Community Ecology: Evolution and Extinction
Evolution
Refers to a process involving gradual changes to explain changes in organisms.
Origin of Life
Inferred to have begun in the oceans.
Earliest remains of plants and animals found in rocks of marine origin.
Freshwater and land.
3 billion year old rocks: Fossils of Eobacterium (resembles bacteria, oldest known organism).
1 billion year old rocks: Fossils of unicellular organisms (blue-green algae, bacteria, fungi).
600 to 700 million year old (1st Cambrian) rocks: Fossils of plants and animals.
Sequence of Appearance
Fishes
Amphibians (transitional phase)
Reptiles
Mammals (examples: Pig, Whale, Dolphin, Lion, Tiger, Dog, Horse, Cat, Deer, Human)
Birds
Theories of Evolution
Life is continuous.
Transmission of traits to the next generation.
Variations due to environment or processes within organisms.
Evolutionary process is slow and difficult to test.
Timeline of Evolutionary Events
4.6 billion years ago: Earth forms and is bombarded by meteorites and comets.
3.8 billion years ago: Replicating molecules (precursors of DNA) form.
3.5 billion years ago: Unicellular life evolves; photosynthetic bacteria release oxygen into the atmosphere.
555 million years ago: Multi-cellular marine organisms are common; bizarre-looking animals like Wiwaxia.
500 million years ago: Fish-like vertebrates evolve; invertebrates (trilobites, crinoids, brachiopods, cephalopods) are common.
450 million years ago: Arthropods move onto land, evolve into scorpions, spiders, mites, and millipedes.
420 million years ago: Land plants evolve, changing Earth's landscape and creating new habitats.
360 million years ago: Four-limbed vertebrates move onto land; seed plants and large forests appear; vast reef systems in oceans.
250 million years ago: Supercontinent Pangea forms; conifer-like forests, reptiles, and synapsids (ancestors of mammals) are common.
248 million years ago: Over 90% of marine life and 70% of terrestrial life go extinct.
225 million years ago: Dinosaurs and mammals evolve; Pangea begins to break apart.
130 million years ago: Earliest flowers evolve; dinosaurs dominate the landscape; bony fish diversify.
65 million years ago: Asteroid hits Yucatan Peninsula; ammonites and non-avian dinosaurs go extinct; birds and mammals survive.
4 million years ago: Early hominid "Lucy" lives in Africa; ice ages begin; many large mammals go extinct.
130,000 years ago: Anatomically modern humans evolve; 70,000 years later, descendants create cave paintings.
Geologic Time Scale
Cenozoic Era (0-65 million years ago)
Quaternary Period: Evolution of humans.
Tertiary Period: Mammals diversify.
Mesozoic Era (65-250 million years ago)
Cretaceous Period: Extinction of dinosaurs; first primates.
Jurassic Period: First flowering plants; first birds; dinosaurs diversify.
Triassic Period: First mammals; first dinosaurs.
Paleozoic Era (250-650 million years ago)
Permian Period: Major extinctions.
Carboniferous Period
Pennsylvanian: Reptiles diversify; seed ferns.
Mississippian: First reptiles; scale trees.
Devonian Period: First amphibians; jawed fishes diversify.
Silurian Period: First vascular land plants.
Ordovician Period: Sudden diversification of metazoan families.
Cambrian Period: First fishes; first chordates; first skeletal elements; first soft-bodied metazoans; first animal traces.
Late Proterozoic
Biological Evolution by Natural Selection
Change in the genetic characteristics of a population from one generation to another.
Explains how life on earth changes over time through changes in the genes of populations.
The Fossil Record
Mineralized or petrified replicas of skeletons, bones, teeth, shells, leaves, seeds, or impressions found in rocks.
Provides physical evidence of ancient organisms and reveals their internal structures.
The world's cumulative body of fossils found.
Uneven and incomplete: Fossils found so far probably represent only 1% of all species that have ever lived.
Genetic Variability in a Population
Populations, not individuals, evolve by becoming genetically different.
The first step is the development of genetic variability in a population.
Occurs through mutations: Random changes in the structure or number of DNA molecules in a cell that can be inherited by offspring.
Sources of Genetic Variation
Mutations: Changes in DNA.
Gene Flow: Movement of genes from one population to another.
Sex: Introduces new gene combinations into a population (genetic shuffling).
Causes of Mutation
Random changes in coded genetic instructions when DNA molecules are copied.
Exposure to external agents:
Radioactivity, X-rays.
Natural and human-made chemicals (mutagens).
Mutation Effects
Affects the looks, behavior, and physiology.
Changes in hereditary material of life (DNA).
How Mutations Occur
DNA copying errors: When a cell divides, the copy of its DNA might not be perfect.
External influences:
Exposure to specific chemicals or radiation.
DNA breaks down, and the cell repairs the DNA, but the repair is not perfect.
Natural Selection
Occurs when some individuals of a population have genetically based traits (resulting from mutations) that enhance their ability to survive and produce offspring with these traits.
Individuals in Populations with Beneficial Genetic Traits Can Leave More Offspring
An adaptation or adaptive trait is any heritable trait that enables an individual organism to survive through natural selection and to reproduce more than other individuals under prevailing environmental conditions.
For natural selection to occur, a trait must be heritable (passed from one generation to another) and lead to differential reproduction (enables individuals with the trait to leave more offspring).
Natural selection acts on individuals, but evolution occurs in populations.
Summary of Biological Evolution by Natural Selection
Genes mutate, individuals are selected, and populations evolve that are better adapted to survive and reproduce under existing environmental conditions.
When environmental conditions change, a population of a species faces three possible futures:
Adapt to the new conditions through natural selection.
Migrate (if possible) to an area with more favorable conditions.
Become extinct.
Natural Selection is the primary mechanism responsible for generating the exceptional diversity and complexity of all living forms, it is conceptually a very simple process.
Components of Natural Selection
Variation: Individuals in a population differ from one another.
Inheritance: Some of these differences are inherited by offspring from their parents.
Differential reproductive success: Individuals with certain traits are more successful than others at surviving and reproducing in their environment.
Historical Figures
Charles Darwin (1809-1882): English Naturalist who observed a struggle for existence; published his theory in 1859 (On the origin of species by means of natural selection or the preservation of favored races in the struggle for life).
Alfred Russel Wallace (1823-1913): Had the same conclusion as Darwin but studied flora and fauna in the Malay archipelago.
Genetic Resistance
Ability of one or more organisms in a population to tolerate a chemical designed to kill it.
Myths about Evolution through Natural Selection
"Survival of the fittest" means "survival of the strongest."
Organisms develop certain traits because they need or want them.
Evolution by natural selection involves some grand plan of nature in which species become more perfectly adapted.
Speciation
Two species arise from one.
For sexually reproducing species: a new species is formed when some members of a population have evolved to the point where they no longer can breed with other members to produce fertile offspring.
The most common mechanism of speciation: Geographic isolation occurs when different groups of the same population of a species become physically isolated from one another for long periods.
Part of a population may migrate or separation of populations can occur because of a physical barrier (mountain range, stream, road), volcanic eruption, earthquake, or when a few individuals are carried to a new area by wind or flowing water.
Geographic Isolation
Leads to reproductive isolation and divergence of gene pools.
Different environmental conditions lead to different selective pressures and evolution into two different species.
Reproductive Isolation
Mutation and change by natural selection operate independently in the gene pools of geographically isolated populations.
If this process continues long enough, members of the geographically and reproductively isolated populations may become so different in genetic makeup that they cannot produce live, fertile offspring if they are rejoined: one species has become two, and speciation has occurred.
Extinction
Is Forever: An entire species ceases to exist.
Endemic species: Species that are found in only one area and are vulnerable to extinction.
Background Extinction
The disappearance of species at a low rate.
An extinction rate is expressed as a percentage or number of species that go extinct within a certain time period such as a year.
Example: One extinction per million species per year would be species per year.
Expressed as a percentage, this is . This is the estimated background extinction rate existing before humans came on the scene.
Based on the fossil record and analysis of ice cores, biologists estimate that the average annual background extinction rate is one to five species for each million species on the earth.
Mass Extinction
Is a significant rise in extinction rates above the background level.
In such a catastrophic, widespread (often global) event, large groups of species (perhaps 25–70%) are wiped out in a geological period lasting up to 5 million years.
Fossil and geological evidence indicate that the earth’s species have experienced five mass extinctions (20–60 million years apart) during the past 500 million years.
For example, about 250 million years ago, as much as 95% of all existing species became extinct.
A mass extinction provides an opportunity for the evolution of new species that can fill unoccupied ecological roles or newly created ones.
Impending 6th extinction, estimated that about 30,000 species were lost/year and a massive lost of species including ourselves.
Levels of Species Extinction
Local extinction: Occurs when a species is no longer found in an area it once inhabited but is still found elsewhere in the world and involves losses of one or more populations of species.
Ecological extinction: Occurs when so few members of a species are left that it can no longer play its ecological roles in the biological communities where it is found.
Biological extinction: Is no longer found anywhere on the earth, is forever, and represents an irreversible loss of natural capital.
Examples of Extinct Species
Passenger pigeons have been extinct in the wild since 1900 because of human activities. The last known passenger pigeon died in the U.S. state of Ohio’s Cincinnati Zoo in 1914.
Lost natural capital: some animal species that have become prematurely extinct largely because of human activities, mostly habitat destruction and overhunting. Passenger pigeon, Great auk, Dodo, Aepyornis (Madagascar).
Endangered vs. Threatened Species
Biologists classify species heading toward biological extinction as either endangered or threatened.
Endangered species: Has so few individual survivors that the species could soon become extinct over all or most of its natural range (the area in which it is normally found).
Threatened species: (also known as a vulnerable species) is still abundant in its natural range but, because of declining numbers, is likely to become endangered in the near future.
Examples of Endangered and Threatened Species
Grizzly bear, Kirkland’s warbler, Knowlton cactus, Florida manatee, African elephant, Utah prairie dog, Swallowtail butterfly, Humpback chub, Golden lion tamarin, Siberian tiger, Giant panda, Black-footed ferret, Whooping crane, Northern spotted owl, Blue whale, Mountain gorilla, Florida panther, California condor, Hawksbill sea turtle, Black rhinoceros
Characteristics of Species Prone to Extinction
Low reproductive rate (K-strategist): Blue whale, giant panda, rhinoceros
Specialized niche: Blue whale, giant panda, Everglades kite
Narrow distribution: Elephant seal, desert pupfish
Feeds at high trophic level: Bengal tiger, bald eagle, grizzly bear
Fixed migratory patterns: Blue whale, whooping crane, sea turtle
Rare: African violet, some orchids
Commercially valuable: Snow leopard, tiger, elephant, rhinoceros, rare plants and birds
Large territories: California condor, grizzly bear, Florida panther
Strategies for Life
Generalist Species
Eats almost anything, have broad niches, they can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions.
Examples: Flies, Cockroaches, humans, Mice, Rats, white-tailed deer, raccoons
Specialist Species
Occupy narrow niches, may be able to live in only one type of habitat, use one or a few types of food, or tolerate a narrow range of climatic and other environmental conditions
More prone to extinction when environmental conditions change
Examples: Tiger salamanders breed only in fishless ponds where their larvae will not be eaten ; China’s giant panda is highly endangered because of a combination of habitat loss, low birth rate, and its specialized diet consisting mostly of bamboo.