Chapter 15: The Theory of Evolution

Big E little e Differences Evolution = belief that one species evolved into a different, more highly developed species Macro-evolution evolution = change over time; changes in the allelic frequency within a population or a species micro-evolution

15.1 Natural Selection and the Evidence for Evolution

Charles Darwin and Natural Selection evolution = the change in populations over time Fossils shape ideas about evolution When geologists provided evidence indicating that Earth was much older than people had thought, biologists began to suspect that life slowly changes over time The first ideas about how a species evolves was published by Charles Darwin

Darwin on HMS Beagle He began in 1831 at age 21 when he took job as a naturalist on the English ship HMS Beagle Sailed to South America and the South Pacific on a five-year scientific journey He studied and collected biological specimens

Darwin in the Galapagos Group of small islands near the equator off the coast of South America Studied many species of plants and animals that are unique to the islands, but similar to species elsewhere

Darwin continues his studies For the next 22 years he worked to find an explanation for how species change over time Thomas Malthus proposed that the human population grows faster than the Earth’s food supply Individuals struggle to survive Darwin observed that the traits of individuals vary in populations Sometimes variations are inherited Breeding organisms with specific traits in order to produce offspring with identical traits is called artificial selection

Darwin explains natural selection Natural selection is a mechanism for change in populations Occurs when organisms with certain variations survive, reproduce, and pass their variations to the next generation Organisms without these variations are less likely to survive and reproduce Alfred Russell Wallace has reached a similar conclusion and wrote to Darwin Jointly presented their ideas

Interpreting evidence after Darwin Evolutionary processes are difficult for humans to observe directly There are many interpretations of the data

Adaptations: Evidence for Evolution Structural adaptations arise over time Adaptations in species develop over many generations Structural adaptations are physical characteristics that enable an organism to better survive Mimicry is a structural adaptation that enables one species to resemble another species A harmless species mimics a harmful species to avoid predators Camouflage is a structural adaptation that enables species to blend with their surroundings

Physiological adaptations can develop rapidly Changes in an organism’s metabolic processes Some medicines and pesticides are no longer as effective

Other Evidence for Evolution Fossils Provide a record of early life and evolutionary history An incomplete record Found throughout the world

Anatomy Structural features with a common evolutionary origin are called homologous structures. Similar structure or arrangement, function, or both Ex: whale and crocodile forelimb

The body parts of organisms that do not have a common evolutionary origin but are similar in function are called analogous structures. Ex: bird and butterfly wing

Vestigial structure is a body structure that has no function in a present-day organism but was probably useful to an ancestor Many organisms have structures with no apparent function

Embryology An embryo is the earliest stage of growth and development of both plants and animals The embryos of reptiles, birds, and mammals all have a tail and gill slits Similarities suggest a common ancestor

Biochemistry Comparisons of the DNA and RNA of different species produce biochemical evidence for evolution Comparing nucleotide sequences More similarities indicate a closer evolutionary relationship

15.2 Mechanisms of Evolution

Population Genetics and Evolution Populations, not individuals, evolve If an organism has a feature that is poorly adapted to its environment, the organism may be unable to survive and reproduce Cannot evolve a new phenotype in response to its environment Natural selection acts on the range of phenotypes in a population Population: all the members of a species that live in an area

Evolution occurs as a population’s genes and their frequencies change over time Gene pool = all of the alleles in a population’s genes Allelic frequency = percentage of any specific allele in the gene pool A population in which the frequency of alleles remains the same over generations is in genetic equilibrium The frequency of alleles for a specific trait is the same in all its generations

Changes in genetic equilibrium A population that is in genetic equilibrium is not evolving One mechanism for genetic change is mutation Lethal mutations are quickly eliminated A useful variation will become a part of the gene pool Genetic drift is the alteration of allelic frequencies by chance events Greatly affect small populations Movement of individuals in and out of a population disrupt genetic equilibrium

Natural selection acts on variations Some inherited variations increase or decrease an organism’s chance of survival Three different types of natural selection that act on variation Stabilizing selection Favors average individuals in a population Directional selection Favors one of the extreme variations of a trait Disruptive selection Individuals with either extreme of a trait’s variation are selected for Tend to eliminate intermediate phenotypes

The Evolution of Species Speciation is the evolution of new species Occurs when members of similar populations no longer successfully interbreed

Physical barriers can prevent interbreeding Geographic isolation occurs whenever a physical barrier divides a population Over time, each small population might adapt to its environment through natural selection and develop its own gene pool

Reproductive isolation can result in speciation Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring Two types Occurs when the genetic material of the populations become so different that fertilization cannot occur Behavioral differences prevent populations from interbreeding

A change in chromosome numbers and speciation Many new species have evolved in the same geographic area as a result of polyploidy Multiple of the normal set of chromosomes Results in immediate reproductive isolation Polyploidy can arise from within a species or from hybridization between species

Speciation can occur quickly or slowly Gradualism is the idea that species originate through a gradual change of adaptations In 1971, Stephen J. Gould and Niles Eldridge proposed the idea of punctuated equilibrium Speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between

Patterns of Evolution Diversity in new environments When an ancestral species evolves into an array of species to fit a number of diverse habitats, the result is called adaptive radiation Occurs in both plants and animals A type of divergent evolution: pattern of evolution in which species that once were similar to an ancestral species diverge, or become increasingly distinct Occurs when populations adapt to different environmental conditions

Different species can look alike A pattern of evolution in which distantly related organisms evolve similar traits is called convergent evolution Occurs when unrelated species occupy similar environments