evolutionary basics

Evolutionary Basics

What was the view of the world and nature before Darwin?

1. Static UniverseThe universe didn’t change through time

Problem - Fossil Evidence


1. Static Universe2. Earth Centred Universe

Galileo - showed that this was wrong


1. Static Universe2. Earth Centred Universe3. Great Chain of Being

Problem - how do you incorporate new species?


1. Static Universe2. Earth Centred Universe3. Great Chain of Being4. Argument from Design

Each species was designed for a specific purpose

Problem:

How did this ‘traditional’ view (or Natural Theology)apply to Biology?

1. Argument from Design

The design of all organisms showed thatthere was an intelligent and benevolent Creator

BUT….How do disease organisms fit into this scheme ?

Ebola



The design of all organisms showed thatthere was an intelligent and benevolent Creator

OR….Why did some species go extinct?



William Paley(1743 – 1805)


1. Argument from Design2. Relationship between Species

(Great Chain of Being)

BUT ..vulnerable to extinction


1. Argument from Design2. Relationship between Species3. Fixed Species and Relationships

How do you incorporate new species?

General Summary:

The world/universe was designed by a benevolent Creator to function perfectly and its form and function were fixed through all time.

But Evolution is about change

What were the pre-Darwinian ideas of change through time?

Pre-Darwinian Ideas of Organic Change

1. Georges Louis Leclerc, Comte de Buffon (1707-1788)

Species - a distinct group maintained by reproduction

Ancestor

Different species

Time

Local Conditions


1. Georges Louis Leclerc, Comte de Buffon (1707-1788)2. Jean-Baptiste Lamarck (1744-1829)



Lamarck’s ideas:1. Spontaneous generation



Lamarck’s ideas:1. Spontaneous generation2. Ascent up the scale of nature

Time

Complexity of the organism

Different species



Lamarck’s ideas:1. Spontaneous generation2. Ascent up the scale of nature3. Acquired characteristics

Originators of Modern Theories of Natural Selection

Alfred Russell WallaceCharles Darwin

Voyage of HMS Beagle

Darwin’s Finches - Geospiza

Galapagos tortoise - Geochelone

Contributing Elements to Darwin’s theory

1. Charles Lyell (1797 - 1875)

Gradualism (Uniformitarianism)

All change through time can be explained by processes at work today

No need to invoke catastrophic events


1. Charles Lyell (1797 - 1875)2. Thomas Malthus (1766 - 1834)

Populations of organism will growfaster than their food supply

Population

Food supply

Number

Time


1. Charles Lyell (1797 - 1875)2. Thomas Malthus (1766 - 1834)3. Plant and Animal Breeders

-showed that the form of a species could be changed over time

Logic of Darwin’s Theory of Natural Selection

(or Descent with Modification)

Observation Deduction1. All organic populations

can exponentially.

2. In spite of Obs. 1, they don’t.

1. There is some kind of struggle for existence.

3. All members of a species are not the same.

4. Differences in individuals are passed to their offspring.

2. Some members of a species are better equipped to survive and reproduce than others.

This differential reproduction/survival is natural

selection

Journal of the Proceedings of the Linnean Society of London. Zoology 3 (20 Aug.): 45-62

Definition of EvolutionChanges over time of the proportion of individuals differing genetically in one or more traits

These changes can occur by:• Changes in frequency of alleles and/or phenotypes in a

population

Evolution is the pattern of change over time.

• Changes in the proportion of different populations in a species

• Changes in the number of species in a larger taxonomic group

Changes over time of the proportion of individuals differing genetically in one or more traits

**PATTERN**

Natural SelectionDifferential success in the reproduction of different phenotypes

resulting from the interaction of organisms with their environment.

**PROCESS**

How Natural Selection Works

For any alleles of a trait:

frequency of the dominant allele is ‘p’

and the frequency of the recessive allele is ‘q’

And p + q = 1

(p + q ALWAYS equals 1)

And if you mate two organisms, you can mathematicallydetermine the expected proportion of offspring of each type

p + qp + q

p2 + 2pq + q2

In a simple organism, p = q = 0.5

and p2 + 2pq + q2

= (0.5)(0.5) + 2 (0.5)(0.5) + (0.5)(0.5)

= .25 +.5 +.25

This is the familiar 1:2:1 genotypic ratio for a simple monohybrid cross

p2 2pq q2

AA Aa aa 1:2:1 – genotypic ratio

3:1 – phenotypic ratioA_ aa

This idea holds true for any value of p or q.

For example:

If p is very common - say 90% of the genes in the population

Then p = .9 and q = .1

And p2 = .81 (the frequency of the AA genotype)

2pq = .18 (the frequency of the Aa genotype)

q2 = .01 (the frequency of the aa genotype)

and 99% will have the A_ phenotype

In the early 1900’s, Hardy and Weinberg used this idea to establish a fundamental idea in the genetic basis of natural selection

The Hardy-Weinberg Equilibrium

Assume that p = .6 and q = 0.4

In Generation 1 p2 + 2pq + q2 = .36 + .48 + .16




•••

Hardy-Weinberg Equilibrium

assumes:

In any population, allelic and genotypic frequencies will remain the same if Mendelian inheritance patterns are the only factors at work

Very large population size

No gene flow

No mutations

Random mating No natural selection


1. Large population sizes

What happens if the population isn’t ‘large’?

Genetic Drift - a statistic consequence of small populations




BottlenecksGenetic Drift




BottlenecksGenetic Drift

Founder effect

How do we model this?

Frequency of AAFrequency of Aa

Frequency of aa

Imagine that ‘A’ mutates to ‘a’ at a rate of m per generation

Frequency of A after one generation of mutation

Frequency of A after a second generation of mutation


2. Mutations - source of all new genetic variation


Imagine that ‘A’ mutates to ‘a’ at a rate of m per generation

Frequency of A after one generation of mutation

Frequency of A after a second generation of mutation Substitute

For any number of generations (x)


3. Random mating

This assumes no preferences in mates

Humans: PreferencesHeight - we tend to mate with people closer to our own height

2. Mutations - source of all new genetic variation



4. Natural Selection

-depends on variability that is heritable-differences must be passed to the offspring

Key idea : Fitness: The contribution an individual makes to the gene pool of the next generation relative to other individuals

3. Random mating2. Mutations - source of all new genetic variation


Lower fitness Higher fitness

Types of Natural Selection

Most traits have a normal (or bell curve distribution)


1. STABILIZING SELECTION


1. STABILIZING SELECTION

Human birth weight


2. DIRECTIONAL SELECTION


2. DIRECTIONAL SELECTION

Salmon fishing - largest fish are taken every year


3. DISRUPTIVE SELECTION

Modelling Natural SelectionSelect against recessive homozygote - aa

Initial Frequency p2 2pq q2 1

AA Aa aa Total

Fitness 1 1 1 - s

Next Generation 1 1 q2(1 – s) 1 – sq2

Normalized

evolutionary basics

Documents

traditional view

lamarcks ideas

predarwinian ideas of

natural theologyapply

jeanbaptiste lamarck

new species

designeach species

speciesfixed species