5.2 Natural Selection
Essential idea: The diversity of life has evolved and continues to evolve by natural selection.
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5.2.U1 Natural selection can only occur if there is variation among members of the same species.
5.2.U1 Natural selection can only occur if there is variation among members of the same species.
Be able to:
Define variation.
Explain why natural selection can only function if there is variation in a species.
The theory of natural selection was posited by Charles Darwin (and also Alfred Wallace) who described it as ‘survival of the fittest’. According to this theory, it is not necessarily the strongest or most intelligent that survives, but the ones most responsive to change. Within a species, different individuals of that species show genetic variation.
Individuals that are best suited for their environment will survive and reproduce. If there was no variation within a species, then all individuals would be the same and no individual would be favored over the other and natural selection would not take place.
The process of natural selection occurs in response to a number of conditions:
Inherited Variation – There is genetic variation within a population which can be inherited
Competition – There is a struggle for survival (species tend to produce more offspring than the environment can support)
Selection – Environmental pressures lead to differential reproduction within a population
Adaptations – Individuals with beneficial traits will be more likely to survive and pass these traits on to their offspring
Evolution – Over time, there is a change in allele frequency within the population gene pool
5.2.U2 Mutation, meiosis and sexual reproduction cause variation between individuals in a species
5.2.U2 Mutation, meiosis and sexual reproduction cause variation between individuals in a species
Be able to:
List sources of genetic variation.
Natural selection requires variation among members of a species in order to differentiate survival (variation needed for selection). This variation can manifest as either discontinuous (distinct classes) or continuous (range across a characteristic spectrum)
Mutation:
Mutation:
New alleles arise from the mutation of existing alleles, usually by changing one or a few base pairs
Genetic mutations are the original source of variation within a species.
Meiosis:
Meiosis:
Crossing over of homologous chromosomes during prophase I results in a recombination of maternal and paternal alleles within chromosomes
Independent assortment as homologous chromosomes randomly orient at metaphase I causing a randomized inheritance of maternal and paternal chromosomes within gametes
During meiosis 50% of the females chromosomes will end up in the egg(haploid gamete) and 50% of the male’s chromosomes will end up in the sperm (haploid gamete).
During meiosis chromosomes will line up or assort independently of each other creating (2n) possible variations of chromosomes in the sex cells.
During meiosis, specifically prophase 1, crossing over might occur in homologous chromosomes where parts of each chromosome are exchanged.
Sexual reproduction:
Sexual reproduction:
New combinations of alleles appear during fertilization
As the unique set of haploid alleles in the egg
Combine with the unique set of haploid alleles in the sperm
Sexual reproduction can produce variation in a species through fertilization and meiosis.
Sexual reproduction occurs when two different members of a species create offspring that have a combination of genetic material contributed from both parents.
Random fertilization through sexual reproduction gives millions of sperms a chance at fertilizing the egg. This allows mutations that have occurred in different individuals to come together in their offspring.
5.2.U3 Adaptations are characteristics that make an individual suited to its environment and way of life.
5.2.U3 Adaptations are characteristics that make an individual suited to its environment and way of life.
Be able to:
Define adaptation.
List examples of adaptations.
Adaptations are features of organisms that aid their survival by allowing them to be better suited to their environment adaptations may be classified in a number of different ways:
Structural: Physical differences in biological structure (e.g. neck length of a giraffe)
Behavioral: Differences in patterns of activity (e.g. opossums feigning death when threatened)
Physiological: Variations in detection and response by vital organs (e.g. homeothermy, color perception)
Biochemical: Differences in molecular composition of cells and enzyme functions (e.g. blood groups, lactose tolerance)
Developmental: Variable changes that occur across the life span of an organism (e.g. patterns of ageing / senescence)
Key points to consider
Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population.
The product of natural selection is the adaptation of populations of organisms to their environment.
Where and how an organism lives is largely due to its specific adaptations that allow it to survive and reproduce in a particular area or habitat
In other words their structure allows them to function in that environment
Polar bears are well adapted to life in the Arctic. They have a large layer of blubber to keep them warm. They are strong swimmers, aided by their strong forearms and layer of blubber for buoyancy. They have hollow fur to aid in insulation as well. For plants, cacti have water storage tissue and spines (prevent water loss) because of the infrequent rainfall in the desert.
5.2.U4 Species tend to produce more offspring than the environment can support.
5.2.U4 Species tend to produce more offspring than the environment can support.
Be able to:
State that species have the ability to produce more offspring than the environment can support.
Use an example to illustrate the potential for overproduction of offspring in a population.
The Malthusian dilemma was proposed by English clergyman Thomas Malthus who identified that populations multiply geometrically (i.e. exponential progression), while food resources only increase arithmetically (i.e. linear progression). In other words, species tend to produce more offspring than the environment can sustainably support.
Populations tend to produce more offspring than the environment can supporter that could survive in a particular community or ecosystem. For example, fish produce thousands of eggs but only few make it to adulthood.
Plants also can produce hundreds or thousands of seeds to be released into the environment. When parents don’t spend a lot or even any time caring for their young, they produce many offspring. This is a reproductive method used to make sure some offspring make it to the next generation. Parents that put a lot of time and energy protecting and raising their young tend to have far smaller litters, i.e. most mammals. The population density that the environment can support is called the carrying capacity. If there are too many organisms, the demand for resources increases. However, there is a limited supply of resources in an ecosystem. Overpopulation and a limited amount of resources creates competition within a population.
All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully observation
Populations tend to remain stable in size except for seasonal fluctuations.
5.2.U5 Individuals that are better adapted tend to survive and produce more offspring the less well adapted tend to die or produce fewer offspring.
5.2.U5 Individuals that are better adapted tend to survive and produce more offspring the less well adapted tend to die or produce fewer offspring.
Be able to:
Outline how a “selective pressure” acts on the variation in a population.
List examples of “selective pressures.”
Explain the effect of the selective pressure on the more and less adapted individuals in a population.
Within a population, there is genetic variation between the individuals in the population. The organisms with the beneficial characteristics will be able to out-compete the other individuals with the less beneficial or harmful genetic traits for limited resources and mates. Therefore, these individuals will survive and reproduce and pass these genetic traits onto the next generation of offspring.
Organisms with less desirable traits will die or produce less offspring
observation 3: Environmental resources are limited.
inference 1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of offspring surviving each generation (= struggle for existence).
5.2.U6 Individuals that reproduce pass on characteristics to their offspring.
5.2.U6 Individuals that reproduce pass on characteristics to their offspring.
Be able to:
Contrast acquired characteristics with inheritable characteristics.
State that only inherited characteristics can be acted upon by natural selection.
Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the surviving individuals. Those individuals whose inherited characteristics best fit them to their environment are likely to leave more offspring than less-fit individuals (= natural selection).
This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with the inheritance of favorable characteristics accumulating over the generations.
In other words: Natural selection is differential success in reproduction ( unequal ability of individuals to survive and reproduce).
These organisms that survive and reproduce, pass these beneficial traits onto their offspring. Over many generations the accumulation of these beneficial genetic traits may result in a change in the population known as evolution.
For another species to develop, these genetically different individuals eventually have to become reproductively isolated (separated from the general population) where they will only reproduce with individuals with similar genetic traits.
Acquired characteristics of an individual such as large muscles are not passed on to an organism’s offspring
5.2.U7 Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
5.2.U7 Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
Be able to:
Compare the reproductive success of better and less well adapted individuals in a population.
Explain the cause of the change in frequency of traits in a population through natural selection.
Evolution is the cumulative change in the heritable characteristics of a population, or the changes in allelic frequencies in the gene pool of a population over time, as a result of natural selection, genetic drift, gene flow, and mutation pressure
Since the better adapted individuals of a species are the ones that survive, reproduce and pass their genes on to the next generation, these alleles will become more frequent within the population. The same would hold true for individuals that are less suited to an environment. These individuals will reproduce less frequently and die more often, thus decreasing the frequency of their alleles within a population. These changes happen over many generations.
5.2.A1 Changes in beaks of finches on Daphne Major.
5.2.A1 Changes in beaks of finches on Daphne Major.
Be able to:
Outline the role of Charles Darwin and Peter and Rosemary Grant in the study of Galapagos finches.
Explain how natural selection leads to changes in the beaks of Galapagos finches with changes in weather conditions.
Adaptive radiation describes the rapid evolutionary diversification of a single ancestral line. It occurs when members of a single species occupy a variety of distinct niches with different environmental conditions. Consequently, members evolve different morphological features (adaptations) in response to the different selection pressures
yearly variation in rainfall in the Galapagos Islands can be extreme due to the weather cycles known as El Nino
in wetter years, all sizes of seeds are available in large numbers
that is, the frequency of the smaller beak alleles increases over time
smaller birds with smaller beak sizes eat more efficiently and have higher rates of surviving and reproducing
smaller beaked birds pass on the trait of smaller beaks to their offspring
as a result, in the following year the average beak size decreases
in dryer years, smaller seeds are quickly consumed, leaving only larger, harder seeds that is, the frequency of the larger beak alleles increases over time
larger birds with larger beak sizes eat more efficiently and have higher rates of surviving and reproducing larger beaked birds pass on the trait of larger beaks to their offspring as a result, in the following year the average beak size increases
As predicted by evolutionary theory, natural selection produces gradual changes in traits in response to changes in the environment
5.2.A2 Evolution of antibiotic resistance in bacteria.
5.2.A2 Evolution of antibiotic resistance in bacteria.
Be able to:
Explain how natural selection leads to changes in antibiotic resistance.
List reasons why evolution of antibiotic resistance has been rapid.
Outline the effect of not completing a full dose of antibiotics on the development of antibiotic resistance.
Antibiotics are chemicals produced by microbes that either kill (bactericidal) or inhibit the growth (bacteriostatic) of bacteria. Antibiotics are commonly used by man as a treatment for bacterial infections (not effective against viral infections). In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation.
Shortly after development of antibiotics (e.g. penicillin) nearly all bacteria were killed during an application of antibiotics
some variants of bacteria had a heritable trait that gives them resistance to antibiotics
the resistant bacteria have higher rates of surviving and reproducing
resistant bacteria pass on the trait of antibiotic resistance to their offspring
in each following generation the percentage of antibiotic resistant bacteria increases
that is, the frequency of the antibiotic resistance alleles increases over time
As predicted by evolutionary theory, natural selection produces gradual changes in traits in response to changes in the environment
Resistance can be passed onto other pathogenic bacteria, creating more species of resistant bacteria.
Some examples of bacteria known to develop resistance are Treponema pallidum which causes syphilis and the bacteria that causes tuberculosis (Mycobacterium tuberculosis)
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