C&C Ch. 6

Question 1

why do different species that coexist in the same locality remain distinct?

Answer 1

because they do not interbreed

Question 2

define reproductive isolation

Answer 2

lack of interbreeding

Question 3

what would happen if there were no barriers to interbreeding between members of different species?

Answer 3

the diversity of life that we see in the world could not exist, and there would be something approaching a continuum of forms

Question 4

what happens when barriers to interbreeding between formerly separate species are broken down?

Answer 4

highly variable offspring are rpdocued

Question 5

state reasons why there may be a barrier preventing closely related species from interbreeding

Answer 5

1. difference in habitats
2. difference in time of breeding of the species
3. use of different breeding sites
4. subtle features of organisms
5. behavioural barriers
6. complete inviability or sterility of hybrids

Question 6

give an example of different times of breeding of species

Answer 6

in plants species with non-overlapping flowering times will be unable to interbreed.

Question 7

give an example of different breeding sites

Answer 7

animals

Question 8

give an example of subtle features of organisms which may prevent interbreeding

Answer 8

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

Question 9

give an example of behavioural barriers to mating

Answer 9

plants have chemical means of detecting pollen from the wrong species and rejecting it

Question 10

explain inviability or sterility of hybrids

Answer 10

• crosses often die at an early stage of development
• even when hybrids are viable, they are often sterile

Question 11

why is it unlikely that the inviability/infertility of interspecies hybrids could be the direct product of natural selection?

Answer 11

there can be no advantage to an individual produce inviable/sterile offspring if hybridised with a different species

Question 12

what is the most likely explanation for the creation of barriers to interbreeding between species?

Answer 12

evolutionary changes that occurred after the populations became isolated from each other by being geographically or ecologically separated

Question 13

under what processes will the genetic compositions of two populations diverge?

Answer 13

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

Question 14

define geographical variation

Answer 14

the tendency of populations of the same species to differ according to their geographical location, often in ways which are clearly adaptive

Question 15

give examples of geographical variation among the human race

Answer 15

numerous minor physical differences between the races, as well as the smaller local differences in features such as skin pigmentation and stature

Question 16

describe how migration affects divergence

Answer 16

• 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

Question 17

what affects the amount by which a species will diverge?

Answer 17

the amount of migration

Question 18

why do gradual geographical changes in traits usually arise?

Answer 18

because migration blurs the differences caused by selection that varies geographically

Question 19

differences between geographically separate populations of the same species do not necessarily require

Answer 19

different types of selection. the same selection can sometimes lead to different responses (eg humans and malaria)

Question 20

on the theory of evolution, there must be —– —- in the development of reproductive isolation

Answer 20

intermediate stages

Question 21

give an example of intermediate stages in the evolution of complete inability to interbreed between geographically separated populations

Answer 21

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

Question 22

use Limulus lewisii and M. cardinalis as an example of failure to reproduce between species

Answer 22

• 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

Question 23

why is it that different species contain sets of genes which are dysfunctional when brought together in hybrids?

Answer 23

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

Question 24

theoretical models, as well as laboratory experiments, show that intense selection can produce profound changes in a trait over —– generations or less

Answer 24

100

Question 25

what two things is strength of reproductive isolation not related two?

Answer 25

• differences in easily observable characteristics between a pair of populations // strength of isolation
• the extent of differences between species // time since isolation

Question 26

when will there be a constancy in traits that an organism displays?

Answer 26

once a species living in a stable environment has had time to adapt to it

Question 27

how do scientists classify species when here is asexual reproduction? here, the criterion of interbreeding is meaningless

Answer 27

they use arbitrary measures of similarity, either based on characters of practical importance (eg composition of bacterial cell walls), or on DNA sequence differences

Question 28

between closely related species, most changes in the coding sequences of genes involve

Answer 28

single changes to individual letters of the DNA sequence

Question 29

how can we quantify level of divergence between species precisely?

Answer 29

comparing the numbers of letters in the DNA by which the sequences of the same gene differ between a pair of organisms

Question 30

replacement changes

Answer 30

differences in the genetic code which alter the protein sequence

Question 31

silent changes

Answer 31

differences in the genetic code which do not alter the protein sequence

Question 32

how does a silent mutation spread through the population if it does not confer any selective advantage to its carrier?

Answer 32

random changes in the frequencies of alternative variants (genetic drift) take place in finite populations

Question 33

describe how genetic drift takes place

Answer 33

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

Question 34

why are smaller populations more susceptible to genetic drift?

Answer 34

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.

Question 35

two effects of genetic drift

Answer 35

• 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

Question 36

at what rate does genetic drift happen?

Answer 36

it is a very slow process; its rate depends on the rate at which new neutral mutations arise (which is faster in larger populations)

Question 37

how can we calibrate the molecular clock?

Answer 37

one needs the sequences from the closest available species whose divergence dates are known

Question 38

why is the molecular clock very slow?

Answer 38

the rate at which single letters in the DNA change by mutation is very low

Question 39

what else can a molecular clock be applied to, other than DNA differences?

Answer 39

protein sequences