Ch 7: Natural Selection and Speciation Flashcards
What are gene pools?
- definition: the range of genes and all their alleles present in a population (group of individuals of the same species that live in the same geographic area and produces fertile offspring)
- Natural selection acts on alleles, which are inheritable. Variation in allels carried by diff individuals that leads to most of the voariation in a population - genetic mutation is the main source
- Variation in a population is restricted to what’s already present in the gene pool. But new variation can arise by mutation.
What is genetic polymorphisms and what are the sources of change in allele frequency?
definition: phenotypes that vary due to genetic differences, it can be affected by
Mutations of alleles
- Mutation of an existing allele can produce a new allele.
- Alleles introduced this way are often recessive and stay hidden, unless individuals breed with related individuals. Recessive alleles are often deleterious, but are also an important source of biological variation i.e. peppered moth
Migration and gene flow
- populations are defined by their reproductive and genetic isolation, generally some migrating will always take place
- gene flow: the transfer of alleles that results from the emigration and migration of individuals between populations
Genetic drift
- definition: changes to the gene pool of a population that occur by chance and are not driven by selection pressures, occurs greater in small populations and occurs because alleles are inherited randomly from parents.
- can give rise to a bottleneck or founder effect
- in a small population, during random fertilisation, some alleles present in a parental group will not be passed on and they may be permanently lost from the gene pool
Bottleneck effect
- definition: when a catastrophic event or a period of adverse conditions drastically reduces the size of a population. The alleles present in the surviving members of the population may not be representative of the original population.
- i.e. cheetahs - endangered species that have survived a drstic genetic bottleneck, with a declining population, inbreeding occurred resulting in resulting generations with similar alleles including ones with negative effects on fertility and lowered resistance to disease
Founder effect
- definition: a type of gene flow that occurs when a few individuals that have become isolated from a larger population do not carry all the alelles that were present in the orginigal population
- the isolated population has less genetic diversity than the original and deleterious recessive alleles may have a higher chance of coming together
- i.e. Amish people have commonly suffered from Ellis van Creveld syndrome as one of the settlers harboured the recessive allele for it
What is adaptive evolution?
changes in populations of organisms that make the population better adapted to its environment over generations
What is the definition for natural selection and what are the causes?
definition: the process when indiiduals with certain inheritable traits survive and reproduce more successfully than other individuals leading to evolutionary change in a population
Basis for natural selection
- individuals differ from one another (variation)
- many of these cariations are caused by mutation in alleles and are inheritable
- in general, more offspring are born than can survive to maturity and reproduce. Because of this, there is a struggle for existence and only some organisms can reproduce
- some individuals have traits that make them more suited to their environmental than others, making them better able to reproduce and pass on their alleles to the next generation
What are selection pressures and give examples?
definition: factors that influence the survival of an individual within a population, resulting in one trait becoming more common i.e.
- Competition for food, water and territories - between individuals within a species and between different species
- sexual selection
- predator-preu relationships
What is sexual selection?
definition: occurs when individual animals with certain inherited characteristics are more successful than other individuals in finding mates. It can produce spectacular effects i.e. tail of peacock, large antlers of moose that do not directly give them any extra survival advantage
* can lead to sexual dimorphism: the situation where males and females of a spcies have different morphologies, often in size and shape
What are the different types of natural selection?
Stabilising selection
- natural selection that tends to advantage organisms similar to their parents, this usually occurs when the environment is very stable and unchanging and selects against extreme phenotypes (deleterious alleles that cause a departure from the optimal phenotype)
Directional selection
- a form of selection that selects against one of two extremes and leads to a change in a trait over time
Disruptive selection
- a form of selection that operates in favour of extremes and against intermediate forms
- i.e. a drought may kill of a species of shrub that produces medium-sized seeds. A seed eating bird species may experience disruptive selection where birds with intermediate sizd bills would not be a well adopted and would be selected against
What is artificial selection?
Artificial selection occurs when humans select individuals with traits they desire to become the breeding stock.
It relies upon human intervention.
What is modern synthesis?
the theory of evolution incorporating our understanding of how traits are inherited, also referred to as neo-Darwinism
What is micro-evolution?
evolution (changes) made below species level. This includes changes in allele frequencies over generations
What is macroevolution?
evolutionary changes above the species level. Involves speciation, is the accumulation of micro evololution and occurs over many genertions, often correlates with major changes in environmental conditions or catastrophic geological events and adaptive radiation
Define speciation
the evolution of one or more new species from an ancestral species
What are the processes contributing to the evolution of species?
- natural selection favours phenotypes that make the population better adapted to its environment. Microevolution occurs as populations change over time due to the accumulation of small changes in their gene pools in response to natural selection
- eventually a population accumulates so many changes that a new species can be identified. This leads to speciation, the multiplication of species
- sometimes a rapid series of speciation events leads to the development of a whole collection of new species, or even higher classification groups (macroevolution)
- extinction
What is the biological species concept?
a species is a reproductively isolated group of organisms - individuals from different species and unable to produce viable offspring under natural conditions. These can be identified through consistent differences in morphological and physiological traits as well as genetic differences
- For species that are now extinct, this concept cannot be tested, so the morphological species concept is used instead - this identifies species based on their morphological and physiological characteristics.
- The morphological species concept is limited by what can be observed in the fossil record.
What is the process of speciation?
- a single population becomes separate populations that one unable to interbreed due to changes that produce physical biological or behavioural barriers (reproductive isolation)
- selection pressures act on the spearated populations to cuase microevolution
- over time their allele frequencies may become so different that the individuals are no longer able to interbreed even if they are reunited
What are reproductive isolating mechanisms?
defintion: a mechanism that prevents organisms from mating or producing viable offspring
Pre-reproductive isolation
definition: a mechanism that prevents organismsfrom being able to interact to reproduce
- Geographical features (seas, mountains, distance, habitat) - determined by size and mobility of individual
- Temporal mechanisms (different breeding seasons or time of day)
- Behavioural mechanisms (different courtship patterns; e.g. mating calls)
- Morphological mechanisms (different reproductive structures)
Post-reproductive isolation
defintion: a mechanism that prevents organisms from being able to interact to reproduce including:
don’t prevent mating but prevent formation of viable, fertile offspring
- Gamete mortality (gametes do not survive after mating)
- Zygote mortality (zygote forms but does not survive)
- Hybrid sterility (adult offspring develop but cannot produce viable gametes)
- Hybrid sterility is not often a reproductive isolating mechanism in plants, where polyploidy is common.
What is allopatric speciation?
definition: speciation that occurs due to physical or geographical isolation, most speciation events seem to occur as a result of this leading to the disruptive of the gene flow and divergence of the population, occurs due to
- water, for terrestrial organisms
- land, for aquatic organisms
- mountains
- continental drift
- rising sea levels
- climate change
i.e. flightless comorant on the Galapagos Islands - orginated most likely from a small pop. of flying species that reached the islands from the South American mainland, the two pops. were completely geographically isolated by 1000km of ocean and there was no gene flow. There were different selective pressure acting on the two pops. while there was pressure for effecient movement underwater but there was no predators so less pressure for efficient flight. This lead to a reduction in the size of the wings in the comorant population to a morphology that was well suited to movement under water but which no longer allowed flight
What is sympatric speciation?
Sympatric speciation refers to the evolution of two or more new species within the same place, without physical or geographic isolation i.e. groups within a single population feed on different things or choose mates based on different characteristcs
For example, some cicadas hatch out and reproduce once every 17 years and others hatch every 13 years. This behaviour may have been the reproductive isolating mechanism that led to their sympatric speciation.
How can we prevent extinction?
Preservation of genetic diversity is key to preventing extinction as having a larger gene pool means having more alleles to draw upon to face the pressures of natural selection.
- Conservation reserves protect endangered species from human influence and predation.
- It is difficult to know how large reserves need to be to be effective. Does there need to be a single large or several small (SLOSS) reserves? What population size is required for long-term survival?
- Wildlife corridors can link otherwise isolated conservation areas and allow animals to move to new areas when resources become scarce.
What is the steps of natural selection?
- there is variation in a population due to mutation and/or sexual reproduction (crossing over - chaismata, random assortment and fertilisation)
- there are more individuals produced in a population than the environment can support (overproduction)
- due to the limited resources, the environmental pressures favour those with better suited/more advantageous characteristics and they out-compete those who do not possess the favourable traits (competition)
- those with the advantageous traits survive and reporduce (survival of the fittest - survive to reproductive age)
- when the survivors reproduce, they pass on their advantageous traits to offspring
- over many generations, the frequency of the alleles for the favourable traits increases, the less favourable alelles will decrease
- eventually the less advantageous traits can disappear from the gene pool, leaving the more advantageous trait to become fixed, leading to speciation
