D4.1 NATURAL SELECTION

Outline evolution

EVOLUTION: The cumulative changes in the heritable characteristics in the frequency of genes of a population with small changes over a long period of time as individuals reproduce or die, resulting in populations to evolve

1. Charles Darwin proposed the idea of evolution by natural selection

2. Widely accepted that species change

3. Darwinism replaced Lamarckism (the idea that species acquired changes)

4. Was a paradigm shift

Outline natural selection

NATURAL SELECTION: The mechanism driving evolutionary change where organisms that are better adapted to their environment survive, reproduce, and pass on their advantageous alleles which over time changes the allele frequency in the population

1. Organisms with advantageous traits have a greater fitness and ability to survive (survival of the fittest) and there is a higher reproductive potential of the genotype

2. Operates continuously over a long period of time (billions of years)

3. Results in biodiversity of living organisms

4. Occurs due to variations in populations

Outline the role of mutation and sexual reproduction in natural selection

1. Natural selection because of variation (differences between species members) that exists among members of a species. Variaiton due to:

2. Mutation in DNA through mutagens, viruses, or replication errors that creates new alleles

3. Meiosis: crossing over in prophase I and independent assortment of homologous chromosomes in Metaphase I that creates new allele combinations

4. Gene flow: Movement of genes between two different populations

5. Sexual reproduction: Random fertilisation of sperm where the combination of gametes is random — the alleles combined from each parent provides variation

Outline overproduction of offspring and competition for resources as factors

1. Overproduction of offspring and competition for resources promote natural selection

2. Populations tend to produce more offspring than the environment can support

3. The population size has increased above the carrying capacity

4. Limiting factors limit population size

5. Competition for limiting factors like food, space/habitat, resources, and mates

Outline abiotic factors as selection presures

1. Selective pressure can be abiotic factors

2. Abiotic factors are non-living and density independent

Outline sexual slection as a selection pressure in animal species

SEXAUL SELETION:

1. Differences in physical and behavioural traits can indicate overall fitness and affect success in attracting a mate and drive the evolution of an animal population. 

2. Reproductive isolation is when allele and phenotypes might decrease chances of reproductive success, isolating the organism

SEXUAL SELECTION VS NATURAL SELECTION:

1. Sexual selection increases reproductive success, but does not increase adaptation to the environment

2. Sexual selection involves competition for mates, natural selection involves competition for limited resources

3. Both can affect the reproductive success of the organisms and drive evolution

Outline an example of sexual selection as a selection pressure in animal species

THE PLUMAGE OF BIRDS OF PARADISE:

1. Sexual dimorphism is the distinct difference between size or appearance of the sexes 

2. Male birds of paradise are brightly colored and can perform courtship

3. Female birds is dull and grey and brown

4. The more brightly colored the male, the more likely females will be attracted and reproduce

Outline the requirement that traits are heritable

1. Advantageous traits are due to advantageous alleles

2. Advantageous alleles are more likely to increase the chance of survival and are more likely to reproduce

3. Advantageous alleles are passed onto offspring which leads to changes in the allele frequency in the population and the evolution of the population

Outline the process for the evolution as change in the heritable characteristics of population

EVOLUTION: The change in allele frequencies in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits

1. Variation exists within populations

2. Variations/adaptations are inherited

3. Populations produce more offspring than the environment can support

4. Environment changes

5. Struggling for survival

6. Disadvantageous variations/less well adapted individuals tend to die — less likely to reproduce and pass on genes, gene frequency decreases

7. Advantageous variations/better adapted survive — more likely to reproduce and pass on genes, gene frequency increases

8. Population evolves to be better adapted to the enviornment

Outline modelling of sexual and natural selection based on experimental control of selection pressures

JOHN ENDLER’S EXPERIMENTS WITH GUPPIES.

1. Investigate the effect of natural and sexual selection on the evolution of guppies

2. In the presence of a predator, substrate type (corse/fine gravel) affects survival — Coloration would decrease

3. Absence of a predator would increase the selection of brightly coloured fish — coloration would increase

GUPPIES:

1. Guppies exhibit variation in color and pattern and sexual dimorphism

2. males are brightly colored and females are dull

3. Coloration and pattern provides them with camouflage from predators

EXPERIMENTAL SET UP:

• Random breeding with no selective pressure for 6 months produced a wide range of phenotypes

• 5 ponds had coarse gravel / 5 ponds had fine gravel

• 2 had no predation

• 4 had a dangerous predator

• 4 had a weak predator

Outline the interpretation of John Endler’s experiments with guppies

1. In ponds with high predation , the mean no. of spots decrease / In ponds with low predation, the mean number of spots increased

2. in course gravel the spots were larger / in fine gravel the spots were smaller

3. spot size mimicked gravel size

4. with no predation the opposite was found - spot size was opposite the background / not matching the background made males more attractive to females

Compare and contrast directional, disruptive and stabilising selection

ALL THREE TYPES: Results in change in allele frequency

DISRUPTIVE:

1. applies selective pressure to the intermediate phenotype

2. occurs in response to environmental change

3. can produce a new species

4. ex.  grass grown in contaminated soil favored fast and slow growing species.

STABILISING:

1. applies selective pressure to the extreme phenotype

2. favors the intermediate phenotype

3. occurs when environmental conditions do not change

4. most common form of selection & results in normal distribution

5. applied to polygenic traits & decreases diversity

6. ex.  human birth weight

DIRECTIONAL:

1. applies selective pressure to the one phenotype

2. favors an extreme phenotype

3. occurs in response to environmental change

4. ex.  Darwin’s finches and drought selected against smaller beak size and selected for a particular size / trait

Outline the concept of the gene pool

A GENE POOL - consists of all the genes and their different alleles, present in a population of interbreeding individuals

Stable gene pools are: 

1. large gene pools 

2. with random chances of mating

3. no selective pressure

4. not evolving

How does antibiotic resistance occur?

1. High number of bacteria — a few of them are resistant to antibiotics

2. Antibiotics kill bacteria causing the illness as well as good bacteria protecting the body from infection

3. The resistant bacteria now have preferred conditions to grow and take over

4. Bacteria can even transfer their drug resistance to other bacteria, cuasing more problems

Outline artificial selection

1. Artificial selection is the deliberate choice of traits

2. Artificial selection is carried out in crop plants and domesticated animals by choosing individuals for breeding that have desirable traits. 

3. traits are heritable

4. over time, offspring will increase the expression of the desired trait

EX. Lactation in cows

1. Chooses animals with beneficial traits — like milk production

2. Offspring are tested and low-yielding with be removed

3. over generations, offspring with have large milk yields where bulls and cows have high lactations

4. Increased from 5kg/yr to 48,000kg/yr

Outline allele frequencies of geographically isolated populations

ALLELES:

1. Polymorphisms (different forms of genes)

2. Can vary with a single nucleotide polyphormism/difference

ALLE FREQUENCY: The relative frequency of alleles in a population

1. Must add up to 1… p+q=1

2. Hardy Weinberg equation can be used to calculate phenotype and allele frequency

GEOGRAPHIC ISOLATION:

1. Geographically isolated populations do not mix/reproduce

2. Isolated populations are exposed to different selection pressures and changes in allele frequencies of the population

ALLELE FREQUNECY NET DATABSE:

1. HLA - Human Leukocyte Antigen

2. Located on Chromosome 6 codes for cell surface antigens responsible for regulation of the immune system

Outline the Hardy Weinbergg equation

1. Use p and q to denote the two allele frequencies

2. p + q = 1

3. Genotype frequencies are predicted by p² + 2pq + q² =1

CONDITIONS:

1. No random mating

2. No gene flow/migration/immigration

3. No natural selection

4. No mutation

5. Large population sizes

6. Not sex linked

7. Organsim is diploid

8. Offspring produced by sexual reproduction