Evolution. 10.1 – Early Ideas About Evolution. Key Concept There were theories of biological and geologic change before Darwin. Early scientists proposed ideas about evolution. Evolution is the biological change over time by which descendants come to differ from ancestors .
10.1 – Early Ideas About Evolution • Key Concept • There were theories of biological and geologic change before Darwin.
Early scientists proposed ideas about evolution. • Evolution is the biological change over time by which descendants come to differ from ancestors. • A species is a group of organisms that can reproduce and have fertile offspring.
Theories of geologic change set the stage for Darwin’s theory. • There were three theories of geologic change: • Catastrophism: natural disasters such as floods and volcanic eruptions have shaped landforms and caused species to become extinct. • Gradualism: changes in landforms resulted from slow changes over a long period of time • Uniformitarianism: the geologic processes that shape Earth are uniform through time
10.2 – Darwin’s Observations • Key Concept: • Darwin’s voyage provided insight on evolution.
Charles Darwin • Known as the father of evolution • Traveled around the world on the HMS Beagle • Observed geological phenomena and adaptations & variation in species • Published findings in his book Origin of Species • 1800’s
Darwin observed differences among island species. • Variation: difference in a physical trait of an individual compared to others in the same group • Galapagos tortoises that live in areas with tall plants have long necks and long legs • Galapagos tortoises that live in areas with low plants have short necks and short legs • Galapagos finches (Darwin’s finches) that live in areas with hard-shelled nuts have strong beaks • Galapagos finches that live in areas with insects/fruit have long, thin beaks
Adaptation: feature that allows an organism to better survive in its environment • Species are able to adapt to their environment • Adaptations can lead to genetic change in a population
Darwin observed fossil and geologic evidence supporting an ancient Earth. • Darwin found fossils of extinct animals that resemble modern animals • Darwin found marine fossil shells high up in the Andes mountains Glyptodon Modern armadillo
He saw land move from underwater to above sea level during an earthquake • Darwin extended his observations to the evolution of organisms(gradual change leads to great change over time)
10.3 – Theory of Natural Selection • Key Concept: • HOW DOES EVOLUTION OCCUR? Darwin proposed natural selection as a mechanism for evolution.
Several key insights led to Darwin’s idea for natural selection. • Natural selection: mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals • Artificial selection: process by which humans change a species by breeding it for certain traits. • Heritability: ability of a trait to be passed down
There is a struggle for survival due to overpopulation and limited resources • Darwin proposed that adaptations arose over many generations
Natural selection explains how evolution can occur. • Variation: heritable differences that exist in every population are the basis for natural selection • Overproduction: Having many offspring increases the chance of survival but also results in competition for resources • Adaptation: certain variation that allows an individual to survive & reproduce better than other individuals it competes against • Fitness: ability to survive and reproduce • Descent with modification: Heritability of adaptations. More individuals will have the trait in every following generation, as long as the environmental conditions remain beneficial for the trait
five digits wrist bone Natural selection acts on existing variation. • Natural selection can act only on traits that already exist. • New alleles (leading to new phenotypes) are not made by natural selection – they occur by genetic mutations. • Structures take on new functions in addition to their original function.
10.4 – Evidence of Evolution • Key Concept: • Evidence of common ancestry among species comes from many sources.
Fossils & the Fossil Record • Shows how species changed their form/shape over time • Ways of dating fossils: • Relative dating: estimates the age of fossils by comparing fossil to others in the same layer of rock • Pro: can be used if there is no other way to tell the age of the fossil • Con: layers of rock can be shifted by natural events (earthquakes, mudslides, etc.) and this can mess up estimate • Radiometric dating: uses the decay of radioactive isotopes (carbon-14 changes into nitrogen-14) • Pro: can give an accurate age • Con: can’t give an age for really old fossils (if all isotopes have decayed)
Biogeography • Island species most closely resemble nearest mainland species • Populations can show variation from one island to another • Example: Darwin’s finches
Larva Adultbarnacle Adult crab Embryology • Similar embryos, diverse organisms • Identical larvae, diverse adult body forms • Gill slits and “tails”as embryos
Homologous Structures • Similar in structure, different in function • Evidence of a common ancestor • Example: bones in the forelimbs of different animals (humans, cat legs, whale fins, bat wings) • Not to be confused withanalogous structures – those that have similar functions but are not made of similar structures. Not evidence of a close evolutionary relationship. Example: bat wings, insect wings.
Vestigial Organs/Structures • Remnants of organs or structures that had a function in an early ancestor but have lost their function over time • Evidence of a common ancestor • Examples: • Human appendix & tailbone • Wings on flightless birds (ostrich, penguins) • Hindlimbs on whales, snakes
Molecular Biology • Common (universal) genetic code (A, T, C, & G) • Similarities in DNA, proteins, genes, & gene products • Two closely related organisms will have similar DNA sequences & proteins
DNA fingerprints will also be very close if the species are closely related
11.1 – Genetic Variation Within Populations • Key Concept: • A population shares a common gene pool.
Genetic variation in a population increases the chance that some individuals will survive. • Genetic variation leads to phenotypic variation • Necessary for natural selection • Genetic variation is stored in a population’s gene pool • Made up of all the alleles of all individuals in a population • Allele combinations form when organisms have offspring • Allele frequency: a measure of how common a certain allele is in a population. Can be impacted by natural selection.
Genetic variation comes from several sources. • Mutations • Can form a new allele • Passed to offspring if in a gamete • Recombination • Usually occurs during meiosis • Parents’ alleles rearranged during gamete formation
11.2 – Natural Selection in Populations • Key Concept: • Populations, not individuals, evolve.
Microevolution • Evolution within a population • Observable change in allele frequencies • Can result from natural selection • Types: • Directional selection • Stabilizing selection • Disruptive selection
Directional Selection • Favors phenotypes at one extreme
Stabilizing Selection • Favors the intermediate phenotype
Disruptive Selection • Favors both extreme phenotypes
11.3 – Other mechanisms of Evolution • Key Concept: • Natural selection is not the only mechanism through which populations evolve.
bald eagle migration Gene Flow • Movement of alleles between populations • Occurs when individualsjoin new populations and reproduce • Their alleles become part of gene pool • Keeps neighboring populations similar • Low gene flow increases the chance that two populations will evolve into different species
Genetic Drift • Change in allele frequencies due to chance • Causes a loss of genetic diversity in a population • Common in small populations • Bottleneck Effect is genetic drift after a bottleneck event • Occurs when an event drastically reduces population size
Founder Effect is genetic drift that occurs after the start of a new population • Occurs when a few individuals start a new population
Sexual selection occurs when certain traits increase mating success. • Sexual selection • Occurs due to higher cost of reproduction for females • Males produce sperm continuously • Females are more limited in potential offspring each cycle • Two types: • Intrasexual selection: competition among males • Intersexual selection: males display certain traits to females
11.5: Speciation through Isolation • Key Concept: New species can arise when populations are isolated.
If gene flow stops between two populations, they are said to be isolated. • Adaptations, mutation, and genetic drift may change the gene pools of the populations, and over time the populations may become more and more genetically different. • Reproductive isolation: when members of different populations can no longer mate successfully with one another. • This is the final step before speciation (the rise of two or more species from one existing species)
Several kinds of barriers can prevent mating between populations, leading to reproductive isolation. • Behavioral isolation: differences in courtship or mating behaviors. • Geographic isolation: physical barriers that divide a population into two or more groups. • Temporal isolation: timing prevents reproduction between populations.
11.6 – Patterns in Evolution • Key Concept: • Evolution occurs in patterns.
Species can become extinct. • Extinction: elimination of a species from Earth • Background extinction • Mass extinction
Background Extinction • Occur randomly, but at a low rate • Usually affect only a few speciesin a small area • Can by caused by local changes in the environment