5.2 Flashcards
List 3 sources of variation among individuals of a species.
Sexual reproduction, mutation, meiosis
What is overpopulation? What are some of its consequences?
Overpopulation occurs when populations produce more offspring than the environment can/will support.
Consequences:
-When the carrying capacity is exceeded, members of the population must compete for resources, because they become increasingly scarce.
Define an adaptation.
An adaptation is a characteristic developed by natural selection that makes an animal more suited to its environment.
What are the results of adaptation?
Adaptations give certain members of the population an advantage and enable them to live longer and have more offspring, which also inherit the adaptation and give them a greater chance of surviving and carrying on the population by reproducing.
-There is progressive change as useful alleles become more frequent. This results in evolution, a cumulative change in heritable characteristics.
Describe bacterial resistance.
Bacteria reproduce quickly, so more mutations appear, leading to more variation. If a form of bacteria inherits a mutation that makes it resistant to antibiotics, the resistant form survives against the antibiotics. It has no competition and will continue to reproduce, passing on the resistant allele to offspring.
- Bacteria can also pass genes to other bacteria species through plasmids.
- The frequency of the resistant allele in the gene pool increases.
Mutation
- Mutations, which produce new alleles and expand the gene pool.
Meiosis
- Independent assortment and crossing-over during Meiosis. Indie. assortment leads to new combinations of chromosomes. Crossing over creates new combinations of alleles.
Sexual Reproduction
- Sexual reproduction, which combines alleles from two different individuals. Mutations can be brought together through sexual reproduction.
Natural Selection Mnemonic
Inherited variation exists within the population
Competition results from an overproduction of offspring
Environmental pressures lead to differential reproduction
Adaptations which benefit survival are selected for
Genotype frequency changes across generations
Evolution occurs within the population
Crossing over
Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I
The exchange of genetic material occurs between non-sister chromatids at points called chiasmata
As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different
Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent
Independent Assortment
When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random
The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I
The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes
Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (2^23) by random orientation
If crossing over also occurs, the number of different gamete combinations becomes immeasurable
Sexual Reproduction
The fusion of two haploid gametes results in the formation of a diploid zygote
This zygote can then divide by mitosis and differentiate to form a developing embryo
As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes
This means that individual offspring will typically show variation despite shared parentage
Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses
Alleles
Alleles encode for the phenotypic polymorphisms of a particular trait and may be beneficial, detrimental or neutral:
Beneficial alleles will better equip the organism to survive and hence produce more offspring (encodes beneficial adaptations)
Detrimental alleles will harm the survival prospects of an organism, leading to fewer viable offspring
Neutral alleles will not affect the organisms survival prospects
Daphne Major
Daphne Major is a volcanic island that forms part of the archipelago that is collectively referred to as the Galapagos Islands
It is the native habitat of a variety of bird species known as Darwin’s finches (subfamily: Geospizinae)
Darwin’s finches demonstrate adaptive radiation and show marked variation in beak size and shape according to diet
Finches that feed on seeds possess compact, powerful beaks – with larger beaks better equipped to crack larger seed cases
In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection
Dry conditions result in plants producing larger seeds with tougher seed casings
Between 1976 and 1978 there was a change in average beak depth within the finch population
Finches with larger beaks were better equipped to feed on the seeds and thus produced more offspring with larger beaks
Antibiotic Resistance
In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation
When treated with antibiotics, the resistant bacteria will survive and reproduce by binary fission (asexual reproduction)
The antibiotic resistant bacteria will flourish in the absence of competition from other strains of bacteria (killed by antibiotic)
Antibiotic resistant bacteria may also confer resistance to susceptible strains by transferring plasmids via bacterial conjugation
The introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)
