Download 1 - WordPress.com

Evolution
Evolution is the gradual change in the characteristics of a population of organisms over successive
generations as a result of variation in the population’s genomes.
Evolution is the result of variation in the FREQUENCIES of genetic sequences (ALLELES of a gene)
Evolution is the development of new species from
existing ones, and this evolution is driven by natural
selection which acts on the variations in a population.
A species is defined as;
A group of individuals who are able to breed together
and have fertile offspring
Evolution is divided into 1. Inheritance
2. Selection
3. Drift
4. Speciation
QUESTIONS
1. Define the term evolution.
2. Describe what causes evolution.
3. Give one example of evolution.
4. Identify the four types of evolution.
INHERITANCE
Genetic material can be inherited by:
1. Vertical transfer
2. Horizontal transfer
1. Vertical transfer
Genes (sequences of protein coding DNA) are transferred from parents down to their offspring.
This can happen by:
(a)
Sexual reproduction
(b) Asexual reproduction
(a) Sexual reproduction
This involves two parents who differ GENETICALLY from
one another. Offspring inherit different combinations of
genes from each parent. This variation increases with
successive generations. Sexual vertical inheritance is shown
as a family tree.
Page 1 of 12
(b) ASexual reproduction
Asexual reproduction from a single Eukaryotic or Prokaryotic parent.
The parent produces offspring who are genetically identical to the parent.
No variation occurs in successive generations.
2. Horizontal transfer of genetic material (prokaryotes)
In prokaryotes (who reproduce using asexual reproduction) – genetic material can be
transferred from one cell to another through horizontal gene transfer.
Horizontal gene transfer can occur in 3 ways:
(a)
Transformation –
when cells are destroyed, bits of their DNA float around and can be picked up by
…..new cells.
(b)
Transduction –
occasionally when viruses replicate some host DNA is packaged up with the virus.
This then enters new cells with the virus.
(c)
Conjugation –
A temporary connection called a conjugation tube forms between touching cells.
Plasmid DNA is then copied from one cell to another.
Page 2 of 12
(1) RAPID EVOLUTIONARY CHANGE IN PROKARYOTES
In early evolution of prokaryotes there was a lot of horizontal gene transfer (HGT)
because obtaining a gene from a neighbour is much faster than waiting for one to evolve.
This allowed rapid evolution of prokaryotes.
However this is a risky strategy as
there are no guarantees the
transferred genetic material will
give an advantage.
It is thought that organisms reached a
certain level of complexity where it was
more beneficial to have distinct lineages
with vertical inheritance of specific
genes. This eventually gave rise to the
tree of life….
…..Common ancestor
(2) SPEED OF ANTIBIOTIC RESISTANCE IN PROKARYOTES
A significant amount of HGT still
occurs in modern day prokaryotes.
Resistance to antibiotics has occurred
through the transfer of plasmids
carrying antibiotic resistance genes
from bacteria to bacteria.
Although less common, horizontal gene transfer can occur in eukaryotes:
(a) From prokaryotes Agrobacterium tumefaciens, is a bacterium, which infects plant
cells with a plasmid that integrates into the genome of the plant.
Page 3 of 12
(b) From viruses
Some viruses can integrate their DNA into
the host’s genome.
Where they remain dormant (as a provirus)
until they reproduce and destroy the cells.
(2) SELECTION
There are 2 types of selection:
(A) Natural selection
(B) Sexual selection
(A) NATURAL SELECTION
Non-random increase in frequency of sequences that increases survival (natural selection) or
successful reproduction (sexual selection).
The non-random reduction in frequency of deleterious sequences.
The differences in outcome as a result of stabilising, directional and disruptive selection.
Darwin’s theory went against the church when it was published, but now it is a cornerstone of biology and
genetics. He noted 6 important facts:
3. Those organisms best
adapted to their
environment survive and
reproduce, passing on
favourable characteristics
to the offspring.
4. Members of a species
show variation in all
characteristics and much
of this is inherited.
5. This process is repeated
in each generation.
Page 4 of 12
Mutations which help animals in natural selection are good
Example 1: Rabbits used to have short ears, but a mutation caused one to get longer ears. These long ear
rabbits could escape from prey more easily, so their numbers increased (see diagram above).
Example 2: Cockroaches which have become flatter and thinner in order to hide and live in the domestic kitchen.
The thin ones escape and have offspring, while the fat ones get squashed
Example 3: When antibiotics are used, 0.1% of bacteria will have a resistance to the antibiotics. They then
survive and live on, which means that next time the antibiotics won’t be effective at all.
Evolution of resistant insects
Example 4: Evolution of resistant insects
DDT poisonous chemical used
as a pesticide
Particularly mosquitoes which
carry malaria.
Mutants with resistance to
DDT enjoy a selective
advantage and increase in
number in this changed
environment.
This is a non random-increase
in frequency of the
resistance gene.
New pesticides must be
developed to combat this
resistance
Example 5: Similar animals exists on different continents (distribution)
Example 6: Homologous structures exist in animals, like skeletal structure and limbs that are similar animal to
animal. Fossils of animals similar to everyday animals but with slight differences exist. He then came to the
conclusion that all organisms must be in a struggle for survival, and the ones which survive pass on their gene –
survival of the fittest.
Page 5 of 12
Evolution theory states that life began from simple unicellular organisms 3 000 million years ago which have
slowly evolved into the multicellular animals of today. Fossils provide evidence and information of the stages in
evolution. However, most animals don’t become fossils, so the records are not complete.
However, species can become extinct, and this happens because of one of three different events:
1. There could be a fast environmental change
2. A new predator could kill them
3. Disease could destroy them
4. They cant compete with a new species for food, and starve
As the environment changes animals adapt to it and it will be better suited to others. However, if the
environment changes too quickly an entire species could be killed.
SEXUAL SELECTION
Sexual selection is a “special case” of natural selection – where selection is driven by the
organism’s ability to get a mate.
Sexual selection is the process of selection for traits that increase reproductive success.
Sexual selection operates by the following mechanisms:
1. Male to male competition
•
•
Males compete aggressively to defend territories and get
access to females.
Larger, stronger males or males with better “weapons” win
mating rights and pass those alleles on.
2. Female choice

Females select males which they consider high quality
depending on the traits they display.

Peacock with large feathers
are the strongest males
Monkey with the brightest colours to attract
female
For the female it is essential that she chooses a robust male who will father a robust male who will father
healthy offspring with a HIGH SURVIVAL RATE. This minimises the vast quantity of energy wasted on production
of offspring which lack advantageous characteristics.
Page 6 of 12
TYPES OF SELECTION FOR A QUANTITATIVE TRAIT
Continuous variables, such as height, mass, skin colour, hair colour etc. are controlled by
many genes and are described as being due to polygenic inheritance.
When you graph data for continuous variable in a
large population you should get a “bell shaped
curve” or normal distribution.
Natural selection can affect the frequency of a
QUANTITATIVE TRAIT in a large population in any one of
3 ways.
(a) Stabilising selection
Page 7 of 12
(b) Directional selection
New Mean
Increasing
number of
individuals
Increasing
number of
individuals
Mean
Increasing value of
inherited characteristic
Increasing value of
inherited characteristic
Common during period of
environmental change.
Selection favours a version
which was initially less
common causing a progressive
shift in the mean value.
e.g. European black bears
increased in mass during
each ice age – as larger
bodies lose relatively less
heat than smaller ones.
(c) Disruptive selection
Mean
Increasing
number of
individuals
Increasing
number of
individuals
Separate means emerge
Increasing value of
inherited characteristic
Increasing value of
inherited characteristic
Selection pressure selects extreme versions of a trait
at the expense of the intermediate versions. Can
result in the population being split into two distinct
groups. This is the driving force behind sympatric
speciation (see later in topic).
NO GENETIC DRIFT
Gene pool:
The total of all the different genes in a population.
If a species is under no selective pressure, frequencies
of individual alleles will stay the same from generation
to generation.
Page 8 of 12
GENETIC DRIFT
Genetic drift is the random increase or decrease in frequency of genetic sequences
(ALLELES OF A GENE).
This occurs due to:
(a) Sampling error
(b) Neutral mutations
(c) Founder effects
(a) Sampling error
In small populations, not all alleles are passed onto the next generation and some may be ‘lost’.
Allele types passed to the next generation are not representative of the population as a whole.
Some alleles are over represented and others under represented.
Wide fluctuations in gene frequencies can occur by chance between one generation and the next.
This provides evolutionary change BUT is NOT NATURAL SELECTION as:
1. It does not allow adaptation to the environment
2. Drive the population to uniformity (by reducing alleles and consequently genetic
diversity).
(b) Neutral Mutation
These change the nucleotide sequence of a gene,
but do not change the amino acids coded for.
They are not subject to natural selection, but are
affected by genetic drift.
Page 9 of 12
(c) Founder Effect
A special case of GENETIC DRIFT.
If a population becomes isolated and is not large enough to contain the entire gene pool,
gene frequencies will be different in that population.
An example of the founder effect can be seen in the different blood group allele frequencies
in different human populations. North America first populated by a small unrepresentative
group of Asian people who migrated across the land bridge, now the Bering strait, and
became separated.
People
Chinese
% population with blood group
A
B
AB
31
28
7
Sioux native 7
americans
2
0
O
34
91
QUESTIONS
1.
Natural (1)_______________ favours those members of a population best
(2)_______________ to an environment.
2.
Rare mutant forms sometimes enjoy a selective (3)_______________ if some
biotic or abiotic factor brings about a change in the (4)_______________
making it favour their survival at the expense of their competitors.
3.
The rapid appearance of bacteria resistant to (5)_______________ is an
example of high-speed (6)_______________.
4.
The frequency of occurrence of a gene in a large population mating at random
remains constant unless the gene (7)_______________ to which it belongs is
affected
by
(8)_______________,
natural
selection,
gene
(9)_______________ or genetic drift.
5.
The members of a (10)_______________ form a natural interbreeding group
which is reproductively isolated from other species.
Page 10 of 12
6.
The process of speciation depends on (11)_______________
to gene exchange
dividing a population into two or more (12)_______________ groups, each of
which takes its own course of evolution.
7.
(13)_______________ radiation is the (14)_______________ over a very long
period of time of a group of related organisms along several different lines by
each becoming adapted to suit a particular ecological (15)_______________.
8.
Evolution is a (16)_______________ process. As new species appear, other less
successful ones become (17)_______________.
9.
Important wild varieties of crop plant and endangered species are often
conserved in cell (18)______________ and (19)_______________ breed farms.
10.
Breeders use (20)_______________ selection to selectively breed organisms
useful to mankind. Loss of genetic (21)_______________ is associated with
(22)_______________ of domesticated plants and animals.
11.
The use of (23)_______________ in plant and animal breeding often produces
offspring which show hybrid (24)_______________.
12.
Genes can be located using gene (25)_______________ or by recognising
(26)_______________ patterns on chromosomes.
13.
Using genetic (27)_______________, scientists are able to take genetic
material from one species and seal it into the (28)_______________ of another
species producing an organism which would never have arisen otherwise.
14.
Sexual (29)_______________ between two species of plant can be overcome by
using (30)_______________ fusion.
Page 11 of 12
----------Word Bank---------Adaptive advantage antibiotics artificial banding
banks barriers continuous divergence diversity
engineering environment evolution extinct
genome hybridisation inbreeding incompatibility
isolated migration mutation niche pool probes
rare selection somatic species suited vigour
Page 12 of 12