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21
Evidence and Mechanisms
of Evolution
Evolution
Evolutionary theory is the understanding
and application of the mechanisms of
evolutionary change to biological
problems.
• Random mutations lead to traits that make
individuals more adapted to dealing with
these biological problems
• Gradual, random
Evolutionary theory has many useful
applications:
 Development of vaccines;
understanding and treating diseases
 Developing better agricultural crops
and industrial processes
 Understanding the diversification of
life and how species interact
Using the term “theory”
• In everyday speech, “theory” means an
untested hypothesis, or a guess.
• In science, “theory” refers to the entire
body of work on the understanding and
application of a field of knowledge.
• Large amount of data, supportive evidence
Darwin
•
The young Charles Darwin was passionately interested in
geology and natural science.
•
In 1831
•
•
•
5 year voyage on the HMS Beagle
Often went ashore to collect rocks and specimens, make observations
In the Galápagos Islands he observed that species were similar to, but
not the same as, species on the mainland of South America. He also
realized that species varied from island to island.
• This variation due to isolation from mainland and changes that occurred
• He wondered what the mechanism for this change was – transmission of
genetics was not yet known
Darwin
These observations, and many others, led
Darwin to propose an explanatory
theory for evolutionary change based
on three propositions:
 Species change over time
 Divergent species share a common
ancestor
 The mechanism that produces the
change is natural selection
Darwin
Darwin published his book, The Origin of
Species, in 1859.
The book provided exhaustive evidence
from many different fields to support
evolution and natural selection.
Darwin
Darwin observed that, though offspring
tended to resemble their parents, they
are not identical.
He suggested that slight variations
among individuals affect the chances
of surviving and producing offspring
Natural selection.
• Humans have been artificially selecting for
thousands of years
• Breeders
• Domestication of animals and crops
• Individuals do not evolve; populations do.
• Population: A group of individuals of the
same species that live and interbreed in a
particular geographic area.
• A population evolves when individuals with
different genotypes survive or reproduce at
different rates.
Population Genetics
Population genetics has three goals:
 Explain the patterns and organization of genetic variation
 Different forms of a gene are known as alleles
 A single individual only has some of the alleles found in a population
 Explain the origin and maintenance of genetic variation
 “Gene Pool” - all the alleles of the population
 How they come together in one individual causes variation
 Understand mechanisms that cause changes in allele
frequencies
 Natural Selection
Artificial selection for different characters in a single
species of wild mustard produced many crop plants.
Allele Frequencies
Locally interbreeding groups are called
Mendelian populations.
Allele frequencies, or their proportion in the
gene pool, are estimated by counting alleles
in a sample of individuals.
Allele Frequencies
Allele frequency:
number of copies of the allele in the population
p
sum of alleles in the population
If a locus has two alleles, A and a, there
could be three genotypes: AA, Aa, and
aa. The population is polymorphic at
that locus.
Figure 21.6 Calculating Allele Frequencies
Allele Frequencies
If p is the frequency of allele A, and q is the
frequency of allele a,
p+q=1
q=1–p
If there is only one allele at a locus, its
frequency = 1. The population is
monomorphic at that locus; the allele is said
to be fixed (not changing)
 Genotype frequencies may not be the
same as allele frequencies.
 Genotype frequencies deal with how many individuals have certain
genotype
 Frequencies of different alleles at
each locus and frequencies of
genotypes in a Mendelian population
make up the genetic structure of the
population.
 How genetic structure of a population
changes over time is a measure of
evolutionary change.
Hardy-Weinberg
• It would be thought that if certain conditions
are met, the genetic structure of a population
does not change over time.
• The Hardy-Weinberg equilibrium describes a
model situation in which allele frequencies do not
change.
• If your data varies from Hardy-Weinberg, then evolutionary
mechanisms are at play with your population
• Genotype frequencies can be predicted from
allele frequencies.
Hardy-Weinberg
Conditions that must be met for Hardy–Weinberg equilibrium:

Mating is random

Population size is infinite
Large populations aren’t affected by genetic drift (random
fluctuation in allele frequency)

No gene flow—no migration into or out of the population

No mutation

Natural selection does not affect survival of any genotypes
Hardy-Weinberg
If these conditions hold:
 Allele frequencies remain constant
 After one generation, genotype
frequencies occur in these proportions:
Genotype AA Aa aa
Frequency p2 2pq q2
Example showing the Hardy-Weinberg to be true
Hardy-Weinberg
For generation 1, probability of two A
alleles coming together is:
p  p  p  (0.55)  0.3025
2
2
Probability of two a alleles:
q  q  q  (0.45)  0.2025
2
2
Hardy-Weinberg
There are two ways of producing a
heterozygote:
p  q or q  p , or 2 pq
The Hardy-Weinberg equation:
p  2 pq  q  1
2
2
in butterfly example:
.3025 + 2(.45)(.55) + .2025 = 1
Hardy-Weinberg
Populations in nature never fit the
conditions for Hardy-Weinberg
equilibrium.
However, it is useful for predicting
genotype frequencies from allele
frequencies.
Also, because the model describes
conditions that would result in no
evolution, patterns of deviation from the
model help identify mechanisms of
evolution.
Hardy-Weinberg
 Hardy-Weinberg equilibrium is a null hypothesis
that assumes evolutionary forces are absent.
 Deviations from Hardy-Weinberg show that evolution is
occurring
 Null hypothesis is opposite of your hypothesis (alternative
hypothesis,what you would expect)
 Do experimentation that results in either retaining null
OR rejecting it in favor of your alternative hypothesis
 Known evolutionary mechanisms:


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Mutation
Gene flow
Genetic drift
Nonrandom mating
Natural selection
Evolutionary Mechanisms
Mutation
• Mutation is the origin of genetic
variation.
• Random
• Most mutations are harmful or neutral
• Few are beneficial
• Mutation rates are low
• Other mechanisms for evolution are at play
if large deviations from Hardy-Weinberg are
seen
Evolutionary Mechanisms
Gene Flow
• Gene flow is a result of migration
• Moving to new locations
Evolutionary Mechanisms
Genetic Drift
• Happens in small populations
• Population bottleneck
• Loss of genetic variation
• Genetic drift results from random changes in
allele frequencies.
• Harmful alleles may increase in frequency, and
rare advantageous alleles may be lost.
Evolutionary Mechanisms
Nonrandom mating
• Nonrandom mating occurs when individuals
choose mates with particular phenotypes.
• If individuals choose the same genotype as
themselves, homozygote frequencies will
increase.
• Sexual selection
Evolutionary Mechanisms
Natural Selection
Natural selection acts on the phenotype
rather than directly on the genotype.
The reproductive contribution of a
phenotype to subsequent generations
relative to other phenotypes is called
its fitness.
Individual fitness is based on
survival (in that particular
environment) to reproductive success
Maintaining Genetic Variation
• Neutral allele
• Mutation that does not affect fitness
• These will accumulate in population
• Molecular techniques allow neutral alleles to be
identified and used to study divergence of
populations and species.
• Sexual reproduction
• Recombining of existing genes
Maintaining Genetic Variation
 The variety of genetic combinations
possible in sexually reproducing species
may be especially valuable in defense
against pathogens and parasites
21.4 How Is Genetic Variation
Maintained within Populations?
Subpopulations in different geographic
regions maintain genetic variation.
The subpopulations may be subjected to
different environmental conditions and
selective pressures.
Constraints
Evolution is constrained in many ways.
•
Lack of genetic variation can prevent evolution of
potentially favorable traits
•
If the allele for a given trait does not exist in a population,
that trait cannot evolve, even if it would be favored by
natural selection
•
Evolution must work within the boundaries of universal constraints
such as:
 Cell size, constrained by surface area-to-volume ratios
 Protein folding, constrained by the types of bonding that can occur
 Laws of thermodynamics that constrain energy transfers
Constraints
Adaptations involve both fitness costs
and benefits.
Benefit must outweigh cost if an
adaptation is to evolve; the trade-off
must be worthwhile.