Download Evolution and Natural Selection (Lecture 2)

Evolution and Natural Selection
Chapters 1.4-1.6, Bush
Introduction to Natural Selection
 History
of Evolutionary Thought
 Theory
of Natural Selection
 Examples
of Natural Selection
Introduction to Natural Selection
 History
of Evolutionary Thought
 Theory
of Natural Selection
 Examples
of natural selection
Paving the way for Darwin
 Charles
Darwin’s theory relied upon the
findings of other scientists
– Casting doubt on Divine Creation
– Cuvier, Georges
– Lyell, Charles
– Darwin, Erasmus
– Contributing to the theory itself
– Lamarck, Jean-Baptiste
– Malthus, Thomas Robert
– Wallace, Alfred Russell
http://goldberg.history.ohio-state.edu/naturalselection/
Georges Cuvier (1769-1832)
 leading
 found
palaeontologist of his time
that many species have gone
extinct
Charles Lyell (1797-1875)
 geologist
 Earth
was way older than the 5000
years or so allowed according to
Biblical chronology
Erasmus Darwin (1731-1802)
 Charles
Darwin’s grandfather
 proponent
of the theory that species
change over time
Jean-Baptiste Lamarck (1744-1829)

Believed scientists like Erasmus Darwin that
life forms could change over time

Lamarckism: acquired traits can be inherited
– e.g. a giraffe with a short neck stretches to get at
vegetation high up a tree and manages to make its
neck longer. This giraffe passes its long neck to its
offspring

got Darwin thinking about inheritance
Thomas Malthus (1766-1834)
 found
that all species have the potential
to create far more offspring than there
are resources to support
Alfred Russell Wallace (1823-1913)
 came
up with the theory of natural
selection independently of Darwin
 spurred
Darwin to publish his own work
on the subject
Charles Darwin (1802-1882)
“I have called this
principle, by which
each slight variation,
if useful, is
preserved, by the
term Natural
Selection.”
(The Origin of Species)
http://www.interaktv.com/Darwin/Darwin.html
Summary of the history of evolutionary thought
Introduction to Natural Selection
 History
of Evolutionary Thought
 Theory
of Natural Selection
 Examples
of natural selection
The “Theory” of Natural Selection
Natural Selection is a “Theory” in
the same way that we consider
gravity or Einstein’s relativity to
be a theory
Theory of Natural Selection

Three conditions for Natural Selection:
1) Variation in traits
2) Heritability
3) Survivorship/Competition

Natural selection  “Survival of the fittest”
Variation and Heritability

Observations from Lamarck and Erasmus
Darwin that offspring are not exactly like
parents (change can occur in a single
generation)

Observed the commonly known facts that:
– all individuals are not alike (i.e., there are
different phenotypes)
– Offspring inherit the majority of their traits from
their parents.
Variation within a species

Variation can be:
– CONTINUOUS: having a
multitude of variants (e.g.,
colour bands in the snail)
– DISCRETE: limited # of
types (such as blood
types)
Heritability in Diploids
 Two
copies of each gene (diploid)
– Humans have 23 chromosomes, 2 copies
of each, for a total of 46 chromosomes)
 Each
egg or sperm has only one copy
of each chromosome
Passing on genes is like tossing coins

Two copies exist for each
gene

Whether you pass on a
certain copy of a gene is an
independent event for each
child

If you have two children,
sometimes you will pass on
the same copy to both
children (leaving the second
copy passed on to neither
child)
Heritability of simple traits
Competition

From Malthus: more offspring are produced
than there are resources to support

Creates a “struggle for existence”

Some offspring will be better at surviving and
reproducing than others (i.e., have higher
fitness)
Fitness
FITNESS:
– the number of offspring an individual
produces that survive to reproduce
themselves
 Fitness
= 1.0 means that individuals of
this phenotype are successfully passing
on 100% of their genes, on average
How is fitness calculated

Fitness = the number of genes passed on to the
next generation

Because diploid organisms (i.e., most organisms)
only pass on half of their genes to each child, they
must have two offspring living to reproductive age to
have Fitness = 1

Fitness = 1 does not exactly mean that you have
passed on 100% of your genes to the next
generation (Remember: sometimes you send two
copies of the same gene and zero copies of the
other)
Outcome

Some phenotypes will be better represented
in the next generation than they are in the
present generation

Could be extended: some entire lineages
may be more successful than others as well
resulting in some lineages going extinct (as
Cuvier had found)
Natural selection will not take place if:

there is no variation
– E.g., No humans have gills, so we cannot select for
them, regardless of how beneficial they might be

If the gene is not heritable
– E.g., Working out and getting a strong heart might
make you live longer and have more children but
selection can not act upon it if is not a genetic trait

If there is no difference in survivorship or
reproductive ability between variants
– E.g., Having attached or free earlobes doesn’t really
matter
“Survival of the fittest”
 This
saying is a bit misleading and
doesn’t quite capture the essence of
what is natural selection
 You
can be as “fit” an individual as can
be but it is the ability to reproduce that
is the key feature for an increase in
representation in the next generation
Aside: Darwin’s nemesis was genetics!
Gregor Mendel – father of genetics
conducted
experiments on pea
plants
 discovered that
most organisms
have two copies of
their genes, one
from each parent.

Darwin never read Mendel’s Paper
Introduction to Natural Selection
 History
of Evolutionary Thought
 Theory
of Natural Selection
 Examples
of natural selection
Ground Finch (Geospiza fortis)

beak size has a lot
to do with how well
a finch feeds on
certain seeds

seeds of Tribulus
have the toughest
seed coat that
requires a large
beak to break
Natural selection in finches
Drought causes
collapse of food supply,
survival plummets
High mortality in
smaller individuals,
strong selection for
large birds that can
crack large, tough
seeds
Human-induced selection
1)
2)
3)
Natural pop’n with
variation for
insecticide resistance
Insecticide appl’n kills
all but those with
resistance
Surviving insects
breed new generation
of insecticide
resistance population
Natural selection can occur rapidly
Rock plants
Summary

Darwin put together a number of ideas from
different disciplines to come up with the Theory
of Natural Selection

Natural selection states that heritable
phenotypes that are well-suited to their
environment will have more offspring and so will
be better represented in the next generation.

Natural selection can operate so quickly that we
can observe it in a single generation
Natural Selection reviewed
Natural Selection – continued
 Characteristics
 Types
of natural selection
of natural selection
 Natural
selection  Evolution
Natural Selection – continued
 Characteristics
 Types
of natural selection
of natural selection
 Natural
selection  Evolution
Characteristics of Natural Selection
 Natural
Selection:
– dependent on the variation present in the
population
– Short-sighted – acts only present selection
pressures
Sources of variation
 Gene
flow: immigration
 recombination
 ultimately,
from mutation
Immigration leads to new variation

Immigration
provides new
genetic material for
selection to act
upon
Recombination creates variation in offspring
Mutation at the Phenotype Level

Mutations can be:
– beneficial
– detrimental
– neutral
Mutation at the DNA Level

A mutation is
caused when the
chromosomal
machinery makes a
mistake
Mutagens

Many things may
increase the
mutation rate:
– radiation
– certain chemicals
(e.g. carcinogens)
Variation is random
 When
a new recombinant or mutant
genotype arises, there is no tendency
for it to arise in the direction of
improved adaptation
 Natural
selection imposes direction on
evolution, using undirected variation
Natural Selection – continued
 Characteristics
 Types
of natural selection
of natural selection
 Natural
selection  Evolution
Types of Natural Selection
 Three
kinds of natural selection:
– Directional selection
– Stabilizing selection
– Disruptive selection
Directional Selection
Larger individuals may have higher fitness
(i.e., produce more offspring) than smaller
individuals.
Directional Selection
Fishing industry
produces
selection that
favours smaller
cod and can
produce a
decrease in
average body
size.
Stabilizing selection
The average members of the population may
have higher fitness than the extremes.
Stabilizing Selection
Babies of
intermediate
birth weight
have higher
survivorship
than very
small and very
large babies
Disruptive selection
Natural selection could favour both extremes
over the intermediate types
Disruptive Selection
In the finch,
Pyrenestes ostrinus
both very large and
very small bills are
beneficial for eating
large and small
seeds, respectively
Natural Selection – continued
 Characteristics
 Types
of natural selection
of natural selection
 Natural
selection  Evolution
Selection pressures may conflict
Other factors in evolution
 If
there is no relation between fitness
and the character in question, then
natural selection is not acting on it
 Chance
events can still make these
traits show change over time =
RANDOM DRIFT
Chance events influence evolution
Summary

Natural Selection acts on whatever variation is
present at the time. This variation is generated
randomly with respect to selection pressures

Selection can be directional, stabilizing or
disruptive

Random factors can also play a part in evolution
"nothing in biology makes sense except in the light of evolution"
-Theodosius Dobzhansky (1900-1975)