chapter 18: diversity and evolution Flashcards
(28 cards)
what is the definition of micro-evolution
microevolution: changes in the allele frequencies in the gene pool of a population from one generation to the next
- ( a tweak or adjustment in traits over a few generations)
population: it is a group of ___ individuals belonging to a particular ___ and sharing the same ___
population: it is a group of interbreeding individuals belonging to a particular species and sharing the same ecological niche
ecological niche: is a term used to describe the sum of a species’ use of ___ and ___ factors
- it is the interaction of the species with its ecosystems
ecological niche: is a term used to describe the sum of a species’ use of biotic and abiotic factors
- it is the interaction of the species with its ecosystems
biotic factors: living parts of the environment
- what it eats, what eats it, diseases it is affected by
abiotic factors: non-living parts of the environment
- temperature
- humidity
- soil type
- water availibility
what are the five principles of natural selection?
observation 1: overproduction of offspring
observation 2: population sizes tend to remain stable in size
observation 3: environmental resources are limited
observation 4: variation exists within a population
observation 5: much of this variation is heritable
principles of natural selection:
observation 1: overproduction of offspring
what does this mean?
- all species have such high reproductive capacity, such that their population sizes would increase exponentially if all individuals that are born reproduce successfully
principles of natural selection:
observation 2: population sizes tend to remain stable in size
what does this mean?
- although organisms are capable of producing large number of offspring
most populations remain relatively constant in numbers - this is because most of the offspring die before they reach reproductive age
principles of natural selection:
observation 3: environmental resources are limited
deduction: struggle for survival
- based on the first three observations, it can be deduced that individuals of a ___ are constantly ____ with one another for ___ environmental resources ( like food, water, light and space)
- hence, not all organisms will survive to reach ___ ___ to reproduce as there are more individuals than the environment can ___
inference 1: production of more individuals than an environment can support leads to a ___ ___ ___ among individuals in a ___
principles of natural selection:
observation 3: environmental resources are limited
deduction: struggle for survival
- based on the first three observations, it can be deduced that individuals of a (species) are constantly (competing) with one another for (limited) environmental resources ( like food, water, light and space)
- hence, not all organisms will survive to reach (sexual maturity) to reproduce as there are more individuals than the environment can (support)
inference 1: production of more individuals than an environment can support leads to a (struggle for existence) among individuals in a population
principles of natural selection:
observation 4: variation exists within a population
what does this mean?
- members of a population often vary in their inherited traits
- each individual has a unique combination of traits
- some traits improve an individual’s chance of survival and reproductive success while others do not
principles of natural selection:
observation 5: much of this variation is heritable
deduction: survival of the fittest by natural selection
- survival in the struggle for existence is not random, but depends on the ____ ____ of the individuals
- within a ___, some individuals who possess ____ traits survive better in the existing environmental conditions than others
- they are better adapted to survive to reach sexual maturity to produce ____ offspring
> they are more ___ (fitness: refers to the ability to produce viable offspring)
- hence, these individuals are said to be selected ___ by the environment and are at a selective ___
- they may pass on alleles ___ these favourable traits to their offspring
- conversely, less adapted individuals die before they reach sexual maturity and are able to reproduce
> they are at a ___ ____
principles of natural selection:
observation 5: much of this variation is heritable
deduction: survival of the fittest by natural selection
- survival in the struggle for existence is not random, but depends on the (heritable traits) of the individuals
- within a (population) , some individuals who possess (favourable) traits survive better in the existing environmental conditions than others
- they are better adapted to survive to reach sexual maturity to produce (viable) offspring
> they are more (fit) (fitness: refers to the ability to produce viable offspring)
- hence, these individuals are said to be selected (for) by the environment and are at a selective (advantage)
- they may pass on alleles (coding) these favourable traits to their offspring
- conversely, less adapted individuals die before they reach sexual maturity and are able to reproduce
> they are at a (selective disadvantage)
evolution is descent with modification
inference 3:
- the ___ ability of individuals to survive and reproduce will lead to a ___ change in a population
- with ___ characteristics accumulating over generations
- over many generations, the proportion of individuals possessing the ____ traits increases in the population whereas the proportion of individuals lacking the favourable traits ___
- eventually, accumulation ( inheritance) of many favourable traits in a population of organisms over time may result in the formation of a new ___
evolution is descent with modification
inference 3:
- the (unequal) ability of individuals to survive and reproduce will lead to a (gradual) change in a population
- with (favourable) characteristics accumulating over generations
- over many generations, the proportion of individuals possessing the (favourable) traits increases in the population whereas the proportion of individuals lacking the favourable traits (decreases)
- eventually, accumulation ( inheritance) of many favourable traits in a population of organisms over time may result in the formation of a new (species)
what does Neo-Darwinism explain?
- the traits which are passed down to offspring are controlled by genes
- the inheritance of traits is through alleles passed down to offspring through gametes
the source of variation found in a population is due to
- gene mutations
- chromosomal mutations
- meiosis: crossing-over and independent assortment
- fertilisation: random mating of individuals and random fusion of gametes
evolution is said to have taken place when….
evolution is said to have taken place when the allele frequency in the gene pool of the population changes ( micro- evolution)
- ___ ___ is the process in which organisms with certain
inherited traits/ characteristics/ phenotypes are most likely to survive and reproduce than
organisms with other traits/ characteristics/ phenotypes, in a certain environment. - ____ variation (variation in alleles in the gene pool of a population) arises
because of gene ___ , chromosomal ___ , independent assortment, and crossing-over
during meiosis and random fusion of gametes during sexual reproduction. - As the trait of an organism is controlled by its genotype, genetic variation results in
___ variation among organisms within the same population - Challenges in the environment (e.g. climate, disease, competition from other organisms,
predation and limitation of food and other resources) will exert selection ___ on
the individuals of the population.
Selection pressure: Factor that reduces reproductive success in a proportion of a population
Reproductive success: The ability to reproduce to produce viable and fertile offspring. - Different selection pressures will select for different forms of the characteristic. Organisms with
traits which enable them to adapt better to the environment are at a selective ____ and they will be ____ for. They are more likely to ____ and reproduce
to pass on the ____ coding for advantageous traits to their offspring/ next generation. - Organisms with traits that cannot adapt well to the environment are at a selective disadvantage
and will be selected ____ . They are less likely to survive and reproduce to pass
on their alleles to their offspring - Over time and many generations, evolution occurs where the frequencies of various alleles
change in the population gene pool. Frequency of alleles coding for ____ traits will
____ in the population, while frequency of alleles coding for less advantageous
traits will ____ . - The population becomes better ____ to the environment.
- Eventually, the gene pool of the descendent population may become significantly different from
the ancestral population, such that it is ____ ____ from the ancestral population, and
this may result in ____
how natural selection brings about evolution:
- NATURAL SELECTION is the process in which organisms with certain
inherited traits/ characteristics/ phenotypes are most likely to survive and reproduce than
organisms with other traits/ characteristics/ phenotypes, in a certain environment. - GENETIC variation (variation in alleles in the gene pool of a population) arises
because of gene MUTATION , chromosomal MUTATION , independent assortment, and crossing-over
during meiosis and random fusion of gametes during sexual reproduction. - As the trait of an organism is controlled by its genotype, genetic variation results in
PHENOTYPIC variation among organisms within the same population - Challenges in the environment (e.g. climate, disease, competition from other organisms,
predation and limitation of food and other resources) will exert selection PRESSURE on
the individuals of the population.
Selection pressure: Factor that reduces reproductive success in a proportion of a population
Reproductive success: The ability to reproduce to produce viable and fertile offspring. - Different selection pressures will select for different forms of the characteristic. Organisms with
traits which enable them to adapt better to the environment are at a selective ADVANTAGE and they will be SELECTED for. They are more likely to SURVIVE and reproduce
to pass on the ALLELES coding for advantageous traits to their offspring/ next generation. - Organisms with traits that cannot adapt well to the environment are at a selective disadvantage
and will be selected AGAINST. They are less likely to survive and reproduce to pass
on their alleles to their offspring - Over time and many generations, evolution occurs where the frequencies of various alleles
change in the population gene pool. Frequency of alleles coding for ADVANTAGEOUS traits will
INCREASE in the population, while frequency of alleles coding for less advantageous
traits will DECREASE . - The population becomes better ADAPTED to the environment.
- Eventually, the gene pool of the descendent population may become significantly different from
the ancestral population, such that it is REPRODUCTIVELY ISOLATED from the ancestral population, and
this may result in SPECIATION
example of environmental factors as forces of natural selection:
FINCHES OF GALAPAGOS ISLANDS
- The Galápagos Islands consist of multiple islands, each offering DIFFERENT food resources and environmental conditions.
- A long time ago, one ancestral species of ground-dwelling, seed-eating finch migrated from the mainland of Central or South America to the Galápagos Islands.
- GENETIC VARIATIONS exist in the ancestral finch population.
- When the finches colonised different islands, the different environmental conditions (e.g. availability of food types) on each island acted as DIFFERENT SELECTION PRESSURES, causing the populations to evolve in various directions.
- Over many generations, finch populations on different islands evolved different beak shapes, each adapted to a different diet — e.g. seeds, insects, flowers, or leaves.
- The GEOGRAPHICAL ISOLATION of the islands PREVENTED GENE FLOW between populations, allowing natural selection to act independently on each island.
Eventually, these populations evolved into 14 separate species, each with distinctive beaks, songs, and feeding behaviours.
🌱 Adaptive Radiation
This process, where multiple species evolve from a common ancestor to fill different ecological niches, is called adaptive radiation.
example of environmental factors as forces of natural selection:
Shell Banding Patterns in Land Snails (Capaea nemoralis)
- The land snail Cepaea nemoralis shows genetic variation in its shell banding pattern and shell colour.
Two main observable traits: - Banding pattern: some snails have dark bands, others have unbanded shells.
- Shell colour: may be yellow, pink, or brown.
- There is a strong correlation between shell traits and habitat type due to predation pressure:
🟨 In Uniform Grassland Habitats:
The background is yellowish (due to dry grass/hay).
- Brown banded shells are more visible to predators → at a selective disadvantage.
- Yellow unbanded snails are better camouflaged → at a selective advantage.
- Natural selection favours yellow unbanded snails, which become more common in grassland populations.
🌲 In Non-Uniform Woodland Habitats:
The background is more varied and dappled with shadows.
- Yellow unbanded shells stand out more and are more easily seen → at a selective disadvantage.
- Brown banded shells blend in better with the surroundings → at a selective advantage.
- Natural selection favours brown banded snails, leading to their predominance in woodland environments.
what are the different sources of variation
- (mutation) gene mutation
- (mutation) chromosomal mutation
- (sexual reproduction) meiosis
- (sexual reproduction) fertilisation
how does gene mutation cause variation in a population?
gene mutation: causes a change in DNA / nucleotide sequence of a gene is brought about by deletion, insertion, substitution or inversion of one or several nucleotides
- gene mutation is the only source that produces new alleles ( variants of the same gene)
- this increases the size of the gene pool for natural selection to operate on
NOTE: mutations will only have evolutionary impact if they occur in germline cells, causing the mutation to be passed on to the next generation
how does chromosomal mutation cause a variation in population?
chromosomal mutation: change in the structure of chromosomes ( through duplication, inversion, deletion and translocation) or number of chromosomes ( through non-disjunction) respectively
- chromosomal mutations can lead to loss, duplication or rearrangement of genes that may result in new gene interactions that produce a new phenotype
non-disjunction: chromosomes do not separate correctly
- one gamete may get two copies of a chromosome (extra)
- another gamete may get none (missing)
how does (sexual reproduction) meiosis cause a variation in the population?
- meiosis: crossing-over between non-sister chromatids of homologous chromosomes during prophase I and independent assortment of homologous chromosomes in metaphase I lead to new combinations of alleles that can produce new phenotypes
how does (sexual reproduction) fertilisation cause variation in the population?
(sexual reproduction) fertilisation: the random mating of individuals of the same species produce progeny with new combinations of alleles at each gene locus
- random fusion of gametes between two mating individuals also increases genetic variation in a population, by producing progeny with different allele combinations
what is neutral variation and what is the result of it?
- neutral variations/ alleles are genetic variations/ alleles which do not alter the ability of an individual to survive and reproduce
- they neither confer a selective advantage nor disadvantage to the individual
- since neutral variations will not come under selection pressure nor natural selection, they will be preserved in a population
for example,
- much of the DNA in eukaryotes is DNA which does not code for a protein or RNA. Hence, variation in non-coding DNA are neutral as they do not affect the phenotype of the organism and hence do not affect the reproductive success of the individuals
- some variations that occur in the coding region of DNA may also be neutral because of the degenerate code. a change in a nucleotide may still lead to the same amino acid coded, hence there is no change in the protein synthesised, hence no effect on the
phenotype.
- Some variations that occur in the coding regions of DNA may lead to a change in amino
acid(s). However, they have no selective effect as they do not occur in the essential regions
of the proteins (e.g. in active site, binding sites). These variations do not affect protein
structure and function and hence do not affect fitness of the individual.
what is the importance of variation in selection?
- natural selection relies on variation to exist in a population before its effect can be exhibited
( genetic variation results in phenotypic variation among organisms within the same population. this makes different phenotypes available for selection, so that under a selection pressure, some phenotypes confer a selective advantage to certain individuals, while others confer a selective advantage to other individuals) - if population of individuals is genetically identical, selection would have no effect, because all the phenotypes are essentially the same and the selection pressure acts equally on all individuals
- populations that are genetically varied will be able to adapt to changing environmental conditions better than those that are genetically identical
how does natural selection influence the distribution of alleles in a population? and what are the three selection trends?
- natural selection can affect the frequency of a heritable trait in a population in three ways
> depending on which phenotypes in a varying population are favoured, and how common these phenotypes are in the population
the three selection trends:
1. directional selection
2. disruptive selection
3. stabilising selection
what does directional selection favour and what is it?
directional selection: favours individuals with initially rare phenotypes
- the selection operates against one extreme of the range of variations of a particular characteristic (this variation is selected against)
- it causes the frequency curve for variations in the phenotype to shift towards the opposite extreme ( this extreme variation is selected for)
- directional selection is most common during periods of environmental change, or when members of a population migrate to a new habitat with different environmental conditions
example: development of antibiotic resistance in bacteria
