Lec 6 slides and reading Flashcards
reproductive isolation
Different species that coexist in the same locality remain distinct because they do not interbreed
This lack of interbreeding–> reproductive isolation
geographical variation
Tendency of populations of the same species to differ according to their geographical location, often in ways that are clearly adaptive → geographical variation
how do species in the same area remain distinct
Different species that coexist in the same locality remain distinct because they do not interbreed
Lack of interbreeding (reproductive isolation) is the best criteria for defining different species
More complex with organisms that don’t reproduce by sexual matings (microbes)
how do barriers to interbreeding affect evolution
Division of living organisms into discrete species is so familiar but not an obviously necessary state of affairs
If there were no barriers to interbreeding between members of different species, diversity of life could not exist
When barriers to interbreeding between formerly separate species have broken down, highly variable offspring are produced
Problem for evolutionists is to explain how species become distinct and why reproductive isolation exists
ways closely related species are prevented from interbreeding
Sometimes main barrier is a simple difference in habitats or in the time of breeding of the species
In plants, there’s a characteristic brief flowering time each year, and species with non-overlapping flowering times will be unable to interbreed
In animals, the use of different breeding sites can prevent individuals from different species from mating with each other
Subtle features of organisms(can only tell from species’ history), prevents individuals from different species from mating (even if in same place)
E.g. there may be an unwillingness to court individuals of the other species because they don’t produce the right smell or sound
Behavioural barriers to mating are obvious in many animals, and plants have chemical means of detecting pollen from the wrong species and rejecting it
Even if mating takes place, sperm from wrong species may be unsuccessful in fertilizing female eggs
how does sterility/inviability affect evolution
Some species are closely related that they occasionally mate (especially if given no choice of a member of their own species)
In many situations first gen hybrids often fail to develop; experimental crosses between individuals belonging to different species often produce hybrids that die at early stages of development
Whereas most offspring of crosses between individuals of same species develop to maturity
Sometimes hybrid individuals can survive but at much lower frequency than non-hybrids
Even when hybrids are viable, they are often sterile, and don’t produce offspring that can pass genes onto future gens (mules→hybrid b/w donkey and horse)
Complete inviability or sterility of the hybrids obviously isolates the 2 species
evolution of barriers to interbreeding
Most barriers to interbreeding between species are by-products of evolutionary changes that occurred after the populations became isolated from each other by being geographically or ecologically separated
-E.g. Darwin’s finches, if a small amount of individuals fly across to island and establish a new pop, the new and ancestral populations will evolve independently of each other
Under the processes of mutation, natural selection and genetic drift, the genetic compositions of the 2 populations will diverge
These changes will be promoted by differences in the environments experienced by the populations, to which they become adapted
E.g. Food plants available to a seed-eating species of bird differ from islands, and even members of the same species of finch differ in beak sizes→ differences in food abundance
geographical variation
The tendency if populations of the same species to differ according to their geographical locations (in ways that are adaptive) → geographical variation
E.g. physical differences between races (skin pigmentation)
