Evolution

EVOLUTION OF POPULATIONS

Three Cases of Variation

• What specifically causes the change in each of these populations of organisms:– Samurai crabs– Galapagos finches– Peppered moth

Example

Change Cause of change

crabs

finches

moth

The Peppered Moth• The Peppered Moth

from England exists in two forms – black and mottled gray.

• Before the industrial revolution (in 1850) most trees were light colored because of a lichen that grew on their bark.

• White moths were common and black moths were rare.

The Peppered Moth…• During the industrial revolution, as

soot and pollution increased, light colored lichens on the bark darkened or died off.

• It was observed that the dark moths increased until they became the more common form.

In order for changes such as these to occur in a

population, what two things need to be present in the

population?

• Variation (between individuals)

• Selection (on those individuals)

Variation of Traits in a Population

• Within a population, individuals vary in observable traits.

• Often these traits are cover a range of appearances (ex. height) that can be represented by a bell curve.

Sources of Variation

• Variations arise in two main ways:– Mutation – the only way to get a new

trait– Genetic Recombination

• Why no two siblings (except twins) are identical

Selection • There are two main

types of selection:• Artificial selection

is used by humans for breeding useful traits into a population.– Example – seedless

fruit; turkeys with more white meat

Breeds of dogs

Natural Selection• Natural selection is the process by

which individuals that are better suited to their environment survive and reproduce most successfully.– Acts only on populations not individuals– Acts only on appearances (phenotype)

not genes (directly)

Examples: pesticide resistance; antibiotic resistance in bacteria

Natural Selection• There are three types of selection on

multiple gene traits:– Directional selection - the range of

shifts toward one extreme. • examples include the peppered moth;

Galapagos finch, antibiotic/pesticide resistance

– Typical of a changing environment

Natural Selection–Stabilizing

selection - the range of phenotypes narrows; extremes are eliminated.• Examples include

birth weight in humans, clutch size in birds, hatching weight in birds

–Typical in a stable environment

Natural Selection– Disruptive selection - the range

of phenotypes “splits” and moves toward extremes; intermediate form is eliminated.• examples include shell color in

limpets, Galapagos finches with large or small beaks (none with medium size!)

Other Causes of Change• Summary: any type of selection will

cause population to change over time.• In addition to selection, there are other

factors that will cause a population to change – Genetic drift – change due to chance in small

populations• Bottleneck effect• Founder effect

– Gene flow = immigration and emigration– Nonrandom mating– Mutation

Bottleneck Effect• Disasters such as fires

may drastically reduce the size of a population reducing the size of the gene pool.– By chance, certain alleles

may be more or less prevalent in the survivors.

– Some alleles may be eliminated altogether.

• This decreases genetic variation in a population.

Founder Effect• Founder effect –This occurs when a

small subunit of a population begins a fresh population (due to migration, or a large die-off). The “founders” don’t have the same allele frequency as the original population.

Immigration & Emigration• Migration - movement of

individuals (and their genes) from one population to another = gene flow

Nonrandom Mating– Nonrandom mating -

any type of mate selection

– Example: sexual selection• Most species have distinctly

male and female phenotypes• Differences include fur,

plumage, antlers• Organisms select mates that

provide resources or “good genes” for the chooser

• It is usually the female that is the “agent of selection” causing males to develop elaborate decorations

Population Stability

• To review: the conditions that WILL cause change are:– natural selection– mutation– immigration or emigration– nonrandom mating

• If none of the previous conditions exist in a large population, the population’s genetic make up should stay the same.

Steps of evolution by natural selection

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