1. How Populations Evolve
Chapter 13
How Populations Evolve
Let’s start with
Darwin’s concept about how new species are evolved with evidence from fossil records, biogeography (geographic distribution of species), comparative anatomy, embryology and molecular biology.
Then we are going to focus on Darwin’s theory of natural selection, particularly, and
the modern view about the natural selection based on genetical studies.
Finally we will explore the mechanisms of evolution.

2. Mosquitoes and malaria
Evolution in Action
Mosquitoes and malaria
The evolution of pesticide-resistant insects is just one of the ways that evolution affects our lives.
DDT may kill 99% of insects
Some mosquitoes have evolved resistance
The alleles code for enzymes that destroy the pesticides
Offspring from the survivors inherit the resistance gene
Proportion of resistant increases each generation, making subsequent sprays less effective
© 2010 Pearson Education, Inc.
Let’s start with Darwin’s concept about how new species are evolved with evidence from fossil records, biogeography (geographic distribution of species), comparative anatomy, embryology and molecular biology. Then we are going to focus on Darwin’s theory of natural selection particularly and the modern view about the natural selection based on genetical studies. Finally we will explore the mechanisms of evoltion.

3. Charles Darwin’s concept Evidence in support of evolution
Evolution in Action
Charles Darwin’s concept
Evidence in support of evolution
Darwin’s theory of Natural Selection
Genetic basis of evolution
© 2010 Pearson Education, Inc.
Let’s start with Darwin’s concept about how new species are evolved with evidence from fossil records, biogeography (geographic distribution of species), comparative anatomy, embryology and molecular biology. Then we are going to focus on Darwin’s theory of natural selection particularly and the modern view about the natural selection based on genetical studies. Finally we will explore the mechanisms of evoltion.

4. CHARLES DARWIN AND THE ORIGIN OF SPECIES
Darwin presented two main concepts:
Life evolves
Change occurs as a result of “descent with modification,” with natural selection as the mechanism
Darwin said that life evolves and the mechanism behind the evolution is natural selection, a process in which organism with certain favorable characteristics are likely to survive and reproduce than the individuals with other characteristics.
As a result of natural selection, the population changes over generation and thus leads to evolutionary adaptation.
A population is a group of individuals of the same species living in the same place at the same time

5. CHARLES DARWIN AND THE ORIGIN OF SPECIES
Natural selection is a process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are individuals with other characteristics.
Natural selection leads to:
A population changing over generations
Evolutionary adaptation
In one modern definition of evolution, the genetic composition of a population changes over time
Darwin said that life evolves and the mechanism behind the evolution is natural selection, a process in which organism with certain favorable characteristics are likely to survive and reproduce than the individuals with other characteristics.
As a result of natural selection, the population changes over generation and thus leads to evolutionary adaptation.
A population is a group of individuals of the same species living in the same place at the same time

6. Camouflage as an example of evolutionary adaptation
A Trinidad tree mantid that
mimics dead leaves
A flower mantid in Malaysia
A leaf mantid in Costa Rica
Figure 13.1 Camouflage as an example of evolutionary adaptation
Related species of insects called mantids have diverse shapes and colors that evolved in different environments

7. Lamarck and Evolutionary Adaptations
In the mid-1700s, the study of fossils began to take form as a branch of science.
Naturalist Georges Buffon noted that:
The Earth may be more than 6,000 years old
There are similarities between fossils and living species
Fossil forms might be ancient versions of similar living species
Jean Baptiste Lamarck suggested that organisms evolved by the process of adaptation by the inheritance of acquired characteristics, now known to be incorrect.
Before we study how natural selection works and how Darwin derived his idea that life evolves, let’s see historical context of other’s ideas and theories.
1. The study of fossils – imprints or the remains of organisms that lived in the past – led Buffon to suggest that Earth may be more than 6,000 years old. By looking at the similarities between fossils and living species, he proposed that fossil forms might be ancient versions of similar living species.
2. Lamarck suggested that the best explanation for this relationship of fossils to current organisms is that life evolves. He suggested that life evolved by the process of adaptation by the inheritance of acquired characteristics. Example, the strong beak of seed-cracking bird are the result of ancestors exercising the beaks during feeding and passing that acquired beak power to offspring – not accepted

8. The Voyage of the Beagle
In December 1831 Darwin left Great Britain on the HMS Beagle on a five-year voyage around the world.
Darwin in 1840
Galápagos
Islands
North
America
South
PACIFIC
OCEAN
ATLANTIC
Pinta
40 miles
40 km
Florenza
Fernandina
Marchena
Genovesa
Equator
Santiago
Daphne Islands
Pinzón
Española
Isabela
Santa
Cruz Fe
San
Cristobal
Great
Britain
Cape of
Good Hope
Europe
Africa
Cape Horn
Tierra del Fuego
Asia
HMS Beagle
Australia
Tasmania
New
Zealand
Andes
This tour had a profound effect on Darwin’s thinking

9. The Voyage of the Beagle
On his journey on the Beagle, Darwin:
Collected thousands of specimens
Observed various adaptations in organisms
Realized that natural forces gradually changed the Earth’s surface and that force still operates
Witnessed an earthquake that raised a part of Chile’s coastal line
Marine snail fossils on mountaintops in the Andes
Darwin was intrigued by:
geographic distribution of organisms on the Galápagos Islands
Similarities between organisms in the Galápagos & S America
He noted the unique adaptations of organisms that inhabited the diverse environment ranging from jungles in brazil, to grasslands of Argentina to frigid land in southern tip of S America. In spite of their unique adaptations, both the plants and animals have a definite south american pattern. Both plants and animals living in the temperate regions of SA are closely related to tropical region of that continent rather than to species living in temperate regions of Europe.
Darwin noticed that Galapagos marine iguanas – with webbed feet and flattened tail that aid in swimming – are similar to, but distinct from, land-dwelling iguanas on the islands and on the S A mainland

10. Descent with Modification
Darwin made two main points in The Origin of Species:
Organisms inhabiting earth today descended from ancestral species
Natural selection was the mechanism for descent with modification

11. Descent with Modification
We will examine five of the many lines of evidence in support of evolution:
The fossil record
Biogeography
Comparative anatomy
Comparative embryology
Molecular biology

12. 1. The Fossil Record Fossils are: The fossil record:
Imprints or remains of organisms that lived in the past
Often found in sedimentary rocks
The fossil record:
Is the ordered sequence of fossils as they appear in rock layers
Reveals the appearance of organisms in a historical sequence
The oldest known fossils are prokaryotes
Younger layer of rocks include fossils of various groups of eukaryotes
When sand and silts were deposited in oceans, they pile up and compressed to form rocks. The dead organism trapped in the deposits left imprints in the rocks. Young layers are on the top of older ones and so the position of fossils in the strata or layer reveals their relative age.

13. A transitional fossil linking the past and present
Paleontologists
Have discovered many transitional forms that link past and present. Example, whales
Series of whale fossils connecting these aquatic mammal to four-legged land mammals
A transitional fossil linking the past and present. Whales living today have forelegs in the form of flippers and small bones that may be the remnants of ancestral hind legs and feet. The picture here is the fossilized bones of ancient whale. The hypothesis that whales evolved from terrestrial ancestors predicts a four-limbed beginning for whales

14. 2. Biogeography
Biogeography is the study of the geographic distribution of species that first suggested to Darwin that today’s organisms evolved from ancestral forms.
Many examples from biogeography would be difficult to understand, except from an evolutionary perspective.
One example is the distribution of marsupial mammals in Australia.
Common
ringtail
possum
Red kangaroo
Common wombat
Australia
Koala
Galapagos animals resembled animals in SA mainland because these sp are evolved form animals that have been migrated form SA, each species adapting to its new environment.

15. 3. Comparative Anatomy Comparative anatomy Homology is:
Is the comparison of body structure between different species
Confirms that evolution is a remodeling process
Homology is:
The similarity in structures due to common ancestry
Illustrated by the remodeling of the pattern of bones forming the forelimbs of mammals
Vestigial structures:
Are remnants of features that served important functions in an organism’s ancestors
Human
Cat
Whale
Bat
The fore limbs of all mammals are constructed from the same skeletal elements.

16. 4. Comparative Embryology
Early stages of development in different animal species reveal additional homologous relationships.
For example, pharyngeal pouches appear on the side of the embryo’s throat, which:
Develop into gill structures in fish
Form parts of the ear and throat in humans
Comparative embryology of vertebrates supports evolutionary theory.
Post-anal
tail
Human embryo
Chicken embryo
Pharyngeal
pouches
All vertebrate embryos have a developmental stage in which structures called pharyngeal pouches appear on the sides of the throat and the tail.

17. All forms of life use the same genetic language of DNA and RNA
5. Molecular Biology
Evolutionary relationships among species can be determined by comparing:
Genes
Proteins of different organisms
All forms of life use the same genetic language of DNA and RNA
The genetic code is universal
Percent of selected DNA sequences
that match a chimpanzee’s DNA
Chimpanzee
100%
96%
92%
Human
Gibbon
Orangutan
Gorilla
Primate
Old World
monkey

18. NATURAL SELECTION
Darwin noted the close relationship between adaptation to the environment and the origin of new species.
The evolution of finches on the Galápagos Islands is an excellent example.
(a) The large
ground finch
(b) The small tree finch
(c) The woodpecker finch
Population on the different islands may diverge more and more in appearance as each population adapts to its local environment. Over many generations, the population on different island could become dissimilar enough to be designated as separate species.
The large ground finch: large beak specialized for cracking seeds
The small tree finch: small beak is used to grasp insects
The woodpecker finch: long narrow beak allows it to hold tools such as cactus spines to probe for wood-boring insects

19. Darwin’s Theory of Natural Selection
Darwin based his theory of natural selection on two key observations:
Observation 1: Overproduction
All species tend to produce excessive numbers.
This leads to a struggle for existence.
Observation 2: Individual variation
Variation exists among individuals in a population.
Much of this variation is heritable.
Inference: Differential reproductive success (natural selection)
Those individuals with traits best suited to the local environment generally leave a larger share of surviving, fertile offspring.
Spore
cloud
Because natural resources are limited, production of more individuals than environment can support lead to a struggle for existence among the individuals of a population. Only a tiny fraction live to leave their offspring.
There is variation among individuals of a population and this variation is heritable
His conclusion about natural selection are derived from these 2 observations that explains how adaptations evolve.
Thousands of acorns hanging from one tree, spores escaping from a puffball, or a salmon spawning thousands of eggs, are all easy examples of overproduction.

20. Natural Selection in Action
Examples of natural selection include:
Pesticide-resistant insects
Antibiotic-resistant bacteria
Drug-resistant strains of HIV
Chromosome with gene
conferring resistance
to pesticide
Reproduction
Survivors
Insecticide application
Additional applications of the same pesticide will be less effective, and the frequency of resistant insects in the population will grow
Here are some more example that evolution and natural selection did not only act in the past but now also.
By spraying the crops with insecticide to kill pests, people unknowingly favored the reproductive success of insects with inherent resistance to the poisons.
Additional applications of the same pesticide will be less effective, and the frequency of resistant insects in the population will grow

21. EVOLUTIONARY TREES
Darwin saw the history of life as analogous to a tree:
The first forms of life on Earth form the common trunk
At each fork is the last common ancestor to all the branches extending from that fork
Tetrapod
limbs
Amnion
Feathers
Lungfishes
Mammals
Amphibians
Lizards
and snakes
Crocodiles
Hawks and
other birds
Ostriches
Amniotes
Tetrapods
Birds
Each branch point represents the common ancestor of the lineages beginning there
Blue dot represents a homologous trait shared by all the groups to the right
An evolutionary tree represents the succession of related species with the most recent at the tips of the branches. Each branch point represents a common ancestor of all species that radiate from it.
Each branch point represents the common ancestor of the lineages beginning there
Blue dot represents a homologous trait shared by all the groups to the right

22. Modern Synthesis Populations as the Units of Evolution
A population is:
A group of individuals of the same species, living in the same place, at the same time
The smallest biological unit that can evolve
The total collection of alleles in a population at any one time is the gene pool.
When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale, which is sometimes called microevolution.
We know that individual organism do not evolve during their life time. The evolutionary changes are apparent only in the population over time.
When researcher studies evolution at population level, they consider what is called gene pool (total of all alleles in all the individuals making up the population). You know that gene exists in 2 or more forms called alleles. For example, in an insect population, there may be one allele that codes for an enzyme that breaks down certain pesticide and another for an enzyme that does not. After spray, the frequency of first allele will increase and the second one will decrease.
When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale, which is sometimes called microevolution

23. Genetic Variation in Populations
Individual variation abounds in populations.
Not all variation in a population is heritable.
Only the genetic component of variation is relevant to natural selection
Variable traits in a population may be:
Polygenic, resulting from the combined effects of several genes or
Determined by a single gene
Polygenic traits tend to produce phenotypes that vary more or less continuously.
Single gene traits tend to produce only a few distinct phenotypes.
Though individual variations are abundant in a population not all of them are heritable. Only those genetic component of the variation that are subjected to natural selection are heritable. Example muscle development by exercise.

24. Sources of Genetic Variation
Genetic variation results from:
Mutations, changes in the DNA of an organism
Sexual recombination, the shuffling of alleles during meiosis
For any one gene, mutation alone has little effect on a large population in a single generation.
Organisms with very short generation spans, such as bacteria, can evolve rapidly with mutations as the only source of genetic variation.

25. Analyzing Gene Pools
The gene pool consists of all alleles in a population at a given time
allele for red flower is symbolized by R
allele for white flower is symbolized by r
p for relative frequency of dominant allele R
q for the frequency of recessive allele r
p + q = 1
(80%) (20%)
Genotype frequencies:
Can be calculated from allele frequencies
Are symbolized by the expressions p2, 2pq, and q2
p2 + 2pq + q2 =1
Frequency of one allele
Frequency of alternate allele
Frequency of one allele

26. The Hardy-Weinberg formula can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles.
Allele frequencies
Genotype frequencies
Sperm
Eggs
p  0.8
(R)
q  0.2
(r) R r RR
p2  0.64 rR
qp  0.16
q2  0.04 rr
pq  0.16 Rr
(RR)
(rr)
2pq  0.32
(Rr)

27. Population Genetics and Health Science
The Hardy-Weinberg formula can be used to calculate the percentage of a human population that carries the allele for a particular inherited disease.
PKU:
Is a recessive allele that prevents the breakdown of the amino acid phenylalanine
Occurs in about one out of every 10,000 babies born in the United States

28. Microevolution as Change in a Gene Pool
How can we tell if a population is evolving?
A non-evolving population is in genetic equilibrium, called the Hardy-Weinberg equilibrium, in which the population gene pool remains constant over time.
From a genetic perspective evolution can be defined as a generation-to-generation change in a population’s frequencies of alleles, sometimes called microevolution.
The main causes of evolutionary change are:
Genetic drift
Gene flow
Natural selection
As mentioned earlier, evolution can be measured as changes in the genetic composition of a population over time

29. Mechanism of Evolution
1. Genetic drift is:
A change in the gene pool of a small population
Due to chance
Only 5 of
10 plants
leave
offspring RR rr Rr
Only 2 of
Generation 1
p (frequency of R)  0.7
q (frequency of r)  0.3
Generation 2
p  0.5
q  0.5
Generation 3
p  1.0
q  0.0
Animation: Causes of Evolutionary Change
This hypothetical wild flower consists of only 10 plants. Due to random change over the generation, genetic drift can eliminate some alleles, as is the case for r allele in generation 3.
But what would cause a population to shrink down to a size where there is genetic drift?
This can occur in 2 ways – the bottleneck effect and founder effect

30. The bottleneck effect:
Results from a drastic reduction in population size
Bottlenecking in a population usually reduces overall genetic variation because at least some alleles are likely to be lost from the gene pool.
Cheetahs appear to have experienced at least two genetic bottlenecks in the past 10,000 years.
Loss of individual variability leads to reduced capacity to adopt to environmental changes
Original
population
Bottlenecking
event
Surviving
Disasters may kill large numbers of individuals, producing a small surviving population that is unlikely to have the same genetic make up as the original population

31. The Founder Effect
The founder effect is likely when a few individuals colonize an isolated habitat and represent genetic drift in a new colony.
The founder effect explains the relatively high frequency of certain inherited disorders among some small human populations.
South
America
Tristan da
Cunha
Africa
The smaller the colony, the less its genetic make up

32. Natural Selection : Darwinian Fitness
Of all causes of microevolution, only natural selection promotes adaptation.
Fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals.
Video: Wolves Agonistic Behavior
For example,
plants in a wild flower population may differ in reproductive success because some attract more pollinators – because of difference in their color, shape or fragrance
Frog may produce more than her neighbors because she is better at catching insects for food.
Thus, the fittest individuals in the context of evolution are those that produce the largest number of viable, fertile offspring and thus pass on the most genes to the next generation.

33. Three General Outcomes of Natural Selection
Directional selection:
Shifts the phenotypic “curve” of a population
Selects in favor of some extreme phenotype
Disruptive selection can lead to a balance between two or more contrasting phenotypic forms in a population.
Stabilizing selection:
Favors intermediate phenotypes
Is the most common
Original
population
Evolved
Phenotypes (fur color)
of individuals
Frequency
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection

34. Sexual Selection
Sexual selection is a form of natural selection in which inherited characteristics or traits determine mating preferences
Sexual dimorphism is:
A distinction in appearance between males and females
Not directly associated with reproduction or survival but used to select their mate
when a female select a mate based on certain appearance or behavior, she perpetuate that particular allele
(a) Sexual dimorphism in
a finch species
(b) Competing for mates
Sexual dimorphism is not directly associated with reproduction or survival but used to select their mate. Every time when a female select a mate based on certain appearance or behavior, she perpetuate that particular allele.
What is the advantage to females being choosy?
Research has shown that traits preferred by females such as bright beaks or long tails are related to overall health of male.