Evolution & Biodiversity

Chapter 5

Origins of life

• Chemical analysis and measurement of radioactive elements in rocks & fossils are studied

• Evolution of life is linked to evolution of earth

• Life evolved over the past 4.7 - 4.8 billion years

• Chemical evolution came first

• Biological evolution - about 2.3 - 2.5 billion years ago

Origins of LifeOrigins of Life

Chemical evolutionChemical evolution

Biological evolutionBiological evolution

How origins are studied

• Fossils

• Fossil record is uneven and incomplete

• Why? Some organisms left no fossils - some have decomposed and some are not yet found.

• It is believed we have fossils representing about 1% of all organisms ever living on earth

• Also use :– chemical and radioactive

dating

– Ancient rocks

– Ice cores

– DNA from living organisms

Evolution & adaptation

• Within limits, populations adapt to changes in their environment - called biological evolution - change in genetic make-up in a population through successive generations

• POPULATIONS, NOT INDIVIDUALS EVOLVE BY BECOMING GENETICALLY DIFFERENT.

• THEORY OF EVOLUTION - all species descended from earlier, ancestral species

• Microevolution – Small genetic changes

that occur in a population

• Macroevolution - – Long-term, large-scale

evolutionary changes among groups of species - new species are formed from ancestral species and other species are lost through extinction.

Microevolution

• Development of genetic variability in a population– Gene pool – set of all

genes in the individuals of the population of a species

• Alleles - genes have two or more different molecular kinds

• These are recombined• Each member of

population gets different combination of alleles

Mutation

• Random change in structure of number of DNA molecules in a cell

• Can occur in two ways:– Exposure to external agents such as radioactivity, x-

rays, natural and man-made chemicals

– Random changes in coded genetic instructions.

Sometimes are beneficial and make survival easier under existing or changing conditions.

Some are harmless and some are lethal.

• Mutations are:– Random and unpredictable– Only new source of genetic material– Rare events

Mutation

• Random changes in structure or number of DNA molecules in a cell

• Can be caused by radiation and chemicals - mutagens

• Random mistakes

• Can occur in any cell but only passed on in reproductive cells.

• Natural selection occurs when some individuals have genetically based traits that cause them to better survive and produce offspring

• Idea developed by Charles Darwin in “On the origin of the species by means of natural selection”

More…

• Must be natural variability for a trait in a population

• Trait must have a genetic bases that can be passed from one generation to another – be heritable

• Must lead to differential reproduction - enable individuals with the trait to leave more offspring than other members of the population.

Adaptations or adaptive traits

• A heritable trait that enables organisms to better survive and reproduce under a given set of environmental conditions

• Selective pressure - a factor in a population’s environment that causes natural selection to occur.

• Environmental conditions do not create favorable characteristics

To summarize

• Genes mutate

• Individuals are selected

• Populations evolve

• Can also change populations by artificial selection (see p. 100)

• Peppered moth - an example

Types of natural selection

• Directional natural selection - conditions shift so that individuals at one end of the normal range become more common than midrange forms - “it pays to be different”

• Most common during periods of environmental change or when members of a population migrate to a new habitat with different environmental conditions.

Second type:

• Stabilizing natural selection - eliminates individuals at both ends of the genetic spectrum and favors average genetic make-up

• “it pays to be average”

• Occurs when an environment changes little and members are well adapted to that environment.

• Individuals with unusual alleles tend to be eliminated

Third type:

• Diversifying natural selection - disruptive natural selection - individuals at both extremes of the genetic spectrum are favored and individuals with normal traits are eliminated.

• “It doesn’t pay to be normal”

• Population is split into two groups

• Occurs when there is a shift in food supply selected against average individuals

Coevolution

• Populations of two different species interact over a long time and changes in one gene pool lead to changes in the gene pool of another species

• Owls become better at hunting mice; certain prey have traits that allow them to escape

• Some of the predators have better eyesight and are more successful hunters and they pass this trait to their offspring.

Ecological niche

• Way of life or role in the ecosystem

• Involves everything that affects survival and reproduction

• Includes: range of tolerance for chemical and physical requirements

• Important because:– Can prevent extinction– Can help assess

environmental changes.

• Types of resources used - food or nutrients

• How it interacts with biotic and abiotic factors

• Role in the flow of energy and matter cycling

• Represents adaptive traits that organisms have acquired through evolution that allow for better survival.

Fundamental niche

• The full potential range of physical, chemical, and biological conditions and resources an organism could theoretically use if there was no competition with other species.– Niches however, often overlab

Realized niche

• In order for an organism to survive and avoid competition for resources it will use only part of its fundamental niche – this is the realized niche.

Generalist species

• Occupy broad niches

• Can live many places

• Eat many types of food

• Tolerate a wide range of environmental conditions

• Flies, cockroaches, mice, etc

Specialist species

• Have narrow niches• Live in only one type of habitat• Eat only one or a few types of food• Tolerate only a narrow range of climactic and

environmental conditions• More prone to extinction during changes in

environmental conditions• Examples: tiger salamander, red-cockaded

woodpecker, spotted owls, giant panda

Limits to adaptation

• Changes in environmental conditions can lead to adaptation only if the traits are already present in the gene pool

• Because each organism must do many things

• Adaptations are usually compromise

• Even if a beneficial trait is present, the population’s ability to adapt is limited by its reproductive capacity

• Adapt, migrate or die

Convergence or convergent evolution

• Species with similar niches tend to evolve similar traits that enable them to survive and reproduce even though they are in different parts of the world

macroevolution

• Evolution that takes place above the level of species and over much longer periods

• Patterns include:– Genetic persistence -

inheritance of DNA molecule through all subsequent lines of descent

– Genetic divergence - long term changes in lineages of species

– Genetic losses - steady loss (background extinction) or abrupt, catastrophic loss (mass extinction) of lineages

– Example: the horse

How do new species evolve?

• Speciation - two species arise from one

• Usually takes place in two phases:– Geographic isolation -

• Populations become separated for long periods of time

• Migration to new area

• Physical barrier such as a road

• Earthquake

– Reproductive isolation -

• Mutation & natural selection act on isolated populations - called divergence - eventually cannot interbreed

– Divergent evolution - arises from speciation

– Usually takes tens of thousands of years

Speciation

Extinction

• Can be caused by large scale movement of the continents (continental drift)

• Gradual climate change

• Rapid climate change - volcanic eruption, etc.

• Extinction is the ultimate fate of all organisms

• Biologists estimate that 99.9% of all species that have ever lived are now extinct.

Types of extinction

• Background extinction - species disappear at a low rate as local conditions change

• Mass extinction - abrupt rise in extinction rates - catastrophic, often global-wipes out large groups of species

• Generally believed to be the result of climate change.

• Five mass extinction s - 20 - 40 million years apart– Last one took place

about 65 million years ago - wiped out the dinasaurs

Adaptive radiations

• Periods of recovery• Numerous new species

evolve over several million years to fill niches vacated by extinct species

• Explosion of mammals after dinosaurs became extinct

Biodiversity =

• Speciation minus extinction• Humans have become a major force in the

premature extinction of species.• We may cause extinction of up to a quarter of

the earth’s current species• On short time scale, new species cannot form

rapidly enough• We are impacting the earth