Topic 1 - Speciation Flashcards
Define parapatric speciation
Speciation that occurs in a continuous population (i.e. not separated by an extrinsic barrier to gene flow) but divergence happens because of reduced gene flow between individuals in one part of the range and others in more distant parts of the population’s range.
What is a potential difficulty with sympatric speciation and to a lesser extent, parapatric speciation?
In both modes, there is no physical barrier to prevent mating between individuals and the exchange of alleles between them. Speciation must therefore occur in the face of gene flow which opposes any divergence that occurs. Both modes therefore require something other than a physical barrier to stop gene flow before populations can diverge sufficiently, so that reproductive isolation and speciation occurs.
List the evolutionary mechanisms which generate divergence.
Natural selection, sexual selection, random genetic drift & mutation.
Explain natural selection as an evolutionary mechanism.
The process by which heritable traits of organisms become more or less common in a population. Natural selection leads to evolutionary change when individuals with certain traits (characteristics) have a greater survival and/or produce more offspring (which inherit the trait) than organisms that lack these traits.
Explain sexual selection as an evolutionary mechanism.
A form of natural selection associated specifically with traits that increase an individual’s reproductive success even at the expense of its survival.
Explain random genetic drift as an evolutionary mechanism.
Evolution arising from random changes in the genetic composition (frequency of alleles or genotypes) of a population from one generation to the next.
Explain mutation as an evolutionary mechanism.
Mutation generates genetic variation on which natural selection, sexual selection and random genetic drift act.
List some possible methods of geographical separation which trigger speciation.
Deforestation splitting two areas of habitat, formation of mountain ranges or colonisation.
Define ring species
A situation in which two populations that do not interbreed are connected by a geographical ring of populations that can interbreed.
Explain ecological speciation
The evolution of barriers to gene flow resulting from ecologically-based divergent selection.
Define divergent natural selection
The evolution, by natural selection, of phenotypic differences between populations that exploit different environments.
Explain sensory drive in terms of speciation
The sensory drive hypothesis of speciation refers to the adaptation in sensory and signalling systems to different environments that results in premating reproductive isolation between populations.
Why is hybridisation between partially diverged populations referred to as being ‘maladaptive’?
Because hybrid offspring have low survival or reduced fertility compared with offspring resulting from matings between individuals from the same population.
Explain reproductive character displacement
The phenomenon that occurs where differences among similar species whose distributions overlap geographically so that they co-occur is greater compared to when their distributions do not overlap.
What is polyploidy?
When cells or organisms contain more than two paired (homologous) sets of chromosomes.
Many eukaryotic organisms are diploid. What does this mean?
They have two sets of homologous chromosomes, one inherited from each parent.
What is an autopolyploid?
Individual that exhibits ploidy (having more than 2 chromosome sets) that arises when multiple chromosome sets are derived from a single species. They can also arise by doubling of the genome so that all chromosomes are derived from a single parent.
What is an allopolyploid?
Individual that exhibits ploidy (more than two chromosome sets) resulting from interspecific hybridisation.
Define hybrid speciation
Speciation that results from hybridisation between two different species which leads to the formation of a new species that is reproductively isolated from both parent species.
What is homoploid hybrid speciation?
Occurs when a stable, fertile and reproductively isolated lineage results from hybridization between two distinct species, without a change in ploidy level. Reproductive isolation between a homoploid hybrid species and its parents is generally attained by chromosome rearrangements, ecological divergence or spatial isolation from the parental species.
Explain the process of fixation in speciation
Where an allele increases in frequency in a population to such an extent that all individuals carry only that allele (it reaches a frequency of 1) and other alleles are lost.
Define epistasis.
A specific kind of genetic interaction between genes at two or more loci, where the phenotype resulting from variation at each locus in combination differs from that which would be expected if the genes were expressed independently. Such interactions may involve one allele negating or enhancing the effects of other alleles or two or more alleles interacting to produce novel phenotypes.
Explain introgression
The movement of alleles from the gene pool of one species into the gene pool of another by repeated backcrossing of interspecific hybrids with one of the parental species.
What are homeobox genes
A large family of genes that direct the formation of many body structures during early embryonic development of multicellular organisms. Most homeobox genes encode proteins that act as transcription factors. They contain a sequence called a homeobox, which encodes a homeodomain, a part of the protein that binds to DNA and has a role in controlling its transcription and hence gene expression.
