Chapter 9 How species evolve

VCE Unit 4 Biology

Gene pool

sum of all alleles possessed by an entire population at any given time. within this pool, we can determine the allele frequencies of different alleles.

allele frequencies

proportion of certain alleles in a gene pool

what leads to increased genetic diversity?

The larger and more diverse gene pool will contain a greater variety of gene and alleles, resulting in increased genetic diversity

mutation

• permanent change in the DNA sequence. they can be advantageous, deleterious or neutral (effects of mutation)

• mutations create new alleles, increasing genetic diversity and can potentially change the frequency of existing alleles by competing with them

advantageous mutation

providing a selective advantage over other phenotypes

deleterious mutation

causes disease

neutral mutation

has no effect on the amino acid sequence (often called silent mutations)

causes of mutations

• mutagens (UV rays, X-rays, chemicals) or can occur spontaneously

• for a mutation to have evolutionary significance, it must be inherited through germline cells

effect of mutations on allele frequencies

• mutations introduce new alleles to a gene pool

  • passing this down to offspring will increase the allele frequency and overtime may replace another allele if it is beneficial

types of mutation

point mutations, frameshift mutations, block mutations

point mutations

changes to one nucleotide in a gene due to substitution

• silent, missense, nonsense

silent point mutations

one nucleotide is substituted for another but still codes for the same amino acid sequence and no effect on the protein due to the degeneracy of the genetic code

missense point mutations

one nucleotide is substituted for another resulting in a different amino acid, altering the protein produced.

nonsense point mutations

one nucleotide is substituted and now codes for STOP leading to a non-functional protein

frameshift mutations

addition or deletion of one or two nucleotides, shifting how the DNA strand is read. this changes the amino acid sequences and the resulting protein. eg. cystic fibrosis, crohn’s disease, certain cancers

insertion (frameshift mutations)

addition of one or two nucleotides which alter the reading frame of the following nucleotides

deletion (frameshift mutations)

deletion of one or two nucleotides which alter the reading frame of the following nucleotides

block mutations

alter the structure of a chromosome by inserting, deleting or swapping nucleotide clusters during meiosis and can impact multiple genes

• duplication

• deletion

• inversion

• translocation

chromosomal abnormalities

aneuploidy and polyploidy

aneuploidy

extra chromosome or is missing one in their karyotype eg. down’s syndrome, klinefelter syndrome, turner syndrome

polyploidy

more than two sets of chromosomes

• not survivable in humans but in plants can result in an altered phenotype

environmental selection pressures

• factors within the enviro that influence the survivability of species and allow for the process of natural selection

• eg. competition, predation, disease and climate change

natural selection

mechanism for evolution where individuals best adapted to their enviro survive and pass on their alleles to their offspring

effects of natural selection

• phenotype that makes the greatest contribution to the gene pool in the next generation has a higher fitness value and is said to be ‘at a selective advantage’. these favourable genes are passed onto the next generation

• phenotype that contributes less has a lower fitness value and is said to be ‘selected against’

four conditions of natural selection

variation, selection pressure, selective advantage, heritability

variation

1.   Phenotypic variation (physical, biochemical, behavioural) exists within a population which is heritable.

selection pressure

2. A selection pressure exists, causing a struggle for survival in some individuals

selective advantage

3. Organisms with favourable phenotypes are selected for and have a selective advantage that helps them overcome the selection pressure. The unfavoured phenotype is selected against and their numbers are reduced in the gene pool.

heritability

4. Those with the favourable phenotype pass their alleles to their offspring, and inherit these traits, increasing the frequency of the favourable allele.

effects of natural selection on genetic diversity

• Environmental selection pressures drive adaptation through natural selection.

  • Phenotypes that are fitter and advantageous are more likely to survive and reproduce, passing on their favourable traits to their offspring.

  • This leads to an increase in the allele frequency of favourable traits in the population.

  • Over time, this can reduce genetic diversity, as favourable alleles rise in frequency while less favourable alleles decrease. This increases chances of extinction due to their inability to adapt to changing environmental selection pressures.

  • Inbreeding will also become more common, leading to a high prevalence of disadvantageous alleles.

genetic drift

involves changes to a population’s allele frequencies due to sudden, random occurrences

eg. natural disasters or random movement to colonize a new population

two types of genetic drift

bottleneck effect and founder effect

bottleneck effect

• A large portion of a population is wiped out due to a random event such as a natural disaster

• Only a small number of individuals remain in the gene pool and does not represent the original population

• causes loss of genetic diversity and increases chances of inbreeding

founder effect

• A small number of individuals colonize a new region and start a new population

• New population is known as colonising or founder population and does not represent original population

• Founding population has a small gene pool and low genetic diversity

• Colonising population is exposed to different selection pressures as the original population

• Mutations may arise forming new alleles

consequences of bottleneck and founder effect

• Bottleneck effect: reduces genetic diversity by removing alleles due to random events eg. Natural disaster

• Founder effect: reduces genetic diversity by establishing a new population with a small, unrepresentative sample of the original population. The colonizing population faces new selection pressures

effect of genetic drift on genetic diversity

• Reduces genetic diversity.

• Increases the chance of inbreeding, keeping harmful alleles in the gene pool.

• Lowers adaptive potential, making the population vulnerable to new selection pressures.

gene flow

the movement of alleles in and out of a population due to migration or interbreeding of individuals between two populations

immigration

movement into a population brings new alleles

emigration

movement out of a population removes alleles

effect of gene flow on allele frequencies

• Immigration increases genetic diversity.

• Emigration decreases genetic diversity.

• It can have a bigger impact on smaller populations as they already have lower genetic diversity

• Interbreeding: when two individuals living in different populations mate and have offspring (increase genetic diversity)

• Reductions in genetic diversity have two major risks:

  • Inbreeding: this keeps harmful alleles in the gene pool

  • Lower adaptive potential: vulnerable to new selection pressures that could challenge and potentially wipe out the entire population due to the absence of advantageous alleles

speciation

process by which populations genetically diverge until they become different species

species

group of organisms that can interbreed to produce viable and fertile offspring. can also be determined by DNA and amino acid sequences

isolating mechanisms

mechanisms that prevent species from interbreeding to produce fertile and viable offspring

• Geographical: physical barrier such as oceans and mountains

• Ecological: different ecological niches or habitats

• Temporal: different breeding cycles

• Behavioural: mating behaviour

• Structural: differences in reproductive organs (physical characteristics)

allopatric speciation

• Occurs when a geographic barrier separates a population, preventing the two from breeding and gene flow.

• Eventually, genetic divergence occurs, creating a new species.

steps of allopatric speciation

1. Geographical barrier separates a population, and no gene flow can occur

2. Different selection pressures act upon each population, favouring different phenotypes

3. Eventually, so much genetic variation has accumulated that the two populations can no longer interbreed even if the geographic barrier is removed

galapagos finches

• Galapagos islands consist of 19 islands in the Pacific Ocean

• Each island represents a specific ecological niche, with its own different selection pressures and species

• Separated by ocean (geographical barrier), preventing gene flow

• 18 Galapagos finches found, each with various beak shapes and sizes to suit their enviro/food source

• Formation of different species of Galapagos finches due to allopatric speciation

• No gene flow, each island contains different selection pressures selecting for different phenotypes and allowing for genetic differences to accumulate.

sympatric speciation

• Involves the formation of a new species in populations located in the same geographical location.

• Where different selection pressures act on different phenotypes within a population, causing individuals with certain phenotypes to diverge from others and form a new species

• Can also be due to genetic abnormalities during gamete formation

• Polyploidy occurs when an organism contains additional sets of chromosomes in its genome (seen exclusively in plants)

howea palms

• Howea belmoreana and Howea forsteriana have distinct soil preferences.

  • H. belmoreana inhabits neutral and acidic soils (low pH)

  • H. forsteriana lives in alkaline soil (high pH)

• Researchers hypothesize that H. forsteriana diverged from its sister species, H. belmoreana, after the initial population colonized alkaline soil, which acted as a selective pressure.

sympatric speciation of howea palms

• Physiological differences, including a change in flowering times (reproductive isolation mechanism), developed in the population in the alkaline soil

• Over several generations, as these differences accumulated, the population in the alkaline soil evolved into a new species, H. forsteriana, which could no longer interbreed with H. belmoreana to produce viable and fertile offspring.

• Given the small size of Lord Howe Island, it is unlikely the palms were ever geographically isolated.

selective breeding

process by which humans select particular traits from a population by directly controlling the breeding of animals or plants to enhance those desired traits. this requires variation in population (just like natural selection) and improves agricultural crops and domestication of animals

process of selective breeding

1. determine desired trait

2. interbreed parents who show desired trait

3. select offspring with the best form of the desired trait and interbreed them

4. continue the process until the population consistently reproduces the desired trait

effect of selective breeding on genetic diversity

• Loss of genetic diversity.

• increased inbreeding, which can increase the prevalence of deleterious alleles.

• Lower adaptive potential – if new selection pressure arises, species may not survive

bacterial resistance to antibiotics

• Inappropriate use and overuse of antibiotics have led to antibiotic-resistant bacteria.

• Bacteria have developed resistance against antimicrobial agents and antibiotics.

• Due to natural variation in bacteria, some are more resistant to antibiotics due to random mutations, some bacteria will be more resistant to an antibiotic than others. These bacteria have a selective advantage

• As a result, natural selection in bacteria has resulted in strains that are antibiotic resistant

antimicrobial agents

agents that kill or slow the growth of microorganisms eg. antiseptics, disinfectants, antifungals, antivirals, antibacterial agents

antibiotics

• chemicals that inhibit the growth of bacteria by interfering with their ability to build cell walls.

steps of bacterial resistance to antibiotics

1. variation: population of bacteria has individuals resistant to an antibiotic as well as individuals susceptible to antibiotic

2. selection pressure: exposure to antibiotic serves as an enviro selection pressure

3. selective advantage: conferred to bacteria with resistance to the antibiotic

4. heritability: bacteria resistant to the antibiotic are able to continue replicating and pass on the allele for resistance to other bacteria via bacterial conjugation, increasing its allele frequency

Factors Contributing to Antibiotic Resistance

• Inappropriate compliance with treatment plans: course of antibiotics is prematurely stopped. May not eliminate all bacteria, which allows them to continue to replicate and time to accumulate mutations

• Inappropriate use of antibiotics: antibiotics prescribed when they are not required. Normal flora is disrupted

• Widespread use of antibiotics: increased use of antibiotics can increase the probability of antibiotic resistance bacteria

antigenic drift

Small, gradual changes to the genes encoding viral surface antigens.

• As mutations accumulate a new subtype of virus can form

antigenic shift

involves sudden and significant changes in the genes encoding for viral surface antigens.

• Occurs when two or more different strains combine when co-infecting a host resulting in viral recombination.

influenza virus

use hemagglutinin and neuraminidase surface antigens. When changes occur in either of these surface antigens, the effectiveness of previous vaccinations and existing medications may be reduced or rendered obsolete