Evolution #101 Flashcards
What is Natural Selection?
environmental or selective pressures act on the phenotype of individuals in a population
lead to changes in allele frequency.
Certain alleles will be favoured and others are selected against.
Variation -
- presence of variation in a population is what allows natural selection to occur.
naturally present in sexually reproducing organisms and then selection allows organisms with certain characteristics to survive and then pass on their genes to the next generation.
The features which are selected to ensure that the organism is well suited or adapted to its environment
Selection: Biotic Factors
- Predators
- Disease-causing organisms
- Competition from members of the same or different species
- Symbiotic relationships with other organisms
Selection: Abiotic Factors
- Rainfall
- Temperature
- Nutrient levels
- Light intensity
- Medical drugs like antibiotics, or chemicals like pesticides
Mutations
permanent changes in the sequence of nucleotides of DNA
source of genetic variation in the species, new forms of a particular gene (alleles).
Mutations in germline cells can be passed on to the next generation.
Variation in Offspring
asexually reproducing organisms = offspring genetically identical, unless mutation has occurred.
sexually reproducing organisms = offspring not genetically identical
Four main factors which contribute to variation among offspring:
Mutations
Crossing over
Independent assortment
Random fertilisation
sexual reproduction allows for the ‘re-shuffling’ of genes from parents to give rise to new combinations in their offspring.
Gene pool
The gene pool of a population is the sum of all of the alleles of all of the individuals in that population.
Population
is a group of individuals of the one species that breed together in a particular habitat.
The genotype of an organism
determines the appearance or phenotype of the individual.
Allele frequency
the relative proportion of a particular allele in the population.
Large/small gene pool
Populations with large gene pools = large diversity of alleles , more likely to survive in harsh conditions.
Populations with; small gene pool = are at greater risk of extinction. less genetic diversity
Changes in the gene pool of a population can be brought about by several factors:
Gene Flow – Movement of alleles between different populations of the same species.
Increased gene flow between populations reduces differences between populations.
Genetic Drift – changes in the frequency of alleles in a population.
Speciation
process that gives rise to the origin of new species.
result from an accumulation of genetic changes influenced by different selection pressures or genetic drift in geographically isolated populations
Allopatric Speciation
involves members of the original population being separated by a geographical barrier.
Sympatric speciation
is where one population gives rise to two or more species while still inhabiting the same region
- no geographical isolation.
If occurs = gene flow still needs to be prevented, leading to reproductive isolation.
Sympatric speciation can occur through hybridisation and…
Asexual Reproduction
Sympatric speciation can occur through Polyploidy
where organisms possess more than two sets of chromosomes.
result of a malfunction during meiosis where chromosomes fail to separate properly.
organism can’t interbreed with an organism with the normal diploid number
- gene flow is immediately stopped.
Convergent Evolution
occurs when different species of organisms develop similar (analogous) features with similar functions.
can occur as different groups of organisms often live in similar habitats with similar selective pressures.
then evolve to have similar structures/ behaviours
Species evolved in particular environments in similar will evolve similar features, undergone convergent evolution.
Analogous Features
have evolved separately in different lines of evolution.
serve the same function in different species but have evolved separately.
has occurred when such features evolve independently in unrelated groups or separate species.
Birds, bats and butterflies are unrelated have all evolved wings for flight independently of one another.
Divergent Evolution
process by which interbreedingspeciesdiverged into two or more evolutionary groups.
these groups of species used to be similar and related. However, they became more and more dissimilar through time.
homologous structures = indicate a species is diverging from its ancestor.
need not have the same function as that of those of the species’ ancestors.
Divergent Evolution: The forelimbs of humans and bats are
homologous structures.
used differently, the basic skeletal structure is the same and they are derived from the same embryonic origin.
indicate plausible evolution from a common ancestor.
Advantages of Divergent Evolution
allows species with common ancestral origin to be able to adapt to their own habitats.
promotes biodiversity.
- leads to speciation,
- and a wide range of different organisms thriving in diverse habitats.
Adaptive Radiation
when a single or small group of ancestral species rapidly diversifies into a large number of descendant species.
Divergent vs Adaptive Radiation
adaptive radiation - small-scale evolution over a shorter span of time
divergent evolution - evolution of species diverging from its ancestors over a relatively long span of time.
adaptive radiation may lead to divergent evolution over time as the species become more and more distinct from its ancestors.
Convergent Comparison
Species evolved from a relatively recent common ancestor.
Gives rise to homologous structures; species diverge.
Species become more different over time.
Species are relatively closely related, as they share a recent common ancestor.
Divergent Comparison
Species have relatively different common ancestors.
Gives rise to analogous structures; species converge
Species appear to become more similar over time.
Species are less closely related, do not have a recent common ancestor.
Issue of reduced gene pool
reduced genetic biodiversity = decreased chance of a population to survive and evolve in the face of changing environmental selective pressures.
large gene pool - more likely that at least some members of the population will survive, pass on favoured genes to the next generation.
Succession
gradual process by which species composition in a community changes.
brought about by modification of the habitat caused by the organisms that live there.
Succession: organisms can change the environment
can provide food or shelter for other organisms. E.g. trees are a source of food and shelter.
change the soil structure by causing the breakdown of rock or increasing the amount of organic matter in the soil.
decompose dead material and make nutrients available for others.
Example of succession
For example, if a lava flow kills all life in a particular area, the order which species begin to grow in the environment can be tracked over time.
The first organisms which establish the area are usually able to survive in harsh conditions e.g. lichens.
These colonizing species can then change the environment to allow other organisms to survive.
There are two main types of succession:
Primary succession and secondary
Primary Succession
Primary succession is where organisms establish themselves in a brand new environment.
Example on Primary Succession
For example, in an area where glaciers are retreating and revealing land with no life present.
Species such as lichens would be able to survive on the harsh rock environment and would slowly change the environment, forming soils.
This soil would then allow species like mosses to survive and over time, with further breakdown and decomposition occurring, the conditions become ripe for grasslands and shrubs.
As these plants begin to survive in the environment, birds and insects can survive and change the environment further, which then allows for other plant species to be introduced.
Over the succession period one type of community is replaced by another, until the ecosystem matures.
At the climax stage of the community, it is dominated by generally larger, longer living and slower growing species.
Secondary Succession
Secondary succession is where changes occur in an environment which has been exposed to damage or a disaster e.g. fire, land clearance.
The same pattern in the emergence of species will occur in secondary succession as in primary succession.
Secondary succession is usually much quicker than primary succession as there is a greater mix of soils and usually a greater mix of species and plants available to recolonize the area.
Succession Characteristics
As the ecosystem matures there is more biomass, but productivity decreases.
There are usually more species in mature ecosystems.
As the ecosystem matures the number of heterotrophic species tends to increase more than the number of autotrophic species.
Mature ecosystems are more efficient at recycling nutrients.
Mature ecosystems have organisms that tend to be more specialized.
Succession, Adaptive Radiation and Divergent Evolution…
When new niches become available to a species, for example as a result of succession or following an environmental change, different selection pressures may lead to divergent evolution or adaptive radiation.
Evidence shows that life has existed on Earth for around…
3.5 billion years.
All living things share many features:
Cell membrane composed of lipids
RNA and DNA
Proteins composed of the same amino acids
Water composition by mass is 50%-80% for all cells and water has the same functions in all cells.
Prokaryotes date back too…
3.5 billion years
The oldest fossils are…
are the cyanobacteria from rocks in Western Australia dated about 3.5 billion years old.
These cyanobacteria formed stromatolites.
Cyanobacteria are photosynthetic, and therefore were important for evolution on Earth as they released oxygen into the atmosphere.
What several processes were necessary for life to begin on Earth?
Synthesis of simple organic molecules e.g. amino acids and ultimately the synthesis of larger organic polymers e.g. proteins.
The origin of molecules that can self replicate. I.e. RNA and DNA.
Packaging of these molecules into membranes with an internal environment different in composition from their surroundings.
These structures are known to form spontaneously.
It is likely that Earth was subjected to a…
a lengthy barrage of meteor showers and emissions from volcanoes which meant the atmosphere contained the precursors needed to make organic molecules.
Evidence also suggests that lfie may have orinagted at the bottom of the sea. How is this possible?
Hydrothermal vents warm ocean water to temperatures suitable for life processes and release hydrogen-rich molecules and minerals that early organisms could have used to obtain energy
What came first DNA or RNA?
DNA is the main hereditary unit of all forms of life today and it could be hypothesized that it was necessary for DNA to be present in the first cells.
The problem with this is that protein enzymes are required for DNA to be copied and replicated. It is likely that RNA appeared before DNA and proteins in the evolution of life.
The key function of RNA and ribosomes in simple cells - RNA can self replicate
RNA canself-replicate
RNA is able to store information in a sequence composed of four recurring nucleotides (similar to DNA)
Short sequences of RNA are able to be accurately duplicated to form identical RNA molecules.
The key function of RNA and ribosomes in simple cells - RNA can act as acatalyst
Modern cells use RNA catalysts (called ribozymes) to remove introns from mRNA and help synthesise new RNA molecules
In ribosomes, rRNA functions as the catalytic component and plays a pivotal role in peptide bond formation
Prokaryotic vs. Eukaryotic Cells
Prokaryotes are the simplest cells which lack a nucleus and membrane bound organelles.
It is thought that prokaryotes evolved before eukaryotes.
One of the pieces of evidence which supports this is the fossil record as the oldest known microfossils are the cyanobacteria found as stromatolites.
Evidence supporting what cell came first.
Fossils – earliest eukaryote fossil found, a multicellular alga – 2.1 billion years old
Biomarkers – eukaryotic biomarkers have been found in sedimentary rocks in the Earth’s crust indicating 2.7 billion years ago. (Sterols – a molecule only found in eukaryotic membranes)
Comparison of cell structure:
DNA in eukaryotes has a higher level of organization
Eukaryotes have membrane-bound organelles
Eukaryotic cells are bigger
Mechanisms for Evolution of Eukaryotic Cells from Prokaryotic Cells
The oldest and most primitive eukaryotes are called Protists.
It is believed that eukaryotic cells probably originated as a collection of prokaryotes.
The evolution of prokaryotic cells would have occurred in two steps:
The first step involved folding of the membrane to form the endo-membrane system. This step is thought to have formed the endoplasmic reticulum and most other organelles.
The second step was the process of endosymbiosis. Chloroplasts and mitochondria seemed to have evolved from prokaryotes which became incorporated after being engulfed by another cell.
Evidence supporting the evolution of prokaryotes
this theory includes that mitochondria and chloroplasts have inner membranes similar to plasma membranes of prokaryotes, they replicate by splitting through binary fission, they have their own circular DNA and have their own protein manufacturing system including ribosomes.
Endosymbiosis
A symbiotic relationship where one organism lives inside the other is known as endosymbiosis. Primary endosymbiosis refers to the original internalization of prokaryotes by an ancestral eukaryotic cell, resulting in the formation of the mitochondria and chloroplasts.
