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Natural Selection and the Evidence for Evolution

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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

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