D4.1 Natural selection Flashcards
What is natural selection?
The process in which diff environments influences survival & reproduction of organisms due to presence of variations or adaptations.
What are the key components of Darwin’s theory of natural selection?
- Variations are seen among organisms in a pop. Heritable or genetic variations are passed on to offspring.
- Due to overproduction, there is competition for resources leading to a struggle for existence.
- In struggle for existence, organisms with traits that are better suited to environment survive & reproduce. This is often referred to as ‘survival of fittest’.
- Organisms that survive pass on these variations to their offspring.
- Over a period of time, frequency of occurrence of favourable variations in pop increases.
- Natural selection eventually causes pop to become better adapted to its environment.
What does the forces of natural selection operate on?
Forces of natural selection operate continuously on genetic variations present in a population.
How does isolation cause genetic variation which leads to natural selection?
Isolation of these pops by geographical or other barriers prevents interbreeding. In isolated pops, environmental differences cause natural selection to favour diff traits. Over generations, accumulation of differences happens to such an extent that isolated pops are considered as separate species.
What is an example where isolation has created genetic variation?
For instance, diff species of Darwin’s finches seen in Galapagos islands have originated from one ancestral species. Isolation & differing environments led to evolution of several similar-looking species with distinct beak shapes.
What do mechanism of natural selection enable? And over what period of time?
Thus, mechanism of natural selection enables evolutionary change. Over a period of bn of yrs, natural selection has led to amazing biodiversity that exists on Earth today.
What is biodiversity?
Biodiversity refers to diversity of life on Earth & encompasses genetic diversity, species diversity & ecosystem diversity
What is genetic variation?
Genetic variation refers to differences in genomes among individuals of same species, & is an essential prerequisite for natural selection.
What are the two reasons for genetic variation?
Genetic variation arises due to mutation & sexual reproduction
What are mutations?
Mutations are errors in copying genetic info during DNA replication, resulting in alleles or many alternate forms of a gene. Thus, mutation introduces new alleles into a pop, resulting in variation.
What are the impacts of mutations?
Most mutations are neutral to an organism; however, some could be harmful & some beneficial. E.g. a chance mutation in a pop of bacteria could make a few individuals resistant to a particular antibiotic. Following exposure to antibiotic, resistant bacteria are more likely to survive, reproduce & pass on favourable trait.
What do mutations in gametes lead to?
It is important to note that only mutations in cells that produce gametes can be inherited. In other cells of body, these mutations would be ‘dead-end’ mutations & have no value in introducing variation into pop
What are the two ways that sexual reproduction introduces genetic variation in a population?
- Formation of gametes involves crossing over & independent assortment of homologous chromosomes, both of which result in unique allele combinations.
- In addition, random fertilisation leads to unique & new combinations of alleles of both parents. In a pop of many reproducing individuals, alleles are mixed again & again each time reproduction occurs, resulting in endless variation.
What is Malthus’ concept around population growth?
Malthus was an economist who stated, in his Essay on Principle of Population, that growth of human pop was exponential whereas increase in food production happened in a linear manner. In other words, food produced would not meet demands of pop, resulting in poverty, famine & even wars
What was Darwin’s concept of population growth?
Darwin stated that, in nature, there is a tendency towards overproduction – plants & animals can produce far more offspring than can possibly survive. E.g. many species of fish lay thousands of eggs. Oysters can lay between 60 & 80 mn eggs at a time. In both these cases, however, only a fraction survives up to adulthood
What do all organisms need? What are these called? And what do they determine?
All living organisms need resources – food, water, shelter, space &, even, mates. These factors are termed limiting factors, as they determine carrying capacity of environment.
Define: Limiting factor
Carrying capacity
Limited factor: A resource in environment that can significantly affect a population’s size if it becomes limited.
Carrying capacity: Maximum pop size of a species that can be sustained by a given environment.
What does overproduction lead to?
Overproduction leads to competition for these limited resources. Organisms who are ‘fit’ – that is, have favourable variations that enable them to meet these needs – survive & reproduce. Others die or produce fewer offspring. This keeps pop size relatively stable. In other words, overproduction & subsequent competition for resources leads to differential survival & differential reproduction, thereby promoting natural selection. Over time, offspring of survivors make up a larger proportion of pop. Pop will now be better adapted to environment & may look entirely diff from ancestral pop. Eventually this would result in evolution of a new species.
What is selective pressures?
Selection pressures are factors (environmental conditions placed on pop) that lead to differential survival or reproduction, which in turn cause a change in the genetic composition of a pop. Selection pressure could include both density-dependent & density-independent factors
What are density-dependent factors?
Density-dependent factors are factors that affect size of pop & depend on density of pop in a given area. Picture a pop of carp living in a pond with access to a fixed supply of food. As long as pop density is low, there will be enough food available for carp. However, as number of carp in pond increases, competition for food may result in starvation & death of some of carp. Pop density becomes too great for fixed amount of food to support.
What are example of density-dependent factors?
- Availability of food
- Availability of water, shelter & other resources
- Presence of predators
- Spread of pathogens
- Finding of mate
What are density-independent factors?
Density-independent, abiotic factors are physical factors that affect size of pop, irrespective of pop density. Factors such as availability of oxygen, temperature & even natural disasters like wildfires, hurricanes & volcanic eruptions would affect all individuals in a pop, regardless of their density. Returning to carp example: a decrease in dissolved oxygen levels of water would affect all carp in pond, irrespective of how many carp are present in pond, & would lead to an overall decline in their pop.
What are example of density-independent factors?
- Natural disasters like wildfires or volcanic eruptions
- Temp
- CO2 & O2 levels
- Pollution
What does variation of individual lead?
Variation in individuals leads to differences in their ability to survive & reproduce allowing populations to adapt over time. These differences form basis of natural selection.