Paving the way for Darwin

Transcription

1 Evolution and Natural Selection Introduction to Natural Selection History of Evolutionary Thought Theory of Natural Selection Examples of Natural Selection Chapters , Bush Introduction to Natural Selection History of Evolutionary Thought Theory of Natural Selection Examples of natural selection Paving the way for Darwin Charles Darwin s theory relied upon the findings of other scientists Casting doubt on Divine Creation Cuvier, Georges Lyell, Charles Darwin, Erasmus Contributing to the theory itself Lamarck, Jean-Baptiste Malthus, Thomas Robert Wallace, Alfred Russell Georges Cuvier ( ) Charles Lyell ( ) leading palaeontologist of his time geologist found that many species have gone extinct Earth was way older than the 5000 years or so allowed according to Biblical chronology

2 Erasmus Darwin ( ) Charles Darwin s grandfather proponent of the theory that species change over time Jean-Baptiste Lamarck ( ) Believed scientists like Erasmus Darwin that life forms could change over time Lamarckism: acquired traits can be inherited e.g. a giraffe with a short neck stretches to get at vegetation high up a tree and manages to make its neck longer. This giraffe passes its long neck to its offspring got Darwin thinking about inheritance Thomas Malthus ( ) Alfred Russell Wallace ( ) found that all species have the potential to create far more offspring than there are resources to support came up with the theory of natural selection independently of Darwin spurred Darwin to publish his own work on the subject Charles Darwin ( ) Summary of the history of evolutionary thought I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection. (The Origin of Species)

3 Introduction to Natural Selection The Theory of Natural Selection History of Evolutionary Thought Theory of Natural Selection Examples of natural selection Natural Selection is a Theory in the same way that we consider gravity or Einstein s relativity to be a theory Theory of Natural Selection Three conditions for Natural Selection: 1) Variation in traits 2) Heritability 3) Survivorship/Competition Natural selection Survival of the fittest Variation and Heritability Observations from Lamarck and Erasmus Darwin that offspring are not exactly like parents (change can occur in a single generation) Observed the commonly known facts that: all individuals are not alike (i.e., there are different phenotypes) Offspring inherit the majority of their traits from their parents. Variation within a species Variation can be: CONTINUOUS: having a multitude of variants (e.g., colour bands in the snail) DISCRETE: limited # of types (such as blood types) Heritability in Diploids Two copies of each gene (diploid) Humans have 23 chromosomes, 2 copies of each, for a total of 46 chromosomes) Each egg or sperm has only one copy of each chromosome

4 Passing on genes is like tossing coins Heritability of simple traits Two copies exist for each gene Whether you pass on a certain copy of a gene is an independent event for each child If you have two children, sometimes you will pass on the same copy to both children (leaving the second copy passed on to neither child) Competition From Malthus: more offspring are produced than there are resources to support Creates a struggle for existence Some offspring will be better at surviving and reproducing than others (i.e., have higher fitness) Fitness FITNESS: the number of offspring an individual produces that survive to reproduce themselves Fitness = 1.0 means that individuals of this phenotype are successfully passing on 100% of their genes, on average How is fitness calculated Fitness = the number of genes passed on to the next generation Because diploid organisms (i.e., most organisms) only pass on half of their genes to each child, they must have two offspring living to reproductive age to have Fitness = 1 Fitness = 1 does not exactly mean that you have passed on 100% of your genes to the next generation (Remember: sometimes you send two copies of the same gene and zero copies of the other) Outcome Some phenotypes will be better represented in the next generation than they are in the present generation Could be extended: some entire lineages may be more successful than others as well resulting in some lineages going extinct (as Cuvier had found)

5 Natural selection will not take place if: there is no variation E.g., No humans have gills, so we cannot select for them, regardless of how beneficial they might be If the gene is not heritable E.g., Working out and getting a strong heart might make you live longer and have more children but selection can not act upon it if is not a genetic trait If there is no difference in survivorship or reproductive ability between variants E.g., Having attached or free earlobes doesn t really matter Survival of the fittest This saying is a bit misleading and doesn t quite capture the essence of what is natural selection You can be as fit an individual as can be but it is the ability to reproduce that is the key feature for an increase in representation in the next generation Aside: Darwin s nemesis was genetics! Gregor Mendel father of genetics conducted experiments on pea plants discovered that most organisms have two copies of their genes, one from each parent. Darwin never read Mendel s Paper Introduction to Natural Selection History of Evolutionary Thought Theory of Natural Selection Examples of natural selection

6 Ground Finch (Geospiza fortis) beak size has a lot to do with how well a finch feeds on certain seeds seeds of Tribulus have the toughest seed coat that requires a large beak to break Natural selection in finches Drought causes collapse of food supply, survival plummets High mortality in smaller individuals, strong selection for large birds that can crack large, tough seeds Human-induced selection Natural selection can occur rapidly 1) Natural pop n with variation for insecticide resistance 2) Insecticide appl n kills all but those with resistance 3) Surviving insects breed new generation of insecticide resistance population Rock plants Summary Darwin put together a number of ideas from different disciplines to come up with the Theory of Natural Selection Natural selection states that heritable phenotypes that are well-suited to their environment will have more offspring and so will be better represented in the next generation. Natural selection can operate so quickly that we can observe it in a single generation

7 Natural Selection reviewed Natural Selection continued Natural Selection continued Characteristics of natural selection Types of natural selection Natural selection Evolution Characteristics of natural selection Types of natural selection Natural selection Evolution Characteristics of Natural Selection Natural Selection: Sources of variation Gene flow: immigration dependent on the variation present in the population Short-sighted acts only present selection pressures recombination ultimately, from mutation

8 Immigration leads to new variation Recombination creates variation in offspring Immigration provides new genetic material for selection to act upon Mutation at the Phenotype Level Mutation at the DNA Level Mutations can be: beneficial detrimental neutral A mutation is caused when the chromosomal machinery makes a mistake Mutagens Variation is random Many things may increase the mutation rate: radiation certain chemicals (e.g. carcinogens) When a new recombinant or mutant genotype arises, there is no tendency for it to arise in the direction of improved adaptation Natural selection imposes direction on evolution, using undirected variation

9 Natural Selection continued Characteristics of natural selection Types of natural selection Types of Natural Selection Three kinds of natural selection: Directional selection Stabilizing selection Disruptive selection Natural selection Evolution Directional Selection Directional Selection Larger individuals may have higher fitness (i.e., produce more offspring) than smaller individuals. Fishing industry produces selection that favours smaller cod and can produce a decrease in average body size. Stabilizing selection Stabilizing Selection The average members of the population may have higher fitness than the extremes. Babies of intermediate birth weight have higher survivorship than very small and very large babies

10 Disruptive selection Disruptive Selection Natural selection could favour both extremes over the intermediate types In the finch, Pyrenestes ostrinus both very large and very small bills are beneficial for eating large and small seeds, respectively Natural Selection continued Selection pressures may conflict Characteristics of natural selection Types of natural selection Natural selection Evolution Other factors in evolution Chance events influence evolution If there is no relation between fitness and the character in question, then natural selection is not acting on it Chance events can still make these traits show change over time = RANDOM DRIFT

11 Summary "nothing in biology makes sense except in the light of evolution" -Theodosius Dobzhansky ( ) Natural Selection acts on whatever variation is present at the time. This variation is generated randomly with respect to selection pressures Selection can be directional, stabilizing or disruptive Random factors can also play a part in evolution