C&C Ch. 6 Flashcards
why do different species that coexist in the same locality remain distinct?
because they do not interbreed
define reproductive isolation
lack of interbreeding
what would happen if there were no barriers to interbreeding between members of different species?
the diversity of life that we see in the world could not exist, and there would be something approaching a continuum of forms
what happens when barriers to interbreeding between formerly separate species are broken down?
highly variable offspring are rpdocued
state reasons why there may be a barrier preventing closely related species from interbreeding
- difference in habitats
- difference in time of breeding of the species
- use of different breeding sites
- subtle features of organisms
- behavioural barriers
- complete inviability or sterility of hybrids
give an example of different times of breeding of species
in plants species with non-overlapping flowering times will be unable to interbreed.
give an example of different breeding sites
animals
give an example of subtle features of organisms which may prevent interbreeding
there may be an unwillingness to court individuals of the other species, because they do not produce the right smell or sound, or their courtship displays may differ
give an example of behavioural barriers to mating
plants have chemical means of detecting pollen from the wrong species and rejecting it
explain inviability or sterility of hybrids
- crosses often die at an early stage of development
- even when hybrids are viable, they are often sterile
why is it unlikely that the inviability/infertility of interspecies hybrids could be the direct product of natural selection?
there can be no advantage to an individual produce inviable/sterile offspring if hybridised with a different species
what is the most likely explanation for the creation of barriers to interbreeding between species?
evolutionary changes that occurred after the populations became isolated from each other by being geographically or ecologically separated
under what processes will the genetic compositions of two populations diverge?
mutation, natural selection, and genetic drift:
- these changes will be promoted by differences in the environments experienced by the populations, to which they will become adapted
define geographical variation
the tendency of populations of the same species to differ according to their geographical location, often in ways which are clearly adaptive
give examples of geographical variation among the human race
numerous minor physical differences between the races, as well as the smaller local differences in features such as skin pigmentation and stature
describe how migration affects divergence
- if migrant individuals can interbreed with members of the population in which they arrive, they will contribute their genes to the population
- migration is therefore a homogenising force, opposing the tendency for local populations to diverge genetically by selection/genetic drift
what affects the amount by which a species will diverge?
the amount of migration
why do gradual geographical changes in traits usually arise?
because migration blurs the differences caused by selection that varies geographically
differences between geographically separate populations of the same species do not necessarily require
different types of selection. the same selection can sometimes lead to different responses (eg humans and malaria)
on the theory of evolution, there must be —– —- in the development of reproductive isolation
intermediate stages
give an example of intermediate stages in the evolution of complete inability to interbreed between geographically separated populations
Drosophila pseudoobscura
- lives on west coast of North and Central America, continuously from Canada to Guatemala
- there is an isolated population living near Bogotá, in Colombia
- these look identical to those from other populations but DNA sequences differ slightly.
- when they mate, first-generation hybrid females are fully fertile, but hybrid males from cross w non-Bogotá mothers are sterile
use Limulus lewisii and M. cardinalis as an example of failure to reproduce between species
- two monkey flower species can be crossed experimentally, and the hybrids are healthy and fertile, yet in nature the species grow side by side without intermingling
- bees and hummingbirds (pollinators) rarely visit both
- changes driven by natural selection as hummingbird pollination evolved could have led to reproductive isolation
why is it that different species contain sets of genes which are dysfunctional when brought together in hybrids?
there is no selection to maintain compatibility of mating behaviour between individuals from geographically or ecologically separated populations, or to maintain interactions that allow normal development, between genes that have come to differ in different populations
theoretical models, as well as laboratory experiments, show that intense selection can produce profound changes in a trait over —– generations or less
100
what two things is strength of reproductive isolation not related two?
- differences in easily observable characteristics between a pair of populations // strength of isolation
- the extent of differences between species // time since isolation
when will there be a constancy in traits that an organism displays?
once a species living in a stable environment has had time to adapt to it
how do scientists classify species when here is asexual reproduction? here, the criterion of interbreeding is meaningless
they use arbitrary measures of similarity, either based on characters of practical importance (eg composition of bacterial cell walls), or on DNA sequence differences
between closely related species, most changes in the coding sequences of genes involve
single changes to individual letters of the DNA sequence
how can we quantify level of divergence between species precisely?
comparing the numbers of letters in the DNA by which the sequences of the same gene differ between a pair of organisms
replacement changes
differences in the genetic code which alter the protein sequence
silent changes
differences in the genetic code which do not alter the protein sequence
how does a silent mutation spread through the population if it does not confer any selective advantage to its carrier?
random changes in the frequencies of alternative variants (genetic drift) take place in finite populations
describe how genetic drift takes place
The sampling process that occurs during fertilization occurs in every population in every generation in every species.
- happens if a new trait is neutral (individuals can have the same no of offspring)
- some individuals with the new trait may have no offspring, while others may happen by chance to have more
- frequency of mutant gene in the progeny generation will not be the same as its frequency among the parents
- over the generations, there will thus be continual random fluctuations in the composition of the population until sooner or later either all members have the gene or it has been lost
why are smaller populations more susceptible to genetic drift?
When producing gametes, it is entirely random as to which gets fertilized. With smaller numbers however, the chance that this causes change increases. With smaller numbers you are very likely to get larger variations in chance and thus more genetic drift.
Larger populations average these differences out.
two effects of genetic drift
- whilst a new variant is drifting to eventual loss or to a frequency of 100% (fixation), the character affected by the gene is variable within the population.
- a selectively neutral variant that is initially very rare has some chance of spreading throughout the whole population. this leads to evolutionary divergence between isolated populations
at what rate does genetic drift happen?
it is a very slow process; its rate depends on the rate at which new neutral mutations arise (which is faster in larger populations)
how can we calibrate the molecular clock?
one needs the sequences from the closest available species whose divergence dates are known
why is the molecular clock very slow?
the rate at which single letters in the DNA change by mutation is very low
what else can a molecular clock be applied to, other than DNA differences?
protein sequences
