Natural Selection (D4.1)

Natural selection

mechanism for evolution

- Overproduction of offspring

- Variation within population (meiosis, sexual reproduction, and mutations)

- Struggle for survival (not enough resources for all members of the population to survive)

- Differential survival (survival of the fittest)

- Reproduction of those who survive (pass genes to their offspring)

Population

Group of individuals that belong to the same species and live in the same area

Population level

Evolution is observed at population level

Carrying capacity

Maximum population size an environment can sustain

Population growth curve

struggle for survival: consequence of supply and demand

Overproduction:

- Species tend to overproduce more offspring than the environment can support for survival

- Increases genetic diversity and resilient to conditions

More individuals lead to competition

J curve - abundance of resources and grow according to it's biotic potential

Environmental resistance - more offspring means less available resources

Population growth slows and plateau - struggle for survival and increase mortality rate

ICEAGE (required conditions for natural selection)

Inheritable variation exists within a population

Competition (struggle for survival)

Environmental pressures (different rates of reproduction)

Adaptations from successful reproduction

Genotype frequencies change across generations

Evolution occurs (change in frequency of alleles)

Malthusian Growth Model

resources in every generation are limited, so individuals in every generation would have to compete for these resources

*human population grows exponentially

*food production grows linearly

Paradigm shift

a fundamental change in approach or underlying assumptions - Darwin's theory replaced Lamarck's theory

Sources of variation

Mutations in DNA , Meiosis, sexual reproduction

Mutations in DNA

one source of variation.

random changes in DNA as a result of errors in DNA transcription

- rise to alleles

- Most mutations don't show in phenotype (in non-coding)

- Genetic diseases result from a mutation on coding DNA

Ex. Lactose Tolerant/Lactose intolerant (from the presence of lactase)

Meiosis

one source of variation: crossing over during prophase I and independent assortment in metaphase I

Sexual reproduction

one source of variation: random fertilization is when a unique sperm will fertilize a unique egg

Alleles

different variations of a gene

Mutations

one source of variation: alleles are a result of mutations and creates variation to reduce vulnerability to threats.

Selection pressure

influences parts of a population and allele frequency

abiotic factors:

- physical environment

- chemical environment

physical environment

temperature, humidity, availability of light

chemical environment

availability of minerals, pH of water/soil, gas concentration in the atmosphere or water

Density dependent factors (PANDA)

Predators

Availability of resources

Nutrient supply

Disease

Accumulation of waste

Density independent factors

Phenomena (natural disasters)

Abiotic factors (temperature, CO2 levels)

Weather conditions

Density independent (Magellanic penguins)

Climate change causes raining that prevents effective thermal insulation in penguin chicks and may die from hypothermia, decreasing population.

Density independent (snow crabs)

Cold water dissolve more oxygen, hence more resources for more individuals results in an increase of population.

intraspecific competition

competition between members of the same species

Fit

organism that is well adapted to the environment and has a higher chance of survival

Heritable characteristics

Traits passed from parents to offspring. An accumulation of changes in heritable characteristics of a population results in evolution.

Survival value

behavioral trait that aids the survival and reproduction of the organism

Ex. plover eggs blend in with the environment to camouflage

Reproductive potential

the maximum number of offspring that an organism can produce

Adaptation

feature that aids in the survival of an organism (structural, behavioral, physiological). Organisms with beneficial adaptations are more likely to survive long enough to reproduce and pass on their alleles

heritable traits

traits are encoded in the organism's DNA that can be passed onto the next generation.

Acquired characteristic

A characteristic acquired during the lifetime of an organism.

Reproductive success

fitness: an individual's genetic contribution to the next generation

Sexual selection

evolved characteristics that favor successful mating over individual survival

Sexual selection examples

Birds: males a typically colorful with courtship behaviors

females are duller to camouflage from predators

Frogs: distinctive mating calls (louder calls are selected)

Deer: females choose males with the biggest antlers

Sexual dimorphism

morphological differences between males and females

intrasexual competition

competition between males for access to females

Guppy characteristics

From natural selection and sexual selection:

Guppies living near predators are dull (camouflage)

Guppies not exposed have brighter colors (preferred by females)

John Endler guppy experiment

Condition:

1. Field (natural)

2. Controlled (artificial ponds)

In either conditions, guppies were kept in water with predators or without predators and observed over generations.

Absence of predators: brighter colors

With predators: less colorful

Selection pressure (guppies)

1. predation: fitness includes cryptic colors for camouflage. The allele frequency for bright colors would decrease over time

2. Sexual selection: males need to be bright to attract females to be fit.

gene pool

Combined genetic information of all the members of a particular population

Inbreeding

when closely related organisms mate with each other and narrows the gene pool

Allele frequency

Proportion of a specific allele in a population (percent)

Allele frequency example

If a certain allele is present in 25% of the chromosomes, this means 1/4 of the loci have that allele and the other 3/4 have a different allele or there is a 25% chance that a chromosome in the population has the allele at the specific locus.

Mutations on allele frequency

due to mutations, new alleles are introduced and natural selection proves some alleles to be more advantageous hence more frequent. alleles that are not advantageous hinders survival and will not be passed down.

Endler's guppy experiment HL

The presence of predators modified the allele frequency for bright colors in males in the gene pool of the isolated populations because they were more likely to be hunted

HLA (Human leukocyte antigen complex)

An example of allele frequency in humans. HLA is a set of genes found on chromosome 6. Class I produces molecules sitting on the white blood cell surface to identify as self or class ii producing molecules that bring nonself antigens that signal an attack by a pathogen. If the genes of a donor and recipient aren't compatible, rejection.

HLA gene of a kidney donor

doctors see the compatibility of the HLA gene by looking at the versions of the genes present in the patients HLA. The more matches, the less chance of rejection.

polymorphism

when genes show multiple variations (alleles) of species within the same population either genetically or environmentally influenced

allele frequencies of geographically isolated populations

1. Endlers experiment with guppies

2. HLA genes in humans

Large variety of polymorphisms in the HLA genes

Human populations have spread out and lived in different parts of the world. Comparing two geographically isolated populations will tend to have fewer polymorphisms in common

Modern synthesis/neo-darwinism

combining Darwin's work with genetics

Change in allele frequency in a gene pool (moths)

the peppered moth during the industrial revolution had two polymorphisms, a light colored speckled grey and a black phenotype. The rarer black moths adapted better and darker alleles become more frequent. However after the clean air act was passed, pollution levels fell and the light colored phenotype increased.

3 main types of selection

1. directional selection: favors one phenotypic extreme over the other

2. disruptive selection: favors both phenotypic extremes

3. stabilizing selection: favors an intermediate phenotype over both phenotypic extremes

Directional selection

-favors one phenotypic extreme over the other

-phenotypic distribution shifting towards beneficial extreme

-from gradual/sustained changes in environmental conditions

-typically followed by stabilizing selection once an optimal phenotype has been normalized

-ex. antibiotic resistance in bacteria

Disruptive selection

-favors both phenotypic extremes

-phenotypic distribution deviates from center and results in bimodal spread

-from fluctuating environmental conditions that favors the two phenotypes (ex. seasons)

-may result in speciation of the two phenotypic variants

-ex. proliferation of black or white moths in regions of sharp contrasts

Stabilizing selection

-favors an intermediate phenotype over both phenotypic extremes

-removal of extreme phenotypes (phenotypic distribution is centrally clustered to reflect homogeneity)

-from stable environmental conditions and low competition

-ex. human birth weights (large=birthing complications, small=risk of infant mortality)

Hardy-Weinberg equation

p^2 + 2pq + q^2 = 1 (p^2=AA, 2pq=Aa, q^2=aa)

p is the frequency of the dominant allele

q is the frequency of the recessive allele

Hardy Weinberg genetic equilibrium

a populations allele and genotype frequency are constant unless theres an evolutionary force that is:

-random mating

-no selection

-no mutation

-no migration

-diploid organisms

-large population

-no sex linked

-no alleles that reduce survival chances

Antibiotic resistance

consequences of artificial selection from misuse or overprescription of antibiotics

Antibiotic resistance procedure

Due to pre existing variation in the bacterial population, some bacteria are more resistant. If the remaining bacteria is not killed and multiplies, it thrives without any competition from other bacteria. Patient will feel sick again and doctor will prescribe a different antibiotic.

antibiograms

A report with antimicrobial susceptibility test results and inhibition zones.

biotic potential

The theoretical maximum rate at which a species can reproduce and grow when all environmental limitations are removed. Combines reproductive potential and survival potential