Isolation & speciation Flashcards
(16 cards)
How does selection affect allelic frequencies?
- any sexually mature individual in a population is capable of breeding with any other meaning that the alleges of any individual organism may be combined with the alleges of any other
- the Allele frequency is affected by selection and selection is due to environmental factors so environmental changes affect the probability of an allele being passed on in a population and hence the number of times it occurs within the gene pool
- environmental factors do not affect the probability of a particular mutant allege arising, they only affect the frequency of any mutant allele that is already present in the gene pool
What is speciation?
- speciation is the evolution of new species from existing ones
- a species is a group of individuals that have a common ancestry and so share the same genes but different alleges and are capable of breeding with one another to produce fertile offspring
- in other words, members of a species are reproductively separated from other species
- it is through the process of speciation that evolutionary change has taken place over millions of years
- this has resulted in great diversity of forms amongst organisms, past and present
what is adaptive radiation?
- the different phenotypes each combination of alleges produced will be subject to selection pressure that will lead to each population becoming adapted to its local environment
- this is known as adaptive radiation and results in changes to the allele frequencies (evolution) of each population, in other words, each population evolves
- as a result of these genetic differences it ,ay be that, even if the populations were no longer physically separated from one another, they would be unable to interbreed successfully
- each population would now be a different species, each with its own gene pool
What are allopathic speciation?
- allopathic means different countries and describes the form of speciation where two populations becomes geographically separates
- geographical separation may be the result of any physical barrier between two populations which prevents them inter breeding
- these barriers include oceans, rivers, mountain ranges and deserts
- what proves a barrier to one species may be no problem to another
- a tiny stream may be a barrier to snails, whereas the whole of the pacific ocean fails to separate populations of certain birds
- if environmental conditions either side of the barrier vary, then natural selection will influence the two populations differently and each will evolve leading to adoptions to their local conditions
- natural selection will then lead to changes in the allelic frequencies of each population which may take many hundreds or even thousands of generations, but ultimately may lead to reproductive separation and the formation of separate species
What is sympatric speciation?
-sympatric means same country and describes the form of speciation that results within a population in the same area leading to them becoming reproductively separated
What is geographical variation?
-populations are isolated by physical barriers such as oceans, mountain ranges, rivers etc.
What is ecological variation?
-populations inhabit different habitats within the dame area and so individuals rarely meet
What is behavioral variation?
-the breeding seasons of each population do not coincide and so they do not interbreed
What is mechanical variation?
- mating is often preceded by courtship, which is stimulated by the colour of markings of the opposite sex, the call or particular actions of a mate
- any mutations which cause variations in these patterns may prevent mating, for example, if a female stickleback does not respond appropriately to the actions of the male, he ceases to court her
What is gametic variation?
-anatomical differences may prevent mating occurring, for example, it may be physically impossible for the pens to enter the vagina of mammals
What is biochemical variation?
- the gametes may be prevented from meeting due to genetic of biochemical incompatibility
- for instance, some pollen grains fail to germinate or grow when they land on a stigma of different genetic makeup
- some sperm are destroyed by chemicals in the female reproductive tract
What does hybrid sterility variation produce?
- hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes
- for example in a cross between a horse 2n=64 and a donkey 2n=62 the resultant male has 63 chromosomes
- it is impossible for these chromosomes to pair up appropriately during mitosis and so the gametes formed are not viable and the mule is sterile
what is an example of allopathic speciation?
- an example of allopathic speciation is the Galapagos finch
- a single ancestral species is thought to have colonised one of the Galapagos islands
- in absence of competition is population increased and populations become established in other habitats on the same and different islands
- each population evolved adoptions to suit its new environment, including available food resources
- physiological and anatomical changes occur in this group e.g. different shapes and sides of beak to deal with different speed types
- being geographically separated from its mainland population, these changes have led to the various populations being so different that they can no longer interbreed and now form separate species
what is a likely example of sympatric speciation?
- a likely example of sympatric speciation taking place is the apple maggot fly
- originally this insect only laid its eggs inside the fruit of hawthorns, which are native to North America
- when apple treks were introduces, the fly started to lay its eggs in apples also
- females tend to lay their eggs on the type of fruit in which they developed and males tend to look for mates on they type of fruit in which they developed
- so flies raised in hawthorns usually mate with each other and flies raised in apples tend to mate with each other
- while the two types of flies are not yet separate species, mutations in each population have led to the evolution of genetic differences
- in time this could result in them being incapable of successfully breeding with one another and therefore being separate species
how does genetic drift affect large vs small populations?
- genetic drift is something that can take place in small populations because the relatively few members possess a smaller variety of alleges than the members of a large population so theres genetic diversity is less
- as these few individuals breed, the genetic diversity of the population is restricted to those few alleles in the original population so there is not an equal chance of each being passed on
- those that are passed on will quickly affect the whole population as their frequency will be high
- the effects of genetic drift will be greater and the population will change relatively rapidly, making it more likely to develop into a separate species
- in large populations the effect of a mutant allele will be diluted because its frequency is far less in the much larger gene pool so effects of genetic drift are likely to be less, and develop into a new species is likely to be slower
what is genetic drift?
-variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce.