speciation and evolution

define evolution

change in allele frequency over time in a population occurring through the process of natural selection

the principle of natural selection

1. Selection pressure (competition, disease, etc)

2. Variation within a population is already present due to MUTATIONS

3. which creates a new ALLELE for …

4. which provides individuals with a particular CHARACTERISTIC

5. These individuals are at a selective ADVANTADGE

6. they’re more likely to SURVIVE and REPRODUCE more often (whereas others are not)

7. this passes on the allele for … to the OFFSPRING more often, and its FREQUENCY increases over many generations

what is genetic drift?

the mechanism of evolution in which allele frequencies in a population change over generations due to chance and not natural selection

explanation of genetic drift

some alleles are passed to offspring more often, regardless of selection pressures and advantadges, due to chance e.g. in small populations with no interbreeding with other populations because the gene pool is small and chance has a greater influence

what is gene flow?

different alleles move between populations when individuals from one populations migrate to another and reproduce

things that decrease genetic diversity

1. genetic bottlenecks/bottleneck effect - population is sharply reduced in size

2. founder effect - a small. new colony forms from a main population

3. selective breeding

how speciation occurs

when a difference in gene pools occurs due to reproductive seperation of 2 populations, new species arise when these genetic differences lead to an inability to produce fertile offspring when interbred

allopatric speciation (GEOGRAPHICAL ISOLATION)

1. population is split due to a physical barrier

2. leading to reproductive isolation, seperating gene pools by preventing interbreeding and gene flow between population

3. random mutations had caused genetic variation within each population

4. so different selection pressures act on each population

5. and different alleles are at a select advantadge and are passed on

6. so the allele frequencies within each gene pool changes over many generations, eventually leading to the inability to produce fertile offspring

types of reproductive isolation

1. behavioural

2. mechanical

3. temporal

Sympatric speciation (NO GEOGRAPHICAL ISOLATION)

1. population not geographically isolated, but mutations had led to reproductive isolation

2. which seperates gene pools by preventing interbreeding and gene flow within the same population

3. so different selection pressures act on each population

4. and different advantadgeous alleles are selected for (disruptive selection)

5. so allele frequencies change within each gene pool over may generations

6. eventually leading to inability to produce fertile offspring once interbred.

examples of factors which can reproductively isolate populations

1. differing courtship behaviour

2. differing body size and shape

3. differing selection pressures

4. differing breeding seasons

5. gamete incompatibility