Evolution and Natural Selection Chapters 1.4-1.6, Bush.

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

Introduction to Natural Selection

History of Evolutionary Thought

Theory of Natural Selection

Examples of natural selection

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

Introduction to Natural Selection

History of Evolutionary Thought

Theory of Natural Selection

Examples of natural selection

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

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

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

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

Directional Selection

Larger individuals may have higher fitness (i.e., produce more offspring) than smallerindividuals.

Directional Selection

Fishing industry

produces selection that favours smaller cod and can produce a decrease in average body size.

Stabilizing selection

The average members of the population may

have higher fitness than the extremes.

Stabilizing Selection

Babies of intermediatebirth weight have highersurvivorship than very small and very large babies

Disruptive selection

Natural selection could favour both extremes over the intermediate types

Disruptive Selection

In the finch, Pyrenestes ostrinusboth very large and very small bills are beneficial for eating large and smallseeds, respectively

Natural Selection – continued

Characteristics of natural selection

Types of natural selection

Natural selection Evolution

Selection pressures may conflict

Other factors in 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

Chance events influence evolution

Summary

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

"nothing in biology makes sense except in the light of evolution"-Theodosius Dobzhansky (1900-1975)