Download Darwin`s Theory of Evolution

Trimester 3
LEAP Biology
2014
Evolution – Chapter 13 Notes
Darwin’s Theory of Evolution
13.1 a sea voyage helped Darwin frame his theory of evolution
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The idea that Earth’s species are descendants of ancestral species that were different from
those living today
Darwin realized:
o that the Earth is very old and
o over time present day species have arisen from ancestral species by natural processes
In 1859 Darwin published On The Origin of Species by Means of Natural Selection
o Presenting an explanation of descendant with modification, evolution by the mechanism
of natural selection and
o Noting that as organism spread into various habitats over millions of years, they
accumulated diverse adaptations that fit them to specific ways of life in these new
environments
13.2 Darwin proposed natural selection as the mechanism of evolution
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Darwin devoted much of On The Origin of Species to exploring adaptations of organisms to their
environment
Darwin discussed many examples of artificial selection, in which humans have modified species
through selection and breeding
Darwin reasoned that
o Organisms with traits that increased their chance of survival and reproducing in their
environment tend to leave more offspring than others and
o This unequal reproduction will lead to an accumulation of favorable traits in a
population over generations
3 Key Points about Evolution by Natural Selection
1. Individuals do not evolve; populations evolve
2. Natural selection can amplify or diminish only heritable traits. Acquired characteristics
cannot be passed on to offspring
3. Evolution is not goal directed and does not lead to perfection. Favorable traits vary as
environments change
13.3 Scientists can observe natural selection in action
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Examples of evolutionary adaption highlight 2 important points of natural selection
1. Natural selection is more of an editing process than a creative mechanism
2. Natural selection is contingent on time and place. Favoring those characteristics in a
population that fit the current local environment
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Trimester 3
LEAP Biology
2014
13.4 The study of fossils provides strong evidence for evolution
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Darwin’s ideas about evolution also relied on the fossil record, the sequence in which fossils
appear within strata (layers) of sedimentary rocks
Paleontologists are scientists that study fossils
13.5 Many types of scientific evidence support the evolutionary view of life
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Comparative Anatomy
o Comparison of body structure in different species
o Was extensively cited by Darwin and
o Illustrates that evolution is a remodeling process
o Homology is the similarity on characteristics that result from common ancestry
o Homologous Structures have different functions but are structurally similar because of
common ancestry
Comparative Embryology
o Comparison of early stages of development among different organisms
o Reveals homologies NOT visible in adult organisms
o For example, all vertebrate embryos have, at some point in their development,
 A tail posterior to the anus
 Pharyngeal throat pouches
Vestigial Structures are remnants of features that served important functions in an organism’s
ancestors
The Evolution of Populations
13.7 Evolution Occurs within Populations
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A Population is a
o Group of individuals of the same species
o Living in the same place at the same time
Populations may be isolated from one another (with little interbreeding)
Individuals within populations may be interbred
We can measure evolution as a change in heritable traits in a population over generations
A Gene Pool is the total collection of genes in a population at any one time
Microevolution is a change in the relative frequencies of alleles in a gene pool over time
13.8 mutation and sexual reproduction produce the genetic variation that makes evolution possible
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Organisms show individual variation
Mutations are
o Changes in the nucleotide sequence of DNA
o Ultimate source of new alleles
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Trimester 3
LEAP Biology
2014
13.9 The hardy-Weinberg equation can test whether a population is evolving
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Sexual reproduction alone does not lead to evolutionary change in a population
o Although alleles are shuffled, the frequency of alleles and genotypes in the population
does not change
o Similarly, if you shuffle a deck of cards, you will deal out different hands, but the cards
and suits in the deck don’t change
The Hardy-Weinberg principle states that
o Within a sexually reproducing diploid population,
o Allele and genotype frequencies will remain in equilibrium
o Unless acted upon by outside forces
For a population to remain in Hardy-Weinberg equilibrium for a specific trait, it must satisfy
these 5 conditions
o 1. Very large population
o 2. No gene flow between population
o 3. No mutations
o 4. Random mating
o 5. No natural selection
Hardy-Weinberg Formula: P2 + 2pq + q2 = 100% = 1.0
o P2 = homozygous dominant
o 2pq = heterozygous
o q2 = homozygous recessive
o The frequency of all 3 genotypes must be 100% or 1.0
How would the Hardy-Weinberg equilibrium be disrupted?
o Small population increases chances that allele frequencies fluctuate by chance
o Individuals moving in or out of population add or remove alleles
o Mutations can change or delete alleles
o Preferential mating can change the frequencies of homozygous and heterozygous
genotypes
o Unequal survival and reproductive success of individuals (natural selection) can alter
allele frequencies
13.10
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PKU = recessive allele
Frequency of individuals born with PKU corresponds to the q2 term in the H-W equilibrium and
would = 0.0001
o Value of q = 0.01
o Frequency of dominant allele = 1 – q, or 0.99
o Frequency of carriers
 = 2pq
 2 x 0.99 x 0.01 = 0.0198 = 1.98% of U.S. population
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Trimester 3
LEAP Biology
2014
Mechanisms of Microevolution
13.11 Natural Selection, Genetic Drift, and Gene Flow can cause microevolution
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If 5 conditions for H-W equilibrium are not met in a population, the population’s gene pool may
change. However,
o Mutations are rare and random and have little effect on the gene pool and
o Nonrandom mating may change genotype frequencies but usually has little impact on
allele frequencies
The 3 main causes of evolutionary are
o 1. Natural selection
o 2. Genetic drift
o 3. Gene flow
1. Natural Selection
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If individuals differ in their survival and reproductive success, natural selection will alter allele
frequencies
Consider the imaginary booby population. Webbed Boobies (ww) might
o Be more successful at swimming
o Capture more fish
o Produce more offspring
o Increase frequency at the w allele in gene pool
2. Genetic Drift
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Change in gene pool of a population due to chance
In a small population, chance events may lead to the loss of genetic diversity
The bottle neck effect leads to a loss of genetic diversity when a population is greatly reduced
o Ex: the greater prairie chicken once numbered in the millions, but was reduced to 50
birds in Illinois by 1993
o A survey comparing DNA of the surviving chickens with DNA extracted from museum
specimens dating back to the 1930’s showed a loss of 30% of the alleles
Genetic drift also results from the Founder Effect, when a few individuals colonize a new habitat
o A small group cannot adequately represent the genetic diversity in the ancestral
population
o The frequency of alleles will therefore be different btwn the old and new populations
3. Gene Flow
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Is the movement of individuals or gametes/spores btwn populations and
Can alter allele frequencies in a population
To counteract the lack of genetic diversity in the remaining Illinois greater prairie chickens
o Researchers added 271 birds from neighboring states to the Illinois populations, which
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Trimester 3
LEAP Biology
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2014
Successfully introduce new alleles
13.12 Natural selection is the only mechanism that consistently leads to adaptive evolution
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Genetic drift, gene flow, and mutations could each result in microevolution, but only by chance
could these events improve a population’s fit to its environment
Natural selection is a blend of
o Chance
o And sorting
Because of this sorting, only natural selection consistently leads to adaptive evolution
An individual’s relative fitness is the contribution it makes to the gene pool of the next
generation relative to the contribution of other individuals
The fittest individuals are those that
o Produce the largest # of viable, fertile offspring and
o Pass on the most genes to the next generation
13.13 Natural selection can alter variation in a population in 3 ways
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Natural Selection can affect the distribution of phenotypes in a population
o Stabilizing Selection favors intermediate phenotypes, acting against extreme
phenotypes
o Directional Selection acts against individuals at one of the phenotypic extremes
o Disruptive Selection favors individuals at both extremes of the phenotypic range
13.14 sexual selection may lead to phenotypic differences btwn males and females
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Sexual Selection
o Is a form of natural selection
o In which individuals with certain characteristics are more likely than other individuals to
obtain mates
In many animal species, male and females show distinctly different appearances, called sexual
dimorphism
Intrasexual selection (within the same sex) involves competition for mates, usually by males
In intersexual selection (btwn) or mate choice, individuals of one sex (usually females)
o Are choosy in picking their mates and
o Often select flashy or colorful mates
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