NATURAL SELECTION Explaining Evolution

THE ORIGIN OF SPECIES

Darwin published “On the Origin of Species” in 1859

He had spent 20 years amassing evidence and developing his theory of NATURAL SELECTION

NATURAL SELECTION The way in which nature favours the

reproductive success of some individuals within a population over others

Over time the population changes as advantageous heritable characteristics become more common generation after generation

Evolution is the result of natural selection occurring over many generations

“SURVIVAL OF THE FITTEST”

A term coined by Herbert Spencer to describe the process of natural selection

The key to natural selection is an individual’s REPRODUCTIVE SUCCESS and its ability to ADAPT

ADAPTATION: a characteristic or feature of a species that makes it well suited for reproductive success and survival

EVALUATING A THEORY

Darwin’s theory of evolution by means of natural selection is able to: Explain how adaptation can arise in a species

Make predictions about the future evolution of a species

Be a testable scientific theory

MODERN THEORY OF EVOLUTION

Evolutionary biology has made tremendous advances since Darwin because of the following: RADIOMETRIC DATING MODERN EVOLUTIONARY SYNTHESIS

PALEONTOLOGY

RADIOMETRIC DATING

Involves the use of radioisotopes to obtain precise estimates of the ages of rocks

RADIOISOTOPE: an atom with an unstable nucleus that is capable of undergoing radioactive decay

HALF-LIFE: the time required for half the quantity of a radioactive substance to undergo decay; the half-life is constant for any isotope

MODERN EVOLUTIONARY SYNTHESIS

The modern theory of evolution that takes into account all branches of Biology

Involves changes in the GENE POOL of a species over time

GENE POOL: the complete set of all alleles contained within a species or a population

RECALL: genetic mutations provide a continuous supply of new heritable information and variation within a species

PSEUDOGENES Vestigial genes that no

longer code for functioning proteins

Are found in virtually all species EXAMPLE: Dolphins

have genes that code for smell however they have no need for a sense of smell

MODERN PALEONTOLOGY

We have made many important discoveries over the last 100 years: Fossils of early human ancestors in

Pakistan Feathered dinosaurs in China

PLATE TECTONICS: the scientific theory that describes the large scale movements and features of Earth’s crust

Explains species distributions

TYPES OF SELECTION There are FOUR types of

selection:1. DIRECTIONAL2. STABILIZING3. DISRUPTIVE4. SEXUAL

DIRECTIONAL SELECTION

A selection that favours an increase or decrease in the value of a trait from the current population average

Common in artificial selection where individuals with an enhanced trait are selected

EXAMPLE: A habitat with long flowers will favour hummingbirds with longer bills, and thus create future generations of birds with longer bills

STABILIZING SELECTION

A selection against individuals exhibiting traits that deviate from the current population average

EXAMPLE: An environment with medium length flowers will select against long billed or short billed hummingbirds

DISRUPTIVE SELECTION

A selection that favours two or more variations of a trait that differ from the current population average

EXAMPLE: An environment with long and short flowers will select against hummingbirds with medium sized bills

SEXUAL SELECTION The favouring of

any trait that specifically enhances the mating success of an individual

Results in males and females of a species differing in behaviour and appearance

SELECTION AT WORK EXAMPLE: Polar bears and white fur

Ability to sneak up on seals on snow covered ice

EXAMPLE: Wolves keen sense of smell Ability to locate and track the

movements of prey EXAMPLE: A human’s large brain

Ability to reason and communicate

GENETIC DRIFT Changes to allele frequency as a result

of chance Such changes are much more

pronounced in small populations Can result in the allele becoming very

common or disappearing entirely over a number of generations

GENETIC BOTTLE NECK

A dramatic, often temporary reduction in population size

Usually results in significant genetic drift and a loss of genetic diversity

EXAMPLE: Cheetahs

HARDY-WEINBERG PRINCIPLE

In large populations in which only random chance is at work, allele frequencies are expected to remain constant from generation to generation

Evolution occurs: If natural selection occurs In a small population If there is a mutation There is immigration or emigration If there is a gaining of new alleles from a

different species

HUMAN INFLUENCE EXAMPLE: Commercial fishing

The alleles that code for large adult sized cod are being lost

EXAMPLE: Insecticide use Bedbugs are becoming resistant to

pesticides EXAMPLE: Antibiotic and antimicrobial use

Many infectious bacteria (MRSA) are becoming resistant to a variety of antibiotics