THE THEORY OF EVOLUTION

Natural Selection & Speciation

Fossils shape ideas about evolution

Scientists used fossils as a basis for evolutionary concepts

When geologists provided fossil evidence of the age of Earth, biologists began to suspect that life changes slowly over time

Evolution = the change in populations over time

Charles Darwin

English Scientist Lived from 1809 – 1882

Took a job as a naturalist on the HMS Beagle (1831) Sailed to South America & the

South Pacific on a 5 yr journey Darwin studied & collected

biological specimens every time the ship stopped He became curious about the

possible relationships between species.

The Galapagos Islands

A group of small islands ~ 1000 miles off the coast of S. America Near the equator

Darwin studied species unique to the islands Similar to species

elsewhere Could not explain how

such changes would occur

Figuring out why

Thomas Malthus – English economist Ideas about human population growth Said that the human population grows

faster than Earth’s food supply Darwin related this to his studies:

Many species produce more offspring than can survive

They compete for resources like food, space, predation, mating, and shelter

Only some individuals survive

Two Kinds of Selection

Natural Selection a mechanism for change in populations –

“survival of the fittest” Organisms with certain variations survive &

reproduce to pass on their variations to offspring Organisms without beneficial variations are less

likely to survive and reproduce Result = each generation inherits beneficial

variations and others die out Artificial Selection

Organisms are purposely bred for desirable traits Examples: Horses, dog breeds

Types of Natural Selection

Stabilizing Selection = favors average individuals Spiders: Large individuals may be easier prey, small

individuals may not be able to get enough food Directional Selection = one extreme is favored

Woodpeckers: short or average beak cannot reach deep enough to get food, long beaks can feed on insects deeper within the tree trunk

Disruptive Selection = either extreme is favored Dark brown & white marine limpets: white limpets

have an advantage on light rocks, brown limpets have an advantage on darker/wet rocks. Tan limpets are easily seen and preyed upon by birds - tan limpets have the least favored adaptation.

Alfred Russell Wallace

Reached a similar conclusion Darwin & Wallace presented their ideas to

the scientific community Darwin published On the Origin of Species in

1859

There have been many advances since Darwin: Any change in the gene pool of a population

is now considered part of evolution

Adaptations

Structural Adaptations: Adaptations = any structure, behavior, or

internal process that makes an organism more likely to survive

develop over generations Examples: Giraffe height, mole-rats

Physiological Adaptations Develop more quickly than structural

adaptations Examples: antibiotics (penicillin) & pesticides

Other Adaptations

Mimicry = one species resembles another “warning” coloration =

red, orange, and yellow Camouflage =

organisms are able to blend into surroundings Insects

Other Evidence of Evolution

1. Fossils – provide a record of early life and evolutionary history

2. Embryology Embryos at early stages of development

are very similar in appearance

3. Biochemistry Comparisons of DNA or RNA of different

species

Other Evidence of Evolution4. Anatomy

Homologous structures – similar structures with different functions Example: animal forelimbs have the same bone

structure even though they have different functions Analogous structures – similar functions with

different structure Example: wings of a butterfly & wings of a bird have the

same function but different structure

15.2

Population Genetics

Darwin developed his theory without an understanding of genes & inheritance

Population genetics = studies of the behavior of genes in populations of plants or animals

Natural selection acts on a range of phenotypes in a population

If a variation of a phenotype is beneficial, it will be more common

Evolution of Populations

All of the alleles of a population’s genes = its’ gene pool

Genetic Equilibrium

Genetic equilibrium = the frequency of alleles remains the same over generations Populations in genetic equilibrium are not

“evolving” Disruptions to genetic equilibrium:

Mutations Genetic drift (chance alterations to allele

frequency) Movement of individuals in or out of the population Size of population – can cause recessive alleles to

become more common

Evolution of a species Speciation = evolution of a new species

Occurs when members of similar populations no longer interbreed to produce fertile offspring

Causes of speciation: Geographic isolation – physical barriers

separate populations & each develops its own gene pool

Reproductive isolation – formerly interbreeding organisms can no longer mate (may be caused by behavior such as breeding seasons)

Chromosomal change – mistakes during cell division may result in polyploidy

Speciation can occur at different rates Gradualism = species originate through

gradual changes and adaptations Punctuated equilibrium = change

occurs in rapid bursts with period of genetic equilibrium in between May be caused by environmental changes or

competitive species Scientists agree that either method can

occur, depending on the circumstances

Patterns of Evolution

Adaptive radiation = when a species evolves into an array of species Example: Hawaiian Honeycreepers Common on islands

Divergent evolution – a diverse group of species share a common ancestor A type of adaptive radiation

Convergent evolution – distantly related species develop similar traits Similar environmental