7.3 Evolution may lead to speciation

Question 1

How and why do organisms vary?

Answer 1

• Mutations - new alleles of a gene
• Independent assortment and crossing over -> new combinations of allele
• Random fertilisation

Question 2

What is discontinuous variation?

Answer 2

Variation that falls into distinct categories
- Usually controlled by a single gene
- Environmental factors have little influence on the phenotype

Question 3

What is continuous variation?

Answer 3

Effected by genes and the environment

• Often polygenic
• Genes can set limits for the phenotype, but it is largely the environment that determines where, within those limits, an organisms lies.
• Therefore will have normal distribution curve

Question 4

How does evolution occur by natural selection?

Answer 4

• There are more organisms born than can survive.
• There is variation amongst the organisms. Some of them are better adapted to their environment than others
• These organisms compete for limited resources.
• The better adapted organisms out compete the less well adapted ones and so survive and breed, passing on the alleles for their advantageous characteristics.
• Over many generations the advantageous alleles become more common in the populations, whilst disadvantageous ones become less common

Question 5

What are the three factors that natural selection depend on?

Answer 5

Organisms must produce more offspring than can be supposed by the available resources
- Leads to intraspecific competition
There is genetic variation (different alleles) within the population
- This genetic variation leads to different phenotypes, which may be advantageous or disadvantageous

Question 6

Define selection pressure:

Answer 6

The environmental factors that limit the size of a population

Question 7

How does selections pressure affect allele frequencies?

Answer 7

• Allele for advantageous characteristic’s become more frequent
• Whilst disadvantageous characteristics become less frequent

Question 8

What is a disruptive selection?

Answer 8

• Selection against the mean
• Population becomes phenotypically divided – favours both extremes of phenotypes
• Most important type of selection for evolutionary change as
  could result in two separate species

Question 9

What is speciation?

Answer 9

• Formation of a new species from an existing species…
• Reproductive separation of two populations (of the same species) can result in the accumulation of differences in their gene pools
• New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring

Question 10

Why can’t different species interbreed?

Answer 10

They have too many genetic differences

Question 11

How does speciation take place?

Answer 11

• A population of organisms is split into two groups so that they cannot interbreed (they are reproductively isolated from each other).
• Different selection pressures (and genetic drift) are exerted on the two different populations, leading to changes in the gene pool
• Over time the two groups become genetically different.
• Eventually they become so different that they can no longer interbreed even if they are brought back together.

Question 12

What is allopatric speciation?

Answer 12

-Geographical isolation
= Separate gene pools; no interbreeding / gene flow (between populations)
- Mutations create genetic variation in each population
- Different selection pressures (e.g. predation, disease, competition) act on each population
- Leading to natural selection of different favourable alleles / characteristics
- Differential survival and reproductive success
- Leads to change of allele frequencies within gene pools (favourable allele increases) over a long time
- Members of different populations can’t interbreed to produce fertile offspring = new species arises from existing species = speciation

Question 13

What is sympatric speciation?

Answer 13

• Populations aren’t geographically isolated / population in the same area
• Genetic variation within the population due to mutations
• Resulting in a mechanism that makes individuals reproductively isolated (gene flow is restricted), for example…
• Different selection pressures operate
• Leads to change of allele frequencies within gene pools / divergence of gene pools
• Members of different populations can’t interbreed to produce fertile offspring = new species
  arises from existing species = speciation

Question 14

What is genetic drift?

Answer 14

Mechanism of evolution in which allele frequencies of a population change over generations due to chance

Question 14

Describe the process of genetic drift:

Answer 14

• Allele frequencies in a population change over time due to chance
• In each generation, some individuals may, just by chance, leave behind a few more descendants than other individuals
• The alleles of these “lucky” individuals become more common in the next generation
• Genetic drift happens in all population, but it’s effect is more marked in small populations