Natural Selection and Variation

Objectives • Define Variation• Define Natural Selection • Explain the importance of heritable variation

to selection.• Environmental factors as forces for natural

selection

Variation• Variation is all the differences which exist

between members of the same species. • Variation of characteristics in a population is

due to both genetic and environmental factors• For each characteristic, the population shows

either continuous or discontinuous variation.

Continuous Variation• Continuous variation has a broad range of

measurements, with many intermediates, from one extreme to the other.

• Characteristics that show continuous variation exists on a continuum.

• E.g. Height; hand span, shoe size; milk yield, leaf length, skin colour.

• Continuous variation is the combined effect of many genes (polygenic inheritance)

Continuous Variation

• The graph showing a characteristic of continuous variation is a bell-shaped curve known as a normal distribution

Discontinuous Variation• This is where individuals fall into a number of distinct

classes or categories, and is based on features that cannot be measured across a complete range.

• An individual either have the characteristic or don't. • A bar graph is used to represent a discontinuous

variation.• Controlled by alleles of a single gene or a small

number of genes. The environment has little effect on this type of variation.

• E.g: blood type, eye colour, tongue rolling,

Types of Variation

Heritable Variation.

• Heritable variation makes possible for• Weeding out negative mutations, passing on

positive and neutral ones. • Without heritable variation, species would

quickly fall victim to parasites who take advantage of identical genetic material in a population and evolution would not occur.

Natural Selection

• Organisms that are best adapted to an environment survive and have greater reproductive success than others.

• Process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits.

The Environment: A force for natural selection

• The key factor for determining survival is adaptation to the environment.

• Environments are not constant, they change with time. The individuals with traits that are able to adapt to these changes have a higher chance of surviving and reproducing.

Selective Survival of Peppered Moths

• Peppered Moth: This species consist of light gray and dark colouration.

• Before the Industrial Revolution, there were far more light gray moths than dark moths.

• With the Industrial revolution, and an increase of industrial pollution of soot, the number of dark moths far outnumbered the light gray moths.

• Why? Before the industrial pollution, moths could easily camouflage in the clean environment, whereas birds could easily catch the dark moths on clean trees.

• Increase in industrial pollution, the dark moths could now camouflage easily against the soot polluted background, while the light moths stood out.

Peppered Moth

• Which moth will the bird catch?

A

B

Rise of Antibiotic Resistant Bacteria• Antibiotics are natural products of bacteria and fungi that

is harmful to other bacteria and fungi.• Antibiotic Resistance: the ability of a microorganism to

withstand the effects of specific antibiotics• How: After the discover y of antibiotics (penicillin),

millions of lives were saved. However, the overuse of antibiotics have led to evolution of strains of bacteria that were resistant to penicillin and other antibiotics.

• Bacteria become resistant in two ways:– Random mutations. Resistant strains expressed enzymes which

could inhibit antibiotics or made impregnable cell walls– Horizontal transfer of resistant gene in plasmid to other

bacterium

Darwinian Hypothesis for Evolution

• Based on 3 primary observations, Darwin proposed that natural selection is the mechanism by which new species arise from pre-existing species (evolution).

• Evolution is descent with modification. The development of a differentiated new species from pre-existing ancestral organisms.

Terms to Understand• Environment: the biotic and abiotic surroundings of

organisms• Species: a group of individuals who can interbreed

to produce viable fertile offspring.• Species name: written in italics, two part scientific

name . E.g. Homo sapiens (humans)• Population: members of the same species that live

in the same area• Instraspecific: between members of same species• Interspecific: between members of different species

Descent with Modification

Cactus-eater. Geospiza scandens Insect-eater Certhidea olivacea)

Seed-eater. (Geospiza magnirostris)v

Although they are of different species, they evolved from a common ancestor species based on their feeding pattern

Darwin’s Observations and Deductions

1. Individuals within a population produce more offspring that are needed to replace themselves.

2. The numbers of a population eventually remain constant

• Deduction A: There exists unequal survival and reproductive success. There is a struggle for existence.

Darwin’s Observations and Deductions

• 3. Variation exists in the population. Members of a population often vary in their inherited traits– Deduction B: Individuals showing variations which

are best adapted to the environment, have a reproductive advantage, producing more offspring than less well adapted members.

– This leads to the accumulation of favourable traits in the population over generations

Darwin’s Observations Explained

Observations 1 & 2: Although all living organisms have reproductive capacity for continuation of the species, population size are limited by environmental factors such as:Food AvailabilitySpaceLight

Deduction A: The struggle for existence is created by intraspecific and interspecific competition.

Darwin’s Observations ExplainedDeduction B: The key factor for determining

survival is adaptation to the environment. Variation, whether physical physiological or

behavioural, which best adapts an organism to an environment provides selective advantage.

Favourable variations will be inherited by the next generation while unfavourable variations will be selected out.

Thus natural selection increases the vigour within a species.

INDIVIDUALS THAT HAVE TRAITS THAT ARE BEST ADAPTED FOR THE CURRENT ENVIRONMENT ARE THE ONES THAT SURVIVE TO BREED AND PASS ON THEIR

GENES TO THE NEXT GENERATION.

NOBODY!!

Organisms not possessing the beneficial traits either die or don’t have as many offspring.

There is no agent involved in natural selection.

Natural selection is a process of elimination

Natural Selection is Survival of the fittest

Selection Pressure• Selection: a process by which those organisms which

appear physically, physiologically and behaviourally better adapted to the environment survive and reproduce.

• Population size and environmental factors produce a selection pressure on which traits (alleles) or phenotypes will pass on to the next generation

• Three modes of selection are: Disruptive, Stabilising and Directional

What acts as a selection pressure on a population?

• Competition for food

• Competition for a mate

• Changes in the environment

• Predators

• Parasites

Directional selection• Directional Selection: occurs when conditions favour

individuals exhibiting one extreme of a phenotypic

range, thereby shifting a population’s frequency curve

for the phenotypic character in one direction or the

other.

• common when a population’s environment changes or

when members of a population migrate to a new (and

different) habitat.

Directional Selection

• Neck of Giraffe• Antibiotic resistance of bacteria• Moth color (melanin) • Camouflage/Mimics• Many sexually selected traits

Disruptive Selection

• Occurs when conditions favour individuals at

both extremes of a phenotypic range over

individuals with intermediate phenotypes.

• Causes species to diverge

Disruptive Selection

• Causes divergence within the species

• Occurs when two different types of resources in one area

• Results in specialization for each branched group

• May lead to formation of new species

• E.g. Darwin’s Finches

Stabilizing Selection• Acts against both extreme

phenotypes and favours intermediate variants.

• This mode of selection reduces variation and tends to

• maintain the status quo for a particular phenotypic character

• Occurs when conditions are optimal/ not severe and competition is minimal

Examples• bird clutch size• Elk Antlers size• Giraffe neck length• Tail length in birds