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 Evolution 4. 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