AP Bio Unit 7 natural selection

Microevolution 

a change over time in the genetic composition of a population.

Macroevolution 

the gradual appearance of all of biological diversity, from the earliest microbes to the enormous variety of organisms alive today.

Descent with Modification

All organism related through descent from an ancestor that lived in the remote past

As the descendants of that ancestral organism spilled into various habitats over millions of years, they accumulated diverse adaptations that fit them to specific ways of life.

Natural Selection

1. Genetic variation 

      Variation of traits in a population

2. Overproduction of offspring

      more offspring than the environment can support

3. Competition for resources

     food, mates, nesting sites

4. Differential Survival and Reproduction

• Individuals with more favorable phenotypes are more like to survive 

• adaptations become more common in population

Genetic variation in a natural population

What contributes to genetic diversity?

• Mutations

• Crossing-over

• Sexual reproduction

Differential Survival and Reproduction

Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations.

Natural Selection in Action
Flowering time – global climate change

Changes in plant reproductive cycles due to shifting climate conditions, affecting pollination and survival.

Insecticide Resistance

1. insecticide didn’t
   kill all individuals

2. resistant survivors reproduce

3. resistance is inherited

4. insecticide becomes less & less effective

Artificial Selection

Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits.

Evidence supporting evolution

Fossil record - transition species 

Anatomical record 

• homologous & vestigial structures

• embryology & development

Molecular record (Chemical)

• protein & DNA sequence

Biogeography

• Plate tectonics

Artificial selection

• human-caused evolution

Fossil record

Layers of sedimentary rock contain fossils 

• new layers cover older ones, creating a record over time

• fossils within layers show that a succession of organisms have populated Earth throughout a long period of time

Fossils can be dated

• Age of the Rocks where the fossil was found

• Rate of decay of isotopes including carbon-14

• The mathematical calculations that take into account information from chemical properties (rate of mutation) and/or geographical data (tectonic plate movement)

Homologous structures

Similarities in characteristics resulting from common ancestry

Convergent evolution

takes place when species of different ancestry begin to share analogous traits because of a shared environment or other selection pressure. 

dolphins and fish have some similar characteristics since both had to evolve methods of moving through the same medium: water.

• Flight evolved in 3 separate animal groups

  • evolved similar “solution” to similar “problems”

  • analogous structures

Vestigial organs

Modern animals may have structures that serve little or no function

• remnants of structures that were functional in ancestral species

• deleterious mutations accumulate in genes for non-critical structures without reducing fitness

  • snakes & whales — remains of pelvis & leg bones of walking ancestors

  • eyes on blind cave fish

  • human tail bone

  • appendix

Embryology

Similar embryological development in closely related species

• all vertebrate embryos have similar structures at different stages of development 

  • gill pouch in fish, frog, snake, birds, human, etc.

Molecular record

Comparing DNA & protein structure

Because all organisms share the genetic code, it is likely that all species descended from a common ancestor.

 compare common genes

• cytochrome C (respiration)

• hemoglobin (gas exchange)

Biogeography – the geographic distribution of species

• Islands are showcases of the influence of geography on evolution

• Most island species are closely related to species from the nearest mainland or neighboring island

What is a population?

A group of individuals belonging to the same species.

Gene pool

the total aggregate of genes in a population at one time.

Allele Frequency

proportion of each   

  allele in the population

Describes a population that is NOT evolving. 

A fundamental principle in population genetics stating that the genotype frequencies and allele frequencies of a large, randomly mating population remain constant provided immigration, mutation, and selection do not take place.

Deviation from Hardy-Weinberg principle indicates the evolution of a species. 

The Hardy-Weinberg Equation

• Describes an existing situation.

• Provides a yardstick by which changes in allele frequency, and therefore evolution, can be measured.

• One can look at a population and ask:  Is evolution occurring with respect to a particular gene locus?

What five factors must be met by a population in Hardy-Weinberg Equilibrium? 

1. Very large population = less chance of fluctuations in the gene pool

2. Isolated from other populations = no new genes enter or old genes leave

3. No net mutations = gene pool stays the same

4. Random mating = no traits are preferred

5. No natural selection = no traits are more beneficial to survival

Hardy-Weinberg Equation

 GENOTYPE FREQUENCIES

           p = dominant allele, q = recessive allele

      p² + 2pq + q² = 1   

                  AA     Aa    aa

          ALLELE FREQUENCIES 

            p   +  q  =  1 

           “A”   “a”

Processes that can alter a population’s genetic composition

• Natural Selection

• Genetic Drift

• Gene flow

Natural Selection

Individuals in a population exhibit variations in their heritable traits, and those with variations that are better suited to their environment tend to produce more offspring that those with variations that are less well suited.

Genetic Drift

The smaller the population, the greater the change of deviation from the predicted result

• Gene frequencies can fluctuate unpredictably from one generation to the next – these fluctuations are called genetic drift

The Bottleneck Effect

Flood, fire, hunting may drastically reduce the size of a population – the survivors have passed through a restrictive “bottleneck,” and their gene pool may no longer be reflective or the original population’s gene pool

The Founder Effect

Isolation of a few individuals from a larger population – they may establish a new populations whose gene pool is not reflective of the source population – colonizing an island

Genetic Drift – KEY EFFECTS

• Significant in small populations

• Can cause allele frequencies to change at random

• Can lead to the loss of genetic variation in a population

• Can cause harmful genes to become fixed

Gene Flow

• Genetic additions to and/or subtractions from a population – resulting from the movement of fertile individuals or gametes

Directional selection

• Favors variants of one extreme – both dark and light colored mice move into an environment with only dark rocks.

Disruptive selection

• Favors variants at both ends for instance different colored mice in a patchy environment.

Stabilizing selection

Removes extreme variants – mice of a light and dark color living in an environment of rocks of intermediate color

Fitness

An organism’s fitness – the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

Speciation

 is the appearance of new species – the source of biological diversity

Macroevolution

Evolutionary change above the species level; the cumulative effect of speciation over vast tracts of time – the appearance of feathers during the evolution of birds from one group of dinosaurs

Biological species concept

A population whose members can interbreed & produce viable, fertile offspring

Reproductive Isolation

The biological species concept hinges on reproductive isolation – biological factors (barriers) that impeded members of two species from producing viable, fertile hybrids

Prezygotic

An obstacle to mating or to fertilization if mating occurs (No zygote forms)

Postzygotic

 Hybrid offspring are unable to  develope into a viable, fertile adult (Zygote).

Habitat Isolation

Species occur in same region, but occupy different habitats so rarely encounter each other  (We also use the term sympatric isolation since they live in the same area)

Temporal isolation

Species that breed during different times of day, different seasons, or different years cannot mix gametes

(We also use the term sympatric isolation since they live in the same area)

Behavioral isolation

Unique behavioral patterns & rituals isolate species

• identifies members of species 

• attract mates of same species 

  • courtship rituals, mating calls

(We also use the term sympatric isolation since they live in the same area)

Mechanical isolation

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators.
These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

Mechanical isolation

For many insects, male &
female sex organs of
closely related species do
not fit together, preventing
sperm transfer

• lack of “fit” between sexual organs:

Gametic isolation

Sperm of one species may not be able to fertilize eggs of another species

• mechanisms

  • biochemical barrier so sperm cannot penetrate egg

    • receptor recognition between egg & sperm 

  • chemical incompatibility 

    • sperm cannot survive in female reproductive tract

Reduced Hybrid Viability

the genes of different parent species may interact and impair the hybrid’s development

Reduced Hybrid Fertility

offspring may be vigorous – but sterile – meiosis may be affected by the number of chromosomes

Hybrid Breakdown

some first-generation hybrids are viable and fertile, but when they mate with one another or with either parent species, offspring of the next generation are feeble or sterile

Reduced hybrid viability

Genes of different parent species may interact & impair the hybrid’s development

Reduced hybrid fertility

Even if hybrids are vigorous
they may be sterile

• chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes

Hybrid breakdown

Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile

Allopatric isolation

geographic separation

Sympatric isolation

still live in same area

Habitat differentiation

factors that enable a subpopulation to exploit a habitat or resource not used by parent population

Sympatric speciation

Can be driven by sexual selection

• Ex. Coloration in cichlids from Lake Victoria in East Africa- females pick mates based on color

Polyploidy

A new species may originate from an accident during cell division that results in extra sets of chromosomes.

• Occasionally seen in animals- gray tree frog

• Generally more common in plants –  estimated that 80% of plant species descended by polyploid speciation

Allopolyploidy

Alloploid is a fertile polyploid

• Can interbreed with other polyploids but not the parent species

• Ex. Goatsbeard plant

Hybrid Zone

Species with incomplete reproductive  barriers come in contact and mate with members of different species producing hybrids with mixed ancestry

Does speciation happen gradually or rapidly?

Survey of 84 species: 

Speciation rate took form 4,000 years to 40 million years – average of 6.5 million years

    (rarely took less than 500,000 years)

Gradualism 

Gradual divergence over long spans of time

• assume that big changes occur as the accumulation of many small ones

Punctuated Equilibrium

Rate of speciation is not constant

• rapid bursts of change 

• long periods of little or no change

• species undergo rapid change when they 1^st  bud from parent population

Speciation genetics

How many genes change when a new species forms?

• Can be a little as one gene, but usually need a change in a larger number of genes 

Monkey flower species

A change in a small number of genes leads to different species of monkey flowers

Simple cells in 4 stages

1. Abiotic synthesis of small organic molecules, such as amino acids and nucleotides

2. The joining of these small molecules (monomers) into polymers, including proteins and nucleic acids

3. The packaging of these molecules into “protocells,” droplets with membranes that maintained an internal chemistry different from that of their surroundings; and

4. The origin of self-replicating molecules that eventually made inheritance possible.

Miller and Urey Expt. 

Proved that organic molecules can form in earth’s early atmosphere

• Formed amino acids

The origin of organic molecules

Two main ideas:

1. Extraterrestrial Origin 

• The original source of organic (carbon) materials was comets & meteorites striking early Earth.

2. Spontaneous Abiotic Origin

• Life evolved spontaneously from inorganic molecules.

Conditions on early Earth

Earth’s early atmosphere

• water vapor (H₂O), carbon dioxide, nitrogen and its oxides, methane, ammonia, hydrogen, and hydrogen sulfide 

Energy sources

• lightning, UV radiation,
  volcanic

Monomers to Polymers

Early polymers were thought to be assembled on solid, mineral surfaces that protected them from degradation

In the laboratory polypeptides and polynucleotides (RNA molecules) containing have been synthesized by dripping solutions of amino acids on to hot sand, clay , or rock.

First Genetic Material

RNA is likely first genetic material

• multi-functional

• codes information

  • self-replicating molecule 

  • makes inheritance possible

  • natural selection & evolution

• enzyme functions (Ribozymes)

• regulatory molecule

• transport molecule (tRNA & mRNA)

Stromatolites

Fossilized mats of prokaryotes resemble modern microbial colonies 

Oxygen atmosphere

Oxygen begins to accumulate 2.7 bya

• reducing → oxidizing atmosphere

  • evidence in banded iron in rocks = rusting

  • makes aerobic respiration possible

• photosynthetic bacteria (blue-green algae)

Theory of Endosymbiosis

Evidence

• structural

  • mitochondria & chloroplasts
    resemble bacterial structure

• genetic

  • mitochondria & chloroplasts
    have their own circular DNA, like bacteria

• functional

  • mitochondria & chloroplasts
    move freely within the cell

  • mitochondria & chloroplasts
    reproduce independently
    from the cell

The Permian mass extinction

Defines the boundary between the Paleozoic and Mesozoic eras, claimed about 96% of marine animal species. Terrestrial life was also affected. For example, 8 out of 27 orders of insects were wiped out. This mass extinction occurred in less than 5 million years, possibly much less—an instant in the context of geologic time. 

Factors include:

• Extreme volcanism

• Changes in global temperatures – ocean temps rise, reduction of oxygen

The Cretaceous mass extinction

Extinction event occurred 65 million years ago, which marks the boundary between the Mesozoic and Cenozoic eras, doomed more than half of all marine species and exterminated many families of terrestrial plants and animals, including most of the dinosaurs.

Factors include:

Layers of iridium – perhaps a large comet or asteroid

Adaptive Radiation

Large scale increase in the diversity of life

• groups of organisms form new species and fill vacant niches in their communities

• these events occur after mass extinctions, or the formation of new islands

3 Domain system

Domains = “Super” Kingdoms

• Bacteria

• Archaea

  • extremophiles = live in extreme environments

    • methanogens

    • halogens

    • thermophiles

• Eukarya

  • eukaryotes

    • protists

    • fungi

    • plants

    • animals

What is the criteria used in Cladistics to classify organisms?

A cladogram is a depiction of patterns of shared characteristics among taxa

A shared derived character

• Is an evolutionary novelty unique to a particular clade

What we cannot learn from phylogenetic trees

• Phylogenetic trees show patterns of descent, not phenotypic similarity

• The sequence of the branching tree does not indicate the absolute ages of particular species

• We should not assume that a taxon on a phylogenetic tree evolved from the taxon next to it

Morphological and Molecular Homologies

In general, organisms that share very similar morphologies or similar DNA sequences

• Are likely to be more closely related than organisms with vastly different structures or sequences

Sorting Homology from Analogy

A potential misconception in constructing a phylogeny

• Is similarity due to convergent evolution, called analogy, rather than shared ancestry