1. Natural Selection and the Evidence for Evolution
Biological Change
Natural Selection and the Evidence for Evolution

2. What is evolution?
The modern theory of evolution is a fundamental concept in biology
Evolution – change in populations over time
Charles Darwin was the first to publish his ideas of how species evolve

3. Common descent
The scientific theory that all living organisms on Earth descended from a common ancestor.
The structures and functions of all living organisms are encoded in the same basic nucleic molecules, DNA and RNA.
Similarities in amino acid sequences between various organisms also suggest common descent
The fossil record also shows cases in which one plant or animal type evolved into different types over time.

4. Charles Darwin
Began his work in 1831 (age 21) as the naturalist on the HMS Beagle
As the ship’s naturalist, it was his job to study and collect biological specimens at each port along the rout
On the Galapagos Islands, Darwin studied many species that were unique to the islands, but similar to species elsewhere. These observations led him to consider the possibility that species can change over time.
It took him 22 years to find an explanation for how species change over time

5. Charles Darwin 1859 published On the Origin of Species
his theory of natural selection to explain how organisms evolve
Darwin developed his ideas while sailing on the Beagle

6. Adaptations: Evidence for Evolution
Structural adaptations arise over time
Mimicry – where one species resembles another species
Camouflage – where a species blends with their surroundings
Physiological adaptations arise over time
Antibiotic resistance of bacteria

7. Other Evidence for Evolution
Fossils
Anatomy
Homologous structures – structures that are similar in anatomy but have different function
Common evolutionary origin
Analogous structures – body parts similar in function but have different structure
No common evolutionary origin
Vestigial structures – body structure that has no function in present-day organisms but was probably useful to an ancestor
Biochemistry
DNA, RNA

9. More Evidence for Evolution
Embryology
As development continues from embryo to a more mature organism, the differences increase, however, in the earliest stages of growth and development, many vertebrate embryos are remarkably similar

12. Biological Change
Species Change

13. Principles of Darwin’s Theory of Natural Selection
Variation
Heritability
Overproduction
Reproductive Advantage

14. Remember… The Principles of Darwin’s Theory
Overproduction of offspring
Variation
Heritabiltiy
Reproductive Advantage (preferential selection of traits)

15. Species Change
Organisms that are best matched to their environment are more likely to survive and reproduce
Adaptation – a trait that improves an organism’s change for survival and reproduction

16. Adaptation
Individual members of a single species exhibit differences in their appearance and function
Difference result from random changes in genetic material from sexual reproduction and mutations

17. Natural Selection
Organisms with traits that help them survive and reproduce pass their characteristics to their offspring.
Helpful traits survive and spread through the population
Harmful traits disappear over time
As a result, a population may evolve into a new species

18. Mismatch with environment
Natural Selection
reproduction
Inherited traits
Random mutations
Variation
Mismatch with environment
Adaptations
Survival
Death; no offspring

19. Populations evolve, not individuals!
Variation exists among individuals within a population
An individual with variations that make them poorly adapted to the environment will not survive and reproduce
Remember genotypes (genes) and phenotypes (expression of genes)
Evolution occurs as a population’s genes and their frequencies change over time

20. Natural Selection Acts on Variation
Some variations increase or decrease an organism’s chance of survival in a particular environment
Three types
Stabilizing selection
Directional selection
Disruptive selection

21. Types of Natural Selection
Directional selection
Occurs when individuals at one end on the frequency distribution are better adapted to the environment than those in the middle
Disruptive selection
Occurs when individuals near the upper and lower ends of the distribution are better adapted than those in the middle
Stabilizing selection
Occurs when individuals near the center of the distribution are more fit than individuals at either end

22. Disruptive Selection
Stabilizing Selection
Directional Selection

23. Determine the type of natural selection indicated by the following examples.
Members of a population of Amazon tree frogs hop from tree to tree searching for food in the rain forest. They vary in leg length. Events result in massive destruction of the forest’s trees. After several generations, only long-legged tree frogs remain alive.
Different grass plants in a population range in length from 8cm to 28 cm. The 8-10 cm grass blades receive little sunlight, and the cm grass blades are eaten quickly by grazing animals.
The spines of sea urchin population’s members vary in length. The short-spined sea urchins are camouflaged easily on the seafloor. However, long-spined sea urchins are well defended against predators.

24. Question
A population of woodpeckers have beak lengths ranging from 2cm-4cm. The woodpeckers with 3 cm beaks are able to reach more insects in the trees in which they feed.
Explain the following:
A.Is longer beak length an advantageous adaptation?
B. How would you expect the population of woodpeckers to evolve after a very long period of time?
C. What type of selection would this be an example of? Explain your answer.

25. Classification of organisms
Organisms are classified based on internal and external characteristics
Species – most specific unit of classification

26. The Evolution of Species
Speciation – the process of the evolution of a new species
Occurs when members of similar populations no longer interbreed and produce fertile offspring

27. Evolutionary History
Phylogeny describes the evolutionary history of a related group of species
All organisms on Earth evolved from a single ancestor
Life on earth began about 3.5 billion years ago; since that time, new species have emerged, lived and died out
New species evolve from pre-existing species

28. Fossil Evidence
Through fossil evidence, physical and molecular similarities between ancient species and modern species have been found.
Physical and molecular similarities between diverse species has also been found.

29. Phylogeny of Humans

30. Phylogeny of Humans

31. Extinction
Organisms try to survive in their environments through adaptation, however many species become extinct
Extinction can occur because of environmental changes, human interference or as a result of failure to adapt to new conditions

32. Extinction Cont.
Extinction is a natural and important part of evolution
It is estimated that 999 of every 1,000 species that have ever lived on Earth have become extinct
The average species survives between 2 and 10 million years
Even the most highly adapted species become extinct.

33. Mass Extinctions
There have been many mass extinctions during Earth’s history
Mass extinction = when more than 50% of species were wiped out
Mass extinctions make it possible for new species to develop
Surviving species are able to diversify

34. Identifying change in species
Biological Change
Identifying change in species

35. Changes in genetic equilibrium
Mutations cause genetic change
Caused by environmental factors such as radiation, chemicals, or can simply occur by change
If a mutation is useful, it persists and becomes part of the gene pool
Lethal mutations cause death and are quickly eliminated from the gene pool

36. Mutations Natural Mutations occur at a regular rate.
The number of differences between the genetic material and different species estimates how long ago two species share a common ancestry

37. Types of Evolution
Macroevolution: The generation of major change in the assemblage of organisms: speciation
Microevolution: Changes in the gene pool of a population that result in changes in allele frequencies; they arise without the influence of selection pressure

38. Mechanisms of Evolution
Populations evolve, not individuals

39. Population Genetics
When Charles Darwin developed his theory of natural selection in the 1800’s, he did so without knowing about genes
The principles of today’s modern theory of evolution are rooted in population genetics.
Today we refer to the Synthetic Theory of Evolution, which included the principled of genetics

40. Genes and Populations
Gene pool: The collection of genes in a population
Because diploids have only two versions of each gene, each has only a small fraction of possible alleles in a population
Genotype: The genetic makeup of an individual at a given locus, taking into account the two possible alleles
Genotype frequency is the proportion of a given genotype in the population
Allele frequency refers to the proportion of a particular allele, such as A or a
Phenotype: the traits of an individual
Phenotype frequency is the proportion of a given phenotype in the population
Phenotype frequency is influenced by the dominance characteristic of an allele

41. Disruption to Genetic Equilibrium
Genetic Drift – alteration of allelic frequencies by chance events
Can greatly affect small populations
Ex. Amish of Lancaster County Pennsylvania – 6 fingers and toes
Individuals in this community have a 1 in 14 chance of having this mutation
Individuals in the larger population of the United States only have a 1 in 1,000 chance of having the mutation

42. Disruption to Genetic Equilibrium
Gene flow – the transport of genes by migrating individuals
When an individual leaves a population, it takes its genes with it
When an individual enters a population, it introduces new genes

43. Alleles and Population Genetics
Although individuals are affected by the process of natural selection, it is the makeup of the population that is critical for determining the subsequent generations
Changes in the gene pool refer to changes in the frequency of the alleles
If the allele frequencies in a population do not undergo change over time, we say that the population is in genetic equilibrium, the population is not evolving.

44. Population Stability
The conventional view might be that dominant alleles would eventually come to dominate the gene pool, and the recessives disappear
They do not necessarily do so; in fact, allele frequencies change only when influenced by other factors.
The stability of populations over time is explained by the Hardy-Weinberg Equilibrium.

45. Hardy Weinberg Principle
This condition can be modeled as the Hardy-Weinberg Equilibrium, which requires:
Large population size
No migration
Random mating
No net mutations
All genotypes have similar selective value
This is idealized and rarely actually occurs, but is a useful tool

46. The Ps and Qs of H&W Imagine 2 alleles, A and a
p is the frequency of A
q the frequency of a
So, p + q = 1
The mathematical equivalent of a random mating can be given by multiplying this relationship by itself
Therefore, (p + q)2 = 1 = p2 + 2pq + q2
p2 = frequency of AA
2pq = frequency of Aa
q2 = frequency of aa
Given this condition, we can always work out the frequencies of each allele in a sexual population that is not evolving.

47. H-W: Example Remember, since (p + q)2 = 1 p2 + 2pq + q2 = 1
Let’s say that a population has the following genotypic and allelic frequencies
Note how all frequencies add up to 1.0

48. A Fun Experiment in Class
Tongue rolling is described by a simple dominant character, T and we can study the HW equilibrium using this trait, in this class
1. Find the frequency of homozygous recessives (q2)in the class
Can you roll your tongue? If so you are either TT or Tt
Note how many can roll tongues ________
If not, you are tt
Note how many cannot roll tongues ________
Take this number and divide by the class total: ______; this is the frequency of homozygous recessives (q2).
2. What is the frequency of p?
Since p + q = 1, then 1 - q = p
Take the root of q2 from above. ___________
We can now calculate p. p = 1 - q

49. 5 Factors Upset Genetic Equilibrium
Mutation
Nonrandom mating
Genetic Drift
Gene Flow
Natural Selection
All of these are conditions that were required by the H-W equilibrium to NOT occur
They cause changes in allelic frequency, and result in microevolution
They all occur routinely

50. Population Genetics Populations evolve not individuals!
All changes in a population occur at the gene level
Variations lead to adaptations
Adaptations that are beneficial become more common in the population
Genotype frequency is the proportion of a given genotype in the population

51. Things that lead to change in populations
Reproductive Barriers
Sexual selection - male/female choice in mates
Reproductive isolation
Prezygotic isolation
Geographic Isolation – when a physical barrier divides a population
Ecological and behavioral barriers
Postzygotic isolation
Hybrid offspring cannot develop or reproduce

52. Speciation
Speciation occurs when a population diverges, and can no longer breed and produce viable offspring, often due to reproductive isolation
Allopatric speciation – when a physical barrier divides one population into two or more populations. The separate populations will eventually contain organisms that, over time will no longer be able to breed successfully
Sympatric speciation – when a species evolves into a new species without a physical barrier. Sometimes based on resource utilization, sometimes caused by polyploidy in plants.

53. Patterns of Evolution Adaptive radiation (aka: divergent evolution)
results when populations adapting to different environmental conditions change , becoming less alike as they adapt, resulting in new species.
In other words, when an ancestral species evolves into an array of species to fit a number of diverse habitats
Ex. Hawaiian Honeycreepers, Galapagos finches, African cichlids
Convergent Evolution – when distantly related organisms evolve similar traits
Occurs when unrelated species occupy similar environments
Similar environmental pressure  Similar pressures of natural selection

54. Patterns of Evolution
Coevolution – When the evolution of one species affects the evolution of another species.
Mutualism is a form of coevolution
Hummingbirds and flowers
Poisonous newts and the predators that can eat them

55. Speciation can occur quickly or slowly…
Gradualism – the idea that species originate through a gradual change of adaptations
Punctuated Equilibrium – hypothesis that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between
Environmental changes, or introduction of competitive species can lead to rapid changes
Happens quickly – in about 10,000 years or less
Both gradualism and punctuated equilibrium are supported by fossil evidence