26-NaturalSelection
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Transcript 26-NaturalSelection
Lecture 26
Natural Selection
How Does Biological Evolution Happen?
How do heritable traits (genes) pass to the next generation?
Theory that individual’s evolve
Inheritance of acquired
characteristics
Somehow passed on to offspring
Theory that populations evolve
Selection of genes already in the
population
Changes the gene frequency of a
population
Evidence for Selection Theory of Change
Human Directed Selection: (demonstrates the reality of selection)
Domestication of plants and
animals
Breeding for extreme variation
Evidence for Selection Theory of Change
Natural Selection (the same process – without human direction)
Drug & pesticide resistance
Analogous anatomy
Ecological equivalents
Homologous anatomy
Common ancestor
Charles Darwin and Natural Selection
Darwin was an ordained Anglican minister who was fully convinced that
species were immutable
In 1831, Charles Darwin took on the role of naturalist of the ship HMS Beagle
The Beagle set sail on a five-year navigational trip around the world
Darwin studied a wide variety of plants and animals across the globe
His observations eventually convinced him that evolution took place
In 1859, he published his book On the Origin of Species
Particularly on the Galapagos Islands
Fossils of extinct species resembled living species in the same area
Galapagos finches differed slightly in appearance but resembled those on the S.
American mainland
In it he proposed that evolution occurs through natural selection
The Theory of Natural Selection
Darwin observed 14 different finch species that differed mainly in beaks
and feeding habitats
He concluded that this resulted from “descent with modification” from a
common ancestor, or Evolution
Darwin was familiar with artificial selection used by breeders to produce
animals/plants with particular traits
Darwin proposed that such trait selection could also occur in nature
which he termed natural selection
Darwin was influenced by Thomas Malthus’
Essay on the Principle of Population (1798)
Populations increase geometrically, while food
supply increases only arithmetically
Thus, food supply will limit population growth
Four Fundamentals of Natural Selection
Proposed independently by
Charles Darwin and Alfred
Wallace
1. Heritability of traits
2. Limiting factors exist in all
environments
3. Overproduction of offspring
4. Reproduction by those with
“best fit”
Adaptation of a population is
the result of natural selection
On the Origin of Species
Darwin drafted a preliminary transcript in
1842
However, he shelved it for 16 years, probably
because of its controversial nature
Alfred Russel Wallace (1823-1913)
independently developed a similar theory
Correspondence between the two spurred
Darwin to publish his theory in 1859
Darwin’s Origin of Species was disturbing
to many
It suggested that humans and apes have a
common ancestor
Darwin presented this argument directly in a
later book, The Descent of Man
How Natural Selection Produces Diversity
Darwin believed that the
Galapagos finches all
evolved from a single
common ancestor
The ancestor came from the
South American mainland
New arrivals occupied
different niches and were
subject to different
environmental pressures
This resulted in a cluster of species
A phenomenon termed adaptive radiation
The 14 finch species that Darwin studied now occupy four types of niches
Ground finches
Tree finches
Warbler finches
Vegetarian finch
The Beaks of Darwin’s Finches
The Grants’ research supported Darwin’s hypothesis
Population Genetics: The Hardy-Weinberg Rule
Population genetics is the study of the properties of genes in a population
Genetic variation in populations puzzled scientists
Dominant alleles were believed to drive recessive alleles out of populations
In 1908, G. Hardy and W. Weinberg pointed out that in large populations
with random mating, allele frequencies remain constant
Dominant alleles do not, in fact, replace recessive ones
Hardy and Weinberg came to their conclusion by analyzing allele
frequencies in successive generations
If a population of 100 cats has 84 black and 16 white
Then the frequencies of black and white phenotypes are 0.84 and 0.16,
respectively
A population that is in Hardy-Weinberg equilibrium is not evolving
The Hardy-Weinberg equilibrium equation
(p + q)2 = p2 + 2pq + q2
Individuals homozygous
for allele b
Individuals homozygous
for allele B
Individuals heterozygous
for alleles B and b
By convention
The more common allele (B) is designated p
The less common allele (b) is designated q
p+q=1
B allele Black color
b allele White color
Calculating Allele Frequencies
Frequency of white (bb) cats = 16/100 = 0.16
=> q2 = 0.16
=> q = √ 0.16 = 0.4
p + q =1 => p = 1 – q = 1 – 0.4 = 0.6
What about genotype frequencies?
Frequency of the homozygous dominant
genotype is
p2 = (0.6)2 = 0.36
36 out of 100 cats are black (BB)
Frequency of the heterozygous
genotype is
2pq = 2(0.6)(0.4) = 0.48
48 out of 100 cats are black (Bb)
Why Allele Frequencies Change
The Hardy-Weinberg equation is true only if the following five
assumptions are met
Large population size
Random mating
No mutation
No migration
No natural selection
Five evolutionary forces can significantly alter the allele frequencies of a
population
Genetic drift
Nonrandom mating
Mutation
Migration
Selection
Why Allele Frequencies Change
1.
Genetic Drift
2.
Nonrandom Mating
3.
4.
Random loss of alleles (more likely to occur in smaller
population)
Founder effect: Small group of individuals establishes a
population in a new location
Bottleneck effect: A sudden decrease in population size to
natural forces
Mating that occurs more or less frequently than expected by
chance
Inbreeding Mating with relatives increases homozygosity
Outbreeding Mating with non-relatives increases heterozygosity
Mutation
Errors in DNA replication
The ultimate source of new variation
Mutation rates are too low to significantly alter allele
frequencies on their own
Migration
Movement of individuals from one population to another (A very
potent agent of change)
Immigration: movement into a population
Emigration: movement out of a population
5. Selection
Some individuals leave behind more
offspring than others
Artificial selection: Breeder selects for
desired characteristics
Natural selection: Environment selects for
adapted characteristics
Selection is a statistical concept
One cannot predict the fate of any single
individual
But it is possible to predict which kind of
individual will tend to become more
common in a population
Three kinds of natural selection
Stabilizing Selection
In humans, infants with intermediate weight at birth
have the highest survival rate
In chicken, eggs of intermediate weight have the
highest hatching success
Disruptive Selection
In the African seed-cracker finch, large- and smallbeaked birds predominate
Intermediate-beaked birds are at a
disadvantage: unable to open large seeds, too
clumsy to open small seeds
Directional Selection
Drosophila flies that flew toward light were eliminated
from the population
The remaining flies were mated and the experiment
repeated for 20 generations
Sickle-Cell Anemia
First detected on December 31st, 1904, sickle-cell anemia is a
hereditary disease affecting hemoglobin molecules in the blood
The sickle-cell mutation changes the 6th amino acid in the b-hemoglobin
chain from glutamic acid to valine resulting in sickled red blood cells
This causes hemoglobin molecules to clump
In normal RBCs, the hemoglobin chains do not clump
Sickle-cell homozygosity leads to a reduced life span
Heterozygosity produces enough hemoglobin to keep RBCs healthy
Why is the defective allele still around?
The disease originated in Central Africa
It affects 1 in 500 African Americans, but it is almost unknown in other racial
groups
People who are heterozygous for the sickle-cell allele have less susceptibility to
malaria
This is an example of heterozygote advantage
Stabilizing selection is thus acting on the sickle-cell allele
It occurs because malarial resistance counterbalances lethal anemia
The Biological Species Concept
Speciation is the species-forming process
It involves progressive change
1. Local populations become increasingly specialized
2. Natural selection acts to keep them different enough
Ernst Mayr coined the biological species concept
“Species are groups of actually or potentially interbreeding natural
populations, which are reproductively isolated from other such
groups”
Reproductively isolated populations
Populations whose members do not mate with each other or who
cannot produce fertile offspring
Isolating Mechanisms
Reproductive isolating
mechanisms are the barriers
that prevent genetic
exchange between species
Prezygotic isolating
mechanisms
Prevent the formation
of zygotes
Postzygotic isolating
mechanisms
Prevent the proper
functioning of zygotes
after they have
formed
Working with the Biological Species Concept
Speciation is a two-part process
1. Identical populations must
diverge
2. Reproductive isolation must
evolve to maintain these
differences
Speciation occurs much more
readily in the absence of gene
flow
This much more likely in
geographically isolated
populations
Populations can become
geographically isolated for
several reasons
Types of Speciation
Allopatric speciation
The differentiation of geographically isolated populations into distinct
species
Sympatric speciation
The differentiation of populations within a common geographical
area into distinct species
Instantaneous sympatric speciation may occur through polyploidy
More than two sets of chromosomes
Autopolyploidy All chromosomes from one species
Allopolyploidy Chromosomes derived from two species, via
hybridization
Much more common in plants than animals
Problems with the Biological Species Concept
The biological species concept has been criticized for several reasons
The extent to which all species are truly are reproductively isolated
It is becoming increasingly evident that hybridization is not that
uncommon in plants and animals
It can be difficult to apply the concept to populations that do not occur
together in nature
It is not possible to observe whether they would interbreed naturally
The concept is more limited than its name would imply
Many organisms are asexual and reproduce without mating
For these reasons, other concepts have been proposed to define a
species; however, none has universal applicability
Because of the diversity of organisms, it may be that there is no single
definition of a species
Natural Selection and Behavior
Adaptive traits confer evolutionary advantages in different ways
Some behaviors reduce predation
Egg-shell removal by gulls reduce predation by crows
Other behaviors enhance energy intake
This allows more offspring to be supported
Other behaviors increase resistance to disease
Still others enhance the ability to acquire a mate
Every behavior that offers a survival advantage for an individual comes
with an associated cost
Thus, for a behavior to be favored by natural selection, the benefits have
to outweigh the costs
Reproductive Behaviors
Reproductive behaviors encompass a variety of animal behaviors,
including courtship
Competition for mating opportunities has been termed sexual selection
Intrasexual selection
Competition between members of one sex (usually males)
Intersexual selection
Essentially, mate choice
The benefits of mate choice for the female
The male that provides the best offspring care
The male that provides the best territory
The male that provides the best genes
The typical number of mates an animal has during its breeding season is
called the mating system
Monogamy – One male and one female
Polygyny – One male and many females
Polyandry – One female and many males
Polygyny is more common than polyandry
Altruism and Group Living
Altruism is the performance of an action that benefits
another individual at a cost to the actor
Helpers at the nest in some bird species
Sentinels that give predator-alarm calls in some mammalian
species
The existence of altruism among animals is rather
perplexing
Natural selection should operate against it
Altruistic behavior may not be truly altruistic after all
The actor may benefit
Nest helpers may get parenting experience or inherit
territory
Sentinels may be able to escape predators in the
confusion following the alarm call
Individuals may benefit directly if there is a mutual
exchange of altruistic acts
In reciprocal altruism, “cheaters” (nonreciprocators) are
discriminated against
These individuals are cut off from receiving future aid
Altruism and Group Living
An altruist compensates for the reduction in its own reproductive success
by increasing that of relatives
Selection that favors altruism directed toward relatives is called kin
selection
The more closely related two individuals are, the greater the
potential genetic payoff
White-fronted bee-eaters
Helpers tend to be close
relatives
Helpers’ assistance
increases with genetic
relatedness