Bio HL1 Test 6 - Kim

Natural Selection, Evolution and Speciation

natural selection

process by which individuals with traits better suited to their environment survive and reproduce to pass those advantageous traits to their offspring (Darwin), also known as ‘survival of the fittest’

process of natural selection

inheritable variation exists in a population

there is competition, or a struggle for survival

number of offspring exceeds what environment can support

selection—diff environmental pressures exist

individuals with the right adaptations (beneficial traits) will most likely survive, reproduce, pass on their genes

evolution occurs when allele frequency of population changes over time

mutation

change in DNA sequence, creates new alleles for natural selection to act on; the only way new alleles can be generated

produces genetic variation in populations

mutation can be…

beneficial—creates successful variation of existing trait

harmful—decreases survival or reproduction

neutral—no immediate effect

sexual reproduction

reproduction involving meiosis and fertilization

creates new allele combinations

increases genetic diversity without producing new alleles

crossing over

exchange of non-sister chromatid segments between homologous CHR during prophase I

produces recombinants, where offspring have gene combos unique from parents

independent assortment

homologous CHR that line up in meiosis I, do so randomly and independently from one another, possibility of different gametes in humans is 2^23 = 8,388,608

random fertilization

any sperm can fertilize any egg

overproduction of offspring

species produce more offspring than ENV can support

ultimately, population > resources

produces competition for resources

resources like food, space, and mates determine carrying capacity, or maximum population ENV can support

allows survival of the fittest, where individuals with advantageous traits (better at getting resources) are more likely to survive and reproduce—therefore, such traits will become more common over time

selective pressures (abiotic)

environmental factors that affect an organism’s chances of survival and reproduction, they may change over time, altering which adaptations are beneficial for survival

abiotic factors can be…

negative—reduce the number of individuals less suited

positive—favor individuals with certain traits survive and reproduce

density-independent factors

factors that impact a population regardless of size, e.g:

high temp—heatwave (coral bleaching)

low temp—sudden frost can kill plants or other food sources

droughts, floods, storms can also impact populations

adaptations

variations among individuals, organisms have different adaptations that allow them to be better suited to their ENV

intraspecific competition and survival

organisms with better adaptations will more likely survive in competition for the same resources; those with higher fitness will pass on their genes to the next generation

fitness is relative, depending on ENV

this is how natural selection changes population over time

evolution

change in heritable traits of a population over time, traits encoded by DNA (genes) and passed from parent to offspring, only changes in DNA like mutations can be inherited

acquired characteristics

like muscle, loss of limb; do not change DNA and thus cannot be passed to offspring

environmental factors

such as diet, sunlight, injury, can only impact the individual, not their genes; Lamarck’s theory is incorrect

sexual selection

type of natural selection where certain traits increase an organism’s ability to attract a mate and reproduce

such traits aren’t directly tied to survival

they act as signals of fitness (health, strength, good genes)

sexual selection can produce significant differences between females and males, known as sexual dimorphism

intersexual selection

individuals of one sex (typically females) choose mates based on traits (color, dance, song)

birds of paradise—males have bright plumage and complex dances that attract females, even though they may increase predation risk

deer—males grow large antlers, despite being energetically costly

natural selection experiments

guppies are a species of fish that evolve traits as a consequence of both natural and sexual selection

John Endler placed guppies into artificial steam tanks with gravel of different colors, independent variables were presence of predators (high, low, none) and color of gravel (for camouflage)

guppies with predators evolved duller colors for camouflage (favored by natural selection—survival), guppies without predators evolved brighter colors due to female preference (favored by sexual selection—mating)

gene pool

total number of alleles for all genes in a population (same species in same area, interbreeding) at a given time

larger ones increase in genetic diversity, fitness, and survival

smaller ones increase in chances of extinction

evolution occurs when allele frequencies in the gene pool change over time

genetic drift

change in the gene pool due to chance or random events, especially smaller isolated populations

larger populations maintain stability due to greater total # of alleles

bottleneck effect (genetic drift)

event that decreases a population size by over 50%

natural disasters (fires, floods) or humans (overfishing)

founder effect (genetic drift)

small group leaves population to setup a new one

differs from bottlenecks, original population stays intact

BOTH populations will have less genetic variability and be more prone to genetic drifts

allele frequency

prevalence of an allele, in comparison to others in a gene pool, represented as a percentile (0-1.0)

as favorable traits are passed on, the frequency of advantageous alleles increases

alleles for less favorable traits decrease in frequency

evidence of evolution via natural selection

Neo-Darwinism

Darwin proposed evolution based on observation, unaware of genes

modern biologists combined natural selection with genetics

Neo-Darwinism is evolution as changes in allele frequencies caused by natural selection acting on heritable genetic variation

stabilizing selection

favors intermediate phenotype and selects over the two extremes

traits become more uniform around the average

e.g. human birth weight

directional selection

favors one extreme phenotype over the other

trait distribution shifts in one direction

e.g. peppered moths, antibiotic resistance in bacteria

disruptive selection

favors both extreme phenotypes over the intermediate

trait distribution splits into two distinct groups

may eventually encourage speciation

e.g. beak size

Hardy-Weinberg

in the HW equation: p=freq. of dominant allele, q=freq. of recessive allele

Sum of the freq. always = 100%, thus p + q = 1

genotype freq. must also = 100%, thus p² + 2pq + q² = 1

p² = AA, 2pq = Aa, q² = aa

using probability, freq. of genotypes in the next generation can be calculated

H-W strategies

if recessive phenotype is given, use the value for aa as q², then find q

use p + q = 1 to find p, use the values for p and q to answer questions

conditions for genetic equilibrium

to maintain genetic equilibrium (keep allele frequencies the same), there are five conditions:

large population size: the smaller the size, the greater the likelihood that small changes produce significant effects (genetic drift)

no gene flow: no individuals moving in/out via migration

no mutations: no new alleles are introduced

random mating: individuals mate without any preference for traits

no natural selection: all genotypes must have equal chances of surviving and reproducing

artificial selection

process where humans choose individuals with desirable traits to reproduce

purpose is to enhance specific traits over generations

e.g. crop plants—select for plants with larger fruits or pest resistance

domesticated animals—breeding dogs for size, temperament, or coat type

antibiotic resistance

bacteria have evolved resistance through natural selection, as an unintended consequence of human action

bacteria became antibiotic resistant through mutation

when exposed to antibiotics (selective pressure), regular bacteria will die

those with resistance will survive and reproduce, and will flourish due to lack of competition

they will further spread resistance by transferring R plasmids via bacterial conjugation

this adaptation has increased the allele frequency of the population

evolution

a change in heritable traits within a population

occurs via natural selection (Darwinism)

before Darwin, leading theory was evolution via acquired traits (Lamarckism), e.g. trees growing asymmetrically due to wind

sequential evidence

evolution is heritable, thus change is visible in DNA, RNA, and AAs

sequence comparison of same genes:

fewer differences in more closely related species

sequence differences in genes

can be accounted for by splitting ancestral species

e.g. gene families like Hox gene, determines body plan during development, found in all ORG. with a clear head-to-tail axis

selective breeding

type of artificial selection, a deliberate breeding to produce desired/favorable traits

purpose: increase freq. of desired traits in a short period of time

mimics and provides evidence of evolution

both domesticated animals and crop plants show diversity…

in between breeds

from original wild species

homologous structures

similar anatomical structures with different functions

inherited from common ancestor but adapted for different environments

pentadactyl limbs

similar bone arrangement of a five-fingered limb shared in mammals, birds, amphibians, and reptiles

each limb, however, shows adaptation to their mode of locomotion

human hands: tool manipulation (power vs. precision)

bird and bat wings: flying

horse hooves: running

whale and dolphin fins: swimming

analogous structures

different anatomical structures that have similar functions

due to convergent evolution

inherited from different ancestor but lived in similar habitats with similar selective pressures

underwent similar adaptations

speciation

creation of a new species from a pre-existing one (only way)

two seperated species begin to evolve independently

can lo longer interbreed even when brought back together

reproductive isolation

when barriers (not always physical) prevent gene flow between gene pools of two populations

geographic separation: physical barriers that are difficult to cross (mountain range, ocean)

divergent selection

different selective pressures cause a population to become different over time

climate—temp, rainfall; affects food supply

predation—more or less, even none

competition—more or less, for resources

differential selection

e.g. Bonobos and chimpanzees—separated by Congo river, at one point water levels fell and chimps crossed, after levels rose again chimps were subjected to same selective pressures and became absorbed into bonobos

types of speciation

both require reproductive isolation via barrier

Allopatric speciation: geographic barrier creates physical isolation

Sympatric speciation: other barriers like behavioral or temporal isolation

behavioral isolation (sympatric speciation)

two populations have different courtship rituals or mating rituals, e.g. blue-footed boobies

temporal isolation (sympatric speciation)

two population breed during different times of day

e.g. crickets, skunks, toads

mechanical isolation (sympatric speciation)

two populations are anatomically incompatible and cannot transfer sperm, e.g. Chihuahua and Great Dane

adaptive radiation

pattern of diversification where species form a common ancestor, occupy different, vacant niches, or ecological roles

minimizes competition and allows most to coexist

leads to great biodiversity

finches at Daphne Major

fairly rapid evolution of a single species

results in great diversification

IND. with adaptations that match their unique selective pressure survive and reproduce

seen in beak shapes

specialized to fit their available food (seeds, insects, nuts, nectar)

eventually occupies different niches

barriers to hybridization

Interspecific hybrids

result from cross breeding different species (mule)

combine useful traits of both (hybrid vigor)

usually sterile even when CHR line up

in evolutionary terms, energy spent on producing a sterile offspring is a waste

to prevent it, ORG use techniques like distinctive courtship rituals

polyploidy

ORG has more than two sets of homologous CHR

CHR duplicate but fail to divide during meiosis

autotetraploid (4n) — all CHR come from same ORG

gametes are diploid (2n)

when they fuse with a haploid (n) gamete, offspring are triploid (3n), which are usually sterile

thus, considered to be different species

Different type of polyploidy

Two species cross breed to produce a hybrid

Contains one set of CHR from each parent

CHR do not form homologous pairs and hybrid is sterile

If CHR duplicate but fail to divide during meiosis, allotetraploid (4n) — CHR are from different species

Can interbreed with other allotetraploids but not with either of parents

Thus, considered to be different species

Smatweed (genus Persicaria)

At least 15 species have originated from allopolyploidy

Persicaria foliosa (2n=22) + Persicaria lapathifolia (2n=22) = Persicaria maculosa (2n=44)

polyploidy is more common in plants due to…

self pollination—plants possess both male and female parts

asexual reproduction—infertile polyploids can reproduce asexually

polyploid crops are desirable:

produces seedless fruits (due to infertility)

shows hybrid vigor by growing larger, showing better disease resistance