D4.1 Natural Selection

Process of Natural Selection

overproduction of offspring

variation within the community

struggle for survival due to lack of recourses

differential survival to different environments

reproduction to pass on adaptations

Main sources of genetic variation

- mutation in DNA gene sequence

- meiosis

- sexual reproduction

Variation effect on a population

increases chances of survival because there is a higher chance of some of the members resisting large amount of change

Gene

a heritable factor that influences a specific characteristic

Allele

different forms of a gene (differ by a few DNA bases)

Mutation

change to base sequence of DNA

Mutations in DNA sequence

new alleles produced my mutations could = change in phenotype, impacting chances of survival

What does a diploid cell produce

four haploid, non-identical cells

Meiosis

production of haploid cells to make gametes so an organism can sexually reproduce

Aspects of meiosis that promote genetic variation

- the crossing-over exchange of genetic material between homologous chromosomes

- the random + independent orientation of homologous chromosomes

Effect of crossing over/random orientation

every single gamete from a parent is genetically unique

- likely to have never existed

Variation in an asexually-reproducing population

all members are genetically identical

Variation in a sexually-reproducing population

wide range because of fusion of gametes from different parents

Examples of species producing more offspring than could survive

Natural selection - overproduction

individuals with the most suitable phenotypes survive

Overproduction - carrying capacity

struggle for survival because of competition for recourses

Individuals that will survive to reproductive age

those better suited to the environment (+ will reproduce)

Main driving force of evolution

change in environment (abiotic factors that are density-independent)

Abiotic factors as selection pressures (Magellanic penguins)

- live on coast of south america, well adapted for cold conditions

- chicks have thick down feathers to retain heat (can shake of snow but aren't waterproof)

- ↑ rainfall due to climate change = ↑ chick death due to hypothermia

Abiotic factors as selection pressures (Snow crabs)

- thrive in cold waters (Atlantic + northern Pacific oceans)

↓ water temperatures = ↑ oxygen absorption = ↑recourses = ↑ crab population

Species fitness

how well-adapted a species is to its environment

Differential survival

organism with high fitness = higher chance of survival over organisms with low fitness

Useful variation examples

hiding from predators, keeping warm/cool, obtaining water, finding food

Harmful variation examples

inappropriate colour for camouflage, large body size (more nutrition req.)

Heritable trait

trait encoded by genes

Phenotype

visible trait

Genotype

combination of alleles

Acquired characteristics example

flamingoes born white, eat prawns, become pink

Sexual selection

reproductive success of an individual

Things that can affect sexual selection

physical + behavioral traits that impact the attraction of mates

Sexual dimorphism

Differences in physical characteristics between males and females of the same species.

Why females in bird species that show sexual dimorphism prefer exaggerated traits

- male has enough energy to grow/maintain it + repeatedly carry out courtship displays = sufficient nutrition

- male can survive in its environment with adaptations = well-adapted in other ways

Which is stronger - selective pressure of predation or selective pressure of attraction?

selective pressure of predation

Selective power of predation example

guppies moved to different stream

- ↑ predator = ↓ spots

- ↓ predator = ↑ spots

Allele frequency

proportion of a particular allele in a population/gene pool

Change in allele frequency =

evidence that evolution is occuring

How allele frequency is determined

average it out

(no. times allele appears/ total number of copies of the gene)

What frequency of alleles must add up to

1.0

Separation + different environments + random changes =

two unique groups (speciation)

When will further mutations occur during speciation

at random - when new alleles are introduced into each of the gene pools

Allele databases

AlFred allele frequency data base

Allele Frequency Net Database

When does natural selection act on individuals

different phenotypes

How are phenotypes mostly determined

genotype

Reproduction impact on gene pool

individuals that reproduce contribute their alleles to the gene pool of the NEXT generation

Natural selection effect on gene pools

some individuals are more likely to survive + reproduce (therefore contribute their alleles) than others

favourable phenotypes

selection pressures in an environment can change to favour one phenotype over another

General patterns of natural selection

- directional selection

- disruptive selection

- stabilising selection

(all types = change in allele frequencies over time)

Directional selection

when natural selection favours one phenotype over another (can occur along a spectrum for a trait ex: length/colour)

When does directional selection occur

change in environment

- overtime frequency ↑ favourable phenotype ↓ unfavourable phenotype

Directional selection example (Indian Peacock)

large tail = favourable bc flight (will still be upper/lower limit based on what's practical)

Directional selection example (Peppered moths)

- naturally occurring as dark or light + mottled

- industrial developments killed pale coloured lichens on near bye trees

- recent efforts to improve pollution brought back lichens, favouring the light phenotype again

Disruptive selection

when two extreme phenotypes are favoured over one intermediate one

Disruptive selection evolution

can be an advantage for accessing recources

- overtime the two extremes become more frequent

Disruptive selection example (red crossbill)

- asymmetric low beaks to extract seeds from conifer cones

- two extremes (left-over-right and right-over-left) vs. one intermediate (normal/symetric)

Disruptive selection example (coho salmon)

- some males mature as much as 50% younger and as small as 30% of the adult body size other males in the population

- small/large gain access to females by sneaking in and fighting, intermediate-sized have a competitive disadvantage

Stabilising selection

intermediate phenotype is favoured over extreme ones

Stabilising selection evolution

different disadvantages of each extreme

- over time intermediate phenotype becomes more frequent

Stabilising selection example (birth weight in humans)

average birth weight favoured

- low = ↓ chance in survival bc pf underdeveloped organ systems

- high = ↓ chance of survival bc of potential complications during pregnancy

Stabilising selection example (clutch size in birds)

medium clutch size (no. eggs) favoured

- low = no offspring survive?

- high = too many offspring to provide adequate care/nutrition to all

Hardy-Weinberg equation for genes with two alleles

p^2 + 2pq + q^2 = 1

and

p + q = 1

p^2 + 2pq + q^2 = 1

P^2 - homozygous dominant

2pq- heterozygous

Q^2 - homozygous recessive

Problem-solving using Hardy-Weinberg equations

1. find q^2 (recessive trait)

2. use q^2 to find p (p+q = 1)

3. p^2 + 2pq + q^2 = 1 to find all genotype frequencies separately

Assumptions of Hardy-Weinberg

- no mutation

- random mating

- no gene flow

- very large population size

- no natural selection

If assumptions not met - Hardy-Weinberg equilibrium

equilibrium not met, species may evolve (allele frequencies may change from one generation to the next)

Natural selection vs. artificial selection

bacteria that is resistant to antibiotics = natural selection, even though antibiotics would be considered artificial selection in this case