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Evolution by Natural Selection - Evolutionary Biology - Lecture Notes, Study notes of Evolutionary biology

Acharya Nagarjuna UniversityEvolutionary biology

Its important key points of lecture notes of Evolutionary Biology are : Evolution by Natural Selection, Rationalistic Approach, Logical Deduction, Darwin Thinking, Groups of Species, Origin of Species, Population Growth, Building Blocks, Darwin Theory

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Charles Darwin and the development of
Evolution by Natural Selection
I. Charles Darwin (1809-1882).
Introduction –Read and digested Lyell’s Principals of Geology, and became involved with Lyell’s empiricism; the
necessity to EXPLAIN FROM OBSERVATION, rather than TO it (as favored by the “rationalistic approach of
“logical deduction.”
a. This may in part explain the mass of detail that is the strength of Darwin’s book, as it was Lyell’s.
b. Both men restricted themselves to empirical data, data that could be observed.
A. CD Biographical sketch.
1. Born the same day as Lincoln, to prominent (but not aristocratic) family in Kent, England.
2. Education - Darwin was not a splendid student.
a. Went to Edinburgh to study medicine, washed out, partly because he couldn't stomach the brutal conditions.
b. Then to Christ College, Cambridge, with intention of studying for the clergy. Natural theology was the
accepted mode.
c. (1831) J.S. Henslow, (botanist sets him up as civilian naturalist on the surveying ship H.M.S. Beagle,
commanded by Capt. Robert FitzRoy.
2. He took only 2 books with him; Paley's Natural Theology, which argued for a literal interpretation of natural
history in light of the bible, and Lyell's Principles of Geology which argued for interpretation from visible
evidence.
a. He left believing implicitly in the former, and returned a convert to the latter.
II. Influences of the voyage on Darwin’s thinking.
A. Human-
1. Darwin got to see the range of "adaptiveness' in humans; Feugean natives educated in England.
B. Geology.
1. Noted that fossils he collected resembled (but were not identical to) living organisms in the same region.
Not consistent with Catastrophism.
D. Biology
1. Galapagos - Tremendous variations among what were clearly similar species.
a. Tortoises on different islands have different shells; so much so the locals could tell at a glance which island
a tortoise came from.
b. Groups of species (like Darwin's finches) which are obviously related, but are ecologically very different.
2. South American flora and fauna from different regions were distinct from the flora and fauna of Europe.
3. He also noted that temperate species of South America were taxonomically closer to species living in tropical
regions of South America than to temperate species of Europe.
4. This extended to fossil record. In Patagonia he collected giant fossil ground sloths, elephants,llamas and
armadillos- all clearly extinct, but very much akin to he living animals of the area.
III. The Origin of Species
1. Discusses population growth with Robert Thomas Malthus.
a. Malthus was among first to recognize that population growth is a multiplicative (or exponential)
b. Darwin recognized that any species has a reproductive capacity high enough to rapidly overpopulate its
environment, BUT THEY DON'T.
2. This combined with his observations of nature gave him the evolutionary mechanism he was looking for - he
eventually called it natural selection. Sits on it. Starts writing in 1856, when….
3. In 1858, he received a letter from Alfred Russell Wallace asking his advice about a paper he (ARW) was
planning to write which outlined a theory of evolution essentially identical to CD's.
5. He nearly gave up, thinking himself scooped. Lyell intervened, persuaded CD to publish his theory
6. CD then put aside his big book on natural selection, and in less than a year, wrote a 513 pp manuscript, called it
"The Abstract". That was published, became an instant bestseller (1st printing sold out in one day). Title: The
Origin of Species by Means of Natural Selection, or the preservation of favoured races in the struggle for life.
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Charles Darwin and the development of Evolution by Natural Selection I. Charles Darwin (1809-1882). Introduction –Read and digested Lyell’s Principals of Geology, and became involved with Lyell’s empiricism; the necessity to EXPLAIN FROM OBSERVATION, rather than TO it (as favored by the “rationalistic approach of “logical deduction.” a. This may in part explain the mass of detail that is the strength of Darwin’s book, as it was Lyell’s. b. Both men restricted themselves to empirical data, data that could be observed. A. CD Biographical sketch.

  1. Born the same day as Lincoln, to prominent (but not aristocratic) family in Kent, England.
  2. Education - Darwin was not a splendid student. a. Went to Edinburgh to study medicine, washed out, partly because he couldn't stomach the brutal conditions. b. Then to Christ College, Cambridge, with intention of studying for the clergy. Natural theology was the accepted mode. c. (1831) J.S. Henslow, (botanist sets him up as civilian naturalist on the surveying ship H.M.S. Beagle, commanded by Capt. Robert FitzRoy.
  3. He took only 2 books with him; Paley's Natural Theology, which argued for a literal interpretation of natural history in light of the bible, and Lyell's Principles of Geology which argued for interpretation from visible evidence. a. He left believing implicitly in the former, and returned a convert to the latter.

II. Influences of the voyage on Darwin’s thinking. A. Human-

  1. Darwin got to see the range of "adaptiveness' in humans; Feugean natives educated in England.

B. Geology.

  1. Noted that fossils he collected resembled (but were not identical to) living organisms in the same region. Not consistent with Catastrophism.

D. Biology

  1. Galapagos - Tremendous variations among what were clearly similar species. a. Tortoises on different islands have different shells; so much so the locals could tell at a glance which island a tortoise came from. b. Groups of species (like Darwin's finches) which are obviously related, but are ecologically very different.
  2. South American flora and fauna from different regions were distinct from the flora and fauna of Europe.
  3. He also noted that temperate species of South America were taxonomically closer to species living in tropical regions of South America than to temperate species of Europe.
  4. This extended to fossil record. In Patagonia he collected giant fossil ground sloths, elephants,llamas and armadillos- all clearly extinct, but very much akin to he living animals of the area.

III. The Origin of Species

  1. Discusses population growth with Robert Thomas Malthus. a. Malthus was among first to recognize that population growth is a multiplicative (or exponential) b. Darwin recognized that any species has a reproductive capacity high enough to rapidly overpopulate its environment, BUT THEY DON'T.
  2. This combined with his observations of nature gave him the evolutionary mechanism he was looking for - he eventually called it natural selection. Sits on it. Starts writing in 1856, when….
  3. In 1858, he received a letter from Alfred Russell Wallace asking his advice about a paper he (ARW) was planning to write which outlined a theory of evolution essentially identical to CD's.
  4. He nearly gave up, thinking himself scooped. Lyell intervened, persuaded CD to publish his theory
  5. CD then put aside his big book on natural selection, and in less than a year, wrote a 513 pp manuscript, called it "The Abstract". That was published, became an instant bestseller (1st printing sold out in one day). Title: The Origin of Species by Means of Natural Selection, or the preservation of favoured races in the struggle for life.

IV. The building blocks to Darwin's theory. A. Variation under domestication.

  1. Individuals of domesticated species are always variable - individuals differ.
  2. This variation is inherited - offspring bear a non-random resemblance to parents.
  3. Artificial selection - proves inheritance; also analogy for evolution in nature. a. Small changes each generation accumulate and can lead to great changes. b. Breeders have caused changes so great different breeds of animals resemble different species. c. Some so great the ancestors of modern varieties are uncertain.
  4. If artificial selection can operate in such a short period of time, then natural selection should be capable of considerable modifications over hundreds of thousands of years.

B. Variation under nature.

  1. He finds an astonishing array of variation in barnacles and had great difficulty in classifying them into species and subspecies.

C. Heritability

  1. Heritability - the proportion of the variation observed in a population that is due to variation in the effects of genes. a. That is, the estimate of heritability attempts to eliminate the influence of environment on phenotypic expression.
  2. Heritability is determined via regression analysis. a. Regression analysis provides an r^2. This tells you how much of the variation in Y is a result of X. E.g. That is, of the variation in beak depth of offspring, how much can be explained by the beak depth of the parentals.

D. The struggle for existence. (And excess offspring)

  1. Darwin recalls Malthus.
  2. He observed nature - not uncontrolled growth. How come? Most individuals born don't survive.
  3. Population size reduced by ecological factors.
  4. On average, the most fit individuals pass their genes onto more offspring than less fit individuals.

E. Is Survival and Reproduction Nonrandom?

  1. Great question by Darwin! He considered the null hypothesis. We the observations he was making solely due to chance alone? a. Natural selection requires more than surviving, it means surviving to reproduce. And it means reproducing more than other phenotypes.
  2. Example of Geospiza fortis – the work of Peter and Rosemary Grant (and students, e.g. Peter Boag)

F. This leads to-: Natural selection.

  1. Natural selection is the differential success in reproduction, and its product is adaptation of organisms to their environment. "This preservation of favorable variations and the rejection of injurious variations, I call Natural Selection."
  2. Even slight advantages of some variations will accumulate in a population over many generations of being perpetuated by natural selection.
  3. Natural selection thus occurs from the interaction between the environment and the inherent variability in a population.
  4. Variations in a population arise by chance, but natural selection is not a chance phenomenon since environmental factors set definite criteria for reproductive success.

VI. Problems with the theory of Natural Selection-What Darwin didn’t know. A. Source of variation.

  1. Darwin did not know the biological basis for heredity or its variations, and his arguments were weakest in these areas. B. Laws of inheritance- the demise of Neo-Lamarckism
  2. If natural selection was to work, traits had to be hereditary. 2 Recognized this as the greatest weakness of theory. He relied on blending inheritance in 1st ed, was shot down - blending inheritance reduces variability.
  3. CD needed something that would maintain variability -" pangenesis " a. pangenesis - all parts of the body produced hereditary units, which traveled via the circulatory system to the reproductive organs for transfer to the next generation. Shot down in face of experimental evidence.
  4. August Weismann (1834-1914) chopped the tails off of 22 generations of mice, and the mice still bore tailed offspring (could be argued that it test Lamarck's hypothesis of disuse). a. Weisman offered the germplasm theory - that the heritable genetic material was sequestered in germplasm , and that the body ( somatic tissue ) had nothing to do with inheritance. Here comes the origins of genotype and phenotype.

C. Gregor Mendel.

  1. Mendel's experiments - used garden pea, successfully deduced laws of heredity by choosing simple discrete characters ( e.g. flower color), keeping detailed records of his experimental results, and mathematical analysis (unusual at the time). Major result: inheritance is particulate. a. Mendel demonstrates that traits are discretely heritable. Disproves blending theory.
  2. Mendel lucked out. Peas had no surprises in their reproductive system. (R.A. Fisher demonstrated that Mendel had a 1:14000 chance of getting the results he reported. a. Mendel also worked with Hawkweed ( Hieraceum ) which was a total failure. Hawkweed is apomictic (asexual).

D. Orthogenesis –straight-line evolution.

  1. Espoused by paleontologists who discovered progressive trends in the fossil record.
  2. The variation that arises is directed toward fixed goals, and that a species evolves in a predetermined direction without the aid of selection.
  3. Such trends need not be adaptive, and could drive a species toward extinction. (contrast with teleology)
  4. E.g. Irish elk ( Megaceros giganteus ) reputably became extinct because it could not stop evolving larger and larger antlers which became too unwieldy for the beast.
  5. No mechanism was proposed for orthogenesis and it was discarded during the Evolutionary Synthesis.

F. Mutationism – based on the principles of Mendelian Genetics.

  1. Darwin argued that slight differences among individual that we see results in continuous variation in a population.
  2. The Mendelians followed Mendel in emphasizing discrete variations, inherited as single factors. Continuous variation had no genetic basis.
  3. DeVries asked where variation came from. a. Worked with evening primrose ( Oenothera lamarckiana ), an apt omen. b. Coined the word mutation for variants of evening primroses that appeared spontaneously and differed substantially from parents. He considered these to be new species. c. Ergo, mutation was sufficient to account for the origin of species, natural selection was superfluous.
  4. Mendelians therefore believed that evolution proceeded in jumps by macromutations.
  5. The snapper was the experimental organism – Mendel was lucky, DeVries was not. a. Oenothera (in hindsight) has heterologus translocations in its chromosomes – portions of chromosome are exchanged, linking the chromosomes together. In Oenothera , all 7 chromosomes of the haploid complement are linked together, and in meiosis, they travel as a unit. b. Two morals here. 1) Beware of extrapolation a theory from a single organism, especially if you don’t know its biology. 2) In hindsight, do population studies – which DeVries did not.

VII. The New Synthesis A. The people

  1. During the early 1900’s, the work of Mendelian genetics laid the groundwork for the understanding of how evolution could operate.
  2. Systematists and paleontologists were at odds with the geneticists
  3. The evolutionary synthesis or modern synthesis was the reconciliation of Mendelian genetics with the mechanisms of natural selection. a. R.A. Fisher, Sewell Wright and J.B.S. Haldane all contributed. b. Theodosius Dobzhansky is given credit because as a practical person, a naturalist and a systematists, he possessed many of the same attributes that made Darwin a great synthesizer. c. He masterfully united the theory of population genetics with the systematists’ and naturalists’ view of the discreteness of species in his 1937 book, Genetics and the Origin of Species. d. The classic works of theoretical population genetics demonstrated that natural selection could work with the kinds of variation observable in natural populations and the laws of Mendelian inheritance. e. No other processes were needed, nor are the inheritance of acquired characteristics, directed variation, or macromutations that produced ‘hopeful monsters’ necessary.
  4. In 1940, these scientists were able to fully integrate genetics and Darwinian theory to argue persuasively that mutation, recombination, natural selection, and other processes operating within species account for the major, long-term features of evolution (macroevolution).
  5. Twenty tenets were set down, involving contributions from laboratory genetics, population genetics, natural history, systematists and paleontologists.