BioSN2Lec2 - Evolution PDF

Biology SN2 Lecture 2 EVOLUTION & Descent with Modification It Is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is most ADAPTABLE to change. - C. Darwin...

Biology SN2 Lecture 2 EVOLUTION & Descent with Modification It Is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is most ADAPTABLE to change. - C. Darwin  Darwin: The Origin of Species 2 main ideas 1) Today’s organisms descended from ancestral species that were different from modern species. – Evolution explains life’s unity and diversity 2) Natural selection is the mechanism for this evolutionary change. – Natural selection results in adaptive evolution – The basic idea of natural selection is that a population can change over time if individuals that possess certain heritable traits leave more offspring. – Results in an accumulation of inherited characteristics that increase the ability of an organism to survive and reproduce in its environment.  Before Darwin: Understanding Life The Greek philosopher Aristotle – Viewed species as fixed and unchanging – Life could be arranged on a scale of increasing complexity “scala naturale” The Old Testament of the Bible – Holds that species were individually designed by God Carolus Linnaeus – Was a founder of taxonomy, grouping similar species into increasingly greater categories – Did not attribute resemblances among species to evolutionary kinship, Attributed similarities rather to the pattern of their creation  Before Darwin: Fossils The study of fossils (Paleontology) helped lay the groundwork for Darwin’s ideas remains or traces of organisms from the past usually in sedimentary rock; in layers (strata)  determines sequence of events - Fossils show that a succession of organisms have populated Earth throughout time. - Cuvier noted that the older the strata, the more dissimilar the fossils from modern life. - Instead of evolution; use ‘catastrophism’ theory; - speculating that boundaries between strata were due to local floods or droughts that destroyed the species present.  Before Darwin: Lamarck Jean Baptiste de Lamarck proposed a theory that organisms evolve over time Organisms pass traits to offspring through use and disuse parts of the body used extensively become stranger & larger, while those not used deteriorate ex: change in environment changes behavior  giraffe developed when a short-necked ancestor stretched its neck to eat leaves on a tree Life force would drive organisms toward greater complexity and perfection over time the mechanisms he proposed unsupported by evidence  Before Darwin: Hutton & Lyell Hutton (Scottish geologist) proposed a theory of gradualism – Gradualism: profound change can take place through the cumulative effect of slow but continuous processes Lyell perceived that changes in Earth’s surface can result from slow continuous actions still operating today – i.e. valleys formed by rivers wearing away at rocks Therefore: - if geologic changes result from slow, continuous processes rather than sudden events, - Then Earth = older than the 6,000 years estimated by biblical inference. - slow and subtle processes persisting for long periods of time can also act on living organisms, - producing substantial change over a long period of time.  Darwin developed his theory based on: i) Malthus’ essay: Struggle for existence ii) Lyell’s Principles of Geology iii) Principles of artificial selection also influenced Darwin’s theories iv) His data (what he saw in the Galapagos) i) Struggle for Existence – (Thomas Robert Malthus (1766-1834) Population growth not always desirable Conflict between population growth and food supply generates famine, disease, war, which inevitably break population growth ii) Lyell’s Principles of Geology Darwin experienced geologic change (earthquakes – rocks thrust upward by several feet – therefore earth not static) Reinforced Darwin’s acceptance of Lyell’s ideas doubt about the traditional view of a young and static Earth. iii) Artificial selection - Humans have modified domesticated plants and animals in just a few generations by selecting individuals with the desired traits for breeding.  If artificial selection can achieve large change in a relatively short period of time, Darwin reasoned that natural selection should be capable of modification over thousands of generations. ex: 400 breeds of dog from Grey Wolf ex: breeding of wild mustard (Canis lupus) in 14,000 years  iv) Darwin’s data from the Galapagos Darwin’s voyage was the basis for his theory of evolution – Voyage on H.M.S. Beagle (1831): 5-year exploration of South America and the South Pacific Darwin observed various adaptations of plants and animals that inhabited the many diverse environments Compared animals on islands (ex: Galapagos) with those of mainland. Impressed by similarities and ALSO distinct differences  Darwinian Evolution: Darwin was fascinated by the unusual organisms found. He hypothesized that the islands had been colonized by plants and animals from the mainland that had subsequently diversified on the different islands. The origin of new species and adaptation of species to their environment are closely related processes. ex: clear differences in the beaks among the 13 species of finches that Darwin collected in the Galapagos are adaptations to the specific foods available on their home islands. Darwin How Does Natural Selection Work? 1) Species are capable of producing more offspring than environment can support – For any species, population sizes would increase exponentially spores However, populations tend to be stable in size How Does Natural Selection Work? 2) Environmental resources are limited – Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, i.e. competition, with only a fraction of their offspring surviving  Competition for limited resources: Food, Water, Light, Space….  Killed by predators, disease, environmental conditions  How Does Natural Selection Work? 3) Members of a population vary extensively in their characteristics – No two individuals are exactly alike – Variations are heritable – Survival depends in part on inherited traits Why do these organisms look different? Genetic Variation Explained Recall: Asexual vs. Sexual reproduction Asexual reproduction: the offspring are identical to the parents (clones). Sexual reproduction: the offspring are not identical to the parents. uses meiosis (a special type of cell division) to produce gametes (i.e. UNIQUE sex cells: egg an sperm): – When these sex cells (egg and sperm) combine during fertilization, the offspring (children): differ genetically from the parents and also from each other. This is because of the DNA (i.e. chromosomes) contained in sex cells is HAPLOID and has undergone mechanisms to increase genetic variation  23 pairs of chromosomes in a human somatic cell (body cell) Chromosome #2 from mom Chromosome #2 from dad Chromosome #1 from mom Chromosome #1 from dad Or XY for males x XX for Same chromosome # from & dad = Homologous Pair females 1 complete set of 23 chromosomes from mom 1 complete set of 23 chromosomes from dad 46 Total Both chromosomes # 1 have the same genes that code for the same trait / characteristic, so together they from a homologous pair Ex. can consider them like this: #1 from mom = chromosome 1 from mom has gene that encodes for hair color (blond) #1 from dad = chromosome 1 from dad has gene that encodes for hair color (brown)  Homologous Chromosomes Homologous pairs of chromosomes are not identical... just similar. Homologous chromosomes code for the same genes (i.e. same trait) eye but may have different alleles for that gene i.e. may have different versions of that gene (ex. brown vs. blue eye color, or blond vs brown hair color, or short vs tall, etc.) height So chromosome 1 is homologous to the other version (from other parent) of chromosome 1 Tongue roll  Alleles Alternative versions of a gene are called alleles – Ex. the gene for flower color in pea plants exists in 2 versions: 1 for purple flowers and the other for white flowers Alleles control the same character (gene) – but do not necessarily contain the same information (ex. purple vs. blue) Ex. There are 2 major genes for eye color: – One that controls for brown or blue – One that controls for green or hazel – There are other genes that modify the above genes The inheritance and expression of genes and various alleles and the resulting Genetic variation of an organism can give it an advantage (or disadvantage) compared with another organism, this variation allows Natural selection ‘choose’ / select the fittest organism Dominant and Recessive Alleles: What traits will appear? A Dominant Allele: – an allele that masks the presence of another allele – always upper case letters ex. B (for brown eyes) A Recessive Allele: – an allele whose presence can be masked by a Dominant Allele – always lower case letters ex. b (for blue eyes) – The allele for brown eyes is dominant over the allele for blue eyes Homozygous vs Heterozygous Since have at least 2 alleles for each Three possible combinations of alleles gene can have: BB = homozygous dominant ex. brown eye & brown eye alleles Bb = heterozygous ex. brown eye & blue eye alleles bb = homozygous recessive ex. blue eye & blue eye alleles Someone who displays the dominant phenotype but is heterozygous for a trait / disorder = carrier i.e. has the recessive allele (heterozygote) but does not have / express the disease (only ‘carries’ it) Sexual reproduction increases Genetic variation by ‘reshuffling’ genes The behavior of chromosomes during meiosis (gamete / sex-cell formation) and fertilization is responsible for most of the variation that arises in each generation The mechanisms that contribute to genetic variation (i.e. which chromosomes and genes end up in a gamete, and therefore which traits the offspring will inherit) are random Ex. One of the mechanisms is Random fertilization (during zygote formation / conception), i.e.:  Mutation = changes in DNA, original source of different alleles  SEXUAL Reproduction= alleles mixed & matched during meiosis (i.e. reshuffling of genes) Note: a Zygote is a fertilized egg  How Does Natural Selection Work? 4) Differential Reproductive success (survival of the fittest) – Individuals whose inherited traits (genes) give them an advantage / high probability of surviving and reproducing: likely to leave more offspring than other individuals – leads to a gradual change in a population, with favorable characteristics accumulating over generations How Does Natural Selection Work? Summary “Environment selects” Increasing adaptation of organisms to environment  How Does Natural Selection Work? Important Notes Natural selection occurs through interactions between individual organisms and their environment, but individuals do not evolve. – A population is the smallest group that can evolve. – Evolutionary change is measured as changes in relative proportions of heritable traits in a population over successive generations. Natural selection can act only on heritable traits,  traits that are passed from organisms to their offspring. Environmental factors vary from place to place and from time to time.  A trait that is favorable in one environment may be useless or even detrimental in another environment  How Does Natural Selection Work? Important Notes  Natural selection doesn't create perfect organisms;  it allows organisms to adapt to their environment (‘edits’ existing variations) Chance and Natural Selection interact  Evidence for Darwin’s Theory a) Natural Selection in action (examples, including ‘artificial selection’) b) Homology (anatomy, development and molecular) c) Convergent evolution (Analogous features) d) Biogeography e) Fossils a) Natural Selection in Action Example – Peppered Moth The peppered moth lives on the trunks of birch trees (white). Pre-Industrial Revolution, birds ate the black moths bec. more easily seen against white tree  inc. proportion of white  Two variants: moths white with black spots black with white spots Industrial Revolution darkened the white birch due to soot. White moths were more easily seen and birds ate more white moths  increase in proportion of black moths a) ‘Artificial selection’ (as seen with human influence on many plants, animals, etc.) Example 3 – Drug Resistance The use of drugs selects for pathogens that, through chance mutations are resistant to the drugs’ effects antibiotic resistant bacteria and bacteria and drug resistant HIV This selection is a cause of adaptive evolution – Researchers have developed numerous drugs to combat HIV using these medications selects for viruses resistant to the drugs  b) Homology: Anatomy Homology: the similarity resulting from common ancestry Homologous structures between organisms – anatomical resemblances that represent variations on a structural theme – present in a recent common ancestor but diverged to have different fucntions (divergent evolution) ex: Human arm, cat forelimb, whale front flipper, bat wing – strikingly similar arrangements of bones, muscles and nerves Vestigial organs are seemingly useless organs or structures (“leftovers from ancestors”) – indicate that the organism evolved from ancestors in which the organ was functional Ex. vestigial hind limb bones seen in large – tend to persist as there is no snakes and whales indicate that they both selective pressure to eliminate them evolved from tetrapod (4-legged) ancestors b) Homology: Development ex: all vertebrate embryos have structures called pharyngeal pouches (gill slits) in their throat at some stage in their development. – These embryonic structures develop into very different (divergent), but still homologous, adult structures, such as the gills of fish and parts of the ear for humans embryonic development of all vertebrates shows remarkable similarities Gill slits  b) Homology: Molecular ex: Universal genetic code of nucleotides in DNA and conserved sequence of amino acids in proteins Triplet (sequence of 3 nucleotides in DNA) codes for a particular codon (mRNA)  codes for a particular amino acid – Example: AAA (DNA)  UUU (mRNA)  which codes for phenylalanine in: shrimp, bacteria, humans, tulips…. Genetic Code: Same Universal code: evidence for a common in almost all organisms! ancestor  b) Homology: Molecular ex: Proteins: The numbers represent the number of amino acid differences between the beta polypeptide chain of human hemoglobin and the beta hemoglobins of the other species. The human polypeptide chain contains 146 amino acids, as do most of the others. In general, the number is inversely proportional to the closeness of kinship.  c) Convergent Evolution (Analogous) When characters are similar, but are not derived from a common ancestor = Analogous Analogous features demonstrate that organisms with separate ancestors may adapt in similar ways to similar environment Independent evolution of similar structures in distantly related organisms Some similar mammals that have adapted to similar environments -similar environments  evolved similar structures (longs sharp claws & elongated snouts) have evolved independently from different ancestors  d) Biogeography Species tend to be more closely related to other species from the same area than to other species that live in different areas  suggests a common ancestor adapts to various habitats Pangea: Continents once joined together, as time passed, each continental plate moved (plate tectonics) This resulted in populations becoming isolated in different environments  evolved ex: Australia's marsupials are a diverse group differently of animals developing in isolation (separate land mass for millions of years)  e) Fossil Record Succession of forms observed in the fossil record consistent with other inferences about the major branches of descent in the tree of life – ex: considerable evidence suggests that prokaryotes are the ancestors of all life and should precede all eukaryotes in the fossil record. In fact, the oldest known fossils are prokaryotes. Stromatolites  Natural Selection … Just A Theory? In science, a theory: – Accounts for many observations and data and attempts to explain and integrate a great variety of phenomena – Massive amounts of data support Darwin’s theory of natural selection The effects of natural selection can be observed and tested in nature Scientists continue to test this theory. Neo-Darwinism is the modern version of Darwinian evolutionary theory which incorporates Mendelian genetics. Neo-Darwinism postulates that natural selection acts on the heritable (genetic) variations within individuals in populations and that mutations (especially random copying errors in DNA) provide the main source of these genetic variations.  Lecture 2: Learning Objectives Evolution by Natural Selection - Define Evolution and Natural Selection - Know the views before Darwin (Aristotle, Old Testament, Linnaeus, Fossils, Hutton, Lyell, Lamarck) - Be able to explain how natural selection works (4 points) - Know where Darwin got his ideas from (4 main sources) - Be able to provide evidence for natural selection (examples, homology, convergence, divergence, anologies, biogeography and fossils) - Understand the importance of genes in evolution and how sexual reproduction (and mutations) contribute to genetic variability - Know the meaning of the scientific terms discussed (ex. chromosomes, gametes / sex cells, somatic cells, homologous chromosomes, alleles, recessive / dominant, genotype / phenotype, zygote, random fertilization, etc.) SUMMARY SLIDES  Sex Cells Somatic (regular body) cells (produced by (divide / produced by mitosis, Meiosis) i.e. a type of cell cloning) ZYGOTE Haploid Diploid cells (2n) = 46 chromosomes gametes (n) = 23 chromosomes  Genotype vs Phenotype Genotype – short hand notation or description for the combination of alleles these alleles present for the characteristic in a particular organism Phenotype – appearance of the characteristic ex. flower color = purple, eye color = blue, hair color = black NB. Phenotype does Ex. Flower Color: not always reveal Genotype Phenotype genotype! PP (Homozygous dominant) Purple pp (Homozygous recessive) White Pp (Heterozygous) Purple