Evolution Flashcards
Evolution
change of allele frequency of heritable traits in the gene pool of a population over time
heritable vs acquired traits, Lamarckism vs Darwinism
Lamarckism says that acquired traits are transferred from one generation to another, Darwinism that only heritable traits get transferred
Population growth curve phases
exponential phase, transition phase, plateau phase, carrying capacity (11 bill for humans)
Mechanism of evolution by means of natural selection:
- Overproduction of offspring – each couple tends to produce more offspring than the environment can support – this increases variability, those better adapted (with genetical advantage) survive (greater survival chances of the population)
- Variation of traits in a population – individuals differ by their level of fitness. Fitness is the different survival and reproductive success, a measure of how well an organism can adapt to the environment
- Competition for resources – survival of the fittest, selection pressures (biotic/abiotic and density dependent/density independent)
- Better adapted individuals produce more offspring so their more beneficial traits/genes are more frequent in the successive generation – after several generations the allele frequency in the population changes
Sexual selection example
male peacocks have larger tail feathers than it would be optimal based on their survival success (natural selection selects male peacocks with shorter tail since those have a larger survival success). However, female peacocks prefer males with longer tails (female choice) because those that survive despite long tails must be the fittest and the strongest.
Generally, males are chosen based on female preferences for male traits. Sexual selection selects individuals based on their mating success while natural selection selects based on fitness/survival capabilities (who gets more food, who can escape predation)
Modelling natural and sexual selection experiment
Biologist Jon Endler conducted an experiment with guppies: female guppies prefer colourful males as mates who are more likely to be eaten by predators. Some parts of the streams where guppies live have less dangerous predators than others, and in these locations the males are more colourful and in locations where predators are more dangerous, they tend to be less colourful. Guppy population in each area has evolved in response to the competing preferences of females and predators present in a particular area
Artificial selection
domesticated species originate from their wild relatives. Humans perform selective breeding (they favour animals with desirable traits and only allow those to reproduce). “Desirable genes” accumulate and after some time create a new species. This shows that artificial selection can cause evolution (e.g. pig and wild boar)
Evidence for evolution
- Comparing sequences (a-a, DNA, RNA) - molecular clock
- Homologous anatomical structures (e.g. Pentadactyl limb in vertebrates (amphibians, reptiles, mammals and birds))
- Selective breeding of domesticated animals and plants
Comparing sequences
The more similar the two organisms are, the fewer differences in sequence there are – species develop over time, gradually diverging from the common ancestor as a result of natural selection
Homologous vs analogous anatomical structures
Basic bone structure is the same in all vertebrates, they originate from the same part of ancestor’s body but look differently and have different functions – e.g. Pentadactyl limb in vertebrates (amphibians, reptiles, mammals and birds), forelimbs in all tetrapods
Adaptive radiation – type of evolution from an ancestral species which changes some aspects of its appearance/behaviour to take advantage of the new environmental opportunities – common after cataclysmic events when new niches open for the surviving species – this is the foundation for development of homologous structures – result of divergent evolution
Analogous structures – those with separate origins but similar structure because they adapted to the same function – e.g. wings of birds and bats – anatomical differences (bat wings have flaps of skin stretched between the bones of the fingers and arm, bird wings have feathers extending all along the arm) – result of convergent evolution
Process of gradual speciation:
- A population thrives on a common habitat, random mutations can be exchanged among members of the population (genes flow) but individual organisms remain similar enough and remain members of the same species
- Some environmental change (barrier) gradually separates population into subgroups, as it grows, it becomes more difficult for them to interbreed – subpopulations become reproductively isolated
- Accumulation of different mutations without the possibility for their exchange among the members (no gene flow) + different selection pressure on different habitats
- Over generations, the reproductively isolated populations will evolve independently and lose the ability to reproduce with each other – new species
What is the key for speciation
Reproductive isolation – when the chromosome number changes or the chromosomes become too different to pair up
Types of barriers:
- Geographic isolation (e.g. squirels in Grand Canyon)
- Temporal isolation (incompatible time frames that prevent populations or their gametes from encountering each other, populations interbreed during different times)
- Behavioural isolation (population’s lifestyle isn’t compatible, courtship is unique to a species, special signals that attract a mate - male fireflies, bird song…)
- Isolation due to infertility caused by mutations – example of abrupt evolution – by polyploidy (polyploid speciation in plant genus Allium) – polyploids can only self-fertilize or mate with another polyploid, they are reproductively isolated from the rest of the (2n) population (2n + n = 3n zygote, unpaired chromosome results in abnormal meiosis) – polyploids usually grow larger, most likely because of larger nucleus
Allopatric vs sympatric speciation
Allopatric speciation – reproductive isolation is geographical, assuming natural selection is different at two habitats there is a divergence of the traits of a population (evolution of Darwin finches)
Sympatric speciation – reproductive isolation is behavioural or temporal, assuming natural selection is different at two habitats there is a divergence of the traits of population (e.g. apple maggot fly) – due to some biological reasons (mutations) a subpopulation changes its behavioral patterns and become reproductively isolated from the original population
Gene pool, allele frequency, interpretation of the change in allele frequency
consists of all the genes and their different alleles in an interbreeding population
the frequency of an allele as a proportion of all alleles of the gene in the population - frequency of the dominant allele is denoted as “p” and of the recessive as “q” => p + q = 1
evidence for the evolution of that population – if there are no mutations, migration or any other occurrences happening, the allele frequency will not change in the next generation (genotype frequency might though)
Hardy-Weinberg equation, principle and population – significance and application
(p + q)^2 = 1
describes the gene pool of a population, stating that allele and genotype frequency of a population won’t change despite sexual reproduction unless some of the mechanisms of evolution act upon that population.
Hardy-Weinberg population:
1. No natural selection (none of the present alleles is better or worse suited)
2. No migration (no new alleles flow into the pool)
3. No mutations (no new alleles are made)
4. Population is very large
5. Individuals mate completely randomly (no sexual selection)
Such a population doesn’t evolve and therefore doesn’t exist in nature. However, the H-W principle is applicable from one generation to the next because there cannot be significant changes to the gene pool in one generation – used to analyse the current gene pool of a population and it sets a “baseline” used to compare new results and estimate the pace of evolution of an analysed population
Task example
frequency of PKU in a population is 1/10 000, what is the percentage of carriers in the population?
Freq (disease) = 0.0001 = q^2, q = 0.01, p + q = 1, p = 0.99, 2pq = 0.0198 = 1.98%
Types of natural selection – depending on what the “better suited” phenotype is:
- Stabilizing NS – average phenotype (extremes are selected against), e.g. mouse fur colour
- Directional NS – one extreme phenotype (the other extreme is selected against), e.g. moths in polluted vs clean area
- Disruptive NS – both extreme phenotypes (average/intermediate phenotype is selected against), e.g. Himalayan rabbits
How does antibiotic (AB) resistance arise through the process of natural selection (steps)?
1) The bacteria have variation in their susceptibility to AB – some are non-resistant, some have intermediate resistance, some are highly resistant (encoded in the plasmid)
2) Presence of AB serves as the selective pressure, killing bacteria that aren’t resistant (even some of the resistant bacteria will be killed by a high enough AB dosage)
3) There is differential survival and reproduction – the resistant bacteria continue to live and divide, the non-resistant die
4) Over generations, the frequency of resistance trait increases in the population
5) Also, AB resistance genes can be passed in form of plasmid from host to host, spreading the population wide AB resistance by horizontal gene transfer
