D4.1 Natural selection Flashcards
Natural selection as the mechanism driving evolutionary change
Process of natural selection:
Within a population lies a variation of traits –> populations tend to produce offspring beyond what the environment can support –> competition of resources arises –> factors that can lead to differential survival rates in a population are known as selection pressures –> individual differences in adaptation, survival and reproduction becomes a basis for natural selection –> only inherited traits leading to such differences will result in evolutionary change
Natural selection operates continuously and over billions of years, resulting in the biodiversity of life on Earth
- the environment of earth can be very diff, ranging from deserts to mountainous terrains to deep oceans
- there is a huge diversity of selection pressures found on earth
- due to differences in selection pressures, and the fact that organisms can move from biome to biome, over billions of years life has evolved into many diff species, resulting in the biodiversity of life on earth
Summary of natural selection & evolution
- populations of organisms will show some genetic variation, particularly so in sexually reproducing species
- Indivs transmit their variations to their descendants
- a popln becomes subjected to selection pressures due to changes in the popln/ environment
- the number of organisms in a popln increases in geometric progression –> supply of food and shelter is limited –> struggle for existence between indivs of the same species
- as a result of struggle for existence, “natural selection” is at work in nature, allowing to survive only those indivs that are suitably adapted to the environment –> this is called “survival of the fittest” (differential survival of some indivs over other)
- these indivs obtain a greater share of limited resources
- improving their ability to reproduce viable offspring
- these surviving indivs have an advantageous characteristic (genes/ alleles) so their offspring will carry the genes/ alleles for these characteristic into the next gen
- useful genes will increase in frequency in the popln, less useful genes will decrease in frequency
- such a process of cumulative change in the heritable characteristics of a popln results in evolution
Summary of natural selection & evolution (part 2)
- there is variation in a popln
- variation is due to mutation, meiosis, and sexual reproduction
- more offspring are produced than can survive, thus there is competition for survival
- leading to survival of those best adapted to the changed environment
- favourable genes are passed on as the best adapted organisms reproduce more
- thus alleles for adaptations to the changed environment increase in the population
- which leads to evolution by natural selection
- evolution is cumulative change in population over time
Notes
- poplns evolve, not indivs
- bc poplns gradually diverge over time, it is natural to see continuous variation across a geographical range –> the greater the geographical separation & the longer the poplns have been separated, the greater the divergence
- the variation in finch beak morphology is a genetically controlled characteristic, it reflects differences in feeding habits –> the beak of finches provided evidence for Darwin’s theory of natural selection
Nature of Science: Larmack vs Darwin
Jean Baptiste Larmack was a scientist who lived around 50 years before Charles Darwin
- Larmack came up w a mechanism to explain evolutionary change, using a theory that was similar to the theory of natural selection
- BUT larmack proposed the inheritance of acquired traits (that had no genetic basis) which was wrong
- Darwin’s theory of natural selection focused on the inclusion of the inheritance of genetic traits, which was correct
This represented a paradigm shift in the understanding of how life evolved -> when a new idea allows us to see a phenomenon in a diff way, it is considered a paradigm shift
Importance of variation in natural selection
Due to genetic differences btw indivs in a popln, there will be a variation of traits
- eg ABO blood groups, presence of genetic disease, height, skin colour
It is impt for a popln to exhibit variation in traits, as it is based on such variation that some indivs will survive better than others
- if there is no variation, a catasrophe/ strong selection pressure can either have no effect/ wipe out the entire popln
Mutation, meiosis & sexual reproduction as a cause of variation
As traits are expressed from the genes within an individual, any processes that results in the change in genes may cause a change in the expressed trait, giving rise to variation
- mutations can arise from mutagens from the environment, errors during DNA replication & from viral infections –> the new resultant changes in the DNA sequence can give rise to diff & new traits
- crossing over & independent assortment that occurs during meiosis can give rise to variation of traits in the offspring
- sexual reproduction also involves the random fusion of gametes, as each gamete is diff from one another & this randomness gives rise to variation
Summary of genetic variation
- mutation in genes generates variation. change to base sequence of gene causes new alleles formed
- radiation & mutagens increase the chance of mutation
- meiosis generates variation as in prophase 1, new combinations of genes are produced by crossing over & in metaphase 1, independent assortment of pairs of homologous chromosomes
- gametes from 2 parents combined in sexual reproduction & random fertilisation increases genetic variation
- in reproductively (eg geographically) isolated populations, natural selection may differ & in small isolated populations, evolution occurs due to natural selection
- disruptive selection can cause diff variatints to diverge
- transfer of genes (between bacteria) in plasmids
THEREFORE, the indiv offspring of parents are not identical & show variations in their characteristics. a particular indiv’s success in reproduction will result in certain alleles being passed on to the next generation in greater proportion than other alleles
*if asked to explain how sexual reprodn leads to variation & evolution –> describe & explain effects of independent assortment, crossing over & random fertilisation on genes, allele combination and gametes [DONT discuss role of mutation bc it is not part of sexual reprodn]
Overproduction of offspring & competition for resources are factors that promote natural selection
species tend to produce more than one/ two offspring in their lifetimes –> impt to ensure that popln/ species will not go extinct as many will die to disease/ predation
- BUT there is a good chance that many offspring will survive & popln numbers exceed what environment can support –> CARRYING CAPACITY is reached –> leads to increased mortality & lower life expectancy due to competition for resources (eg food)
Variation between members of population exist & the better adapted are more likely to survive & thus better adapted to reproduce to pass on favourable genes
natural selection leads to evolution
Carrying capacity of an Environment
Carrying capacity = max number of individuals that an environment can provide for
In a specific environment there are too many factors that can become limited when the popln living inside them grows too large
- there is a limit to how much food (energy from food web) is available to organisms in the environment
- space, esp habitable ones, is a potential limited resource
Consequence of supply & demand is competition for resources to stay alive = STRUGGLE FOR SURVIVAL [key component of natural selection]
competition for resources as a factor which promotes natural selection
when there are limited resources like food & space, indivs in a popln come under competition –> indivs may possess traits more advantageous than others
EG giraffes with long necks can reach higher levels on trees in the savannah & potentially obtain more nutrients while other with shorter necks will have less access to less food
- those with longer necks with more nutrients will be healthier & survive longer & have more offspring
abiotic factors as selection pressures
A selection pressure = a factor that can influence the success of parts of a population & thereby influence changes in allele frequencies
- density dependent selection pressure
- refers to those heavily influenced by density
- tend to be biotic factors
- eg predation (density of predators exert a stronger pressure), disease (higher density results in higher rate of disease transmission) & competition for space - density independent selection pressure
- refer to those that would be applied equally to the entire population regardless of density
- tend to be abiotic factors (non-living)
- eg high/ low temperatures (MUST KNOW)
- eg seasonal changes (monsoon rains) & pollutants in environment
EG (MUST KNOW) Magellanic penguins
- live along the coasts of the southern tip of South America & are well adapted to surviving in snow & cold
- penguin chicks are covered w down feathers that allow them to maintain a healthy body temp by trapping warm air near their bodies
- if snow lands on them, they can shake it off & keep dry and warm
- but there are not as adapted for rain –> it causes the downy feathers to stick to the chick’s bodies & the thermal insulation effect is lost bc no more warm air surrounds their bodies –> chicks then die of hypothermia bc of their inability to keep warm
- with global climate change, higher temperatures in winter means that it now rains more frequently in parts of the range of Magellanic penguins, causing a decrease in some poplns
