D4.1 - natural selection and sexual selection (5q) Flashcards
What is an example of experimental control of selection pressures?
John Endler’s studies on guppies, which are a tropical fish, native to Trinidad
- there is variation in the colouration of male guppies, including number, colour and size of spots
- he completed observational studies, field studies and lab-experiments to study the evolution of this colouration
what was Endler’s hypothesis and initial observations?
John Endler spent years mapping the location of pools containing guppies and categorizing the types, forming the following observation:
there were 2 types of selection pressures:
- sexual selection leading to an increase in colorful males within a population
- predation, as part of natural selection, leading to a decrease in colorful mates within the population
how did Endler investigate his hypothesis using field studies?
what could he conclude from this?
- he found a pool containing the non-guppy predator and no guppies
- he transferred the dull-colored guppies out of a pool containing their predator into the pool with the non-guppy predators
- after 2 years (15 generations), he observed that the guppies remained dull in the pool with the predator, but in the non-guppy predator pool, they evolved to be colorful
- this data suggests that the selection pressure of predation leads to a decrease in the number of colourful guppies
what lab experiments did Endler carry out?
he carried out experiments using ponds within a laboratory greenhouse, which allowed him more control over the conditions in which the guppies were kept in
how did Endler investigate the evolution of spots on the number of guppies?
what does this experiment provide evidence for?
- he initially stocked 3 ponds with guppies and no predators
- despite variation, the mean number of spots was 10 - after 6 months, the mean number of spots increased to 11.8
- each pond was then given a particular condition:
- pond 1 - no species were added; this was a control condition to check whether the changes observed were due to predation
- pond 2 - non-guppy predators were added to look at whether the presence of other fish species affected the number of spots
- pond 3 - guppy predators were added; this was the experimental condition - after 5 months, the mean number of spots increased in pond 1 and 2 to 12.5 and 13, while in pond 3, the mean number of spots decreased to 10.5
- after a further 9 months, the ponds continued to diverge, with ponds 1 and 2 having a mean of 13 spots and pond 3 having a mean of 9.5 spots
- this experiment provided evidence for sexual selection driving an increase in number of spots for the times when no predator was present
- this experiment provided evidence for predation driving a decrease in the number of spots when the predator was present
which color of spot did not show evolution due to sexual selection?
yellow as there is a lower mean number of spots across both natural and artificial ponds compared to the other colors
- less spots is beneficial to hide away from predators
which color of spot did show evolution due to sexual selection and predation?
blue and iridescent as they have a higher mean number compared to other groups
- they also had mean number of spots decrease as predation levels increased
how did Endler investigate the evolution of size of spots on guppies?
- in the initial condition, there were no predators and 2 types of gravel; course and fine
- as there was no predation, the fish developed spots that were opposite to the gravel size so that they could be seen easily
- however, when predators were added, the fish developed spot sizes similar to the size of the gravel in order to camouflage
what is sexual selection?
the concept that many animals choose with whom they mate
what type of traits affects the success in attracting mates?
differences in physical and behavioural traits and can affect success in attracting a mate and reproducing, and so drive the evolution of an animal population
how does sexual selection acts as a selection pressure in animal species?
some traits are more preferred by mates as they can be used as signs of overall fitness, indicating their offspring would likely have the traits needed to survive also
- therefore sexual selection increases the changes of survival and reproduction of individuals with the favoured traits, meaning those organisms will leave more offspring and pass the allele(s) for the trait to those offspring
- the alleles become more frequent in the population
what is the basis for variation that will impact evolution?
- both physical and behavioral variations within a population have a genetic basis
- sexual selection occurs when one individuals has more reproductive success than another
why are these traits selected for?
differences in behavioral and physical traits may be preferred by mates as they can be used as signs of overall fitness, indicating their offspring would likely have the traits needed to survive also
what are behavioral trait examples?
an individual will only have enough energy to perform courtship behaviors such as a mating ritual dance (eg, Peacock spiders) or building a structure (eg, Bower birds) if they have gained enough resources to exceed their survival needs
why are these behavioral traits a suitable way of selecting a mate?
this is a suitable way of selecting mates as impressive behavioral traits indicate that the individual has good enough resources to exceed their survival needs, so are fit
