
For animals, examples include blood group,
eye colour and inherited disorders like cystic fibrosis and
haemophilia.
Many flower colours of plants are
determined solely by their genetic make-up.
See
Introduction to the inheritance of characteristics and
genetic diagrams
Genes and how
natural selection comes into play!
From the above discussion on the origin of
genetic variation, we can see that a population has a pool of
genetic variants (variations on genes are called alleles).
Most genetic variants have little or no
effect on the phenotype.
However, a genetic variant can have a
significant effect on the phenotype.
Such a variant might give an organism
some advantage within its habit, making it better suited to
survive and breed in the environment of the population.
These advantageous phenotypes will
be passed to future generations, increasing the prevalence of
these 'advantageous alleles' in the population.
If e.g. competition for food or climate
change becomes a survival factor, the organisms with the best
adapted phenotypes are most likely to survive and reproduce -
the process of 'natural selection' - survival of the fittest.
The process of selecting the most
advantageous phenotypes, derived from the pool of genes, can be
repeated so that a species can become more and more adapted
to live in its environment.
This process can eventually to a
new
species evolving - see
speciation notes
and
adaptations.
Key points
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
Key
ideas for genetic studies and a modern view of evolution theory
Modern
developments in evolutionary theory, with a focus on
genetics, variation, adaptation, and natural selection.
Modern Developments in
Evolutionary Theory
1.
The Foundation: Darwin and Natural Selection
-
Charles Darwin proposed
that organisms evolve through natural selection—the
process where individuals with advantageous traits survive and reproduce
more successfully.
-
However, Darwin didn’t
know how traits were inherited. This gap was later filled by
genetics.
2.
The Modern Synthesis
- more sources data leads to more complex and advanced theories
-
In the 20th century,
Darwin’s ideas were combined with Mendelian genetics to
form the Modern Evolutionary Synthesis.
-
This theory explains
evolution as a change in the frequency of alleles (gene
variants) in a population over time.
-
It incorporates:
-
Mutation:
Random changes in DNA that introduce new alleles.
-
Gene flow:
Movement of genes between populations.
-
Genetic drift:
Random changes in allele frequencies, especially in small
populations.
-
Natural
selection: The main
driver of adaptation.
3.
Genetic Variation and Variants
4.
Adaptation Through Natural Selection
-
If a variant gives an
advantage (e.g. better camouflage, resistance to disease), individuals
with that variant are more likely to survive and reproduce.
-
Over generations, the
advantageous variant becomes more common—this is adaptation.
-
Example: Bacteria
developing resistance to antibiotics due to genetic mutations.
5.
Scientific Evidence from Genetics
-
DNA sequencing
allows scientists to compare genes across species, showing how closely
related they are.
-
Fossil DNA
(e.g. from Neanderthals) reveals evolutionary relationships and gene
flow between species.
-
Case study:
The Mexican tetra fish has both surface-dwelling and blind cave-dwelling
populations. When surface fish are raised in darkness, they develop
traits similar to cave fish—suggesting that developmental
conditions can influence gene expression, which may then be
selected for over time.
6.
Extended Evolutionary Synthesis
-
Some scientists now argue
that developmental biology (how organisms grow and
develop) also plays a key role in evolution—not just DNA mutations.
-
This view suggests that
environmental factors can influence gene expression,
which may lead to new traits that are later inherited.
7.
Why It Matters
Summary of learning objectives and key words or phrases
Know that modern developments in evolution theory in biology
have come from scientific evidence from genetics and the role of genes and
DNA mutations in explaining variation and advantages evolutionary
adaptations.
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