TOPIC O: BIOLOGICAL EVOLUTION Flashcards
What is Darwin’s Theory of descent with modification?
All organisms are derived from common ancestors by a process of branching. Over time, populations evolve into different species, which are related because they descend from a common ancestor.
It also explains why similar species tend to occur in the same geographic region. As the descendants of that ancestral organism occupied into various habitats over millions of years, they accumulate diverse modifications or adaptations that fit them to specific ways of life. The changes are gradual and slow, taking place over a long time.
What are the two requirements for natural selection to occur?
- There must be heritable variation for a particular trait. If all the organisms within a species are identical, there would be no choice for selection pressures to act upon.
- There must be differential survival and reproduction abilities associated with the possession of that trait.
What are the essential features of Darwin’s theory of evolution?
- Overproduction of offspring. All individuals of a population are capable of producing large numbers of offspring. Many of these offspring will fail to survive and reproduce.
- Constancy of numbers.
- Variation in traits between individuals within a population.
- Struggle for survival and survival of the fittest.
- Individuals of a population are constantly competing with one another for limited resources like food and shelter.
- Individuals who are better adapted will survive till maturity and produce viable offspring compared to others. - Like produces like.
Individuals who can survive to maturity are likely to produce offspring who will inherit the beneficial traits, which give them the edge over others. May lead to speciation
What is adaptive radiation and how do Darwin’s finches exhibit it?
Adaptive radiation is the development of a variety of species from a single ancestral form as a result of novel adaptations when a new habitat becomes available to that population.
On the Galapagos, the finches who descended from one common ancestor led to many species because other birds were few or absent, leaving empty niches to fill, and because the numerous islands of the Galapagos provided ample opportunity for geographic isolation.
What are the three types of selection?
- Directional selection.
- When one extreme of the trait is disadvantageous, natural selection acts against it, favouring the rest of the individuals.
- Average population shifts over time, moving away from the disadvantageous extreme. - Stabilising Selection.
- The extremes are disadvantageous and eliminated, leaving the intermediate phenotypes favoured and preserved by comparison.
- The population becomes more uniform as variation in the trait is reduced. - Disruptive Selection.
- When intermediate phenotypes are at a disadvantage, selection is disruptive, producing mostly individuals with traits at the two extremes of the bell curve distribution.
- This is polymorphism, as two or more conspicuous and distinctive forms occur in abundance in the same species.
How does diploidy preserve genetic variation in natural populations?
The diploid nature of most eukaryotes hides a considerable amount of genetic variation from selection in the form of recessive alleles in heterozygotes.
Harmful recessive alleles can persist in a population through their propagation by heterozygotes.
It is only exposed to selection only when each parent carries the same recessive allele and combines the two copies in their zygote.
Heterozygote protection maintains a huge pool of alleles that may not be suitable for present conditions but could bring new benefits when the environment changes.
How does balanced polymorphism preserve genetic variation in natural populations?
Natural selection can maintain stable frequencies of two or more phenotypic forms in a population.
Heterozygote Advantage:
If individuals who are hetzyg at a particulr gene locus have greater fitness than any type of homozyg, then two or more alleles will be maintained at that locus by natural selection.
Frequency Dependent Selection:
Survival and reproduction of any one morph declines if that phenotypic form becomes too common in the population.
What are the advantages of molecular methods in classification?
Evolutionary changes between species can be comapred via similarities or differences in nucleotide or amino acid sequences even though they may differ vastly in terms of morphology.
Degree of divergence between different species can be quantitatively measured by comparison of nucleotide or amino acid sequences.
Molecular comparisons transcend barriers among organisms whose relationships cannot be evaluated by traditional experimental techniques.
What is direct comparison of DNA sequences and its dis/advantages?
short DNA sequences are sequenced. Scientists obtain the DNA sequences of a segment of a particular gene for comparison between species.
Adv: Comparisons of direct molecular sequences allow evolutionary rates to be estimated more quantitavely
Relatively quick and inexpensive.
Dis: Alignment between the sequences can sometimes be difficult if two species have diverged significantly.
Relatively large quantity of purified DNA is needed for the procedure. PCR can be employed to amplify the DNA for sequencing.
What is mitochondial DNA sequence comparison and its advantages?
Mitochondrial DNA is inherited only from the maternal parent to offspring.
Adv: Direct phylogeny via the maternal lineage can be deduced as the sequence is not confounded by genetic recombination.
mtDNA also mutates at a higher rate compared to nuclear DNA, so it gives researches a more useful, magnified view of diversity and phylogeny.
mtDNA mutations occur at a constant rate, time taken for species divergence from their ancestors can be calculated.
What is DNA-DNA hybridisation and the dis/adv?
Estimates the ease and accuracy by which homologous DNA strands from two different species can form a double helix.
DNA extracted from each species are dissociated into single strands and then allowed to re-anneal with the single strands of another species to form double strand hybrids by incubating them at appropriate temperatures.
Adv: Allows an estimate of overall divergence in DNA sequences between species.
Disadv: Through yielding a single hybridisation value, a wide variety of sequence information is lumped together, thus the estimation of divergence between the species lack precision
What is the direct comparison of amino acid sequences and the dis/adv?
Amino acids are sequenced by biochemical methods and compared against the same proteins in different species.
Measuring divergence in protein sequences of proteins commonly found in various species is used as a method of estimating evolutionary divergence.
Adv: Comparisons of direct molecular sequences allow evolutionary rates to be estimated more quantitatively
Dis: Underestimates degree of evolutionary change because of the redundancy and degeneracy of the genetic code. Silent mutations may not be detected because these do not occur in a change to the amino acid sequence.
What is the biological concept of species and its limitations?
The BCS defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but do not do so with members of other such groups.
They are defined in terms of reproductive compatibility.
Limits:
Does not apply to organisms that reproduce asexually.
Does not apply to extinct organisms
Overemphasises on gene flow, downplaying the role of natural selection as it can cause morphologically and ecologically distinct pairs of species to remain so and yet have gene flow between them.
What is the ecological concept of species and their limitations?
ECS defines one in terms of its ecological niche, the sum of how members of the species interact with the non-living and living parts of the environment.
Adv: Accommodate asexual as well as sexual species.
Emphasises the role of disruptive natural selection as organisms adapt to different environmental conditions.
Limits: Niches are generally difficult to identify because a niche involves the interactions between the organism and its environment.
What is the morphological concept of species and their limitations?
Defines a species in terms of its body shape and other structural features.
Adv:
Can be applied to a/sexual organisms
Does not require any info on the extent of gene flow.
Easiest and fastest concept to apply
Limit:
Relies on subjective criteria and researchers may disagree on which distinguish a species.
