AP Review Part 3:

• AP Review Part 3:– Evolution

• Natural Selection• Speciation

– Plants• Reproduction in Flowering Plants• Plant Tropisms and Hormones (esp. auxins)

AP Review

• Evolution– Natural Selection (Darwin’s “survival of that fittest”)

• Some individuals possess alleles (genotypes) that generate traits (phenotypes) that enable them to cope more successfully with their environment. These individuals survive to produce more offspring.

– If environment favors a trait, that trait is considered adaptive

– If environment is unfavorable for that trait, it is maladaptive

AP Review

• Darwin’s arguments (postulates) for Natural Selection:– Populations have enormous reproductive potential– Population sizes remain stable– Resources are limited– Individuals compete for survival– There is variation among individuals in a population– Much variation is heritable– Only the most fit will survive– Evolution occurs as advantageous traits accumulate

AP Review

• Types of natural selection– Stabilizing selection– Directional selection– Disruptive selection– Sexual selection– Artificial selection

AP Review

• Stabilizing selection– Eliminates individuals with extreme or unusual traits

AP Review

• Directional selection– Favors extremes that are at one end of the spectrum

and selects against the ones at the opposite end.

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• Disruptive selection– Environment favors extreme or unusual traits and

selects against the common traits.

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• Sexual selection– Male competition

• Antlers, horns, heavy musculature selected for

– Female choice• Chooses male she finds attractive

– Colorful feathers etc

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• Artificial selection– Form of directional selection carried out by

humans on plants and animalsThe BEST Breed!

AP Review

• Genetic equilibrium– is when allele frequency in a population remains

constant, also called Hardy-Weinberg equilibrium– At equilibrium no evolution is occurring, so can use HW

formula to show evolution has occurred.– In order for equilibrium to occur, the following must be

true:• All traits are selectively neutral• Mutations do not occur• Isolated population – no gene flow• Large population• Random mating

AP Review

• Hardy Weinberg Equation

AP Review– Hardy Weinberg Equation

• States that an equilibrium of allele frequencies in a gene pool will remain in effect each successive generation of a sexually reproducing population, as long as these five conditions are met:

– 1. No mutations: allelic changes do not occur, or changes in one direction balanced by changes in the opposite direction.

– 2.. No gene flow: migration of alleles into or out of the population does not occur. No immigration of emigration.

– 3. Random mating: pair by chance.– 4. No genetic drift: the population is very large, and changes in allele

frequency due to chance alone are insignificant.– 5. No selection: no selective agent favors one genotype over another.

• Are these conditions likely to be met?• So what good is this formula?

– Tells us what factors cause evolution (opposites of the five listed conditions)

– Use equation as a baseline to measure whether evolution has occurred.

AP Review

• Speciation– Species = a group of individuals capable of interbreeding– Speciation = splitting of one species into two or more species, or

the transformation of one species into another over time. Speciation is the final result of change in gene pool allele and genotype frequencies.

– Processes by which speciation can occur:• Allopatric speciation

• Sympatric speciation

• Adaptive radiation

AP Review

– Allopatric speciation• Populations geographically isolated from each other

(mountains, rivers etc) – gene flow stops.• Gene frequencies of the 2 populations can diverge due to

– Natural selection of different environments

– Mutations

– Genetic drift

AP Review

– Sympatric speciation• Population develops into two or more species

without prior geographic isolation. Often occurs in plants

– Plants often polyploid – leads to reproductive isolation– Balanced polymorphism

» Reproductive isolation due to some trait within the species, such as color

– Hybridization» Hybrid may possess greater genetic variations

than either parent

AP Review

– Adaptive radiation• Several different species develop rapidly from one

ancestor due to diverse geographical or environmental conditions

– Marsupials of Australia all from common ancestor– 14 species of finches on Galapagos Island all evolved

from one mainland species

AP Review

• Plants – Reproduction in Flowering Plants

Plant divisions

Bryophyta - nonvascular Tracheophyta - vascular

Coniferophyta –Gymnosperms (seeds)

Anthophyta -Angiosperms – Flowering Plants (seeds)

Monocots

Docots

Lycophyta Sphenophyta Pterophyta

Horsetails FernsClub mossesConifers

In all but Bryophytes, the diploid sporophyte generation is dominant.

Mosses (not Club mosses), Liverworts, Hornworts

AP Review

• Reproduction in Flowering Plants– Life Cycle of flowering Plant

• Alternation of generations– Sporophyte

» dominant in flowering plants – plant we recognize

» Diploid – produces haploid spore by meiosis

– Gametophyte» Haploid- produces diploid

zygote» Small and not

independently living (is independent in mosses and ferns)

» (Dominant in mosses)

AP Review

• Flower parts– “Female”-ovary and egg

• Pistil (some texts call a carpel)– Stigma– Style– Ovary

– “Male” – produces pollen grain

• staMEN– Anther (MANther)– filaMENt

– Petals– Sepals

AP Review• Pollination and Fertilization

– Pollen (microgametophyte) usually wind blown (in flowering plants). One pollen grain contains 2 haploid cells.

– Pollen (male) sticks on sticky stigma (female)– Pollen grain contains one generative cell and

one tube cell (each haploid)– Generative cell divides by mitosis to produce

two sperm cells– Tube cell forms a tube that grows down to the

ovule and sperm cells enter through the micropyle. The ovule contains the egg (egg developed from the megagmetophyte).

– One sperm joins with egg to form zygote.– Other sperm joins with both polar nuclei to

form triploid (3n) endosperm which will provide nutrition to the embryo.

– This double fertilization is unique to flowering plants

Zygote becomes sporophyte embryo

Ovule matures into seed and seed coat

Ovary becomes the fruitFertilization animation

..\..\Biology\Biology Clipart Movies Animations Sounds\Biology animations\PlantFertilization.mov

AP Review• Embryo Development

– Endosperm nucleus divides to become endosperm tissue

(seed development animation)

– Zygote divides into embryo (which will develop cotyledons) and suspender which will transfer nutrients to the embryo

AP Review

• Plant tropisms and hormones– Plant hormones

• Auxins• Gibberellins• Cytokinins• Ethylene• Abscisic acid (ABA)

– Plant tropisms• Phototropism• Gravitropis/geotropism• thigmotropism

AP Review• Plant hormones

– Auxins• Indolacetic acid (IAA) most common

auxin• Promotes plant growth by facilitating

elongation of cells• Produced at tips of roots and shoots

– Promotes adventitious roots and apical dominance

• Promotes phototropism – Cells on dark side grow faster

• Promotes geotropism/gravitropism– Auxin moves to lower side of stem or

root in response to gravity» Auxin inhibits growth of root

cells on lower side root grows down

» Stimulates growth of stem cells on lower side stem grows up

• Spraying fruits and leaves with auxins keeps them from falling off

AP Review• How auxins work

– Auxin (red sphere) binds to receptors on cell membrane– Activates ATP driven proton (H+) pump

• Needs energy, process of chemiosmosis

– Pumps H ions into space between cell membrane and cell wall.– H ions (acid) weaken cell wall.– Gradient set up that pulls solutes then water into cell causing it to swell– Pushes on cell wall causing cell to elongate

AP Review• Plants hormones

– Gibberellins• Group of over 60

hormones that also promote cell growth

• Synthesized in young leaves, roots and seeds, but often transported to other parts of plant

• Promote stem elongation especially in dwarf plants

• Can break dormancy in seeds and buds

AP Review

• How Gibberellins work– Gibberellin binds to a

receptor on the cell membrane

– Activates a second messenger inside the cell – Ca++

– Ca++ combines with a protein called calmodulin

– This complex activates the gene that codes for amylase

– Amylase acts on starch to release sugars

AP Review• Plant hormones

– Cytokinins• Zeatin and synthetic kinetin• Promote cell division and differentiation• Derivatives of adenine• Prevent senescence

– Often sprayed on cut flowers to prolong

• Varying ratios of cytokinins and auxins can effect how plant differentiates

• Stimulate growth of lateral buds, weakens apical dominance• Produced in roots and transported• Variety of effects depending on target organ and other hormones

AP Review• Plant hormones

– Ethylene• Inhibits growth

• CH3 - gas that promotes ripening of fruit

• Involved in stimulating the production of flowers

• Ethylene in combination with auxins– Inhibits root, stem and leaf elongation– Influences leaf abscission (aging and dropping of leaves)

– Abscisic acid• Inhibits growth

• Maintains dormancy

• Influence on abscission is controversial

AP Review

Functions of Plant Hormones

Hormone Function – all of these hormones work together

Auxins Stimulate growth

Involved in stem and root cell elongation in phototropism and gravitropism

Gibberellins Stimulate growth especially stems, especially in dwarf plants.

Can break seed and bud dormancy

Cytokines Stimulate growth by causing cell division

Abscisic Acid Inhibit growth

Causes stomates to close

Maintains seed and bud dormancy

Ethylene Inhibit growth

Causes fruit to ripen and fall

AP Review

• Plant tropisms– Phototropism

• Response to light caused by auxin– Auxin accumulates on shady

side causing increased growth, so plant bends toward light

– Gravitropism• Response to gravity by roots and

stems, involves auxins and gibberellins

• Action depends on relative hormone concentration and type of tissue (root vs stem – roots grow down, stems grow up)

– Thigmotropism• Response to touch, such as in

vines