Evolution happens not in a single individual but in a group of individuals — a population. The genetic make-up of a population changes from generation to generation and eventually leads to a new species. This process is called microevolution. The exam asks about species criteria, population characteristics, the elementary factors of evolution and the ways species form, and sometimes includes a Hardy–Weinberg calculation.
§1. The species and its criteria
A species is a group of individuals that are similar in structure, interbreed freely and produce fertile offspring, occupy a particular area (range) and are reproductively isolated from other species.
Species criteria: morphological (external and internal structure), physiological (similar life processes, ability to interbreed), genetic (number and structure of chromosomes — the karyotype), biochemical (composition of proteins and nucleic acids), ecological (ecological niche), geographical (range).
No single criterion is enough to identify a species; they are used together:
Sibling species look almost identical but do not interbreed (for example, black rats with different chromosome numbers, several species of malaria mosquito) — the morphological criterion is not enough.
Polymorphism and sexual dimorphism — members of one species may look very different.
The ranges of many species overlap — the geographical criterion is not enough either.
Some interspecific hybrids (the mule, from a horse and a donkey) are born but are usually sterile.
§2. The population — the elementary unit of evolution
A population is a group of individuals of one species that live for a long time in a particular area, interbreed freely, and are relatively isolated from other populations of that species.
Characteristics of a population:
size and density (number of individuals per unit area or volume);
birth rate and death rate;
age and sex structure;
spatial structure (how individuals are distributed in the area);
gene pool — the sum of the genes of all individuals in the population.
The population is the elementary unit of evolution: an individual’s genotype does not change during its life, and a species is too large and complex a system. It is the population’s gene pool that changes from generation to generation — the elementary evolutionary event.
The merging of Darwinism and genetics in the first half of the 20th century produced the synthetic theory of evolution; S. S. Chetverikov made a major contribution to population genetics.
§3. The elementary factors of evolution
The mutation process supplies new alleles — the raw material of evolution. Mutations are random and undirected.
Population waves (S. S. Chetverikov) — periodic fluctuations in population size (for example, sharp rises in the numbers of hares or mouse-like rodents in some years, followed by crashes). When numbers fall, some alleles disappear or become common by chance.
Genetic drift — random change in allele frequencies in small populations. When a few individuals settle a new area (an island, say), the new population’s gene pool differs from the original by chance — the founder effect.
Isolation — barriers to interbreeding between populations. Geographical (mountains, rivers, seas, deserts) and biological: ecological (different habitats or breeding seasons), behavioural (courtship rituals), morphological (differences in reproductive organs), genetic (sterility of hybrids).
Natural selection — the only directing factor: it preserves well-adapted genotypes and raises their frequency.
Mutations, population waves and genetic drift change the gene pool randomly; only natural selection gives these changes a direction.
The Hardy–Weinberg law: in a large, freely interbreeding population with no mutation, selection or migration, allele frequencies stay constant from generation to generation. If the frequency of allele A is p and of allele a is q, then p + q = 1 and p² + 2pq + q² = 1, where p² is the frequency of AA, 2pq of Aa, and q² of aa.
Worked example
Individuals with a recessive trait (aa) make up 16% of a population. Find the proportions of dominant homozygotes and heterozygotes.
The key condition for a new species to form is the appearance of reproductive isolation between populations.
Allopatric (geographical) speciation — the range is split by natural barriers, or the species spreads into new areas; different mutations accumulate in the isolated populations and selection acts in different directions. Examples: subspecies of lily of the valley, whose range was broken up during the Ice Age; Darwin’s finches on the Galápagos Islands.
Sympatric (ecological) speciation — within one range, populations occupy different ecological conditions or breed at different times. Example: races of Sevan trout that spawn at different times and places; early- and late-flowering races of plants in frequently mown meadows.
Rapid (abrupt) speciation by polyploidy — individuals with a multiplied chromosome number are immediately reproductively isolated from the parent species; this is common in plants.
Worked example
After a mountain range rose, a population of one lizard species was split into two groups. Many thousands of years later, when they met, they could not interbreed. What kind of speciation is this, and which factors acted?
The barrier is mountains → geographical isolation.
Mutations accumulated independently in the separated populations, selection acted in different directions in different conditions, and drift operated in small groups.
As a result, reproductive isolation arose.
Answer: allopatric (geographical) speciation.
§5. Mechanisms of reproductive isolation
Species living in the same area do not merge, because several “barriers” separate them. These fall into two groups.
Before a zygote forms (they prevent mating itself):
ecological — different habitats (one species in water, another in drier places);
temporal (seasonal) — different breeding or flowering times;
behavioural — different courtship behaviour, songs, displays or scent signals (birds, frogs);
mechanical — reproductive organs or flowers do not fit;
gametic — gametes do not “recognise” each other (important in species with external fertilisation in water).
After a zygote forms:
death of the hybrid embryo;
hybrid sterility — a mule (horse × donkey) is viable, but in meiosis chromosomes from the two species cannot pair, so almost no gametes form;
weakness of second-generation hybrids.
Once reproductive isolation has arisen, two populations become separate species: their gene pools no longer mix.
Sometimes natural selection itself strengthens isolation: if hybrids survive poorly, individuals that mate with their “own” kind leave more offspring, and the differences deepen.
§6. Estimating population size and population dynamics
When a population cannot be counted completely, its size is estimated by sampling:
quadrat sampling — individuals are counted in several small quadrats and the mean density is multiplied by the whole area (plants, slow-moving animals);
mark–recapture — for mobile animals: N = M × C / R (M — marked, C — caught the second time, R — marked among them).
Age structure shows a population’s future: a large share of young individuals means growth, a large share of old ones means decline. With unlimited resources numbers grow fast — exponentially; with limited resources growth slows and levels off near the carrying capacity.
Worked example
In a forest, 10 quadrats of 1 m² contained 45 mushrooms in total. The forest covers 2 ha. Estimate the number of mushrooms.
Mean density: 45 / 10 = 4.5 per m².
2 ha = 20 000 m².
Population: 4.5 × 20 000 = 90 000.
Answer: about 90 000 (the estimate is reliable if the quadrats were placed at random).
Worked example
40 frogs in a pond were marked and released. Later 60 frogs were caught, 8 of them marked. What is the population size?
N = M × C / R.
N = 40 × 60 / 8 = 300.
Answer: about 300 frogs.
Check yourself
Why is the morphological criterion alone not enough to identify a species?
What is the elementary unit of evolution?
Which factor of evolution is the only directing one?
In which populations is genetic drift strongest?
The frequency of allele a is 0.3. What proportion of the population is heterozygous?
A new species arose from populations in one range that breed at different times. What kind of speciation is this?
Why is a mule sterile?
Different songs in two bird species are an example of which kind of isolation?
Common mistakes
Treating the individual or the species as the unit of evolution — the elementary unit is the population.
Calling genetic drift a directed process — it is random.
Identifying a species by a single criterion.
Confusing allopatric and sympatric speciation: allopatric involves a geographical barrier; sympatric happens within one range.
In Hardy–Weinberg problems, taking q instead of q² as the frequency of the recessive phenotype.
Treating mutations as the directing factor of evolution — they only supply the material.