CHAPTER 19. Descent With Modification

Transcription

1 CHAPTER 19 Descent With Modification

2 REALLY Looking at Organisms Differences from other organisms Features that show the relationship of an organism to where it lives, what it does Ex: marine iguanas to land iguanas (claws, tail = diet)

3 Who Thought What Prior to Darwin Early Greek: (2500 years ago) life began in water w/simpler forms then more complex forms Aristotle Greek: ( years ago) life was fixed and didn t evolve Judeo-Christian: all species were created in a single act of creation about 6000 years ago Lamarck: (French, early 1800s) supported evolution but believed in acquired characteristics as mechanism for change

4 Darwin & His Boat Trip Spent 4 ½ years traveling to different places Had awareness of how each organism s adaptations allow it to fit into its environment Came to see how species change over time & that living species arose from earlier species Based on ideas of interrelatedness dating back to over 2500 years ago

5 The Man of Controversy Charles Darwin Published The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle For Life in 1859 based on ideas of those before him, evidence he had accumulated, & publishing pressure

6 Natural Selection As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. Charles Darwin, The Origin of Species

7 Natural Selection In a varied population, individuals whose inherited characteristics adapt them best to their environments are more likely to survive and reproduce leaving more offspring than less fit individuals do

8 Natural Selection = Differential Reproduction Indivduals in populations vary Some are more suitable to a given environment & reproduce more easily and abundantly Favored characteristics are passed on to next generation (less-favored are not) Gradual accumulation of favored characteristics among individuals in a population occurs over vast amounts of time

9 Natural Selection: The Prominent Force in Nature Regional and timely Populations tend to adapt to local environments during one time period These same adaptations may be pointless in other locales or times

10 2 Examples: Cepaea nemoralis (land snail) & Biston betularia (peppered European land snail s shell patterns camouflage in different areas (striped for welllit, dark for shady areas) moth) Light & dark variety Light was predominant before Industrial Revolution Darkened trees w/pollution allowed for dark variety to become predominant

11 Fossil Evidence for Evolution Hard parts (skeletons, shells) fossilize easy Some fossils retain organic matter (leaves) that can be analyzed molecularly Fossilized tree amber trapped organisms (protecting them from decomposition)

12

13 Fossil record is an array of fossils w/in strata of sedimentary rock - each stratum has deposits of material that has accumulated - younger strata sit on top of older strata Fossil record shows historical sequence of organisms from earliest prokaryotes (3.5 billion years ago) to eukaryotes through many intermediate forms to modern forms

14

15 Mass Evidence for Evolution 1. Biogeography-observations about distribution of different but related forms around the world & in neighboring geographical regions Island forms are more similar to forms found on the closest mainland than those found on similar but more distant islands Can give rise to a new species found only there (endemic)

16 2. Molecular Biology- demonstrates universality of genetic code Conservation of amino acid sequences in proteins (hemoglobin) are compared Some processes virtually the same in all organisms (glycolysis)

17 Genome Sequences & Divergence comparing distantly related species (evolutionary trees) comparing closely related species (classification) comparing within a species (diseases, immunity)

18 3. Comparative Anatomycomparison of homologous structures All mammals have same basic limb structures Structure not function Leads to descent w/modification Vestigal organs

19 4. Comparative Embryologycomparison of embryonic stages of different organisms All vertebrates have pharyngeal pouches in their throat regions

20 Chapter 21: The Evolution of Populations Population group of individuals of the same species living in the same place at the same time Evolution is measured by the frequency of a given characteristic w/in a population over successive generations not an individual

21 Darwin realized this but didn t know the genetic mechanism which is?? population genetics (1920s)- rediscovery of Mendel s genetic principles & Darwin s natural selection (how populations change genetically over time)

22 Modern synthesis (1940s) incorporates population genetics & ideas from paleontology, taxonomy, biogeography Focuses on populations as the units of evolution & that central role of natural selection

23 biological species a group of populations whose individuals have the same potential to interbreed & produce fertile (viable) offspring Opportunities for breeding among populations will vary, depending on species and extent of isolation of populations

24 Microevolution A change in a population s gene pool all of its alleles in all individuals making up a population that can be inherited by the next generation Most gene loci have 2 or more alleles in a population During microevolution, the relative frequencies of alleles can change (light to dark peppered moths)

25 Hardy-Weinberg Principle In a large population, the gene pool will remain constant unless acted upon by some outside agent Separation of alleles during meiosis, recombining during fertilization has no effect on frequency of alleles in a population Is this the case in nature?

26 5 Conditions Required by Hardy- Weinberg 1. Very large population 2. Isolated population (no migrations into or out) 3. Mutations do not alter gene pool 4. Mating is random 5. All individuals are equal in reproductive success (no natural selection)

27 Then why use it? Public health agencies: estimate the frequency of harmful alleles in a population establish programs to deal w/genetic diseases real populations are not the ideal (static) Hardy-Weinberg, stats must be constantly reevaluated.

28 4 Potential Causes of Microevolution 1. Genetic Drift change in a gene pool of a small population due to chance (effect of losing a few individuals is much greater than in a large population) Bottleneck effect: an event that drastically reduces population size (earthquake, fire, flood, humans) Founder effect: small group moves locations, genes not representative

29 2. Gene Flow gain or loss of alleles in a population due to immigration or emigration of individuals or gametes - must consider average heterozygosity (2 different alleles)

30 3. Mutations rare events that it changes the whole population beneficially 4. Nonrandom Mating more often the cause, particularly among animals (choosing mates w/particular traits, ex: healthiest) -differential success in reproduction is probably always the case in natural populations (leads to adaptive evolution)

31 Adaptive Evolution results when natural selection upsets genetic equilibrium degree of adaptation that can occur is limited only by amount and kind of genetic variation in a population

32 - difficult to show an allele benefits in one environment but not in another endangered species often have less variation because of a small population not all genetic variation is due to natural selection: - some neutral variation shows no apparent selective advantage (fingerprints) - frequency of characteristics may be due to genetic drift

33 Extensive Variation in Most Populations a single characteristic can be caused by one or more genes or by environmental action causing a phenotypic change population is polymorphic for a characteristic if 2 or more morphs (contrasting forms) are present

34 populations can exhibit geographic variation (preference of flower color by the local pollinator) Cline- graded change in an inherited characteristic due to geographic location

35 Processes Generate Variation mutations normally are harmful but could be adaptive in a changing environment (point mutation, changing gene # or position) rapid reproduction sexual recombination shuffles mixture of alleles (such as?) Independent assortment, crossing over, random fertilization

36 How does Natural Selection Affect Variation? starts w/varied ancestral population over successive generations, those w/characteristics best suited for environment leave more offspring behind than ill-suited recessive alleles (and their effects) can remain in population over long periods of time

37 Overall Effects of Natural Selection Perpetuation of Genes Defines Evolutionary Fitness Natural Selection s 3 Modes of Action Sexual Selection & Sexual Dimorphism Natural Selection Acts on Whole Organisms Resistant Populations of Pests & Parasites

38 Perpetuation of Genes Defines Evolutionary Fitness emphasis is survival of genes over time, not individual organisms Relative fitness - the relative combination that an individual makes to the gene pool of the next generation (passes on the greatest number of genes)

39 Natural Selection s 3 Modes of Action Stabilizing Selection - tends to narrow ranges in population variability toward some intermediate form (in stable environments, most common) (1)

40 Directional Selection - tends to move the modal form toward one of the extremes (most common during times of environmental change) (2)

41 (3) Disruptive Selection - occurs when environmental factors are varied in a way that favors extremes over the intermediate form disruptive

42 Sexual Selection & Sexual Dimorphism Secondary sexual characteristics (size, male adornment) Differences between male & female Intrasexual selection 1 lion w/pride Intersexual selection - peacocks

43 Natural Selection Acts on Whole Organisms acts on phenotypes each phenotype is the sum of the organism s genotype there is no way for natural selection to select individual gene loci (it favors the whole organism)

44 Resistant Populations of Pests & Parasites directional selection is the mode for pesticide, antibiotic, and drug resistant pests and disease causing organisms these life forms are adaptable because they are numerous, multiply rapidly, and have short generation times

45 So Why Aren t We Perfect? 1. Selection w/variations populations can only use what is available 2. Historical Constraints we can only use what our ancestors gave us (see # 1) 3. Adaptations & Compromises no rain, no rainbows 4. Chance & Natural Selection nature can change things, sometimes good, sometimes bad

46 The key is man s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to have made for himself useful breeds...for hardly any one is so careless as to breed from his worst animals. - Charles Darwin The Origin of Species