Evolution Flashcards
Define evolution
Evolution is the cumulative change in the heritable characteristics of a population
evidence for evolution provided by the fossil record
The totality of fossils (both discovered and undiscovered) is known as the fossil record
The fossil record reveals that, over time, changes have occurred in features of organisms living on the planet (evolution)
Moreover, different kinds of organisms do not occur randomly but are found in rocks of particular ages in a consistent order (law of fossil succession)
This suggests that changes to an ancestral species was likely responsible for the appearance of subsequent species (speciation via evolution)
Furthermore, the occurrence of transitional fossils demonstrate the intermediary forms that occurred over the evolutionary pathway taken within a single genus
* fossil record is incomplete*
Outline the evidence for evolution provided by selective breeding of domesticated animals
example of artificial selection, which occurs when man directly intervenes in the breeding of animals to produce desired traits in offspring
domesticated breeds can show significant variation compared to the wild counterparts, demonstrating evolutionary changes in a much shorter time frame than might have occurred naturally
Outline the evidence for evolution provided by homologous structures
Comparative anatomy of groups of animals or plants shows certain structural features are basically similar, implying a common ancestry
Homologous structures are those that are similar in shape in different types of organisms despite being used in different ways
An example is the pentadactyl limb structure in vertebrates, whereby many animals show a common bone composition, despite the limb being used for different forms of locomotion
illustrates adaptive radiation (divergent evolution) as a similar basic plan has been adapted to suit various environmental niches
The more similar the homologous structures between two species are, the more closely related they are likely to be
State that populations tend to produce more offspring than the environment can support
The Malthusian dilemma states that populations tend to multiply geometrically, while food sources multiply arithmetically
Hence populations tend to produce more offspring than the environment can support
consequence of the potential overproduction of offspring is a struggle for survival
However, with more offspring there will be less resources available to other members of the population (environmental resistance)
This will lead to competition for available resources and a struggle for survival
Intraspecific competition occurs when members of the same species compete for the same resources in an ecosystem (e.g. light, food, water)
It is density dependent, as the available resources must be shared among members of the species
Competition that occurs between different species for resources is interspecific
members of a species show variation
Members of a species show variation, which can manifest itself in one of two forms:
Discontinuous variation: A type of variation usually controlled by a single gene, which leads to distinct classes (e.g. ABO blood group in humans)
Continuous variation: A type of variation controlled by many genes, which leads to a range of characteristics (e.g. skin pigmentation in humans)
three primary sources of variation within a given population Gene mutations (a permanent change to the genetic composition of an individual) Gene flow (the movement of genes from one population to another via immigration and emigration) Sexual reproduction (the combination of genetic materials from two parental sources)
reproduction promotes variation within a species
Independent Assortment
crossing over
Random Fertilisation
Independent Assortment
During metaphase I, when homologous chromosomes line up at the equator, the paired chromosomes can randomly arrange themselves in one of two orientations (paternal left / maternal right OR maternal left / paternal right)
When the chromosomes separate in anaphase I, the final gametes will differ depending on whether they got the maternal or paternal chromosome
Independent assortment of chromosomes creates 2n different gamete combinations (n = haploid number of chromosomes)
Crossing Over
During prophase I, when homologous chromosomes pair up as bivalents, genetic information can be exchanged between non-sister chromatids
The further apart two genes are on a chromosome, the more likely they are to recombine
Crossing over greatly increases the number of potential gamete variations by creating new genetic combinations
Random Fertilisation
Fertilisation results from the fusion of gametes from a paternal and maternal source, resulting in offspring that have a combination of paternal and maternal traits
Because fertilisation is random, offspring will receive different combinations of traits every time, resulting in near infinite genetic variability
natural selection leads to evolution
The theory of natural selection was postulated by Charles Darwin (and also independently by Alfred Wallace) who described it as ‘survival of the fittest’
There is genetic variation within a population (which can be inherited)
There is competition for survival (populations tend to produce more offspring than the environment can support)
Environmental selective pressures lead to differential reproduction
Organisms with beneficial adaptations will be more suited to their environment and more likely to survive to reproduce and pass on their genes
Over generations there will be a change in allele frequency within a population (evolution)
Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria
Example 1: Staphylococcus aureus (associated with a variety of conditions, including skin and lung infections)
Variation: Antibiotic resistance (some strains have a drug-resistant gene ; other strains do not)
Environmental change: Exposure to antibiotic (methicillin)
Response: Methicillin-susceptible S. aureus (MSSA) die, whereas methicillin-resistant S. aureus (MRSA) survive and can pass on their genes
Evolution: Over time, the frequency of antibiotic resistance in the population increases (drug-resistant gene can also be transferred by conjugation)
Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria
Example 2: Peppered Moth (Biston betularia)
Variation: Colouration (some moth have a light colour, while others are a darker melanic colour)
Environmental change: Pollution from industrial activities caused trees to blacken with soot during the Industrial Revolution
Response: Light coloured moths died from predation, whereas melanic moths were camouflaged and survived to pass on their genes
Evolution: Over time, the frequency of the melanic form increased (with improved industrial practices, the lighter variant has become more common)
D.4.1 Explain how the Hardy-Weinberg equation is derived
The Hardy-Weinberg equation predicts the frequency of two alternate alleles in a population
It is used for traits that show classical Mendelian inheritance:
Only two alleles for a gene (one dominant and one recessive allele)
Follows autosomal inheritance (not sex-linked traits)
For two alleles of a given genetic characteristic, three genotypes will exist: AA, Aa and aa
Dominant allele is A, with a frequency of p
Recessive allele is a, with a frequency of q
The total frequency of both alleles will be 1 (i.e. p + q = 1)
Because genotype frequencies consist of two alleles, the equation must be squared: (p + q)2 = 1
This gives the expanded form of the Hardy-Weinberg equation: p2 + 2pq + q2 = 1
Frequency of AA= p^2
Frequency of Aa=2pq
Frequency of aa= q^2
