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Natural Selection and Evolution

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1 Natural Selection and Evolution
Chapter 16: Darwin’s Theory

2 The eight characteristics of living things?
Living things are based on a universal genetic code (DNA) Living things grow and develop Living things respond to their environment (stimulus) Living things reproduce Living things maintain a stable internal environment (homeostasis) Living things obtain and use material and energy (metabolism) Living things are made up of CELLS Taken as a group, living things evolve over time

3 Making Inferences and Forming Hypotheses:
Hypothesis A logical interpretation based on what scientists already know. A scientific explanation for a set of observations that can be tested in ways that support or reject it. A possible explanation for what you expect to happen Ex. Plants will grow toward a source of light.

4 Scientific Theory A well-tested explanation that unifies a broad range of observations and hypotheses Explains a broad set of observations about the natural world. Is supported by evidence and data Provides a set of principles that can be used to make testable predictions and hypothesis. The meaning of the word theory in daily life is different from its meaning in science A scientific theory is NOT a hunch

5 Cell Division and Reproduction
Asexual Reproduction Sexual Reproduction A single parent Genetically identical offspring Advantage: Simple, efficient, & effective way for an organism to produce a large number of offspring. Disadvantage: may not be able to adapt to change Fusion of two sex cells – one from each of two parents Genetically diverse offspring Advantage: Genetic diversity helps ensure survival of species when environment changes Disadvantage: Process takes a long time

6 Genes and Alleles Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting varieties. These gene variations produced different expressions, or forms, of each trait. The different forms of a gene are called alleles. Dominant = Form of a gene that is expressed even if present with a contrasting recessive allele. Example: BB or Bb A = dominant allele Recessive = Form of a gene (allele) that is only expressed in the homozygous state. Example: bb

7 Homozygous (Purebred) vs. Heterozygous (Hybrid):
Homozygous = Organism that has two identical alleles for a particular trait. (Ex: AA, bb, CC, dd) Heterozygous = Organism that has two different alleles for the same gene. (Ex: Aa, Bb, Cc, Dd)

8 Genotype vs. Phenotype:
Genotype: Genetic make-up of an organism (Ex: Aa, BB, cc) Phenotype: Physical characteristics of an organism (Ex: flower color, eye color)

9 Types of Cells Affected
Germ Mutation - affects a reproductive cell (gamete or sperm/egg) Does not affect the organism Passed to offspring Somatic Mutation – affects body cells (all cells except gametes) Not passed to offspring

10 Polygenic Traits Traits controlled by two or more genes are said to be polygenic traits. Polygenic means “many genes.” Polygenic traits often show a wide range of phenotypes. The variety of skin color in humans comes about partly because more than four different genes probably control this trait.

11 Segregation During gamete formation, the alleles for each gene segregate from each other, so that each gamete carries only one allele for each gene.

12 Synapsis and Tetrads During synapsis, the homologuous chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.

13 Crossing Over As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over. First, the chromatids of the homologous chromosomes cross over one another. Then, the crossed sections of the chromatids are exchanged. Crossing-over is important because it produces new combinations of alleles in the cell.

14 Darwin’s Epic Journey Charles Darwin was born in England on February 12, He grew up at a time when the scientific view of the natural world was shifting dramatically. Geologists were suggesting that Earth was ancient and had changed over time, and biologists were suggesting that life on Earth had also changed. The process of change over time is called evolution. Darwin developed a scientific theory of biological evolution that explains how modern organisms evolved over long periods of time through descent from common ancestors.

15 Darwin’s Epic Journey Darwin was invited to sail on the HMS Beagle’s five-year voyage mapping the coastline of South America. Left from England, sailed around South America, across Pacific, around Australia and Africa, and back to England Darwin was the ship’s naturalist – he planned to collect specimens of plants and animals on the voyage. Most famous for observations made at Galapagos Islands No one knew it, but this would be one of the most important scientific voyages in history.

16 16-1 Darwin’s Voyage of Discovery
Darwin noted 3 patterns of biodiversity: Species vary globally Species vary locally Species vary over time

17 1. Species vary globally Different, yet ecologically similar animal species inhabited separate, but ecologically similar, habitats around the globe Ex) flightless birds: rheas, ostriches and emus Rhea Ostrich Emu Native to Africa Largest of 3 Native to Australia Slightly Larger than Rhea Native to S.America

18 (tortoise shell shape - Galapagos)
2. Species vary locally Different, yet related animal species occupied different habitats within a local area (tortoise shell shape - Galapagos)

19 3. Species Vary Over Time Darwin also collected fossils, which are the preserved remains or traces of ancient organisms. Darwin noticed that some fossils of extinct animals were similar to living species.

20 Fossils found in Towamencin
While construction crews from Allan A. Myers (AAM) have been building stormwater basins and doing excavations by Exit 31 in Lansdale as part of the ongoing Northeast Extension road-widening project, geologists from Kutztown University have uncovered something even bigger hidden among the massive slabs of rock. Fossils Found in Turnpike Excavation article on The Reporter Fossils Found in Turnpike Excavation pictures

21 Lamarck’s Theory of Evolution
Inheritance of Acquired Traits Acquired traits could be inherited For example: If you spent your life lifting weights, then your children would inherit large muscles.


23 Lamarck’s Theory of Evolution
Evaluating Lamarck’s Theory He was wrong in several ways He didn’t know how traits were inherited He did realize that organisms adapt to their environment.

24 Artificial Selection To find an explanation for change in nature, Darwin studied change produced by plant and animal breeders. Breeders knew that individual organisms vary, and that some of this variation could be passed from parents to offspring and used to improve crops and livestock – selective breeding. For example, farmers would select for breeding only trees that produced the largest fruit or cows that produced the most milk. Over time, this selective breeding would produce trees with even bigger fruit and cows that gave even more milk.

25 Artificial Selection Darwin called this selective breeding process artificial selection, a process in which nature provides the variations, and humans select those they find useful. Darwin put artificial selection to the test by raising and breeding plants and fancy pigeon varieties. Darwin's pigeons described by Randal Keynes (7:49) BBC Darwin's Pigeons (2:18)

26 Artificial Selection Darwin had no idea how heredity worked or what caused heritable variation, but he did know that variation occurs in wild species as well as in domesticated plants and animals. Before Darwin, scientists thought variations among individuals in nature were simply minor defects. Darwin recognized that natural variation was very important because it provided the raw material for evolution. Natural variation = differences among individuals of a species When Darwin published his scientific explanation for evolution, it changed the way people understood the living world.

27 Population Growth Malthus observed that babies were being born faster than people were dying. If population continued to grow there would be insufficient food and space. In plants or animals most of the offspring die Only those who survive can reproduce What determines which ones will survive and reproduce?

28 The Struggle for Existence
After reading Malthus, Darwin realized that if more individuals are produced than can survive, members of a population must compete to obtain food, living space, and other limited necessities of life. Darwin described this as the struggle for existence.

29 Variation and Adaptation
Darwin knew that individuals have natural variations among their heritable traits, and he hypothesized that some of those variants are better suited to life in their environment than others. Any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment is called an adaptation.


31 Three types of adaptations that involve body parts or structures :
Structural – physical features of an organism Ex: long tongue to get food, sharp teeth, mimicry, camouflage Behavioral – actions an organism takes Ex: migration, tracking prey, storing nuts, growing towards light Physiological – functioning or biochemical processes Ex: venom, ink of octopus, protein in web, respiration rate, digestive enzyme, blood clotting

32 Mimicry Scarlet king snake exhibits mimicry - an adaptation in which an organism copies, or mimics, a more dangerous organism. Although the scarlet king snake is harmless, it looks like the poisonous eastern coral snake, so predators avoid it, too. Non poisonous snakes will also rattle their tale and flatten their head to look poisonous to a predator.

33 Mimicry Monarch butterfly - is toxic and very nasty to eat.
Its bright orange coloration is a warning to birds to leave it alone. Non-toxic viceroy butterfly has developed colors and wing patterns that are very similar to those of the monarch most birds won’t take a chance by taste-testing it

34 Camouflage An adaptation that allows an organism to blend into its background and avoid predation.

35 Variations vs. Adaptations
Variations – differences that exist within a population that may have no effect on fitness Ex: length of your thumb Adaptations – a variation that all members of a population have inherited because that trait improves fitness Ex: an opposable thumbs

36 Sources of variation Sources of variation:
Sources of variation Sources of variation: Mutations – individual genes change Ex: ATC  AGC Events during meiosis – during meiosis, chromosomes get mixed up (crossing over) in making eggs & sperm Random fusion of gametes – which sperm fertilizes which egg – determined by chance

37 Survival of the Fittest
According to Darwin, differences in adaptations affect an individual’s fitness. Fitness describes how well an organism can survive and reproduce in its environment. Individuals with adaptations that are well-suited to their environment can survive and reproduce and are said to have high fitness. Individuals with characteristics that are not well-suited to their environment either die without reproducing or leave few offspring and are said to have low fitness. This difference in rates of survival and reproduction is called survival of the fittest. In evolutionary terms, survival means reproducing and passing adaptations on to the next generation.

38 Natural Selection Darwin named his mechanism for evolution natural selection because of its similarities to artificial selection. Natural selection is the process by which organisms with variations most suited to their local environment survive and leave more offspring. In natural selection, the environment — not a farmer or animal breeder — influences fitness.

39 Natural Selection Well-adapted individuals survive and reproduce.
From generation to generation, populations continue to change as they become better adapted, or as their environment changes. Natural selection acts only on inherited traits because those are the only characteristics that parents can pass on to their offspring.

40 Natural Selection Natural selection does not make organisms “better.”
Adaptations don’t have to be perfect—just good enough to enable an organism to pass its genes to the next generation. Natural selection also doesn’t move in a fixed direction. There is no one, perfect way of doing something. Natural selection is simply a process that enables organisms to survive and reproduce in a local environment.

41 Natural Selection If local environmental conditions change, some traits that were once adaptive may no longer be useful, and different traits may become adaptive. If environmental conditions change faster than a species can adapt to those changes, the species may become extinct.


43 Common Descent Natural selection depends on the ability of organisms to reproduce and leave descendants. Every organism alive today is descended from parents who survived and reproduced. Darwin proposed that, over many generations, adaptation could cause successful species to evolve into new species. He also proposed that living species are descended, with modification, from common ancestors—an idea called descent with modification. According to the principle of common descent, all species—living and extinct—are descended from ancient common ancestors.

44 16-3 Darwin Presents His Case
Darwin published his findings in 1859 in a book entitled The Origin of Species by Means of Natural Selection. He was motivated to publish his book in 1859 because Alfred Wallace had independently come up with the same conclusions

45 The Age of Earth Evolution takes a long time. If life has evolved, then Earth must be very old. Hutton and Lyell argued that Earth was indeed very old, but technology in their day couldn’t determine just how old. Geologists now use radioactivity to establish the age of certain rocks and fossils. Radioactive dating indicates that Earth is ~4.5 billion years old - plenty of time for evolution by natural selection to take place.

46 16.4 Evidence of Evolution Fossils Anatomy Analogous structures
Homologous structures Vestigial structures Embryology Biochemistry Direct observation

47 1. Fossil Evidence Fossils - the remains of past life
Ex: shells, bones, teeth, imprints Tell us - age, habitat, diet, & lifestyle of organisms. Record is incomplete – many organisms leave no fossils behind

48 2. Anatomical Evidence – Homologous Structures
Darwin proposed that animals with similar structures evolved from a common ancestor with a basic version of that structure. Structures that are shared by related species and that have been inherited from a common ancestor are called homologous structures. Homologous structures are similar in structure because they develop from same tissues early in development may or may not have different functions. Bones of vertebrate forelimbs

49 Homologous Structures


51 2. Anatomical Evidence – Analogous Structures
The clue to common descent is common structure, not common function. A bird’s wing and a horse’s front limb have different functions but similar structures (homologous structures). Body parts that share a common function, but not structure, are called analogous structures. Analogous structures are used for the same purpose but are not due to a common ancestor. The wing of a bee and the wing of a bird are analogous structures.

52 Analogous vs. Homologous Structures

53 Cormorant – flightless bird
Vestigial Structures Not all homologous structures have important functions. Vestigial structures are inherited from ancestors, but have lost much or all of their original function due to different selection pressures acting on the descendant. The hipbones of bottlenose dolphins are vestigial structures. In their ancestors, hipbones played a role in terrestrial locomotion. However, as the dolphin lineage adapted to life at sea, this function was lost. The human tailbone and appendix are vestigial structures. Cormorant – flightless bird

54 Vestigial Structures

55 Vestigial Structures Vesitgial Structure Video Evolution Evidence
in dropbox Evolution Evidence

56 Chicken, Fish, Human, Rabbit, Salamander, Turtle??????
3. Embryology Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor. the more similar the embryos are at certain stages of development, the more closely related they are thought to be. Which one is a… Chicken, Fish, Human, Rabbit, Salamander, Turtle??????

57 Embryology - the more similar the embryos are at certain stages of development, the more closely related they are thought to be

58 4. Biochemical Evidence Universal genetic code – organisms use the same triplet code and the same 20 amino acids in proteins All organisms have certain organic molecules in common. Hemoglobin - carries oxygen in blood Cytochrome c - protein for cell respiration found in almost all living cells HOX genes – control development Genes Control Body Segmentation PBS - 13:45

59 Significance of biochemical differences

60 5. Direct Observation Direct observation – we’ve seen evolution occur in cases like: The Grant’s studies of finches on Daphne Major in the Galapagos (35 yrs) Bacteria becoming resistant to antibiotics Insects that become resistant to pesticides

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