Download evolution_natural_selection_2011

Evolution as “descent with
modification”
Evolution refers to the changes that life forms have
undergone over long periods of time
• Evolution means that all present-day forms of life …
–
–
–
–
have descended from, and are
related to, those that lived in the past
may look different because they became
modified from one generation to another
Sometimes the modifications are so great that it is difficult
to see the similarities between the modern form and its
ancestor
http://evolution.berkeley.edu/evolibrary/home.php
http://evolution.berkeley.edu/evolibrary/home.php
www.darwinday.org/englishL/life/beagle.html
www.darwinday.org/englishL/life/beagle.ht
Used by permission of Darwin Day
Celebration (at DarwinDay.org), 2006
I have called this principle, by which
each slight variation, if useful, is preserved,
by the term Natural Selection.
—Charles Darwin from "The Origin of Species"
http://evolution.berkeley.edu/evolibrary/home.php
Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.
Natural Selection ….
• IS a process
• IS the simple result of variation,
differential reproduction, and heredity - it
is mechanistic.
• It just selects among whatever variations
exist in the population.
What is natural selection all about?
A
B
This population has many
beetles of the same species –
some brown, some green - variation.
C
2nd
Birds eat the beetles;
for some reason they prefer
the green beetles – don’t
survive.
D
Generation
The young of the brown
beetles survive; the green
ones are eaten.
Gene pool of next generation?
3rd Generation
The green beetles disappear
from the population; only
brown beetles remain.
Natural selection
Key concepts:
• Variation between individuals from the same spesies
• Environmental conditions change – shortage of water,
nutrition, shelter, etc.
• Individuals with favourable/desired (genes) trait/s will
survive
• Individuals with unfavourabletrait/s became distinct/die
• The favourable trait/s are inherited from generation to
generation and the composition of the population change
– gene pool of population has change
• Over time all the members of the population inherited the
favourable trait/s
Examples of variation for evolution
• Phenotypic variation is a result of genetic
variation e.g. Finches of Galapagos,
Cheetah or the White lion
List the sources of variation in
populations.
• Random
assortment/segregation/
recombination of chromosomes
during meiosis in the formation of
gametes
Crossing over
• Chance/random fertilisation of
gametes/sexual reproduction
• Mutation
• Outbreeding/Gene flow
OR meiosis
SOURCES OF GENOTYPIC VARIATION
Meiosis
Crossing over
during phase
Prophase
Random
arrangement of
chromosomes
during phase
Metaphase
Reproduction
Mutations
Outbreeding
Fusion of many types of
sperm cells and egg cells
can produce many different
types of offspring
Gene mutations
Alteration in the sequence of
nitrogenous bases on DNA
Chromosome mutations
• Extra chromosomes added
 One or more sets of
chromosomes are added
(a)Lethal: the mutated organism dies and the harmful characteristics are not passed
on to the next generation
(b) Neutral: has no effect on the structure and functioning of the organism
(c) Fixed: advantageous/sometimes the advantageous mutation wipes out all the
other alleles controlling the same characteristic within the population
1. Different types of mutations
5. Polyploidy
Normal 2n chromosomes
A
Genes
A…F
B
C
D
E
...
..
C-G
A-T
T-A
G-C
A-T
T-A
...
..
F
1. Point
mutation
C
A
T
T
A
T
Substitution
C
A
T
A
T
Deletion
4. Deletion or
addition of
chromosome
1. Gene
multiplication
C
A
T
G
A
A
T
Duplication
4. Rearrangement
3.
G H I
A
B
X Y
Z
W
C
D
E
E
F
X
W
J
G
J
G H
I
K
Y Z
I
H K
J
K
Finches of Galapagos
Leaves
Buds and fruit
Insects
Larvae/worms
Ancestral seedeating species from
the mainland
Natural selection example 1
Natural selection: Example 2
A
B
C
White
mice
Black mice
January
February
March
A population of mice in an environment with light sand as background
Natural selection: Example 3
A
B
C
Roots
Offspring
Time
The structural adaptation of cacti over time
Micro-evolution
Using examples, explain how variation over a period of time can
lead to: micro-evolution (variation within a species) – small
changes that occur within a single species
• Example : Peppered moths of Manchester
• In the early 19th century, both dark-coloured and light-coloured
moths lived in Manchester
• The light-coloured moths were in greater numbers
• Manchester became industrialised and black smoke from the
factories collected as soot on the tree trunks