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Objectives
Evolution
•Darwin’s contribution to science
Dr. Mario A. Fares
Evolutionary Genetics and Bioinformmatics
Laboratory
•Significance of Darwin’s theory of evolution
Department of Genetics, Smurfit Institute of
Genetics, TCD
•Darwin and society
Phone number: 8913521
Email: [email protected]
Web page: http://bioinf.gen.tcd.ie/~faresm/
Evolution before Darwin
Evolution before Darwin
•Platon: All the variable phenomena on the
world have a fixed essence
•Plato ruled out evolutionary thinking because
everything remains essentially unaltered under
his philosophical view
•Aristotle also supported that the essential
properties of things are fixed
•Categorised all living things in the world into
gradated classes. These classes were sorted in
increasing complexity. His view summarised
the concept of Scala Naturae (Great Chain of
Being)
•Under this view species are fixed natural
entities (and therefore do not evolve
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Evolution before Darwin
•Christian thought was very much based on the Platonic and
Aristotelian philosophy and defended the immutability and
perfection of design of living things
•This design was perfect because it came from God and was
therefore unchangeable
•Implicit in this idea was the idea of purpose, only known by
God
•Evolution was then negatively stigmatised
Lamarck and the Transformism
Evolution before Darwin
By the early 19’s century the basis over which to build the
idea of organic evolution were present
•Geologists conceived the age of earth in millions of years
•The discovered fossil record was a witness of previously
extinct species
•Systematists and comparative anatomists realised about the
similarities among species
•The idea of inheritance was accepted by most if not all of
the scientists
•The mechanism underlying this change among organisms
was still a mistery
Postulates of Lamarck’s Theory
•Evolution was driven by perfection which Lamarck
expressed as the tendency towards greater organism
complexity
•Lamarck assumed that the closer organisms from
perfection the more adapted they were to the
environmental conditions
•Those part of the body most used to overcome the
environmental challenges became larger and stronger, while
those not used deteriorated (Use and Disuse)
•Acquired characteristics are heritable
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Charles Darwin (1809 – 1882)
•Darwin presented his theories in the “Origin of Species” and
these can be summarised into two main points:
-Descent with Modification: All species descend from
previous ones with no interruption
- Darwin discovered and explained the mechanism of
evolution by natural selection. In summary this theory has
three main points:
- Changes observed over time in a population of
organisms are the consequence of an unbalance in the
number of offspring left by individuals with different
heritable traits
- Survival depends on the heredity contribution of
individuals
- Survival of the fittest against of the flattest
Lamarck’s view versus Darwin’s
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HERITABLE VARIATION IS SUFFICIENT TO LEAD TO
EVOLUTION section 3.3 (F&H)
-set up experiment so that we know we have heritable variation (in this
case, flower colour= variation, and we know that it is controlled by
genes, so it is heritable)
- let bees pollenate naturally (bees are performing the natural selection)
- after several generations, look again at the “population” of flowers
- - observe that the frequency of yellow and white flowers has changed
white flowers increased in number, so were apparently selected more often
by bees
Darwin and the Galápagos
Darwin’s observations
• During Charles Darwin’s five-year voyage
– (1831-1836) on the HMS Beagle,
– he visited the Galápagos Islands
– where he made important observations
– that changed his ideas about
– the then popular concept called the fixity of
species
• an idea holding that all present-day species
• had been created in their present form
• and had changed little or not at all
– Fossil mammals in South America are similar yet
different from present-day
– llamas, sloths, and armadillos that the finches and
giant tortoises living on the Galápagos Islands vary
from island to island and still resemble ones from
South America, even though they differ in subtle ways
• He concluded that organisms descended with
modification
– from ancestors that lived during the past
– the central claim of the theory of evolution
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Galápagos Finches
• Darwin’s finches from the Galápagos Islands
– arranged to show evolutionary relationships
Insect eaters
Berry
eater
Darwin and society
• Theologists claimed to have been given a new
theological insight
• Scientists did not support the idea of change, descent
with modification and considered Darwin’s theory
incomplete or ambiguous not explaining concepts
such as, complexity or the role of design
Seed Cactus
eaters eaters
Insect eaters
•In fact Darwin’s theory could not explain:
- How is variation generated
•Despite the limitations, Darwin’s theory was very
successful, because:
- How is this variation passed to the next generations
a) The synthesis of the inheritance theory of Mendel with
the Darwinian evolution
His ideas were based on blending of parental
characteristics
b) New research directions
c) The Darwinian’s theory can explain a lot:
•This however would not work
because blending leads to loss
of variation
- Patterns of biological diversity
- Geographical distribution of organisms
- Anatomical anomalies
- Design or chance?
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Anatomical convergence
Absence of design
Anatomical anomalies
Respiratory and digestive systems
for fishes and mammals
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Mendel’s Peas
•Around the same time as Darwin was doing his work, Gregor
Mendel was doing some very careful and insightful experiments
with plants (published 1866)
•Monk in Brno in present day Czech Republic
• Mendel used Peas for his inheritance experiments because:
- True breeding commercial strains were available
- easy to cultivate
- easy to observe traits:
Seed colour and seed shape
Pod colour and Pod shape
Flower color and Flower position
Plant size
Two Alternative Traits of the Seed Shape Character
Note that each of
seed is a new
individual of a
different
generation –
seeds are not of
the same
generation as the
plant that bears
them.
Mendel’s Monohybrid Cross –
P to F1
•Progeny were all from one parental
type.
•The idea of Dominance: one allele
may conceal the presence of
another in a heterozygote
• Yellow is dominant over green
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Genes, Alleles, and Chromosomes
Staying the Course –
Mendel Continued
Crosses to the F2 (the
grandchildren)
What was learned?
The green trait was not lost or
altered, even though it
disappeared in the F1.
One trait is dominant to the
other in its expression.
The reappearance of the recessive trait in ¼ of the F2, suggests genes
come in pairs that separate in the formation of sex cells.
Principle of Segregation
A cross between individuals differing in single
character is a monohybrid cross.
The analysis of monohybrid crosses allowed
Mendel to deduce the Principle of
Segregation ....
Independent assortment
•Mendel performed dihybrid crosses
to find out.
•Mendel’s conclusion: Different
characters are inherited
independently.
Genes come in pairs that separate in the
formation of sex cells (and these sex cells
unite randomly at fertilization).
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Why Did Mendel Conclude That The
Inheritance of one Trait is Independent of
Another?
The alternative
and incorrect
hypothesis:
dependent
inheritance.
Because it’s the only way to explain
the pattern of inheritance?
Evolutionary Theories
The Punnett Square for a Dihybrid
Cross
Note that we’re
simultaneously applying
the Principles of
Segregations and
Independent
Assortment.
The Modern Synthesis
August Weissman
• Weissman’s major breakthrough was in distinguishing somatic and
germ line cells
•Distinguished somatic cells (all around your body) from germ line cells
(cells that go on to form gametes [sperm and egg])
• Argued that only changes in the germ line were heritable
• Changes in the rest of the body (phenotype) were not heritable
• Proposed that spontaneous and random changes can occur in genes
during reproduction
• Offspring will have a changed genotype and phenotype
•Even though evolution by common descent was commonly
accepted, the mechanism of natural selection was not. For example,
some people believed that mutation was the only mechanism of
evolution (mutationists).
•Reconcillation of Darwin’s theory with facts of Genetics by Fisher,
Haldane and Wright.
•Developped a mathematical theory of population genetics which
showed that mutation and natural selection together cause evolution
•Mutation, recombination and natural selection operating within
species account for the origin of new species and long-term features
of evolution.
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Key ideas:
1. Gradual evolution results from small genetic
changes that are acted upon by natural
selection
2. The origin of species (macroevolution) can be
explained in terms of natural selection acting
on individuals (microevolution)
Genes are the key to evolution
• Genes are both the material of evolution, and the markers
of evolution
• [material] They are the heritable characters that change and
result in evolutionary changes
• [markers] They are also our major source of information for
understanding the process of evolution
• Mutations occur in DNA sequences giving rise to variation
= source of heritable variation
• Without mutation there is no evolution
Mutations are mistakes in the copying of DNA.
When a mutation occurs in the DNA sequence of a gene
it may have one of several possible effects:
1. Deleterious – e.g., breast cancer gene BRCA2
2. Advantageous – e.g., herbicide resistance
nightshade chloroplast gene
3. Neutral (or, we don’t really know!) – e.g., ABO blood
group alleles
A, B, and O alleles all persist
One is not obviously more “successful” than the others
Mutations generate new “versions” of the gene = alleles
Allele = variant form of a gene.
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