Trends in Skulls
This part of the skull has become more rounded and larger over time in hominins.
The brain case / cranium
These limbs became longer in relation to the arms as hominins adapted to walking upright.
The legs
This type of speciation happens when a population is separated by a physical barrier like a river or mountain.
Allopatric speciation
This term describes the process where one ancestral species rapidly gives rise to many new species, often linked with divergent evolution.
Adaptive radiation
This prezygotic barrier happens when two species use different mating calls or rituals.
Behavioural isolation
The name given to the first skeleton found of the extinct hominin species Australopithecus afarensis.
Lucy
As bipedalism evolved, this bone became more angled inwards to keep the body’s centre of gravity balanced when walking.
The femur
This type of speciation occurs without physical separation—organisms live in the same area but evolve differently due to other factors.
Sympatric speciation
This type of evolution occurs when unrelated species develop similar traits due to similar environmental pressures.
Convergent evolution
This barrier occurs when physical differences prevent mating between species.
Mechanical / structural isolation
The name for the hole in the base of the skull through which the spinal cord passes.
Foramen magnum
Unlike apes, hominins developed a pelvis shaped like this, making upright walking more efficient.
Short and broad
Though both lead to the formation of new species, this key factor distinguishes allopatric from sympatric speciation.
Geographic isolation / barrier
This model of evolution suggests that species change slowly and steadily over long periods of time.
Gradualism
This type of isolation occurs when species live in the same area but breed at different times of the year or day.
Temporal isolation
This feature, once used to anchor large jaw muscles, has reduced or disappeared in modern humans.
The sagittal crest
This major trade-off of bipedalism made childbirth more difficult for hominins.
Narrower birth canal
This is one cause of sympatric speciation in plants, where the number of chromosomes suddenly changes.
Polyploidy
This type of evolution occurs when a common ancestor gives rise to different species with different traits.
Divergent evolution
This form of isolation occurs when sperm and egg from different species can't fuse to form a zygote.
Gametic isolation
The face of hominins has become this shape over time, becoming less protruding.
Flatter / less prognathic
This term refers to the inward angle of the femur that brings the knees closer together, helping hominins walk upright efficiently.
The valgus angle.
This error in cell division results in gametes with too many or too few chromosomes and can lead to instant speciation via polyploidy.
Non-disjunction
This model proposes that species stay the same for long periods, with short bursts of rapid change.
Punctuated equilibrium
This postzygotic barrier occurs when offspring of two species are produced but cannot reproduce themselves.
Hybrid sterility