History of natural selection

-natural theology

-Aristotle 382-322 BC

• scale of nature

James Hutton and Charles Lyall developed the theory of Gradualism which is supported by the fossil record.

Fossil record

Newer rocks have different shape to older rocks. Gradual changes were observed.

Jean Baptiste Lammarck

-developed the first major theory of evolution

• organisms could adapt to their environment

• changes in individuals were passed on from generation to generation (red bit turned out to be wrong)

Charles Darwin

-published “origin of species“

• collected data from his trip on HMS Beagle

• examples he found were “Darwin’s finches“

• Over 13 different species had evolved from a common ancestor and adapted to different ecological niches.

Darwin’s theory of natural selection: Descent by modification

1.Principle of Variation

• inherent variability in population

2.Principle of Heredity

• variation is passed down through generations

3.Principl of selection

• organisms tend to over multiply

• some individuals have a greater capacity to survive or reproduce

Evidence for natural selection

1. Biogeography

   • the adaptation of organisms to their environment e.g. Darwin’s finches

2. Fossil record

   • insert fossil picture

3. Comparative anatomy

   • evolve from common structures e.g. mammalian forelim

   • insert arm photo

   • evolve from common structure

   • insert second arm photo

4. Comparative embryology

   • common development structures e.g. human versus chick embryo

5. Similarities in DNA sequence show common origins

   • chimps and humans share 98% similarity in their DNA sequence

   • dogs and humans share 85% similarity in their DNA sequence

Principle of variation

1. Genetic polymorphism

   • changes to the DNA sequence

2. Phenotypic polymorphism

   • changes in the appearance or measurable characteristics.

poly = many

morphism = types of forms

Genotype - DNA sequence

Phenotype - function or appearance

variability due to changes in the DNA

Mechanisms that generate variation

1. Gene mutation

   • changes in DNA sequences in single or multiple genes e.g. eye colour mutants in Drosophilia

2. Chromosome mutation

   • changes in the umber of chromosomes e.g. down syndrome

3. Recombination

   • new arrangements caused by sexual reproduction

Principle of heredity

1. A species

   • all members of the same species can interbreed

   • not all have the opportunity

2. A population

   • A group of organisms within the same species that do have the opportunity to interbreed

Principle of selection

All organisms tend to over-reproduce

2. the best adapted out-compete others

   • “selective pressure“ is the term given to the factors that cause this competition.

Selective pressures

1. Evasion of predators

   • these best adapted evade predators more effectively e.g. the Pennered moth

2. Competition for food e.g. giraffe

3. Resistance to disease

   • some animals/people are more resistant to disease than others

   • e.g. African cattle more resistant to sleeping sickness parasites than European cattle

4. selection for metabolic fitness

   • some organisms utilise resources more efficiently than others

   • e.g. micro-organisms can adapt to a wide range of food sources

5. Sexual selection

   • partner choose mate

   • e.g. all types of mating displays

6. Resistance to toxins or drugs

   • organisms develop resistance to chemicals

   • e.g. antibiotic resistance in bacteria in hospitals

7. Artificial selection

   • humans have deliberately selected animals/plants

   • e.g. all domestic breeds of animals and crops

Principle of selection: changes in gene frequencies

Modes of selection

In a field of mice with natural variation in coat colour some mice may have very light colouration, other very dark but the majority have an intermediate colour. This can be depicted as a frequency distribution

Balanced selection

• sometimes more than one selective pressure acts at the same type but has opposite effects

• e.g. sickle cells Anaemia and malaria in Africa

• Sickle cell is a fatal human disease - negative selection but provides partial protection against malaria

• Malaria is a fatal human disease - negative selection

Modes of reproduction

1. Asexual organisms

   

• rapid reproduction

• limited variability - genetically “clonal“

• limited adaptability

• Higher variability

• Higher adaptability

• Lower reproductive rate

Speciation

formation of new species

• estimated about 100 new species are formed each year

• one mechanism involves diversifying selection

This is known as cladogenesis

clade = a group

genesis = birth of

cladogenesis = birth of groups

Speciation

• formation of new species

• one mechanism is called Anagenesis

  • A or An = without

  • “without the birth of groups“

Mechanisms of separation of populations

• often referred toas isolation of populations

• mechanisms that cause barriers to mating between populations

Genetic isolation of populations

1. Geographical isolation

• Geographical isolation causes the formation of new species in a different place this is known as ALLOPATRIC speciation

• When speciation occurs at the same place this is known as SYMPATRIC speciation

• (patric = place; allo = differeny; sym = same)

2. Habitat isolation

   • populations evolve to occupy the same space but use different resources

   • this is an example of sympatric speciation

   • e.g. Darwin’s finches

3. Behavioural isolation

• organisms evolve different behaviours to prevent mating between different species

• e.g. fireflies, males have a flashing light pattern and there are different patterns for different species

4. Temporal isolation

   • organisms evolve different timing to prevent mating between different species

   • e.g. western USA spotted skunks mate in summer and eastern USA spotted skunk mate in winter

5. Mechanical isolation

   • organisms evolve different mating apparatus to prevent mating between different species

   • e.g. spiders - male sex organs (the pedipalps) have to be the perfect shape… or else…

   • if the grooves on the male pedipalps are not the right shape he will get eaten

6. Gametic isolation

   • organisms evolve sperm and egg incompatibility to prevent mating between different species

   • e.g. fish - female lay eggs on a river bed and males spread sperm over the eggs, lots of possibilities for wrong sperm to meet wrong egg.

7. Hybrid sterility

   • sometimes different species can interbreed but they usually produce sterile off springs

   • e.g. mules are infertile off springs of female horse x male donkey

rth of groups