Evolution and Natural Selection

Evolution and Natural Selection Chapters 1.4-1.6, Bush

Introduction to Natural Selection • History of Evolutionary Thought • Theory of Natural Selection • Examples of Natural Selection

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 http://goldberg.history.ohio-state.edu/naturalselection/

Georges Cuvier (1769-1832) • leading palaeontologist of his time • found that many species have gone extinct

Charles Lyell (1797-1875) • geologist • Earth was way older than the 5000 years or so allowed according to Biblical chronology

Erasmus Darwin (1731-1802) • Charles Darwin’s grandfather • proponent of the theory that species change over time

Jean-Baptiste Lamarck (1744-1829) • 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 (1766-1834) • found that all species have the potential to create far more offspring than there are resources to support

Alfred Russell Wallace (1823-1913) • came up with the theory of natural selection independently of Darwin • spurred Darwin to publish his own work on the subject

Charles Darwin (1802-1882) “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.” (The Origin of Species) http://www.interaktv.com/Darwin/Darwin.html

Summary of the history of evolutionary thought

The “Theory” 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

Passing on genes is like tossing coins • 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)

Heritability of simple traits

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)

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

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 pop’n with variation for insecticide resistance • Insecticide appl’n kills all but those with resistance • Surviving insects breed new generation of insecticide resistance population

Natural selection can occur rapidly

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

Natural Selection reviewed

Natural Selection – continued • Characteristics of natural selection • Types of natural selection • Natural selection  Evolution

Characteristics of Natural Selection • Natural Selection: • dependent on the variation present in the population • Short-sighted – acts only present selection pressures

Sources of variation • Gene flow: immigration • recombination • ultimately, from mutation

Immigration leads to new variation • Immigration provides new genetic material for selection to act upon

Recombination creates variation in offspring

Mutation at the Phenotype Level • Mutations can be: • beneficial • detrimental • neutral

Mutation at the DNA Level • A mutation is caused when the chromosomal machinery makes a mistake

Mutagens • Many things may increase the mutation rate: • radiation • certain chemicals (e.g. carcinogens)

Variation is random • 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

Natural Selection – continued • Characteristics of natural selection • Types of natural selection • Natural selection  Evolution

Types of Natural Selection • Three kinds of natural selection: • Directional selection • Stabilizing selection • Disruptive selection

Evolution and Natural Selection